Cable and Wire Harness Assembly Handbook Ground Rules · IPC Midwest September 2008 Cable and Wire Harness Assembly Handbook Ground Rules: 1) Why do we do what we do 2) Technical

IPC Midwest September 2008

Cable and Wire Harness Assembly Handbook Ground Rules: 1) Why do we do what we do 2) Technical History 3) Some How, but only for that which SHOULD not be in the standard 4) Further explain the processes already in the standard 5) Lessons Learned to be as part of each section it applies to. 6) Not be a J/STD-001 or A-610 book, more like the A&J Handbook or Conformal Coating but referencing back to the standard to

see the end results. 7) Structure to follow TECHNOLOGY 8) Include ancillary technology. 9) Audience – WIRE AND CABLE MANUFACTURES and others who are craving information, Designer, ME, QA, Assemblers. 10) This is stand alone with the A620. 11) If the Information for a section resides in another document it’s OK to reprint into this document. Must add statement that if

they want to learn more than what is applicable to this document you see the base reference. 12) Always think about new technology insertion 13) Pb-Free issues. 14) Include basic material types and use selection information in each section. 3 TYPES OF HARNESS (INFO FOR INTRO SECTION) Type 1 - Unprotected Harness No overbraiding, jacketing etc, held together by lacing/ties Type 2 - Protected Harness Has Jacket/Cloth overbraiding, no overall shielding Type 3 - Protected/Shielded Harness Jacketed/Cloth overbraid and overall shielded


TABLE OF CONTENT for IPC-HDBK-620 1 Cable and Wire Harness Assembly Handbook $Scope $Purpose $Approach To This Document $Uncommon or Specialized Designs $Terms And Definitions #Shall or Should #Classes of Product #Document Hierarchy #Tool and Equipment Control #Observable Criteria #Defects and Process Indicators #Inspection Conditions #Measurement Units and Applications #Verification of Dimensions #Visual Inspection #Contamination #Materials and Processes

Subjects marked with ‘$’ are to be looked at and where necessary expand or explain the category. Those subjects marked with ‘#’ will be rewritten to agree with actual handbook use and ground rules. Teresa Rowe?

Covers Section 1

2 APPLICABLE DOCUMENTS IPC STAFF Covers Section 2 3 Cable and Wires Layout and Length measurement

wire measurement termination type measurement points and

process strip allowances. wire types and selection of (important) <Apr

08>

Brett Miller – USA Harness<Sept 08> John Laser – L3 <Sept 08>

Covers Section 11

4 Wire Preparation Stripping Tinning

Richard Rumas – Honeywell <Apr 08> Covers Sections 3, 4, 13.1

5 Wire Termination Methods a) Mechanical Crimp Terminations (Contacts and Lugs) Insulation Displacement Connection (IDC) b) Thermal Splices (including hot air melt) Soldered Terminations (solder cups, turret,

pierced, J-Hook) c) Other Technology Ultrasonic Welding Wire Wrap d) Coaxial and Twin Axial Cable Assemblies

Subsection & Author a)Richard Rumas – Honeywell <Apr 08> d))John Laser – L3 <Apr 08> c)Brett Miller – USA Harness <Sept 08> b)Teresa Rowe ????

Covers Sections A) Section 5, 6, 8 B) Section 4, 8 C) Section 7, 18 D) Section 13

6 Connectorization Richard Rumas – Honeywell <Apr 08> Covers Sections 9, 13 7 Molding/Potting Brett Miller – USA Harness <Sept 08>

Gordon Sullivan <Sept 08> Covers Section 10

8 Marking/Labeling Les Bogart – Bechtel <Apr 08> Covers Section 12 9 Securing

Lacing tape vs plastic types knots, anti-knot loosening location of ties, use of lacing tape as securing of tapes and sleeving

Randy McNutt - Northrop Covers Section 14

10 Harness/Cable EMI/RFI Shielding EMI/RFI shielding theory methods of shielding shield jumpers [with how to of using shrink

sleeves in wire term methods/splices)

John Laser – L3 <Sept 07> Covers Section 13, 15


11 Harness Jacketing Methods Mechanical Braiding (discussions of machines

and braiding materials). Open loose Sleeving Closed Sleeving Tapes

Randy McNutt - Northrop Covers Sections 15, 16

12 Finished Assembly Installation wire routing rules box installations rules hardware termination Terminal Blocks clamping ESD caps

Les Bogert <Sept 07> Covers Sections 14, 17

13 Measurement/Testing Les Bogert<Sept 07> Section 19 14 METHODS OF STD REPAIR & MODIFICATION

(proposed) On Hold NEW

APP A A-620A to this HDBK pointer Chart Leadership prior to publishing

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This handbook is a companion reference to and was prepared using IPC/WHMA-A-620A. Format of this Handbook The section and paragraph numbers in this handbook refer and correspond to the section and paragraph numbers in Revision A of IPC/WHMA-A-620. Where used verbatim, text of IPC/WHMA-A-620 is identified by being boxed. Foreword 1.1 Scope The scope of the IPC/WHMA-A-620A provides visual, electrical and mechanical acceptability requirements. This document can be used by manufacturers or as a stand-alone for purchasing products. Activities such as in-process and end product inspection are not defined in the document. 1.2 Purpose This document does not address assembly methods. 1.3 Approach to This Document The document is organized such that the title of each section includes the criteria for that topic. In some cases, the same or similar figures are shown throughout the document, and the user is advised to select the correct section when reading the document. When product is compliant to Class 3, the manufacturers are required to use a documented process control system. Documentation may occur in any format compliant with a user’s internal requirements. For all Class 3 product and where a document process control system is used for Class 2, process control and corrective action limits are required. There is no requirement for Class 2 to have a documented process control system, however when one exists, these additional requirements apply. The focus in this section is on the process control. As stated in the document, there is no requirement for a statistical process control system. The concern is about managing the processes to produce hardware that meets the requirements. The user may decide that statistical process control is necessary for a particular situation, and in these situations, they may select this as the type of process control system to use. Class 2 and Class 3 manufacturers are required to use process control methodologies in the planning, implementation and evaluation of the processes. Unlike the earlier requirement in this section that may result in documentation depending on the product class, the approach to using process control methodologies is more of a technique used to achieve an end result. 1.4 Shall or Should The word “shall” is used throughout the document for mandatory requirements. In some cases, the requirement is applicable to all classes as a process-related requirement, but in some cases, the requirement is not applicable to all Classes. In each case, a text box with the associated requirement is listed near the paragraph in which the word appears. Each Class requirement is stated in the text box, and the user will select the appropriate class to determine whether a hardware defect exists for the relevant product. Conditions such as “Defect,” “Process Indicator,” Acceptable” and “Not Established” could be stated in the text boxes. Where the condition is “Acceptable,” no further action is required by the user when the condition exists. Where the condition is “Not Established,” the document does not provide any criteria. If a condition exists where the criteria is “Not Established,” the user is encouraged to determine if additional action is necessary for the particular product. Where it is used, the word “should” is providing guidance to the user. Even though no requirement exists, the document developers provide this as useful information to the users. 1.5 Uncommon or Specialized Designs The document developers recognize that industry consensus documents typically address common technologies. There may, however, be times when the user needs to have additional requirements definition for particular applications. Users are encouraged to develop these additional criteria for their application and to include the definition for acceptance of each characteristic. Users are also encouraged to provide this information, where feasible, to the IPC Technical Committee for consideration in future revisions of the document.

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1.6 Terms and Definitions Definitions for some of the terms used in the document can be found in the Terms and Definitions section of the standard or in Appendix A of the standard. 1.7 Classes of Product The product classes are provided in this section of the standard. Definition of the product class is necessary in order to determine which requirements are applicable. The standard provides guidance to the manufacturer where the manufacturer and user have not established the product class requirements. In these situations, the manufacturer is permitted to select the product class. 1.8 Document Hierarchy There are many documents that may be invoked when manufacturing this product. The document hierarchy or order of precedence is established in this section of the standard. There are various standards available to the industry that include topics also discussed in the IPC/WHMA-A-620A, including J-STD-001, “Requirements for Soldered Electrical and Electronic Assemblies” and IPC-A-610, “Acceptability of Electronic Assemblies.” IPC/WHMA-A-620A users are not required to use these documents unless contractually required. Although information provided may appear to be similar, the documents have different scope statements and conflicts with the requirements of IPC/WHMA-A-620A may be introduced it the documents are used incorrectly. The user does have the option to select alternate acceptance criteria. Where such criteria are specified, however, procurement documentation must also include the order in which the documents are used. This provides a standard hierarchy that, where a conflict exists, eliminates confusion on which takes precedence. 1.9 Tool and Equipment Control Manufacturers are required by the standard to have tool and equipment control processes in place. This is to ensure that tools are in good working condition and are used as intended. These processes are in addition to calibration requirements that are also defined in this section. Manufacturers are required to have a documented calibration system as stated in the standard. Where a National or International standard other than ANSI/NCSL Z540-1 is used, the standard selected for the calibration system is required to meet minimum criteria as established in IPC/WHMA-A-620A. 1.10 Observable Criteria This document establishes acceptance criteria for the subject matter. Measurements are not typically required, however, they may be made to supplement an inspection. There is no requirement for this. Not every condition stated in the standard can be shown in the figures provided. Many times, the conditions shown are worse-case conditions in order to over-emphasize the condition. This is an aid to the user of the standard in understanding the requirement as stated. Hence, the written requirements always take precedence over the figures in the standard. 1.11 Defects and Process Indicators Defects are defined in the standard as conditions that fail to meet the acceptance criteria of the document and affect form, fit or function of the assembly in its end use environment. Process indicators also fail to meet the acceptance criteria, but they do not affect the form, fit or function of an assembly. Since process indicators do not affect the form, fit or function of an assembly, disposition is not required. The recommendation, however, is that process indicators be monitored. Since a process may be unique to a manufacturer or type of product, there are situations where defects or process indicators may exist that are not listed in the standard. The manufacturer is responsible for identifying those situations. In many cases, the user is more knowledgeable of the product and its end use. For this reason, the user is tasked with the responsibility for identifying any defects that are unique to the product. 1.12 Inspection Conditions When inspecting a product, the inspector will need to know the product class of the product under inspection in order to appropriately evaluate the product. The standard requires documentation be provided to the inspector which identifies the product class and states the inspector can not select it.

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1.12.1 Target Definition 1.12.2 Acceptable Definition 1.12.3 Process Indicator IPC/WHMA-A-620A requires processes for Class 3 products where the number of process indicators indicates an abnormal variation in the process, an undesirable trend or conditions that indicate the process is nearing or is out of control to be analyzed. This implies that the number of process indicators be counted as part of the process control system even though no requirement for this exists. The manner in which these situations are identified is the responsibility of the manufacturer. 1.12.4 Defect Manufacturers are required to document and disposition each defect for all three product classes. There is no requirement for when this documentation is to be prepared, and the manufacturer is responsible for determining this as part of process definition. 1.12.5 Disposition The most common dispositions, i.e., the way the product with defects is handled, are rework, repair, scrap and use-as-is. Where the disposition is “repair,” Class 3 manufacturers are required to conduct the repairs in accordance with documented procedures. 1.12.6 Product Classification Implied Relationship Some criteria given in the document are for either Class 2 or Class 3. By implied relationship, these criteria are also applicable to any lower classes. 1.12.7 Conditions Not Specified Not all conditions can be included in the standard. For this reason, where conditions are not specified as either a defect or process indicator, the condition is considered acceptable. Where this decision leads to a condition that affects the user-defined form, fit, function or reliability, the manufacturer is responsible for identifying those conditions. 1.13 Electrical Clearance Electrical clearance measurement is defined by the design activity and may be on the design documentation. Although the design will consider the minimum electrical clearance measurement and provide clearances to include this amount, there may be instances where an assembly process causes a violation of this clearance. Violation of electrical clearance is a defect condition in all cases. Where no electrical clearance measurement is defined, the value can be calculated using Table 1-1. This number takes into account the operating volt-ampere dating and the voltage. 1.14 Measurement Units and Applications The dimensions in the document are given in SI (System International) units. These units are commonly referred to as “metric.” The IPC policy is to provide the Imperial English units to three decimal places, and these are presented in the document in brackets following the dimensions in metric units. Because the conversion is not one-to-one, there are instances where a conflict exists between the two numbers. In these situations, the procurement documentation defines the order of precedence between the two documents. 1.15 Verification of Dimensions Dimensions provided in this standard are need definition of absolute limits per ASTM E29.

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1.16 Visual Inspection 1.16.1 Lighting The lighting recommendation for 1000 lm/m2 at the surface of workstations is a practice used throughout the industry. This is not a requirement, and lighting may be altered depending upon the working conditions and the product. This does not address lighting in an area other than at the workstation surface. 1.16.2 Magnification Aids and Lighting Magnification aids may be required for assembly inspection. Table 1-2 provides the magnification power requirements, and the manufacturer selects the magnification power based on the item to be inspected. Two magnification power ranges are provided for each wire size. The first is the “inspection range,” and it is used when the product is being inspected. If there is a question about whether or not a defect condition exists that cannot be determined at this magnification, the referee magnification power may be used. If, after inspection at the referee magnification power, no defect is identified, the product is considered acceptable. Where mixed wire sizes are found in an assembly that would require two different magnification powers, the greater magnification is allowed, but not required, for the entire product. This is an exception to the requirements for magnification power when only one magnification power is required. 1.17 Electrostatic Discharge (ESD) Protection The introduction of an electrostatic event to an unprotected component can lead to failures. These failures may lead to component degradation (latent failures) or immediate failure. ESD protection, as defined in the required ANSI/ESD-S-20.20-1999 or an equivalent document, will provide protection for the components when implemented. ESD protection is required for all product classes when the assemblies contain ESD sensitive components. 1.18 Contamination When this standard is used, a defect condition exists when assemblies with materials that are not a part of the assembly are present. This may require process development to provide this level of control, but such processes are not a requirement. Where soldered assemblies are present, a separate requirement for cleanliness is found in section 4.2. 1.19 Materials and Processes When using this document for assembly or manufacturing purposes, selection of the materials and processes that are used is important. The desired end result is an acceptable assembly, and the items selected that comprise this activity need to work together to produce that assembly. From time to time, major changes may be required for an assembly process. These changes may be a result of many things, including technology changes or process improvement efforts. For Class 3 products, where major changes are made to proven processes, it is important to determine if the changes will affect the end product. IPC/WHMA-A-620A requires validation of the changes for this product. The manufacturer may select the method of validation.

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8 Marking and Labeling of Cable and Wire Harness Assemblies IPC/WHMA-A-620 provides accept/reject criteria and process information for the manufacture of various types of cable and wire harness assemblies. Section 12 of the document entitled, Marking and Labeling, provides the accept/reject criteria for the various types of marking and/or labeling that one may encounter. For simplicity, the terminology “marking and/or labeling” is referred to herein as “marking”. This Handbook section provides tutorial information on the most commonly used methods for marking of cable and wire harness assemblies and discusses the importance of implementing this methodology in a manner that provides marking that is legible and permanent, and that the marking methods do not damage or otherwise degrade the performance of the product the markings are applied to. This Handbook section does not mandate any marking of cable and wire harnesses. When marking is required it is normally specified by the customer and identified on the applicable documentation (e.g., assembly drawing, electrical schematic, wiring diagram, wire list, etc.) The following topics are addressed in this section: 8.1 Scope 8.1.1 Reference Designation Marking 8.1.2 Part Number Marking 8.1.3 Drawing Revision Level Marking 8.1.4 Manufacturer Identification Marking 8.1.5 MIL-STD-130 Marking 8.1.5.1 Unique Identification (UID) Marking 8.1.5.2 Machine Readable Information (MRI) Marking 8.1.6 Serial Number and Other Traceability Marking 8.1.7 National or International Regulations Marking 8.1.7.1 CE Marking 8.1.7.2 Underwriters Laboratories (UL) Marking 8.1.7.3 Canadian Standards Association (CSA) Marking 8.1.8 Environmental Markings 8.1.8.1 WEEE Marking 8.1.8.2 RoHS Marking 8.1.9 Temporary Marking 8.1.10 IPC/JEDEC J-STD-609 Marking 8.1.11 National Stock Number (NSN) Marking 8.2 Wire Marking Methods 8.2.1 Hot Stamp Marking 8.2.2 Inkjet Marking 8.2.3 Dot Matrix Marking 8.2.4 Laser Marking 8.2.5 Hand Ink Pen Marking 8.2.6 Label Marking 8.2.7 Heat Shrink Marking 8.2.8 Thermal Marking 8.3 Fundamental Marking Principals 8.3.1 Correctness of required Marking 8.3.2 Marking Process (s) 8.3.3 Marking Robustness

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8.1 Scope Markings, when required, are normally provided for the applications noted below; however, reference to the type of marking herein does not mandate that the marking be provided unless otherwise required by the applicable contract/documentation. 8.1.1 Reference Designation Marking This marking provides an easy method of identifying where the completed wire and/or cable harness terminates to when installed in the next higher assembly, or otherwise within itself. For example, J1 designates a connector receptacle, P1 designates a plug that mates with a connector, F1 designates a fuse, XF1 designates a fuse-holder for the F1 fuse, R designates a resistor termination point, K designates a relay contact termination, TB1-1 designates the wire is connected to terminal 1 of terminal board TB1, etc. 8.1.2 Part Number Marking This marking identifies the part number (if assigned) for the completed cable and/or wire harness assembly. For example, part number (P/N) 123456 (G1, GR1, GP1 or dash 1) designates the complete assembly; 123456G2(G2, GR2, GP2 or dash 2) designates one branch of the completed assembly. Some assemblies may not have a unique P/N assigned since they may be manufactured on a wire harness board or otherwise manufactured at the next higher assembly (e.g., within a chassis, enclosure or cabinet), by using the parts/materials specified on the bill-of-material (BOM) or parts list of the assembly drawing. 8.1.3 Drawing Revision Level Marking The revision level of the assembly drawing may be marked adjacent to the assigned P/N for configuration control. For example, the assembly is marked with Revision A to designate that it was manufactured to drawing Revision A. If a subsequent change was implemented via drawing Revision B, future manufactured assemblies would be marked Revision B to maintain configuration control. However, when the change impacts form, fit or function such that the Revision A assembly is no longer suitable for use, most users require that a new P/N be assigned to the assembly. Other methods of providing configuration control marking are sometimes used. For example, the assembly may be marked with P/N 123456G1, Revision A, EO1. The EO1 marking designates that the Revision A assembly was modified by Engineering Order # 1 (EO 1). 8.1.4 Manufacturer Identification Marking The manufacturer may mark the manufacturer name, LOGO or Commercial and Government Entity (CAGE) code on the assembly. The CAGE code is a five-position alphanumeric code with a numeric in the first and last positions (e.g., 27340, 2A345, 2AA45 OR 2AAA5, etc.) used extensively within the federal government. The CAGE Code is used to support a variety of mechanized systems throughout the government and provides for a standardized method of identifying a given facility at a specific location. A listing of CAGE Codes may be found at http://www.dlis.dla.mil/cage_welcome.asp. Non U. S. companies can obtain a North Atlantic Treaty Organization (NATO) CAGE (NCAGE) Code form the appropriate source. The NCAGE Code can be obtained directly from the Codification Bureau in your country. For most countries contact the following site for the NCAGE Code information; http://www.dlis.dla.mil/Forms/Form_AC135.asp. For a list of addresses go to: http://www.dlis.dla.mil/nato_poc.asp. The CAGE CODE and/or the NCAGE Code are required for the Central Contractor Registration (CCR) discussed below. Manufacturers interested in doing business with the U. S. Federal Government must be registered in the Central Contractor Registration (CCR). Information on the CCR may be obtained at: http://www.ccr.gov. In some instances, a Data Universal Numbering System (DUNS) number may be assigned. This is a nine-digit number assigned by Dun & Bradstreet to each business in their global database.

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8.1.5 MIL-STD-130 Marking If the cable and/or wire harness assemblies are destined for ultimate shipment to a military customer, the marking requirements of MIL-STD-130 may have been mandated on the contract/documentation. MIL-STD-130 may be obtained from: http://assist.daps.dla.mil. 8.1.5.1 Unique Identification (UID) Marking The UID number included in MIL-STD-130 is a system of establishing globally unique and unambiguous identifiers within the Department of Defense, which serves to distinguish a discrete entity or relationship from other like and unlike entities or relationships. A UID may apply to a cable or wire harness assembly. Marking of a UID symbol normally takes up little space. A 50-character concatenated UID symbol only requires a square space of from 0.25 inch square to a 0.50 inch square depending of the size of the Data Matrix “cell”. Most concatenated UID’s contain less than 35 characters. 8.1.5.2 Machine-Readable Information (MRI) Marking The MRI referenced in MIL-STD-130 is a pattern of bars, squares, dots, or other specific shapes containing information interpretable through the use of equipment specifically designed for that purpose. The patterns may be applied for interpretation by digital imaging, infrared, ultra-violet, or other interpretable reading capabilities. MIL-STD-130 requires that items be marked with a machine readable 2D Data Matrix bar code. 8.1.6 Serial Number and Other Traceability Marking A serial number, lot number, batch number, manufacturing number, or other form of traceability marking may be required by contract/documentation, or otherwise may be used by the manufacturer. This type of marking provides for traceability to manufacturing, inspection and test records for the item. The marking can also assist in root cause determination and corrective action for any defects found on shipped product. In some cases, a unique serial number may have been assigned by the customer. Date code marking and bar code marking are also sometimes used as traceability marking. The manufacturer should employ a positive system for precluding duplication of serial number markings. It is recommended that serial numbers be assigned from a computer generated listing/database that creates a new unique serial number each time the program is accessed. 8.1.7 National or International Regulations Marking Cable and/or wire harness assemblies may require certification markings of one or more of the types discussed below. Other types of these markings also exist but are not discussed herein. 8.1.7.1 CE Marking CE marking is a European marking of conformity that indicates that a product complies with the essential requirements of the applicable European laws or Directives with respect to safety, health, environment and customer protection. Generally, this conformity to the applicable directives is done through self-declaration. The CE Marking is required on products in the countries of the European Economic Area (EEA) to facilitate trade between the member countries. Unlike the UL mark, the CE Marking: a) Is not a safety certification mark; b) Is generally based on self-declaration rather than third party certification, and c) Does not demonstrate compliance to North American safety standards or installation Codes. 8.1.7.2 Underwriters Laboratories (UL) Marking The UL Mark on a product means that the UL has tested and evaluated representative samples of that product and determined that they meet UL’s requirements. In addition, products are checked by UL at the manufacturing facility to make sure they continue to meet UL requirements.

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8.1.7.3 Canadian Standards Associating (CSA) Marking A CSA marking, like the UL marking, deals with the issue of safety and safe use of products with a focus on the Canadian market. CSA Marks appear on over one billion products worldwide. Each mark tells you that an authorized testing laboratory has evaluated a sample of the product to determine that it meets applicable national standards.

8.1.8 Environmental Marking Because of environmental concerns, various environmental regulations have been issued. These include the EU RoHS (Restriction on the use of Certain Hazardous Substances) in electrical and electronic equipment, and the WEEE (Waste Electrical and Electronic Equipment). The main environmental concern that applies to cable and/or wire harness assemblies pertains to the elimination of lead (Pb). Manufacturers who design their products to be Pb-free may choose to provide Pb-free markings, or otherwise the contract may mandate such markings. The following discussion reviews some, but not all, of the various environmental markings that exist:

8.1.8.1 WEEE Marking Electrical and electronic equipment (EEE) plays an ever-increasing role in our daily lives. Our kitchen appliances, mobile phones and computers offer us many benefits during their working lives but when this equipment is thrown away it affects the environment. Waste electrical and electronic equipment (WEEE) is one of the fastest growing waste streams in multiple countries. Some WEEE contains hazardous substances and parts such as mercury in some switches, lead in solder, and cadmium in batteries. Recycling rates for most types of WEEE (other than large ‘white goods’ such as fridges and washing machines) are very low. The WEEE Directive covers a wide range of electrical and electronic products, although some are exempt from certain requirements. The types of products covered are: a) large and small household appliances; b) IT and telecommunication equipment; c) consumer equipment such as TVs, videos, hi-fi; d) lighting, electrical and electronic tools (except large stationary industrial tools); e) toys, leisure and sports equipment; f) automatic dispensers; g) medical devices (these are exempt from the WEEE recycling and recovery targets); h) monitoring and control instruments.

Many of these products may have cable and/or wire harness assemblies installed and as such, they may be subject to the WEEE requirements.

If the cable and/or wire harnesses fall within the scope of the WEEE Directive then products must be marked with the “crossed out wheelie bin” symbol designed to specifications set forth in EN 50419:2005.

8.1.8.2 RoHS Marking The restriction of the use of Certain Hazardous Substances in Electrical and Electronic Equipment Directive (RoHS), which went into effect on July 1 2006, places restrictions on the use of mercury, lead, and other materials. The most significant impact for cable and/or wire harness assemblies involves the prohibition on the use of lead (Pb). Many cable and/or wire harnesses contain lead-bearing solder which must be phased-out if a manufacturer intends to sell product to the EU market, or otherwise where a customer mandates Pb-free. Other countries are adopting their own version of the RoHS requirements. Manufacturers may be required by contract/documentation, or otherwise may elect to mark their cable and/or wire harness assemblies to designate RoHS compliance. 8.1.9 Temporary Marking Manufacturers may elect to use temporary markings for ease of manufacturing and/or for traceability during manufacturing, inspection and testing. For example, a wire number may be marked on a temporary label affixed to individual wires on a harness board.

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Whenever possible, it is recommended that the wire markings/labels installed on the wire harness board are the same as required by the assembly documentation, in lieu of using a temporary marker, to help reduce the potential for wiring errors during installation of the harness into its next higher assembly. A label or indication of inspection/test status stamp may be applied to designate completion of required inspections and/or tests. Such markings are normally removed prior to shipment. However, unless otherwise prohibited by the customer (Class 3 product), some temporary markings, such as stamp marking indicating acceptance by Quality Assurance and/or Test, may remain on the product. The manufacture should select a temporary marking method that will not result in damage to the item or otherwise result in functional problems. 8.1.10 IPC/JEDEC J-STD-609 Marking Normally, cable and/or wire harness assemblies will be shipped as stand alone assemblies for ultimate installation into a next higher assembly. These next higher assemblies may be marked in accordance with IPC/JEDEC J-STD-609, Marking of Labeling of Components, PCBs and PCBAs to Identify Lead (Pb), Pb-Free and other Attributes. It is recommended that manufacturers who include soldered terminations on their cable and/or wire harness assemblies include the following “e” markings, as applicable, from IPC/JEDEC J-STD-609: e0 – Designates intentionally added lead (Pb) (≥ 3 percent by weight) e1 – Tin-silver-copper (SnAgCu) e2 – Tin (Sn) alloys with no bismuth (Bi) nor zinc (Zn), excluding tin-silver-copper (SnAgCu) e3 – Tin (Sn) e4 – Precious metal [e.g., silver (Ag), gold (Au), nickel-palladium (NiPd), nickel-palladium-gold (NiPdAu)

(no tin (Sn)] e5 – Tin zinc (SnZn), tin-zinc-other (SnZnX) [all other alloys containing tin (Sn) and zinc (Zn) and not

containing bismuth (Bi)] e6 – Contains bismuth (Bi) e7 – Low temperature solder (≤150ºC) containing indium (In) [no bismuth (Bi)] e8 and e9 symbols - unassigned 8.1.11 National Stock Number (NSN) Marking If the cable and/or wire harness assembly is designated for shipment for military use by the USA and/or NATO countries; it may have been assigned a National Stock Number (NSN). The contract/documentation may require marking of the NSN on the cable and/or wire harness assembly, or otherwise as part of the marking provided on the shipping containers. The NNSN is a 13-digit number that is assigned by the Defense Logistics Information Service (DLIS) in Battle Creek Federal Center in Michigan. The 13-digit number is composed as follows: 6150-00-014-3579 The first four digits (6150) represent the Federal Supply Class (FSC). The first two digits (61) are assigned as the Federal Supply Group (FSG). The “61” identifies

that the item is listed as; “Electric Wire, Power and Distribution Equipment.” The next two digits (50) identify the specific type of item classification within the “61” FSG. The

“50” designation represents “Miscellaneous Electric Power and Distribution Equipment.” Digits 5 and 6 (00) represent the Country of origin, as noted below. USA 00 and 01 Germany 12 France 14 Canada 20 and 21 Slovenia 40 Australia 66

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The remaining digits seven through 13 (014-3579) are a unique number that is assigned to the item. In this example, the number shown is a bogus number. These seven digits are also referred to as the NIIN (National Item Identification Number). 8.2 Wire Marking Methods The following paragraphs identify the most commonly used methods for marking of cable and/or wire harness assemblies. 8.2.1 Hot Stamp Marking Hot stamp marking is still the most inexpensive method for wire or cable identification and it can be used to mark over Teflon insulation. In order to ensure a quality marking you should have the correct air pressure, dwell time, wheel temperature and foil. The air pressure is the pressure in which the wheels make contact with the wire or cable. Control of this process is important to preclude affecting the integrity of the wire or cable insulation. The dwell time is the length of time in which it takes to complete the whole stamping cycle. The wheel temperature and foil types are chosen together. The foil consists of a backing and a pigment color. The pigment is transferred to the wire or cable insulation via the heat from the character wheels. The backing of the foil should be able to withstand the required temperature range and can be made of different materials such as Mylar or Nylon. Certain pigments will stick to certain substrates and will require different temperatures to transfer them. For example, one cannot use a foil that will mark on PVC at 275 degrees for Teflon that may require temperatures of 350-400 degrees. Hot stamp marking is specifically prohibited for some wire types; specifically, wire conforming to AS 81044 and AS50881. This wire is used for aerospace application and the concern is that this marking method could sufficiently penetrate the wire insulation and result in a dielectric breakdown of the insulation. 8.2.2 Inkjet Marking Inkjet technology has improved greatly over the last few years. With less maintenance and quicker start-ups, inkjet marking systems have grown much more reliable and user friendly. For the wire and cable industry, it is usually a dye or pigmented ink, with an MEK (Methyl Ethyl Ketone) base. Although rare, alcohol based inks can be used; however, drying time is increased. Depending on the interfacing wire processing equipment and software, one can mark on the fly and vary text strings throughout the length of the wire or cable. You can also vary font sizes and bold font, tower print, and invert the text. Inkjets are dot matrix printers, and the ink is directed onto the wire or cable via deflector plates once it is electrically charged. Inkjet marking, however, does have its limitations. Certain substrates (insulations) require certain ink types, and marking Teflon is not an option. In addition, if you have an automated printer with black ink, you normally cannot clearly mark on black wire. Since changing ink is normally not a possibility, one would need to purchase an additional printer to mark using a different color ink. 8.2.3 Dot Matrix Marking A dot matrix printer is used to apply marking information to a label. The Dot Matrix terminology refers to the method the printer uses to create the marking images. This is accomplished by several small pins, aligned in a column. These pins strike an ink ribbon positioned between the pins and the label, creating dots on the label. Characters are formed by patterns of these dots by moving the print-head laterally across the page in tiny increments. The pins are contained in the print-head and are driven by solenoid actuated small hammers which force each pin to contact the ink ribbon and label. A Dot Matrix printer has an advantage in being able to print letters in italics or bold by changing the way dots are arranged on the label. These printers are more economical than laser printers.

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8.2.4 Laser Marking Laser wire marking provides a permanent, non-contact, permanent mark for wire and multi-core cable identification. It is the preferred, and most often specified, method of marking wire and cable today for aerospace and military applications. Equipment is available that is capable of marking wire in accordance with the AS 50881 International Standard. Although laser marking is the commonly preferred marking method, there are certain mil-spec wire and cable that cannot be laser marked. This includes Kapton insulated wire and cable. Ink Jet marking may be used for marking this type of wire and cable. Laser marking is not a viable option for some manufacturers because of the high cost ($125-$400K), and/or the time it takes to apply the marking. The following different laser types are available: a) Vector Based Laser – The wire needs to stop and the laser uses x-y coordinates. b) Mask Type Laser – The mask acts as a type of stencil. c) Carbon Dioxide Laser – Destructive and not typically used for wire and cable. d) UV Laser – Ideal for marking Teflon. However, the Teflon jacket must contain Titanium Dioxide in order for a color change to occur. 8.2.5 Hand Ink Pen Marking Hand marking of information using a commercially available ink marker pen is sometimes used to modify markings in fielded product. However, this marking method is not recommended for initial marking because of legibility and longevity concerns. 8.2.6 Label Marking Applying marking using a label is especially useful when hot stamp or inkjet cannot provide satisfactory results. Labels are printed and then applied automatically, or manually. Some labels can also withstand harsh environments such as gasoline and oil. Some labeling systems allow you to program the text to be printed via a PC and use a master machine to send a print signal for the label location. Wrap-around markers, including a self-laminating marker, are an easy method for marking that does not require the termination to be removed in event a wire marker replacement is needed. These markers have a clear portion that will wrap around and laminate the marking legend. This protects the marking from damage. The gauge (size) of the wire or cable determines the length of the self-laminating/wrap-around marker or the diameter of the sleeve to be used. Normally the length of the label should be five times the outer diameter of the wire or cable to be marked. 8.2.7 Heat Shrink As with labeling, heat shrink can also be an effective method when hot stamp or inkjet marking is not an option. Heat shrink can be marked prior to it being applied to the wire or cable and then heated to shrink. However, once the wire is terminated, heat shrink cannot normally be used in event a marking change is needed unless the termination is removed and replaced with new heat shrink. 8.2.8 Thermal Marking Thermal marking requires a foil and heat, but unlike hot stamp marking, it does not require “impacting the insulation”. This method uses heat to transfer the pigment from the foil to the wire while it is rolled on. It can mark on both flat and round cables with either black or white markings. Color foil changeover is quick and it can mark logos and other bitmaps. 8.3 Fundamental Marking Principals – The process(s) involved in the marking of cable and/or wire harness assemblies should be robust and under process control to the extent necessary to assure that the following marking principals are met to achieve compliance with the accept/reject criteria of Section 12 of IPC/WHMA-A-620.

8-8

8.3.1 Correctness of Required Marking – The manufacturer should ensure that all applied marking contains the correct information (text, numbers, color, font size, etc.) specified in the contract/documentation, and that the marking was provided in the correct location, using the specified materials (e.g., shrink sleeve, labels, ink, etc.). 8.3.2 Marking Process (s) – The manufacturer should ensure that the contractually specified marking process (s), and/or applicable process (s) specified by the manufacturer were used to apply the marking and are under acceptable process control. 8.3.3 Marking Robustness – Completed marking should be permanent and free from damage, including any evidence of damage that may have occurred from use of the incorrect marking process (s).

10 Harness/Cable EMI/RFI Shielding. 10.1 EMI/RFI Shielding Theory Electro-magnetic interference can be composed of both magnetic fields and electric fields that cause a disruption in the desired signals to and from an electronic device. Radio Frequency Interference is by virtue of its higher frequency is primarily an electric field causing the disruption of the desired signals. These fields can come from the signals with in the cable or harness and from outside source. 10.1.1 Magnetic Field shielding Magnetic fields are not easy to shield, magnetic fields must be contained by continuous metal enclosures of magnetically permeable materials, and may include highly permeable rare earth metals and alloys. The effects of a magnetic field on a cable or harness can be reduced by increasing the distance between the field generator and the cable and harness or by twisting the conductors. When the cable is generating the field as in a high current power cable, the magnetic field is mitigated by running the source and return together in a twisted wire, the sum of the opposite magnetic fields is zero where the currents are equal. 10.1.2 Electric Field Shielding Electric fields are a much more common problem and can be shielded by placing a terminated conductive shield around the cable or harness. The electric field induces currents in the shield and those currents must be dissipated into ground so effective grounding of the shield is also required. The higher frequencies of RFI may be reflected off an un-terminated shield but non grounded shields only partially reduce the effects of the electric fields. The shielding may be terminated on one or both ends and may be terminated to the ground at intermediate points as determined by engineering. A round wire terminating the shield is acceptable only at low frequencies because the impedance of the round wire goes up with frequency do to the self inductance of the round wire, the low resistance path for DC current is now a high impedance path for the high frequency currents making a ineffective ground path. 10.1.3 Shield Braid Condition Braided wire should be uniform over the wire bundle with at least 80% braid coverage.

Photo 46 (less than 80% coverage)

Unacceptable

Photo 47 (minimum of 80% coverage)

Acceptable

Photo 48 (90% + coverage)

Preferred

10.2 Shield Termination 10.2.1 Terminating Over Shield, Adapter The shield must be terminated to the Chassis ground by a very low impedance path to be the most effective. There are cable to connector adapters that are designed to terminate the shield around the cable circumference (360 degrees). When using an adapter the shield terminating surfaces must be cleaned and free of oils or other contaminants. The shield material must also be cleaned and adapter assembled per manufacture’s instructions. The DC resistance between the connector body through the adapter to the shield should be less than 2.5 milliohms.

(Compression Fitting)

(Bandit ®) (Crimp Ring) 10.2.2 Terminating Over Shield, Molded The molded part should be wrapped in a copper foil or other shielding material. The foil must be connected to the connector body and the cable shield and seams in the foil soldered or over lapped at least 50%. The DC resistance between the connector body through the foil mold to the shield should be less than 2.5 milliohms.

Solder foil to connector body completely around and patch any openings in the foil with a small piece of foil and solder.

Clean

Clean Clean

Patch

Solder foil around connector body

10.2.3 Terminating Over Shield, Other Methods Connectors with strain reliefs or other rear appliances may terminate the shield by the methods specified on the engineering drawing. Power cable shields may be terminated to the chassis ground and drain wires may be connected to the shield and terminated on a ground lug or clamp screw, but shield drain wires terminating the shield are not effective at higher frequencies and may not have the desired results. 10.3 Sub-Cable Shield Termination Components of a cable may be also be shielded. When shielded cable components like twisted shielded paired wires are used the shields must also be terminated for best results. These shields may be terminated to chassis ground at the cable connector adapter, or they may terminate in a pin or pins of the connector to be carried continuously through the cable. 10.3.1 NEED TITLE OR NOT A SUBCLAUSE To terminate the component shields to the connector adapter the inner conductors may be pulled through the braid and the braid terminated in the adapter. There are specialized adapter designs for terminating component shield to the adapter and installation shall follow the manufactures instructions. The component shields may be soldered to the over braid to terminate to the adapter, caution must be used to prevent wire damage from excessive heat. 10.3.2 NEED TITLE OR NOT A SUBCLAUSE When terminating the component shield to pins, a wire must be soldered to the shield and terminated with a pin either directly or by using a leaded solder ferrule to terminate the shield. The lead must be kept short, preferably the same length as the signal leads and going in the same direction.

Not Preferred

Pick off wire is too long

Preferred

Pick off wire is oriented to be as short as possible

10.3.3 NEED TITLE OR NOT A SUBCLAUSE When multiple shields are terminated together in a pin the leads may come out the rear side of the ferrule except the lead with the pin. Wiring should be done to keep the lead length short and minimize the additive wire length.

Figure 4 Excess Length

Excessive length in daisy chain, wire twisting not maintained

Figure 5 Preferred

Short daisy chain wires, wire twisting maintained

Not preferred

Pick off wire exits from the end of the daisy chain

Preferred Pick off wire exits from the center one of the daisy chain.. .

10.3.4 NEED TITLE OR NOT A SUBCLAUSE Component cables may use a foil shield with low coverage braid or drain wire. The braid is terminated as any braid in the cable. The drain wire is terminated the same way the pick off wires are terminated, sleeving may be required by the drawing to cover the drain wire.

Section 11 – Harness/Jacket Method

11-1

Covers Sections 15 (machine braiding method only), all of Section 16 General Jacketing and overall harness shielding materials are applied either by machine, molding/extrusion, or by hand. Environment, harness materials, type of connectors and cost drive the method and type of construction that should be specified by the Designer. When electrical shielding is applied the requirements for its application is more controlled. Here shielding effectiveness is the objective. It is normally specified in percentage (%) coverage since its effectiveness is based on the amount and size of gaps of the applied material. Formulas based on wire gauge, number of yarn ends and the number of carries verses the print shield percentage coverage requirement will drive the setup. Sometimes a total resistance value may be specified, this is for …………. See xxxx for converting % coverage to machine setups and from the setup to calculate D.C. resistance of the braid per unit length. Separation tape is required over the wire between machine braided protective or shield coverings. The spiral of the tape wrapping is always in the opposite direction from the wire twist to maintain bundle configuration and twist. For braiding non-metallic protective materials (protective coverings) overlapping of the separation tape is not required. Its function is to hold the bundle shape during the braiding operation. For metal shield braid the separation tape also provides additional abrasion resistance. Here tape application require a minimum of a 25% overlap of the separation tape, however, overlapping of the tape which exceeds 50% will lead to decrease flexibility and should be avoided. Where the wire bundle reaches a braid stop, the spiral wrap shall be stopped and terminated with 1-1/2 to 2 straight wraps of tape. Separation tapes should be a thin smooth surface tape, i.e. Teflon, Mylar, Impregnated Fiberglass, etcetera, with or without a weak adhesive backing. Strong adhesive backing will decrease flexibility by preventing the tape from sliding over itself during flexure. Adhesive bond failure after assembly is not an issue since its function during assembly has been fulfilled. For cut ends of braid materials, any material, apply a layer of a thin smooth surface adhesive backed tape, i.e., Teflon, Mylar, etcetera, to stop fraying of the end. Definitions Pick. A Pick is the point in a braid at which one carrier goes over another carrier along the long axis of the cable. Picks per Inch. Picks per inch is the number of picks in a distance of 1 inch. Cordage. Cordage is the product formed by twisting together two or more ply yarn. Yarn. Yarn is the product formed by two or more single continuous filament threads when twisted together. Monofilament. Monofilament is a single continuous filament. Ply yarn. Ply yarn is the product formed by twisting together two or more yarns. Denier. Denier is the unit weight of yarn or cordage based upon a skein 450 meter. long, weighting 0.05 gram. It is numerically equal to the number of grams per 9,000 meters. Mechanical Braided Materials

o Machines Wardwell, Steeger, etc(Insert pics of different machine and how each works in general) Number of carries for most machines are 8, 16, 24, 32, 36, 46, 54, 64 and xxx. The diameter

of the finished cable and the number of ends of the yarn being used normally determine what size of machine to use. Here are some general guidelines: (see my specs on typical limits).

The braider drives on what type of bobbins are used, and how much yarn can be put on the bobbin. Bobbins with multiple yarn ends need be laid flat and parallel to each other with equivalent tension to create a smooth flat surface. The layers also must be evenly applied and free of loose turns or turns that could come loose during braiding operations. The starting and ending thread must not be tied together.


11-2

o o Materials

Polyester Aramid Glass/Ceramic fibers Plated High Strength Synthetic Fibers Metal/Plated Fiber Blend a mixture of plated yarn and metal yarn to achieve a complete

protection to the entire EMI spectrum with a reduced weight over all metal braid. Metal

o Shield Braiding

Braided Shielding verses Carriers Formulas

The shield braid shall be applied in such a manner as to provide the percentage coverage as shown on the engineering drawing/model. In lieu of testing, the value may be determined by calculation from the listed formulas below. The intended result is to determine a minimum number of picks per inch as a quick measure for production to achieve the minimum required shielding.

Tan = [2(D+2d)P]/C

Where: When:

= braid angle C = number of carriers in braiding machine d = gauge diameter of shield wire D = Diameter of unbraided harness F = shield over K = percent coverage N = number of wire ends per carrier P = Picks per inch

K = (2F-F2)100

and

F = (NPd)/Sin

(Values for P shall always be chosen so that F is always less than or equal to 1.0.)

D.C. Shield Resistance Calculations R = dR/[Cos(NC)]

Where:

= braid angle C = number of carriers in braiding machine dR = D.C. resistance of 1 strand end, ohms/unit length N = number of wire ends per carrier R = D.C. resistance in ohms/unit length

Insert picture of what this means, see Alpha wire technical data sht on Shielding.

Hand Applied Coverings

o Open/loose Sleeving Expandable sleeve Metallic braid sleeve Metal Mesh Tape

o Closed Coverings,. Shrink Sleeving

Most Common Materials, other materials can be used depending on the capability to be manufactured and meet end item requirements. Shrink material is normally an extruded tube of material that when heated will return to its original diameter. This is a beneficial material property that will allow materials to be hand applied over the wire and cable easily to create a protective jacket, and then shrunk down to create a slimmer and neat package, at lower cost than either over-molding or braiding. Materials commonly used are:

o Polyolefin


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o Polyethylene Terephthalate o Polyvinylidene Fluoride (Kynar) o Fluorosilicone Rubber o Silicone Rubber o Fluorinated Ethylene Propylene (FEP) o Polytetrafluoroethylene (PFE) o Ethylene-Tetrafluoroethylene Fluoropolymer (ETFE) o Tetrafluoroethylene Fluoropolymer (TFE)

Many of the materials listed above can have an adhesive coating to the interior of the

expanded tube normally called a melt liner. Conductive lined

Shrink Boots – w & w/o Sealant Tapes that are used for protective coverings of harness assembles are normally made from

Silicone, though other materials such as Neoprene and fiberglass have been used, but the most common type is self bonding silicone.

What is meant by self bonding silicone? It is where the tape is supplied with one side rough

and one side ultra smooth and clean. The tape roll comes with a release paper liner to kept the clean side uncontaminated and away from the rough exposed side. When the two sides are put in contact under a small compressive force, that created when the tape is wrapped over itself tightly, the two surfaces will bond to each other in about 24 hours.

Silicone Tapes, i.e. – Self-Sealing, Guideline, square cut, taper cut Electrician’s Tape which many people think can be used on any commercially

producted harness is made from PVC with an adhesive backing and is normally black, but can be any color. Due to most commercial and military regulations is not allowed to be used because of hazardous outgassing and burn products. All existing military and industry specifications have been canceled to preclude its use.

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12 Securing and Finished Assembly Installation 12.1 Scope IPC/WHMA-A-620 provides accept/reject criteria and process information for the manufacture of various types of cable and wire harness assemblies. Section 14 of the document entitled, Securing, provides criteria applicable to cable and wire harness manufacture rather than criteria applicable to installation of the completed cable or wire harness assembly into the next-higher assembly (e.g., chassis, drawer, panel, cabinet, etc.). Section 17 of IPC/WHMA-A-620 entitled, Finished Assembly Installation, provides criteria that apply to the installation of the completed cable and wire harness assemblies into the next-higher assembly (e.g., chassis, drawer, panel, cabinet, etc.). This handbook Section 12 provides tutorial information on the methodology most commonly used for securing (e.g., application of tie-wrap/lacing, breakouts and routing) cable and wire harness assemblies during their manufacture. It also provides tutorial information on the methodology most commonly used for installing the completed cable and wire harness assembly into a next higher assembly. This handbook section was developed as a companion document to IPC/WHMA-A-620 Sections 14 and 17, to assist the manufacturer in manufacturing and/or subsequently installing completed cable and wire harness assemblies into a next higher assembly in a manner that assures wiring system safety, performance, reliability, maintainability, service life and minimizes life cycle costs. The information provided herein can be used for all Product Classes of equipment. This handbook section, by design, does not discuss criteria for manufacture of the next higher assembly the completed cable and/or wire harness will be installed in, except in general terms. IPC/WHMA-A-620 is primarily structured to provide acceptance and/or rejection criteria for cable and wire harness assemblies similar in structure to IPC-A-610, Acceptability of Electronic Assemblies. However, unlike IPC-A-610, IPC/WHMA-A-620 also includes process information. The majority of cable and wire harness assemblies manufactured to IPC/WHMA-A-620 requirements are sold as individual completed assemblies. However, in many instances, the manufacturer of the cable and/or wire harness assemblies is the same manufacturer for the next-higher assemblies (e.g., chassis, drawer, panel, cabinet, etc.) the cable and/or wire harness assemblies will be installed in. Therefore, this handbook section provides tutorial information pertaining to the next-higher assemblies as related to the installation of cable and/or wire harnesses, and discusses related topics (e.g., personnel safety, electrical bonding, etc.) that are not directly addressed in IPC/WHMA-A-620. Since this document is a handbook, as such, non-mandatory terminology “should” is used throughout this section of the handbook rather than mandatory terminology “shall”.

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However, the user of this handbook is cautioned that much of the information contained in this section of the handbook may be invoked as a mandatory requirement by other contractual documents such as the applicable design specifications, other IPC documents, or via the applicable contract documentation such as drawings and parts lists. Much of the information herein is based on existing recognized military and aerospace documents that are also acceptable for use for non-military or non-aerospace applications as they contain information of a generic, primarily tutorial, nature. Reference to these documents herein should not be construed as a mandate for their use, or otherwise as a mandate to use military and/or aerospace mandated parts. Parts and materials required for manufacture of wire and cable harnesses, and for installation of these harnesses into the next higher assembly are controlled by the applicable contractual design requirements and/or as mandated by the user. The following documents, in addition to IPC/WHMA-A-620, were used in creating this handbook section and may be consulted for additional information: (a) MIL-E-917, Electric Power Equipment Basic Requirements (b) MIL-W-5088 (Superseded by AS50881), Wiring Aerospace Vehicle (c) MIL-HDBK-419, Grounding, Bonding, and Shielding for Electronic Equipments and Facilities

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The following topics are addressed in this section: 12. Securing and Finished Assembly Installation 12.1 Scope 12.2 Selection of Parts, Materials and Tools 12.2.1 Standard Parts and Materials 12.2.2 Non-Standard Parts and Materials 12.2.3 Sealing Materials 12.2.4 Fastening Devices 12.2.4.1 Nuts, Bolts, and Screws 12.2.4.2 Self-Locking Threaded Fastener 12.2.4.3 Flat-Head Screws 12.2.4.4 Blind Fasteners 12.2.5 Thread-Cutting Screws (Self-Tapping Screws) 12.2.6 Washers 12.2.6.1 Lock Washers 11.2.6.2 Flat Washers 11.2.7 Utilization of Standard Tools 12.2.8 Materials 12.2.8.1 Non-Recommended Materials (May be Specifically Prohibited by the User) 12.2.8.1.1 Toxic Pyrolytic Materials 12.2.8.1.2 Flammable Materials 12.2.8.1.3 Fragile or Brittle Materials 12.2.8.1.4 Mercury 12.2.8.1.5 Asbestos 12.2.8.1.6 Silicone 12.2.8.1.7 Polychlorinated Biphenyls (PCB) 12.2.8.1.8 Polyvinyl Chloride 12.2.8.1.9 Cadmium and Cadmium Plating 12.2.8.2 Other Non-Recommended Materials (May be Specifically Prohibited by the User) 12.2.9 Metals 12.2.9.1 Selection of Metals in Direct Contact 12.2.9.2 Corrosion-Resisting Metals 12.2.10 Plastics 12.2.11 Insulation Materials 12.2.11.1 Arc and Tracking Resistance 12.2.11.2 Laminated Plastics 12.2.11.3 Molded Thermosetting Plastics 12.2.11.4 Thermoplastics 12.2.12 Classes and Definitions of Insulating Materials 12.2.12.1 Class 90 12.2.12.2 Class 105 12.2.12.3 Class 130 12.2.12.4 Class 155

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12.2.12.5 Class 180 12.2.12.6 Class 200 12.2.12.7 Class 220 12.2.12.8 Class 240 12.2.12.9 Class Over 240 12.2.12.10 Electrical Tape 12.2.12.11 Sleeving 12.2.12.12 Straps and Clamps 12.2.12.13 Lacing Cord 12.2.13 Terminal Lugs 12.3 Wiring Selection 12.3.1 Conductor Degradation 12.3.1.1 Tin-plated Conductors 12.3.1.2 Silver-plated Conductors 12.3.1.3 Conductor Solderability 12.3.2 Aluminum Wire 12.3.3 Insulation Compatibility with Sealing and Servicing 12.3.3.1 Wire Diameter 12.3.3.2 Potting Seal on Wire or Cable 12.3.3.3 Insulation Degradation 12.3.4 Wire Size and De-rating 12.3.5 Wire and Cable Identification 12.3.5.1 Wire Size Color Code System 12.3.6 Wire for Electromagnetic Interference (EMI) 12.4 Service Life

12.5 Safety and Personnel Protection 12.5.1 Electric Shock 12.5.1.1 Levels of Electric Shock 12.5.1.2 Exposed Metal or Other Conductive Parts 12.6 Electrical Creepage and Clearance Distances 12.6.1 Distance from Enclosure 12.7 Accessibility

12.8 Maintenance and Repair

12.9 Smoke and Fire Hazards

12.10 Cable and Wire Harness Installation 12.10.1 Arrangement and Harnessing 12.10.2 Bundle and Group Size 12.10.3 Dead Ending 12.10.4 Splicing 12.10.5 Routing

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12.10.6 Stress Relief and Mechanical Support 12.10.7 Slack in Cable and Wiring 12.10.7.1 Connector Termination 12.10.7.2 Lug Termination 12.10.7.3 Strain Prevention 12.10.7.4 Free Movement 12.10.7.5 Cable and Wire Shifting 12.10.8 Inspection and Maintenance 12.10.9 Protection and Support 12.10.10 Bend Radius 12.10.11 Drip Loop 12.10.12 Routing Near Moving Parts or Controls 12.10.13 Routing Near Fluid Lines 12.10.14 Ground Return 12.10.15 Shielded Wire Grounding 12.10.16 Multiple Grounds 12.10.17 Connectors 12.10.17.1 Environment Resisting Connectors 12.10.17.2 Contacts 12.10.17.2.1 Spare Contacts 12.10.17.3 Connector Installation 12.10.17.3.1 Circular Connector Installation 12.10.17.3.2 Rectangular Connector Installation 12.10.17.4 Potting 12.10.17.5 Safety Wiring 12.10.17.6 Dust Protection 12.10.17.7 Connector Accessories 12.10.18 Splices 12.10.19 Terminal Lugs 12.10.20 Terminal Boards and Terminal Junction Modules 12.10.21 Wiring Mockup 12.10.22 Screw Thread Standards for Fastening Devices 12.10.22.1 Fastening of Harnesses and Associated Parts 12.10.22.2 Threads in Aluminum 12.10.22.3 Threads in Plastic 12.10.22.4 Inserts 12.10.22.5 Thread Projection 12.10.22.6 Bolt and Screw Thread Engagement 12.10.22.7 Thread Locking of Mechanical Assemblies 12.10.22.8 Flexible Wiring 12.10.22.9 Wire Connections and Terminals 12.10.22.10 Spare Terminals 12.10.22.11 Manufacturing Processes 12.11 Bonding 12.11.1 Purposes of Bonding

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12.11.2 Resistance Criteria 12.11.3 Direct Bonds 12.11.3.1 Contact Resistance 12.11.3.1.1 Surface Contaminants 12.11.3.1.2 Surface Hardness 12.11.3.1.3 Contact Pressure 12.11.3.1.4 Bond Area 12.11.4 Direct Bonding Techniques 12.11.4.1 Welding 12.11.4.2 Brazing 12.11.4.3 Soft Solder 12.11.4.4 Bolts 12.11.4.5 Rivets 12.11.4.6 Conductive Adhesives 12.11.5 Indirect Bonds 12.11.5.1 Resistance 12.11.5.2 Frequency Effects 12.11.5.2.1 Skin Effect 12.11.5.2.2 Bond Reactance 12.11.5.2.3 Stray Capacitance 12.11.6 Surface Preparation 12.11.6.1 Solid Materials 12.11.6.2 Organic Compounds 12.11.6.3 Plating and Inorganic Finish 12.11.6.4 Corrosion By-Products 12.11.7 Completion of the Bond 12.11.8 Bond Corrosion 12.11.8.1 Chemical Basis of Corrosion 12.11.8.1.1 Electrochemical Series 12.11.8.1.2 Galvanic Series 12.11.8.1.3 Relative Area of Anodic Member 12.11.8.1.4 Protective Coatings 12.11.9 Workmanship 12.12 Electrostatic Discharge (ESD) Control Program

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12.2 Selection of Parts, Materials and Tools Normally, the parts and materials needed to manufacture cable and wire harness assemblies, or otherwise needed to install the assemblies into their ultimate next-higher level assembly, will have been specified by the user activity, or otherwise, will have been defined on the applicable documentation (assembly drawings, wire lists, parts lists, etc.), which may or may not require user approval. Consult Section 7 of this handbook for additional information pertaining to materials. However, this handbook section will provide some general guidance that may be useful when parts and materials have otherwise not been previously selected or otherwise mandated. Tools needed are normally selected by the manufacturer; however, some tooling may have been selected or otherwise mandated by the user. 12.2.1 Standard Parts and Materials Whenever possible, standard parts and materials (items purchased to recognized commercial, industrial or military/NASA specifications) should be used. Such items should be suitable for their intended purpose. 12.2.2 Non-Standard Parts and Materials Any parts and materials not selected in accordance with paragraph 12.2.1 are considered non-standard and as such user approval may be required for their use. 12.2.3 Sealing Materials Some cable and wire harness assemblies may need to function in a humid environment or otherwise in applications where water or other liquid can exist. In such cases, it may be necessary to provide a means of sealing of components such as connectors, etc. Materials used for sealing should be elastomeric and reversion resistant. Materials should also be fully cured at the time of delivery of the assembly. If not fully cured, the material may out-gas unacceptable chemicals, or otherwise may not have developed the necessary mechanical and sealing properties needed for the application. Various sealing materials are available. Examples include cured synthetic rubber (operating range -60ºF to + 200ºF)(MIL-PRF-8516) which acts as a deterrent to fatigue, corrosion, and contamination, as well as an aid in reducing arc-over between electrical connector pins. Another example is a silicone rubber sealing compound with accelerator (operating range -80ºF to +400ºF) (MIL-PRF-23586), for use in applications where tear resistance is not critical. Proper curing is essential and in some cases, vacuum de-airing may be needed to eliminate air bubbles. Volatile by-products formed during cure should be removed. Factors affecting the rate and degree of cure are numerous, such as; catalyst concentration, humidity, diluents (if used to lower the viscosity), thickness of section area exposed for the release of volatiles and post-cure employed. Because of the high volume expansion of RTV silicones they are not recommended for use for the total filling of

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confined items. Pressures developed during heating are of a high order. Literature pertaining to the manufacturer’s product should be thoroughly understood prior to consideration of RTV silicones, and all applications for use of this material should be thoroughly evaluated prior to use. 12.2.4 Fastening Devices Fastening devices (e.g., nuts, bolts, screws, lock washers, flat washers, clips, pins, lock wire, etc.) should be made of corrosion-resisting material, or otherwise treated to resist corrosion, without using paint. Spring type locking devices, such as lock washers and retaining rings, that are made of precipitation hardened semi-austenitic corrosion-resisting steel, do not require additional protection against corrosion.

Aluminum alloy fasteners are not normally considered to be corrosion resistant. Fastener galling is an important consideration as certain fastener combinations such as using a stainless steel fastener in a tapped hole in stainless steel material can create galling of the fastener. In many cases, this galling causes the fastener to bind up in the machined hole and it eventually may break when trying to remove the fastener. Acceptable methods exist to prevent galling. These include, but are not limited to, plating the fastener (e.g., chrome plating, etc.), or using an anti-galling compound. If anti-galling compound is used, it is important that the compound selected be compatible with the fastener material. Additionally, the recommended fastener torque value (if specified) may require adjustment to the “lubricated”, rather than “dry” thread value if anti-galling compounds are used. 12.2.4.1 Nuts, Bolts and Screws Nuts, bolts and screws should be selected from recognized federal, military or industry standards; examples of such standards include, but are not limited to; FF-S-85, FF-S-86, FF-S-92, FF-S-200, MIL-DTL-1222, NASM17828, NASM17829, NASM17830, NASM21250, etc. When lock washers are required, they are normally supplied as stand alone items, or otherwise as part of an assembled fastener. 12.2.4.2 Self-Locking Threaded Fastener Self-locking bolts and screws are sometimes used in a design. These are standard UNC, UNJC, UNF, UNJF, UNRC or UNRF threads that have a special self-locking feature provided in the fastener. A recommended specification is MIL-DTL-18240. This provides for three types of self-locking features; Type N (Plug/Pellet), Type L (Strip) and Type P (Patch). The self-locking elements are incorporated in external screw used in applications where maximum temperature does not exceed 250°F. Type N element, plug/pellet configuration is installed via a hole drilled into the fastener. Type L element, strip configuration is installed via a strip cut through the threads parallel to the length of the fastener.

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Type P element, patch configuration is installed without removal of any material of the fastener. Normally, a specific type should only be specified when required by design or application requirements. The plug/pellet is not recommended for sizes below 0.190. Self-Locking fasteners are not recommended for the following applications:

1. Temperatures above 250ºF. 2. Applications where the item being fastened must be removed and/or reinstalled

and/or opened and closed multiple times (e.g., panels, doors, drawers, plug-in modules, etc.). Normally self-locking fasteners are only qualified to withstand five removal and/or reinstallations.

3. Safety related applications where the failure of the fastener can cause injury or death to personnel or product damage.

4. Applications where the self-locking mechanism will encounter keyways, slots, cross-holes or thread interruptions.

5. Fasteners that have had the self-locking mechanism reworked or reprocessed 6. Electrical connections. 7. Other considerations as indicated in the applicable self-locking fastener

specification. 12.2.4.3 Flat-Head Screws Flat head screws should not be used in material of a thickness less than one and one-half times the height of the screw head. Flat-head screws should be properly and completely seated in the material. 12.2.4.4 Blind Fasteners A blind fastener is the type of fastener used when only one side of an assembly is accessible for installation of the fastener. Blind fasteners are generally provided in two different types; Type I - pull type-positive mechanically locked, and Type II - threaded-self-locking type. See NASM8975 for one type of blind fastener that may be used. The Type I fastener is installed by the spindle being pulled into the sleeve, forming a blind head on the back side of the assembly, and by subsequently removing the pulling portion of the spindle. The Type II fastener is a multiple piece construction furnished as an integral assembly which consists of a nut body, a core-bolt, and a sleeve. Special commercially available tooling is used to install these fasteners.

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12.2.5 Thread-Cutting Screws (Self-Tapping Screws) These types of fasteners should only be used for non-critical and non-structural applications such as for mounting information and/or identification plates. 12.2.6 Washers Two types of washers are used; lock washers and flat washers. Washers are used to distribute the load over a larger area, and to provide a hardened bearing surface. Additionally, lock washers compensate for developed looseness between component parts of an assembly. 12.2.6.1 Lock Washers Lock washers are normally available in two different types, helical spring-lock washers, and tooth-type. Helical spring-lock washers include: regular, heavy, extra duty, and high-collar types. Tooth-lock washers include internal tooth, external tooth, countersunk external tooth, internal/external tooth, and others. ASME B18.21.1 covers one type of acceptable lock washer; Lock Washers (Inch Series), and NASM35338, is an example of a Regular Medium Series Helical-Spring Lock Washer. Split ring (helical spring) are the preferred type. External tooth lock washers (tin-brass, copper alloy 425) are preferred for electrical connections (see MS35335 for one example that conforms to ASSME B18.21.1). External-tooth type lock washers are used to bite through protective coatings of aluminum parts if they are grounded or electrically bonded through the fastening device. Internal-tooth lock washers are used instead of external-tooth lock washers only where necessitated by space limitations, appearance, or other special conditions. If internal-tooth lock washers are used, it is important that the size of the washer and diameter of the fastener are chosen so that the serrations make satisfactory contact. 12.2.6.2 Flat Washers One recommended specification for procurement of flat washers is NAS1149 (supersedes MS16208, FF-S-92, and AN960). Flat washers are recommended for use for the following applications:

a) Between screw heads and soft materials, unless a washer head screw or similar type is used to provide a bearing surface equivalent to the bearing surface of the appropriate flat washer.

b) Between a nut or lock washer and a soft material. c) Where lock washers are used for securing a soft material, flat washers are

used to prevent marring or chipping of the material and the applied protective coating, except in areas where an electrical ground is required.

d) Between an organically finished material and lock washers, bolt and screw heads, or nuts, except where their use conflicts with electromagnetic interference considerations.

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12.2.7 Utilization of Standard Tools Whenever possible, only standard tools should be needed to install and/or to maintain cable and wire harness assemblies. A standard tool is defined as wrenches, crimp tools, soldering equipment, and other types of tooling that is normally available as listed in the Federal Supply Catalog. All other tooling is normally considered special tooling, and as such, the user may require the manufacturer to provide this tooling, including a place to store the tooling in the delivered equipment, when appropriate. 12.2.8 Materials Various types of materials are used during the manufacture of cable and wire harness assemblies, or otherwise, when installing these assemblies in their next-higher assembly. Normally, the materials will have been mandated for use via the applicable documentation (e.g., drawings, parts lists, etc.), or otherwise as specified by the user. The following guidance is provided to aid the manufacturer in event that materials have not yet been selected, or otherwise mandated for use. 12.2.8.1 Non-Recommended Materials (may be specifically Prohibited by the User) Various materials exist that are not recommended for use because of potential personnel hazards, restrictions by Government regulations, or for other reasons. Some of these materials may have been specifically prohibited from use by the user. The following are examples of non-recommended materials. This list may not be all inclusive. (a) Toxic pyrolytic materials. (see 12.2.8.1.1) (b) Flammable materials (see 12.2.8.1.2). (c) Fragile or brittle materials (see 12.2.8.1.3). (d) Mercury (see 12.2.8.1.4). (e) Asbestos (see 12.2.2.8.1.5). (f) Silicone (see 12.2.8.1.6). (g) Polychlorinated biphenyls (PCB) (see 12.2.8.1.7). (h) Polyvinyl chloride (PVC) (see 12.2.8.1.8). (i) Cadmium and cadmium plating (see 12.2.8.1.9). (j) Freon solvents. (k) Radioactive materials . (l) Magnesium or magnesium base alloys 12.2.8.1.1 Toxic Pyrolytic Materials Toxic pyrolytic materials include those materials which emit toxic gases or other harmful products when exposed to high temperatures, including fire. Toxicity of materials can be determined by performing a pyrolysis test. One such test method is DTIC AD 297457, Procedure for Determining Toxicity of Synthetic Compounds. 12.2.8.1.2 Flammable Materials Flammable materials include any material in a form which will ignite or explode from an electric spark, flame, or from heating, and which, if so ignited, will independently support combustion in the presence of air. Material

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flammability can be determined by various test methods. Some example test methods include; DTIC AD 297457, ASTM D5948, Standard Specification for Molding Compounds – Thermosetting, and ASTM D 635, Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position. 12.2.8.1.3 Fragile or Brittle Materials Fragile materials include any materials which are fragile in the form, size, and manner in which they would be used. Brittle materials, in general, fall within this category from the point of use as structural members. However, certain brittle materials may be used in small quantities, within a part, when the materials is so mounted, constrained or otherwise disposed within the part that it will not be strained under any processing, environmental, and handling conditions to which the part reasonably may be subjected. (For example, glass and ceramic terminals seals and bushings have been employed successfully in packaging certain semiconductor devices). Any material in a frail form which is not positively protected against mechanical damage as used in a part or subassembly is considered fragile. Cast iron, semi-steel, porcelain, and similar brittle materials are not recommended for use as frames, brackets, mounting panels, spacers, or enclosures. 12.2.8.1.4 Mercury Mercury is highly toxic to humans and is corrosive to metals. Therefore, equipment, components and cable and wire harness assemblies should be free of mercury. Mercury should not be used in the manufacture or testing of cable and wire harness assemblies. Manufacturers should evaluate materials to confirm that they do not contain intentionally added mercury. For example, some potting compounds may use a catalyst that contains mercury as a method of minimizing the presence of air bubbles during the potting process. 12.2.8.1.5 Asbestos Materials containing asbestos should not be used since asbestos is a known human health hazard. 12.2.8.1.6 Silicone This material is acceptable when used in conjunction with the manufacture and/or installation of cable and wire harness assemblies. Silicone can also contaminate a solder joint in event it is inadvertently allowed to come in contact with surfaces designated for soldering. 12.2.8.1.7 Polychlorinated Biphenyls (PCB) PCBs are a type of organic compound that should not be used because they are an environmental pollutant. Historically, their main use was within components such as certain types of capacitors, and transformers. 12.2.8.1.8 Polyvinyl Chloride (PVC) PVC, commonly referred to as vinyl, is hazardous to human health and the environment throughout its entire life cycle. PVC is useless

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without the addition of a plethora of toxic additives, which can make the PVC product itself harmful to consumers. These chemicals can leach out of PVC, posing risks to children and consumers. New car smell? New shower curtain smell? That’s the smell of poisonous chemicals out-gassing from the PVC. One of the most toxic additives is DEHP, a phthalate that is a suspected carcinogen and reproductive toxicant readily found in numerous PVC products. When heated in a fire, PVC releases toxic hydrogen chloride gas, forming deadly hydrochloric acid when inhaled. PVC cannot be effectively recycled due to the many different toxic additives used to soften or stabilize PVC, which can contaminate the recycling batch. Based on the above, PVC, in particular, PVC insulated wire is not recommended for use. 12.2.8.1.9 Cadmium and Cadmium Plating Cadmium and its compounds are extremely toxic even in low concentrations, and will bio-accumulate in organisms and ecosystems. When working with cadmium, it is important to do so under a fume hood to protect against dangerous fumes. For example, silver solder, which contains cadmium, should be handled with care. Serious toxicity problems have resulted from long-term exposure to cadmium plating baths. Based on the above, manufacturers should evaluate the components, fasteners, and other items they intend on using to verify that they do not contain cadmium. One common source of cadmium is fasteners that have been cadmium plated. 12.2.8.2 Other Non-recommended Materials The following other types of materials should not be used:

(a) Linen (b) Cellulose acetate (c) Cellulose nitrate (d) Regenerate cellulose (e) Wood (f) Jute (g) Leather (h) Cork (i) Paper and cardboard (j) Organic fiberboard (k) Hair or wool felts (l) Plastic materials using cotton, linen or wood flour as a filler

12.2.9 Metals Metals should be selected or processed and applied in a manner that provides corrosion-resistance. Metals that are not inherently corrosion resistant should be processed (treated, plated, or painted) to provide corrosion-resistance.

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12.2.9.1 Selection of Metals in Direct Contact Equipment should meet guidelines for minimizing attack due to electrolytic action between dissimilar metals in contact with each other. MIL-STD-889 can be used for guidance. See paragraphs 12.11.8.1 and subparagraphs thereto for additional information. Metal-to-metal contact is not normally considered to exist if one of the contact surfaces is hard-coat sulfuric acid anodized aluminum that has not been previously exposed to a corrosive environment. If a metal is coated or plated, the coating or plating metal rather than the base metal should be considered. 12.2.9.2 Corrosion-Resisting Metals The following commonly used metals, when properly applied, are considered to be inherently corrosion-resistant without further processing when the service environment precludes immersion, condensation, or periodic wetting of the surface. These metals are suitable except where otherwise specified for severe environmental conditions. (a) Brass – Brasses containing 20 to 40 percent zinc are highly susceptible to stress corrosion cracking in marine environments when highly stressed (b) Bronze (c) Copper (d) Copper-nickel alloy (e) Copper-beryllium alloy (f) Copper-nickel-zinc-alloy (g) Nickel-copper-alloy (h) Nickel-copper-silicon alloy (i) Nickel-copper-aluminum alloy (j) Aluminum alloys, types 3003, 3004, 5052, 5056, 5083, 5085, 5086, 5154, 5456, 6061 (k) Titanium (1) Austenitic steels, corrosion-resisting types 202, 301, 302, 303, 304, 304L, 309, 310, 316, 316L, 321, 324A, 347 – Austenitic stainless steels are susceptible to stress corrosion cracking in marine environments when service temperatures exceed 150ºF. 12.2.10 Plastics It is recommended that plastics which serve as electrical insulation be selected in accordance with 12.2.11. It is recommended that plastics which do not serve as electrical insulation (structural parts, and so forth) meet all physical and mechanical properties required for plastic insulating materials, including non-flammability and non-toxicity; however, these plastics may not need to meet the arcing and tracking resistance requirements. 12.2.11 Insulation Materials Various types of materials are suitable for use in the manufacture of cable and wire harness assemblies, and when installing these assemblies into their next-higher assembly. The following guidance is recommended regarding insulation materials:

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12.2.11.1 Arc and Tracking Resistance Structural insulators, such as laminates, molding compounds, encapsulating materials, bus bar coverings, and similar materials subject to arcing conditions should have an arc resistance of not less than 130 seconds and a track resistance of not less than 70 minutes for low voltage (< 2000 V) equipment. For equipment rated at 2000 V and higher a minimum track resistance of 300 minutes is recommended. Arc resistance may be determined via ASTM D495, and tracking resistance may be determined by ASTM D3638. 12.2.11.2 Laminated Plastics. Laminated plastics in the form of sheets, rods, or tubes are recommended for use where rigid materials with dielectric properties are needed. Such laminates should meet the temperature, mechanical and electrical requirements of each application. Other forms of plastics should be used only when suitable for the particular application. It is recommended that laminates be chosen to meet the minimum requirements for toxicity, flame resistance, and arc and tracking resistance. Machined edges on glass based laminates may require sealing to prevent moisture infusion. 12.2.11.3 Molded Thermosetting Plastics Molded thermosetting plastics are generally used in electrical equipment where a rigid dielectric is needed and where the form or shape is such that fabrication of the part out of sheet stock is too costly, or the part too complex in design. It is recommended that the molding compound meet minimum requirements for toxicity, flame resistance and arc resistance and the mechanical and electrical requirements for each application. See ASTM D5948 for some recommended molding compounds. Consideration should be given to specifying the color of thermosetting plastics depending on the voltage rating. For example, red color is normally used for voltages rated at 2000 V or higher and gray color is used for lower voltages. 12.2.11.4 Thermoplastics In general, thermoplastics are not recommended for any molded part because of temperature rating considerations. However, if the application is such that only a thermoplastic material can be used, then polyamide (nylon) (ASTM D4066), or polycarbonate (ASTM D3935), molding compounds are recommended as long as they are suitable for the application. 12.2.12 Classes and Definitions of Insulating Materials Temperature classes of insulating materials have traditionally been established by definition based on a chemical composition of the materials. Methods of temperature classification based on the results

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of thermal evaluation tests are coming into use. Since the temperature classification of a material that has been accepted for a long time will have been established by field experience, its life temperature characteristics determined by test provides a basis for comparison with the thermal life of a new material. The purpose of assigning each material to a definite temperature class, therefore, is to facilitate comparisons between materials and to provide a single number to designate each class for purposes of standardization. The life expectancy under the test conditions may be shorter than, and has no direct relation to, the life expectancy of the material in actual service. The classes and definitions of insulating materials are grouped according to the classifications noted below. New classifications may be created, and new materials may be developed for existing classifications. Therefore, the listing in the subsequent paragraphs may not be all inclusive. 12.2.12.1 Class 90 Materials or combinations of materials such as cotton, silk, and paper without impregnation. Other materials or combinations of materials may be included in this class if by experience or accepted tests they can be shown to have comparable thermal life at 90ºC. 12.2.12.2 Class 105 Materials or combinations of materials such as cotton, silk, and paper when suitably impregnated or coated or when immersed in a dielectric liquid such as oil. Other materials or combinations of materials may be included in this class if by experience or accepted tests they can be shown to have comparable thermal life at 105ºC. 12.2.12.3 Class 130 Materials or combinations of materials such as mica, glass fiber, and so forth, with suitable bonding substances. Other materials or combinations of materials may be included in this class if by experience or. accepted tests they can be shown to have comparable thermal life at 130ºC. 12.2.12.4 Class 155 Materials or combinations of materials such as mica, glass fiber, and so forth, with suitable bonding substances. Other materials or combinations of materials may be included in this class if by experience or accepted tests they can be shown to have comparable thermal life at 155ºC. 12.2.12.5 Class 180 Materials or combinations of materials such as silicone elastomer, mica, glass fiber, and so forth, with suitable bonding substances such as appropriate silicone resins. Other materials or combinations of materials may be included in this class if by experience or accepted tests they can be shown to have comparable thermal life at 180ºC. 12.2.12.6 Class 200 Materials or combinations of materials such as mica, glass fiber, asbestos, and so forth, with suitable bonding substances. Other materials or combinations of materials may be included in this class if by experience or accepted tests they can be shown to have comparable thermal life at 200ºC.

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12.2.12.7 Class 220 Materials or combinations of materials which by experience or accepted tests can be shown to have the required thermal life at 220ºC. 12.2.12.8 Class 240 Materials or combinations of materials which by experience or accepted tests can be shown to have the required thermal life at 240ºC. 12.2.12.9 Class over 240 Materials consisting entirely of mica, porcelain, glass, quartz, and similar inorganic materials. Other materials or combinations, of materials may be included in this class if by experience, or accepted tests they can be shown to have the required thermal life at temperatures over 240°C. 12.2.12.10 Electrical Tape Electrical tape is sometimes used in lieu of sleeving for protection from chaffing and/or for maintaining required electrical clearances between non-common conductors. Any tape that provides the necessary electrical properties may be used. However, the tape used should be suitable for the maximum operating environment, and the adhesive should have a shelf life and robustness to remain attached without unraveling or otherwise coming loose over the expected life time of the equipment in the worst case operating environment. Some available tapes include MIL-I-631, MIL-I-24391, MIL-I-19166 and A-A-59770. Tapes selected should be non-flammable. Tape specifications should be carefully evaluated for flammability ratings since a specification may cover both flammable and non-flammable materials. 12.2.12.11 Sleeving Sleeving is commonly used to protect against chaffing and for maintaining required electrical clearances between non-common conductors. Any sleeving that provides the necessary electrical properties may be used. However, the sleeving used should be suitable for the maximum operating environment. Some available sleeving includes MIL-I-3190, MIL-I-631, MIL-I-22129 and AMS-DTL-23053. Sleeving selected should be non-flammable. Sleeving specifications should be carefully evaluated for flammability ratings since a specification may cover both flammable and non-flammable materials. One disadvantage of sleeving is that it normally cannot be easily replaced after the conductors are terminated with components such as terminal lugs or connectors unless the sleeving is cut and subsequently tied back together after installation; or unless an alternate type of sleeving such as “zipper” or “spiral” sleeving is used. Heat-shrink sleeving can also be an issue in event it is damaged via a nick or tear since in time, the damage can eventually cause the sleeving to completely tear and separate from the conductors due to the internal stresses from the heat shrink process.

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12.2.12.12 Straps/Clamps Various types of straps and/or clamps are normally used to restrain and/or route cable and wire harness assemblies. Their primary function is to preclude damage during a shock and vibration environment and to facilitate routing in an organized fashion. They also preclude interference with moving components such as cooling fan blades, etc. Straps and clamps that include a non-metal insert should be non-flammable. The applicable specifications should be carefully evaluated for flammability ratings since a specification may cover both flammable and non-flammable materials. The materials of the inserts should be verified for chemical compatibility with the conductor insulation since some materials may interact chemically and leave a residue on the conductor insulation or sleeving. It is also recommended that insert materials have an unlimited shelf-life (i.e., 20 years) whenever possible and appropriate for the end-item application. See SAE-AS23190 and SAE-AS21919 as examples. 12.2.12.13 Lacing Cord Lacing cord and/or lacing tape should be selected based on the maximum operating/environmental temperature range since some materials are restricted to 105ºC whereas other materials are available for 200ºC or greater temperature rating. Materials selected should not have a wax coating. Typical materials include; MIL-T-713, MIL-I-3158, MIL-Y-1140 and A-A-52080 through A-A-52084. 12.2.13 Terminal Lugs Whenever possible, it is recommended that terminal lugs be of the insulation restricting ring-tongue type such that the lug is designed to accommodate only a single wire size (the inside diameter of the terminal lug barrel in which the bare wire is to be inserted is smaller than the outside diameter of the insulation of the wire). This precludes crimping down over the wire insulation and also helps ensure the correct wire size has been used with the correct size lug. Normally these type lugs have the wire size embossed in the lug barrel and the insulation portion of the lug is color coded with a bi-color stripe for identification purposes. See SAE AS7928/1 or SAE AS7928/2 for this type of lug. SAE AS7928 ring type lugs are recommended for larger wire sizes not included in the other two specifications. Class 1 ring type terminal lugs per the cited specifications are preferred for reference on drawings; however, Class 2 lugs may be used by the manufacturer. Other types of lugs may be used when specified or approved. When required, terminal lugs should be of the insulated type as long as the temperature rating of the terminal lug is capable of withstanding the worst case equipment operating temperature in the worst case environment. Another alternative is to select an un-insulated lug and add properly temperature rated insulation over the lug.

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Terminal lugs should be plated with either tin or silver; however, tin whisker concerns should be considered if a pure tin plated lug is selected. Only tin plated lugs are recommended for connections to aluminum material. Un-plated copper or copper alloy lugs are not recommended and may require specific user approval. 12.3 Wiring Selection Wiring should be of a type suitable for the application. Wire should be selected so that the rated maximum conductor temperature Is not exceeded for any combination of electrical loading, ambient temperature, and heating effects of bundles, conduit and other enclosures. Typical factors to be considered in the selection are voltage, current, ambient temperature, mechanical strength, abrasion, flexure and pressure altitude requirements, and extreme environments such as Severe Wind and Moisture Problem (SWAMP) areas or locations susceptible to significant fluid concentrations. The wire selection should take into account all requirements of the application and the following design considerations: 12.3.1 Conductor Degradation Degradation of tin and silver plated copper conductors wI1l occur If they are exposed to continuous operation at elevated temperatures. These effects should be taken into account in the selection and application of wiring. 12.3.1.1 Tin Plated Conductors Tin-copper intermetallics will form resulting in an increase in conductor resistance. The increase is inverse to size, being up to 4 percent for the smallest gage. 12.3.1.2 Silver Plated Conductors Degradation in the form of inter-strand bonding, silver migration, and oxidation of the copper strands will occur with continuous operation near rated temperature, resulting in 1oss of flexibility. Due to potential fire hazard, silver plated conductors should not be used in areas where they are subject to contamination by ethylene glycol solutions. These potential problems should be considered in the application of silver plated copper wire. 12.3.1.3 Solderability Both tin plated and silver plated copper conductors will exhibit poor solderability after exposure to continuous elevated temperature. In event of a maintenance action, it may be necessary to re-tin the conductors to restore their solderability. 12.3.2 Aluminum Wire Aluminum wire should be avoided and may require user approval for use. If aluminum wire has been authorized for use, it should be terminated only by terminations specifically compatible with the application.

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12.3.3 Insulation Compatibility with Sealing and Servicing Wiring terminations in devices where the wiring must be sealed to provide an environment-resistant joint should have insulation compatible with the sealing feature of the device. After installation, the integrity of the sealing features of all such devices should be intact, and able to perform their function. A device is considered as sealed if the outermost sealing feature (web) is in full contact with the device when visually inspected. The wiring should be installed so that transverse loads will not destroy the integrity of the sealing feature of the wire. 12.3.3.1 Wire Diameter The finished wire outside diameter should be within the limits specified for the grommet specified in the appropriate component specification and should not exceed the capability of contact servicing tools to insert and release contacts. 12.3.3.2 Potting Seal on Wire or Cable The potting should be bonded to the outermost surface of the wire or cable in such a way to ensure an environmental seal. 12.3.3.3 Insulation Degradation Wiring should be handled, stripped and installed so as not to distort, roughen or damage the insulation on which sealing is to be effected. Methods-of marking and identification should be applied so as not to provide a track for moisture entry. The impression left on the insulation of shielded and twisted wires can also cause unacceptable degradation of the insulation in relation to the elastomer seal. Caution should be used to avoid this condition. 12.3.4 Wire Size and De-rating The required minimum wire size (AWG or conductor diameter) depends on the maximum operating and/or transient current it will need to carry in the application. It also depends on the worst case operating environment and whether the wiring is in an open or closed enclosure, with or without added air flow or cooling, and whether or not the wires are bundled together, how many wires are in a bundle, and whether the bundle is covered with a protective material. The impact of altitude should also be considered when determining the minimum wire size Including specific minimum wire current rating information herein for all available types of wire is beyond the scope of this handbook. However, generally wire ratings for aerospace application are more conservative that for other applications and therefore, the following wire de-rating values from NASA publication EEE-INST-002 are reproduced in Table 12-1 below. The reader should consult applicable design standards for other wire current ratings.

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Table 12-1 Wire and Cable De-rating Requirements (Notes 1 and 2)

De-rated Current (Amperes) Wire Size (AWG) Single Wire Bundled Wire or Multi-

conductor Cable 30 1.3 0.7 28 1.8 1.0 26 2.5 1.4 24 3.3 2.0 22 4.5 2.5 20 6.5 3.7 18 9.2 5.0 16 13.0 6.5 14 19.0 8.5 12 25.0 11.5 10 33.0 16.5 8 44.0 23.0 6 60.0 30.0 4 81.0 40.0 2 108.0 50 0 147.0 75.0 00 169.0 87.5

Note 1: De-rated current ratings are based on an ambient temperature of 70ºC or less in a hard vacuum 10-6 torr. For de-rating above 70ºC ambient, consult the NASA project parts engineer. Note 2: The de-rated current ratings are for 200ºC rated wire, such as TeflonTM insulated (PTFE) wire, in a hard vacuum of 1 X 10-6 torr.

a. For 150ºC wire, use 80% of the values shown in Table 12-1. b. For 135ºC wire, use 70% of the values shown in Table 12-1. c. For 260ºC wire, use 115% of the values shown in Table 12-1.

Small wire sizes can save weight but require special handling in order to prevent breakage. Conductors smaller than 26 AWG are discouraged for use and may require specific user approval for use. When conductors smaller than 24 AWG must be used, it is recommended that the conductor material be high strength copper alloy. The number of different wire sizes should be minimized. For example, it is not desirable to mix 22 AWG and 20 AWG wires unless some means such as color coding is used to differentiate between the wires. It is relatively easy to visually mistake 22 AWG wire for 20 AWG wire. Standardizing on just a few different wire sizes minimizes the amount of different tooling (e.g., crimp tools, contact insertion tools, etc.) needed thus saving costs. 12.3.5 Wire and Cable Identification In some applications, it may be necessary to mark each wire and cable with an identification code on the jacket or sleeving of the wire

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and cable via a method that will not damage the conductor or insulation. See Section 8 of this handbook for additional guidance on marking and labeling. Identification may include component reference designations (e.g., J for connector receptacles, P for connector plugs, TB1-1for terminal 1 of terminal board # 1, etc.), harness part numbers, drawing revision level, RoHS markings, and other types of markings. In some applications it may be necessary to identify the criticality of the circuit the wire or cable is a part of to ensure proper harness separation and protection from damage, fire, etc. It is recommended that identification marking be readable horizontally from left to right and vertically from top to bottom. Marking characters should be legible and permanent and the method of applying the marking should not impair the characteristic of the wiring. Hot stamp marking directly on wire or cable insulation is not recommended. Identification should be located close to the termination end such that it is readily visible on installation. Long wire or cable runs may require identification at periodic intervals (e.g., every 12 inches or shorter). 12.3.5.1 Wire Size Color Code System Use of color coded wire provides an easy means for differentiating between different wire sizes, as well as for facilitating separation of wiring for critical versus non-critical circuits. However, color coding may require user approval. When wire color coding has been authorized, the color code scheme shown in Table 12-2 is recommended, unless otherwise specified.

Table 12-2 Wire Size Color Code Size Color Size Color 26 Black (Not a recommended wire size) 10 Brown 24 Blue (Not a recommended wire size) 8 Red 22 Green 6 Blue 20 Red 4 Yellow 18 White 2 Red 16 Blue 1 White 14 Green 0 Blue 12 Yellow

The color code can be assigned by using solid colored wire, wire with a distinctively color band or distinctively stripped; however, the methods should not be mixed in the harness or assembly for consistency and understanding. The use of black and green color coding per table 12-2 also needs to consider whether this same color scheme is being applied to ground wires and/or common return wires to avoid any confusion.

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12.3.6 Wire for Electromagnetic Interference (EMI) Some wires and cables may connect to circuits that are vulnerable to noise pickup or incorrect equipment operation when subjected to external (e.g., lightning, operation of welding equipment, lighting fixtures, magnetic components, etc.) or internal (e.g., within the chassis) electromagnetic interference. In military applications, such as for Naval ship-based aircraft, wiring must be such as to facilitate proper operation in EMI fields caused by the ship’s radar, communication, and tracking equipment. Vulnerability to EMI can be mitigated by using wires with shields, or by adding shielding over the wire or cable harness. Twisting the conductors can also help alleviate EMI concerns, and routing wires away from EMI sources (e.g., magnetic components, switching power supplies, etc.) can also help mitigate EMI concerns. In addition to providing shielded wires, or adding shielding over completed wire and cable harnesses, special electrical cable that acts as distributed low-pass filters is sometimes used in critical applications where EMI interference cannot be tolerated. One such filter cable is MIL-C-85485. Filter line wires must have a metallic shield surrounding them in order for the expected filtering action to occur. The method of installing this type of cable is critical to performance of the product for its intended use. Consult SAE AIR4465 for details on how to install this type cable. 12.4 Service Life Wire and cables and associated components used for the wire and cable installation should be selected and installed to provide ease of maintenance and high reliability over the expected service life of the equipment. The user normally defines the required service life. 12.5 Safety and Personnel Protection Wire and cable harness installation can have a significant adverse impact on personnel safety unless steps are taken to address personnel safety considerations. When the harnesses are properly installed and the enclosure is grounded, there should be no accessible way for operating personnel to receive an electric shock even though an internal fault that may exist between any two circuits, between any circuit and a structural member, or between any circuit or ground. Installation should be such as to minimize the possibility of maintenance personnel being exposed to electrical shock while servicing, adjusting, or checking out the equipment. For access to such circuits, further positive action should be required to remove a cover or open a portion of the guard means. A warning plate is recommended for prominent display to remind personnel of appropriate precautions to ensure the circuit has been de-energized. With regard to personnel protection from mechanical safety hazards, external moving parts should be avoided. If their use is unavoidable, positive protection in the form of a guard should be provided. Sharp corners and projections which may cause injury or catch on clothing should be avoided. Excessive fastener length for fasteners used to secure clamps, etc. can act as sharp surfaces and as such may cause personnel injury.

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12.5.1 Electric Shock Electric shock occurs when the human body becomes a part of an electric circuit. It most commonly occurs when personnel come in contact with energized devices or circuits while touching a grounded object or while standing on a damp floor. The major hazard of electric shock is death. The effects of an electric current on the body are principally determined by the magnitude of the current and the duration of the shock. The current is given by Ohm’s Law, which, stated mathematically, is I= V/R where V is the open circuit voltage of the source and R is the resistance of the total path including the internal source resistance, and not just the body alone. In power circuits, the internal source resistance is usually negligible in comparison with that of the body. In such cases, the voltage level, V, is the important factor in determining if a shock hazard exists. At the commercial frequencies of 50-60 Hz and at voltages of 120-240 volts, the contact resistance of the body primarily determines the current through the body. This resistance may decrease by as much as a factor of 100 between a completely dry condition and a wet condition. Thus, perspiration on the skin has a great effect on its contact resistance (For calculation purposes, the resistance of the skin is usually taken to be somewhere between 500 and 1500 ohms). At voltages higher then 240 volts, the contact resistance of the skin becomes less important. At the higher voltages, the skin is frequently punctured, often leaving a deep localized burn. In this case, the internal resistance of the body primarily determines the current flow. 12.5.1.1 Levels of Electric Shock The perception current is that current which can just be detected by an individual. At power frequencies, the perception current usually lies between 0 and 1 milliamps for men and women, the exact value depending on the individual. Above 300 Hz, the perception current increases, reaching approximately 100 milliamps at 70 kHz. Above 100-200 kHz, the sensation of shock changes from tingling to heat. It is believed that heat or burns are the only effects of shock above these frequencies. The reaction current is the smallest current that might cause an unexpected involuntary reaction and produce an accident as a secondary effect. The reaction current is 1-4 milliamps. The American National Standards Institute limits the maximum allowable leakage current to 0.2 milliamps for portable two-wire devices and 0.75 milliamps for heavy movable cord-connected equipment in order to prevent involuntary shock reactions. Shock currents greater than the reaction current produce an increasingly severe muscular reaction. Above a certain level, the shock victim becomes unable to release the conductor. The maximum current at which a person can still release a conductor by using the muscles directly stimulated by that current is called the “let-go” current. The “let-go” current varies between 4-21 milliamps, depending on the individual. A normal

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person can withstand repeated exposure to his “let-go” current with no serious after effects when total duration of each shock lasts only for the time required for him to release the conductor. Shock currents above about 18 milliamps can cause the muscles of the chest to contract and breathing to stop. If the current is interrupted quickly enough, breathing will resume. However, if the current persists, the victim will loose consciousness and death may follow. Artificial respiration is frequently successful in reviving electric shock victims. Above a certain level, electric shock currents can cause an effect on the heart called ventricular fibrillation. For all practical purposes, this condition means a stoppage of the heart action and blood circulation. Experiments on animals have shown that the fibrillating current is approximately proportional to the average body weight and that it increases with frequency. In Table 12-3, the various hazardous current levels for ac and dc are summarized along with some of the physical effects of each.

Table 12-3 Summary of the Effects of Electric Shock Alternating Current

(60 Hz)(ma) Direct Current (ma) Effects

0-1 0-4 Perception 1-4 4-15 Surprise (Reaction Current) 4-21 15-80 Reflex Action (Let-Go Current)

21-40 80-160 Muscular Inhibition 40-100 160-300 Respiratory Block

Over 100 Over 300 Usually Fatal

12.5.1.2 Shock Prevention Most shock hazards can be divided into two categories: unsafe equipment and unsafe acts. The most common hazards in each category can be controlled as follows:

a. Power cords and drop cords with worn and/or broken insulation should be routinely replaced.

b. All spliced cords should be removed from service. c. Exposed conductors and terminal strips at the rear of switchboards and

equipment racks should be enclosed and warning labels installed. d. Rubber mats should be installed on the floor of all enclosures containing

exposed conductors and on the floor in front of high voltage switches. e. High voltage switches should be of the enclosed safety type. f. All wiring should comply with recognized electrical codes and it should be

large enough for the current being carried. g. Temporary wiring should be removed as soon as it has served its purpose. h. The noncurrent-carrying metal parts of equipment and power tools should be

grounded. i. The main power switch to all circuits being worked on should be locked open

and tagged.

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j. Power switches should be opened before replacing fuses and fuse pullers should be used.

k. Fuse boxes should be locked to prevent bridging or replacing with a heavier fuse.

l. Care should be taken to prevent overloading of circuits. 12.5.1.2 Exposed Metal or Other Conductive Parts Design and construction of the equipment should be such that all exposed parts or panels of metal or other electrically conductive material are at ground potential at all times. Exposed metal portions of electrical parts (switches, and so forth) or other parts located near electrical circuits (including parts inside enclosures where access is required for operation or adjustment) should be in intimate physical contact with the frame of the equipment or electrically connected to the frame if these parts could touch the electrical circuits as a result of deformation, wear, insulation failure, and so forth. 12.6 Electrical Creepage and Clearance Distances Wire and cable harness assemblies should be designed and/or installed in their next higher assemblies in a manner that precludes direct electrical short-circuits, or otherwise an electrical breakdown due to insufficient spacing between non-common conductors, or via insufficient spacing along insulating materials. This is assured by specifying a minimum electrical creepage and clearance distance. Electrical creepage and clearance distances are defined as follows: (a) Clearance distance is the shortest point-to-point distance in air between un-insulated energized parts or between an energized part and ground. (b) Creepage distance is the shortest distance between energized parts, or between an un-insulated energized part and ground, along the surface of an insulating material. When necessary, insulating barriers may be used to interrupt continuous electrical creepage paths. Cemented or butted joints should not be accepted as techniques to obtain the minimum creepage distances in Table 12-4. Creepage and clearance distances between electrical circuits, between each electrical circuit and ground, and across lines and between circuit elements that operate at significantly different potential levels within each circuit should be not less than those values shown in Table 12-4. It is emphasized that the values shown in Table 12-4 represent the minimum acceptable limits for non-arcing rigid construction based on normal volt-ampere (product of the normal voltage applied to the circuit times the current carried) ratings and that they take into consideration only the average degree of enclosure and service exposure. Therefore, the designer should employ creepage and clearance distances in excess of these minimums where-it is probable that structural features, contaminants, lack of maintenance, environment, exposure or application overstress will create service conditions more severe than normal.

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12.6.1 Distance From Enclosure Exposed non-arcing current-carrying parts within enclosures should have an air space between them and the un-insulated part of the enclosure of not less than 0.75 inch. However, the values shown in Table 12-4 may be applied to the creepage and clearance distances between un-insulated parts of enclosures and exposed non-arcing current-carrying parts of devices whose mounting is sufficiently rigid and so designed to prevent decrease of the clearance distance through a blow on, or distortion of the enclosure.

Table 12 -4 Electrical Creepage and Clearance Distance (Note 1) Creepage (Note 3) Voltage

(ac or dc) Set (Note 2) Clearance (Inches)

Open (Inches) (Note 4)

Enclosed (Inches) (Note 5)

A 1/16 1/16 1/16 B 1/8 1/8 1/8

Up to 64

C 1/8 3/8 1/2 A 1/16 1/16 1/16 B 1/8 ¼ 1/8

Over 64-150

C 1/4 ¾ 3/8 A 1/16 1/16 1/16 B 1/8 ¼ 1/8

Over 150-300

C 1/4 ¾ 1/2 A 1/16 1/8 1/8 B 1/8 ¼ 1/4

Over 300-600

C 1/4 ¾ 1/2 A 1/8 ½ 3/8 B 1/4 1 3/4

Over 600-1000

C 1/2 2 1.5 Over 1000-3000 C 2 4 2 Over 3000-5000 C 3 5 3

Note 1: Use of electrical parts or assemblies such as potentiometers, connectors, printed wiring assemblies, and :similar devices having lesser creepage and clearance distances is permissible provided these parts and assemblies conform with applicable specifications, and their energized portions are enclosed to protect against entry of dust and moisture. For example, wire connections to a power semiconductor may require additional protection since the clearance distance between the semiconductor terminals may be less than listed in Table 12-4. Note 2: Set A - Normal operating volt-ampere rating up to 50. Set B - Normal operating volt-ampere rating of 50 to 2000. Set C - Normal operating volt-ampere rating over 2000. Note 3: For top curved surfaces having a radius greater than 3 inches and for top flat surfaces, surface creepage distance should be increased 33 percent where these surfaces have irregularities which permit the accumulation of dust and moisture. Note 4: Open is defined as equipment or parts with open enclosures in accordance with MIL-STD-108.

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Note 5: Enclosed is defined as equipment or parts with enclosures in accordance with MIL-STD-108, except open enclosures. 12.7 Accessibility All wire and cable harness assemblies, as installed, which may require servicing or replacement during the life of the equipment, should be readily accessible for such actions without major disassembly of the equipment and without removing the equipment from its foundation. Access to wire and cable harness assemblies should be from the front of the enclosure. 12.8 Maintenance and Repair Cable and wire harness assemblies, and installation into the next higher assembly, should be designed for ease of maintenance and repair Whenever possible, the design should be capable of being repaired either by replacement of defective individual parts or by utilizing commonly available bulk materials (wire, varnish, insulation, etc.). 12.9 Smoke and Fire Hazards Wire, cable and associated installation components, should be selected and installed in such a manner to minimize the danger of smoke and fire hazards. Adequate protective means, both physical and electrical, should be employed to provide reliability and safety commensurate with this expectation. 12.10 Cable and Wire Harness Installation Design of wire and cable installation should conform to the following order of precedence:

(a) Safety (b) Cost (c) Ease of maintenance, removal and replacement of cable and wire.

Cable and wire harnesses should be fabricated and installed to achieve the following:

(a) Maximum reliability. (b) Minimum interference and coupling between systems. (c) Prevention of damage.

12.10.1 Arrangement and Harnessing Wiring should be neatly formed into groups which are locked, sleeved, tied, or clamped in a manner that provides support and prevents chafing of the wire insulation due to vibration and shock. There should be no splices in the wire (unless such splices have been specifically authorized for repairs), and all connections should be made at the terminals of the devices, at terminal blocks, or at part mounting boards. Wire groups running from hinged panels and doors should be flexible and covered with protective material, secured in place, as appropriate.

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Finished harness diameter should not restrict flexibility requirements where necessary. The use of preformed cables and wiring harnesses is preferred to the point-to-point method of wiring. Conductors combined into a harness should be securely held together by means of lacing, ties, or clamps, or be permanently mounted in cabling ducts. Individual conductors which are thus combined should lie parallel to one another and should not entwine other conductors. This is not intended to preclude the use of twisted pairs or triads where required for electrical reasons. The combined heating of bundled wires or proximity heating by components should not cause maximum temperatures of harnessed wire insulation to be exceeded. 12.10.2 Bundle and Group Size As a design objective, bundles and groups within clamps should be no more than 2 inches in diameter. Wiring to high density connectors may be run as a single group, provided all of the wiring in the group is pertinent to a single item, equipment or system. The number of electrical wires in high density harnesses should be limited only by efficient and good design. The use of wire sizes larger than 16 is discouraged unless there are also smaller electrical wires in the same harness. 10.10.3 Dead Ending Each un-designated wire end should be dead ended with a suitable protective cap such as an AS25274 crimp cap, or other suitable means of protection. 10.10.4 Splicing Splicing of conductors should be avoided and the use of a splice connection may require user approval. If splicing is allowed, the splice should be made using standard crimp and/or solder splices made for this purpose. It is recommended that each lot of splice connections have a sample pull test performed to verify the integrity of the splice connection.

12.10.5 Routing In addition to 12.10.1 above, wires and cables should be routed to ensure reliability and to offer protection from the following hazards:

(a) Chafing – Chafing is defined as repeated relative motion between wiring system components, or between a wiring system component and structure of equipment, which results in a rubbing action that causes wear which will likely result in mechanical or electrical failure during the equipments specified service life.

(b) Use as handhold or as support for personal equipment.

(c) Damage by personnel during use or storage.

(d) Damage by anticipated environmental conditions, including any harsh

environmental conditions (moisture, high temperatures, fluids, etc.).

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(e) Damage by moving parts (e.g., cooling fans, etc.).

(f) Crushing, kinking or stretching fiber optic cable. These cables should be

routed to avoid the application of axial and lateral loads to the cable end terminations. When not being used, the fiber optic connectors should be covered with a temporary protective cap to preclude damage to the termination.

(g) Routing through the equipment mounting base should be avoided unless

specifically required (e.g., such as when using “stuffing tubes” to facilitate wire and cable entry from the bottom of an enclosure).

(h) Fire hazard may require special wire or cable bundle protection, or otherwise

routing in a separate bundle such that a fire in a single bundle will not impact wires or cables in a redundant circuit.

12.10.6 Stress Relief and Mechanical Support Wire and cable assemblies should be routed such that they are not under tensile or compressive stress in order to prevent damage to the terminations under a shock or vibration environment, or when the wire and cables connect between movable sections of the equipment. Electrical connections should be designed and provided with supports to prevent breakage and minimize changes in performance due to vibration, inclination, or shock. Where electrical connections are constructed of members in firm contact, such as parts held together by bolts, the contacts should not depend on force transmitted through plastic spacers or other deformable parts. Only metal parts should be so employed and these electrical connections should not rely on the clamping screw, bolt or fastener threads to carry current; however, stud type semiconductor devices may be mounted separated from their heat sinks or other mounting surfaces by insulators when direct metallic contact is incompatible with the circuit requirements, provided the method of mounting conforms to the device manufacturer’s recommendations.

12.10.7. Slack in Cable and Wiring In addition to slack provided for drip loops (See 12.10.11), slack should also be provided as noted below. In production, wire harness fabrication provisions should be incorporated into the harness design and fabrication process to ensure that the installed harness meets the criteria without the need for straining, forcing or modifying the harness. Slack should be provided so as not to impair movement or put undue stresses on the wires or parts in those places where movement of parts may be expected. Slack should also be provided to prevent undue-stresses on terminal connections due to shock or vibration. Where soldering is used to connect hook-up wires to the terminals of replaceable parts, -sufficient slack should be provided for at least one replacement of the part in the event that the wires are damaged or have to be clipped at the terminals during disassembly.

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Where solder-less lug terminals are used, sufficient slack should be provided for one replacement of the terminals on 14 AWG and smaller wires. 12.10.7.1 Connector Termination When wiring is terminated in a connector or terminal junction, a minimum of 0.5 inch of slack for complete connector replacement should be provided. This slack should be between the connector and the second wiring support clamp. The 0.5 inch slack requirement is interpreted to mean that with the connector unmated and the first wiring support clamp loosened, the wiring will permit the front end of the connector shell to extend 0.5 inch beyond the point normally required to properly mate the connector. Slack for replacement of potted connectors should be, as a minimum, the length of the potting plus one inch. Connectors terminating size 8 and larger electrical wires, RF cables, and fiber optics cables are normally not subjected to re-termination slack requirements. 12.10.7.2 Lug Termination At each end of a wire terminated by a lug, a minimum length of slack equal to twice the barrel length of the lug should be provided. For copper wire, size 2 and larger, and aluminum wire, size 4 and larger, the minimum length of slack should be equal to one barrel length of the lug. The slack should be in the vicinity of the lug and available for replacement of the lug by maintenance personnel. 12.10.7.3 Strain Protection The wiring and cable installation should be designed to prevent strain on wires, junctions and supports. 12.10.7.4 Free Movement The wiring and cable installation should permit free movement of shock and vibration mounted equipment. 12.10.7.5 Cable and Wire Shifting The cable and wire installation should permit shifting of the wire, cable, and equipment to perform maintenance. 12.10.8 Inspection and Maintenance In open wiring, groups should be installed to permit replacement of the group without removal of the bundle. High density harnesses should be designed so that they are readily replaceable in sections. Fiber optic cables should be installed so they are accessible for periodic inspection, and replacement, if needed, without the need to disassembly any riveted or bonded attachments. Wires or cables should not be routed such that they impinge on, or otherwise impede component safety provisions (e.g., do not block the pressure-relief plug on power can type capacitors, etc.). 12.10.9 Protection and Support Wiring and cable should be supported to; (a) Prevent chafing as previously defined. (b) Secure wiring and cable where routed through bulkheads and structural

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members. (c) Properly group, support and route wiring and cables in junction boxes, panels and bundles. (d) Prevent mechanical strain or work hardening that would tend to break the conductors and connections. (e) Prevent arcing or overheated wiring from causing damage to mechanical control cables, and associated moving equipment. (f) Facilitate reassembly to equipment terminal boards. (g) Prevent interference between wiring and other equipment. (h) Provide support for wiring to prevent excessive movement in areas of high vibration. (i) Dress the wiring at connectors and terminating devices in the direction of the run without deformation of grommet seals. Primary support of wiring and cable should be provided by metal cushion clamps and plastic clamps spaced at appropriate intervals. In addition, where wiring is routed through cutouts in any metal structure, clamps should be installed as necessary to preclude damage, including chafing. Open wiring contained in troughs, ducts or conduits is exempt from the need for clamps. Clamps for harnesses other than round should be shaped to fit the contour of the harness and should provide a snug fit. Plastic clamps should not be used to support rigid portions of harnesses. Plastic cable straps should not be used as primary supporting devices unless allowed by the user. The primary support of wiring should not be attached to adjacent wiring. Clamps should be of a size that holds the wiring and cable in place without damaging the insulation or degrading its performance. Wire or cable bundle diameter should be adjusted (built-up) with a suitable material such as authorized tape or sleeving so the harness fits snug (but not overly compressed) within the clamp such that the clamp provides the desired clamping action.

Holes in metal that are not chamfered to prevent wire or cable chafing should be provided with a protective grommet. The grommet should be installed with the grommet seam facing away from the cable or wire and should be selected for compatibility with the maximum expected temperature. Normally these grommets are made from plastic material requiring an adhesive to secure the grommet, or an epoxy coated spring type stainless steel grommet that does not require an adhesive (See NASM22529 and NASM21266 for acceptable types of grommets). Ring grommets per NASM3036 are also acceptable for use. An epoxy rather than RTV material is recommended for securing the plastic type grommets. Harnesses can also be protected using tape suited for the purpose. One such acceptable tape is silicone based self-adhering tape per A-A-59163.

Terminals may be protected using a terminal nipple specifically made for this purpose. One such acceptable terminal nipple is silicone conforming to A-A-59178.

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12.10.10 Bend Radius See IPC/WHMA-A-620, paragraph 14.3.2 for recommended minimum bend radius. However, these values are not conservative for aerospace application where AS50881 may apply. For aerospace application, the following minimum bend radius is recommended:

(a) Individual wires, cables and harnesses, the recommended minimum bend radius is 10 X the diameter of the largest wire or cable in the harness. If wires used as shield terminators or jumpers are required to reverse direction in the harness, the minimum bend radius should be three times the diameter at the point of reversal providing the wire and cable are adequately supported.

(b) For semi rigid coaxial cable the minimum diameter should be ten times the outside diameter.

(c) For fiber optics cable the minimum bend radius should be in accordance with the cable manufacturer’s recommendations and be sufficient to avoid excessive losses or damage to the cable.

12.10.11 Drip Loop When wiring is dressed to a connector, terminal block, panel or junction box, in addition to the slack provision addressed above, a trap or drip loop should be provided in the wiring and cable to prevent fluids or condensate from running into the aforementioned devices. Potted connectors and connectors containing only fiber optics do not require a drip loop. 12.10.12 Routing Near Moving Parts or Controls Wiring or cables attached to assemblies where relative movement occurs (such as at hinges and rotating items) should be installed or protected in a manner to prevent damage or deterioration caused by flexing, pulling, abrasion and other effects of frequent removal and replacement of equipment. Whenever possible, it is recommended that bundles be installed to twist instead of bending across hinges. Wires and cables that must be located close to operating controls should be installed in a manner that allows for proper operation of the controls in the event of failure of any single point of attachment.

12.10.13 Routing near Fluid Lines Wiring and cables should be routed independent and separate from gas and fluid containing lines whenever possible. When this is not possible, it is recommended that the routing be above, and at an angle to, rather than parallel to the lines containing gas or fluids. Terminating devices should not be placed under any lines, and unless otherwise required, the wiring and cables should not be tied or otherwise supported by the lines. If wiring is installed in locations where fluids may be trapped and the wires and cables contaminated, the wires and cables should be properly routed and protected against fluid damage. Wiring and cables should not be routed through fuel tanks except where there is no alternative. It is recommended that if wiring and cables must be routed in a fuel tank, that the routing be via a separate dry access space to preclude fuel contact.

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12.10.14 Ground Return The electrical power source ground terminals should be connected to the primary metallic structure of the equipment using terminal lugs and threaded fasteners. However, for large size ground wires (AWG-4 or larger diameter), it is recommended that a separate ground tab sized to handle the ground current and fastened to the equipment structure by a suitable method be used. The shortest possible ground wire connection is recommended. Ground return wiring should not be connected to magnesium. Where EMI is a consideration, see MIL-STD-464 for additional guidance. 12.10.15 Shielded Wire Grounding Equipment EMI requirements may dictate the need to shield wiring and cable. Shields should be terminated as close as practicable to connectors, and specifically within any booted areas of breakout terminations. AS83519 shield terminations is one acceptable method for terminating tin and silver coated shields except when the operating temperature of the equipment exceeds the part rating. It is recommended that the un-terminated end of a shield be covered with sleeving or other means to preclude un-wanted local shield shorts to ground or to each other. In some applications, shields may need to float at one end since tying both ends of a shielded wire to ground may cause un-wanted noise signals. 12.10.16 Multiple Grounds The total number of ground connections to any single ground stud should not be greater than the stud is designed to handle. In some situations, it may be necessary to separate the grounds depending on the circuit application. 12.10.17 Connectors Connectors should be selected so that contacts on the “live” or “hot” side of the connection are socket type rather than pin type to minimize personnel hazard and to prevent accidental shorting of live circuits if the connector is unmated with power still applied. 12.10.17.1 Environment Resisting Connectors It is recommended that connectors be sealed against the ingress of water and water vapor under all service conditions including changes in altitude, humidity and temperature. The connectors should have an interracial seal as well as sealing at wire ends. Environment resisting connectors having wire sealing grommets are preferred; however, potting may be used where a grommet seal connector would not be suitable. 12.10.17.2 Contacts Connectors using removable crimp contacts are preferred to solder contact types. One acceptable contact is AS39029, or otherwise as specified by the base connector specification. Wire size should be within the crimp barrel size range as identified in the contact specification. Crimp tools used by the manufacturer should be such as to meet specified performance in accordance with the applicable connector specification and the crimp connections should be replaceable in the field using standard crimp tools available in the field.

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12.10.17.3 Spare Contacts In some applications, the user may require a certain number (e.g., 10%) of spare contacts to be provided within a connector for future growth. These spare contacts may require sealing grommets be installed if the connector is environmentally sealed. In some cases, unused contacts may need to be terminated with a dead-ended pigtail lead that can be connected to in the future without damaging the connector. In other applications, the connector insert may be provided with unused holes such that the contacts can be added later if needed. However, if the connector manufacturer recommends that all un-used insert holes be filled with contacts or otherwise where this is needed for shock and vibration considerations, then this is preferred over leaving the holes un-filled. 12.10.17.3 Connector Installation It is recommended that connectors be used to join harnesses to equipment or other harnesses when frequent disconnection is required to remove or service equipment, components or wiring. Connectors should be located and installed so that they will not provide hand holds or foot rests to operating and maintenance personnel, or be damaged. Fasteners should be used in all holes of flange mounted connectors. Both plug and receptacle should be visible for engagement and orientation of polarizing key(s). Mated plugs should not be strained by the attached wiring. Connectors used to provide separation of or connections to multiple electric circuits in the same location should be installed so that it will be impossible to mate the wrong connector in another mating unit. It is preferred that wiring be routed and supported such that an improper connection cannot be made. 12.10.17.3.1 Circular Connector Installation Adequate space should be provided for mating and un-mating connectors without the use of tools. A one inch minimum spacing for removal of the connectors by hand is recommended. Unless otherwise specified, circular connectors, when installed with the axis in a horizontal direction, should be positioned so that the master keyway is located at the top. When Installed with the axis in a vertical direction the master keyway should be located forward in relation to the equipment. 12.10.17.3.2 Rectangular Connector Installation Since rectangular connectors are normally provided with jack screws in lieu of a circular locking ring; much less hand space is needed for removing the connectors. Therefore, the rectangular connectors can be installed closer to each other. However, sufficient access for use of connector removal and re-installation tools is needed. 12.10.17.4 Potting Unless otherwise specified, potting is an acceptable method of providing sealing or stress relief protection for electrical connectors. The type of potting material should be compatible with the worst case equipment operating temperature in its worst case environmental temperature. Potting materials should also be chosen for compatibility with the various materials they will come in contact with. In some applications, the user may specifically prohibit potting of connectors. Potting compound

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should be given a sufficient cure cycle (time and temperature) to preclude out-gassing after delivery. 12.10.17.5 Safety Wiring Non-self-locking threaded coupled connectors located in areas of high vibration, and in areas which are normally inaccessible for periodic maintenance inspection, should have the coupling nut safety-wired or otherwise mechanically locked to prevent opening of the connector due to vibration. 12.10.17.6 Dust Protection During production manufacturing, un-mated connectors should be suitably covered with a protective material to preclude the entry of dust. This is of particular importance for fiber optic connectors. The protective material should remain installed until the wire or cable harness is installed in its next higher assembly. Plastic caps may be suitable as a protective material. 12.10.17.7 Connector Accessories Connectors should be provided with strain relief accessories when necessary to preclude damage to the wire or cable conductors. It is recommended that these accessories not be used for ground wire connections unless the accessories were specifically designed for this purpose. 12.10.18 Splices The use of wire or cable splices should be avoided whenever possible, unless their use has been specifically authorized by the contract documentation. When splices are allowed for use, the following use criteria is recommended: a. Limit the total number of splices to one per conductor. b. Install splices such that they do increase the size of the bundle so as to prevent its fitting in its designated space or which would adversely affect maintenance. c. Do not use splices to salvage scrap lengths of wire or cable. d. Don’t use splices within 12 inches of a termination device except when otherwise specifically approved. e. The engineering documentation should identify all allowable use of splices. f. Splices may be used for repairs when authorized. g. Splices should not be used for critical circuits such as firing or control circuits associated with ordnance or explosive sub-systems. h. Spliced wires in current carrying circuits should be of a size adequately protected by the circuit protection devices. Splices installed for assembly or subassemblies should be contained in splice areas identified as such on the applicable drawings. Splice areas should be selected so that they are readily accessible for maintenance and inspection including splices contained in the center of the bundle. 12.10.19 Terminal Lugs Terminal lugs should be used to connect wiring to terminal board studs, equipment terminal studs and ground studs. The maximum number of terminal lugs used on any single terminal should not exceed the maximum allowable number based on the stud design and need to maintain full engagement of the stud

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fasteners. Normally, this is four total (three lugs and one bus strip connection, or four lugs and no bus strip connection). When the terminal lugs attached to a stud vary in diameter, the greatest diameter should be placed on the bottom and the smallest diameter on top. Terminal lugs should be selected with a stud hole diameter which matches the diameter of the stud. Tightening terminal connections should not deform the terminal lugs or the studs. Whenever possible, it is recommended that straight terminal lugs not be bent for installation. However, bending of the lug tongue up to 90 degrees is acceptable as long as; the bend radius is not less than twice the thickness of the lug tongue; the distance from the tip of the tongue to the beginning of the bend is not less than the diameter across the lug; bending is not required to remove the fastening screw or nut; and the bend does not cause a crack or other damage to the lug. The position of the terminal lug should be such that movement of the lug will tend to tighten the fastening screw or nut. Spacers or washers should normally not be sandwiched between the tongues of terminal lugs. 12.10.20 Terminal Boards and Terminal Junction Modules Terminal boards or terminal junction modules should be used for junctions of wiring requiring infrequent disconnection or for joining two or more wires to a common point. Terminal boards and their associated terminals should be assigned a reference designation as shown on the electrical schematic or wiring diagram. For example, TB1-12A designates terminal board # 1, terminal 12, side A, if the terminal board contains two rows of terminals. The identification should be marked on the assembly adjacent to the terminal board such that it can be easily read; left to right, or top to bottom. Removal of the terminal board should leave the identification intact. Terminal boards should be secured with machine screws so they can easily be replaced if needed. 12.10.21 Wiring Mockup Consideration should be given to manufacturing a wiring mockup when complex harness assemblies are involved. This would facilitate clear understanding of how the harnesses must be installed within an assembly to establish necessary clearances between other components, to preclude potential chafing issues, to establish minimum bend radius for harnesses, and to address periodic inspection for tightness of electrical connection to terminal boards, and other factors. This mockup could also be used to determine the most cost effective manufacturing methods and to facilitate review and buy-in by the user when required.

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12.10.22 Screw Thread Standards for Fastening Devices Screw threads for all threaded fastening devices should be in accordance with ANSI B1.1. The threads should be the coarse-thread series, unified form, class 2A/2B unless the component design indicates a necessity for the use of the fine thread series. 12.10.22.1 Fastening of Harnesses and Associated Parts Through bolting should be used wherever practicable. For electrical panels and other applications where frequent disassembly is required, blind nuts and captive fasteners should be used when practical. Similarly, these types of fasteners should be used when practical to prevent a loose fastener from dropping into electrical equipment. 12.10.22.2 Threads in Aluminum Threads in aluminum or aluminum alloys should be avoided, where practicable, by use of through bolting. Where through bolting is not practicable, and screws are removed for routine equipment maintenance or where high stress in the screw is needed for alignment of a vital part, metal inserts for the fastenings should be cast or screwed into the aluminum or aluminum alloy. Inserts should be given a corrosion-resistant treatment, except where bushing type inserts of corrosion-resisting steel are cast into the aluminum or aluminum alloy. Inserts need not be provided for securing identification plates, terminal boards or other items that are removed only when the equipment is overhauled or modified. 12.10.22.3 Threads in Plastic. Metal inserts should be used where threads in plastic are used. 12.10.22.4 Inserts Metal inserts, where required in aluminum alloys or plastics, should be the bushing type, or the helical-coil. See MS22076 through MS122115 and NASM21209 for examples of the helical-coil inserts. Care should be exercised when installing helical-coil inserts, since if they are not properly installed, the inserts can easily back out of the hole when the mating fastener is removed. These inserts also contain a tang which is normally removed after installation. Attention should be given to ensure the inserts are installed in the correct orientation and that non-locking inserts are not inadvertently installed when a locking insert is required. Visual inspection should confirm that the insert has been installed since one can mistake a tapped hole as containing an insert when it in fact may not. The bushing type is recommended. The use of helical-coil type inserts should be limited to applications where the threaded hole permits full engagement of the insert. Bushing type inserts should be the cast-in, molded-in, or screwed-in types. Screwed-in types should be pin, key-, swage-, or ring-locked to prevent backing out. 12.10.22.5 Thread Projection Except for threading into blind holes or in

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thick material, bolts, and machine screws should be of such length that when tightened, at least one thread and preferably not more than four threads should project beyond the outer face of the nut or bolted part. With plastic insert self-locking nuts, the thread projection should be measured from the crown of the plastic insert. 12.10.22.6 Bolt and Screw Thread Engagement For materials having similar mechanical properties, the full thread engagement should be no less than one major diameter (ID). For materials having dissimilar mechanical properties, the minimum thread engagement should be in accordance with FED-STD-H28; part, 1, appendix 5, using the maximum tensile strength of the stud material and minimum specified tensile strength of the body material, plus one thread; but in no instance less than the root diameter. Where helical-coil type threaded inserts are used, the length of the thread engagement should be not less than 1-1/2 times the major diameter (nominal) of the bolt thread. 12.10.22.7 Thread Locking of Mechanical Assemblies Bolts, nuts, and screws, used for mechanical connections, where the specified operation under all anticipated conditions, including shock, vibration, and heating, depends upon maintaining tight connection of parts, or where a holding screw, bolt, nut, or fastened part may fall into the equipment, should be secured by one of the following means: (a) Lock Washer. (b) Lock Nut. (c) Castellated nut with cotter pin or safety wiring. (d) Self-locking screws. This method should only be used where removal for maintenance is very infrequent. (e) Deformation of screw or bolt threads projecting from nut or secured, part. This method should only be used in cases where disassembly is never required for maintenance or repair. (f) Locking wire for use to lock bolts when only bolt heads are available to apply a locking device. (g) Self-locking nut. (h) Blind nuts and captive fasteners. 12.10.22.8 Flexible Wiring Where flexible wiring is required by hinged doors, panels or sliding, subassemblies, abrasion and chafing should be minimized by use of flexible plastic sheaths on wiring. Wire groups running from hinged panels or doors should be formed and clamped so that sharp bends do not occur with the panel or door in either the open or closed position; and, if more than three wires are contained in the group and the panel or door is required to be

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removable, a terminal block (or the receptacle portion of a multi-pin connector, where permitted) mounted on a stationary part of the structure within the enclosure or on & the hinged panel or door should be used for connections. Flexible harnesses should be broken down into individual bundles. 12.10.22.9 Wire Connections and Terminals The ends of each conductor (except for conductors requiring solder connections to a terminal or stud) should be connected to terminals on the part or to terminal boards by means of solder-less lug terminals, or by forming the conductor around a part terminal and retaining the-loop in a cup or crimped washer. If a wire loop is used, strands of the conductor should be secured together by soldering. No more than three connections (or three connections with a bus strip) should be made to each terminal unless the terminal is specifically designed to accommodate additional conductors. Pins or conductors should not be paralleled for the purpose of increasing current capacity except where capacity above 220 amperes is required or where specifically allowed by the user. Nuts, bolts, studs, and screws used for electrical connections should be secured by lock washers, except lock washers may not be required when certain types of terminal boards, having a barrel-nut locking capability are used. External tooth flat lock washers are recommended for electrical connections, where practical. 12.10.22.10 Spare Terminals Terminal boards or cable connectors should have not less than 10 percent unused terminals when used for connections in the equipment and when used for the connection of assemblies with enclosures. There should be not less than two such terminals, except that no spares are required where a total of six, or less, active terminals are involved. Spare terminals in connectors should be in the outermost row of terminals. Where connectors or terminal boards are used only for primary power connections, no spare terminals need be provided. If more than one terminal board or connector is needed at a common place, only 10 percent of the total number of terminals at this place are required as spare terminals. 12.10.22.11 Manufacturing Processes Consult the applicable sections of this handbook and/or IPC/WHMA-A-620 for manufacturing processes such as crimping, soldering, weld connections, wire wrap connections, wire marking, etc. 12.11 Bonding As used herein, bonding refers to the process by which a low impedance path for the flow of an electric current is established between two metallic objects. Other

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types of bonding which involve simply the physical attachment of one substance or object to another through various mechanical or chemical means are not discussed herein. 12.11.1 Purposes of Bonding In any realistic electronic system, whether it be only one piece of equipment or an entire facility, numerous interconnections between metallic objects are made in order to provide electric power, minimize electric shock hazards, provide lightning protection, establish references for electronic signals, etc. Ideally, each of these interconnections should be made so that the mechanical and electrical properties of the path are determined by the connected members and not by the interconnection junction. Further, the joint should maintain its properties over an extended period of time in order to prevent progressive degradation of the degree of performance initially established by the interconnection. Bonding is concerned with those techniques and procedures necessary to achieve a mechanically strong, low impedance interconnection between metal objects and to prevent the path thus established from subsequent deterioration through corrosion or mechanical looseness. In terms of the results to be achieved, bonding is necessary for the: a. protection of equipment and personnel from the hazards of lightning discharges, b. establishment of fault current return paths, c. establishment of homogeneous and stable paths for signal currents, d. minimization of rf potentials on enclosures and housings, e. protection of personnel from shock hazards arising from accidental power grounds, and f. prevention of static charge accumulation. With proper design and implementation, bonds minimize differences in potential between points within the fault protection, signal reference, shielding, and lightning protection networks of an electronic system. Poor bonds, however, lead to a variety of hazardous and interference-producing situations. For example, loose connections in ac power lines can produce unacceptable voltage drops at the load, and the heat generated by the load current through the increased resistance of the poor joint can be sufficient to damage the insulation of the wires which may produce a power line fault or develop a fire hazard or both. Loose or high impedance joints in signal lines are particularly annoying because of intermittent signal behavior such as decreases in signal amplitude, increases in noise level, or both. Poor joints in lightning protection networks can be particularly dangerous. The high current of a lightning discharge may generate several thousand volts across a poor joint. Arcs produced thereby present both a fire and explosion hazard and may possibly be a source of interference to equipment. The additional voltage developed across the joint also increases the likelihood of flashover occurring to objects in the vicinity of the discharge path. Degradation in system performance from high noise levels is frequently traceable to poorly bonded joints in circuit returns and signal referencing networks. As noted previously, the reference network provides low impedance paths for potentially incompatible signals. Poor connections between elements of the reference network increase the resistance of the current paths. The voltages developed by the currents

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flowing through these resistances prevent circuit and equipment signal references from being at the same reference potential. When such circuits and equipments are interconnected, the voltage differential represents an unwanted signal within the system. Bonding is also important to the performance of other interference control measures. For example, adequate bonding of connector shells to equipment enclosures is essential to the maintenance of the integrity of cable shields and to the retention of the low loss transmission properties of the cables. The careful bonding of seams and joints in electromagnetic shields is essential to the achievement of a high degree of shielding effectiveness. Interference reduction components and devices also must be well bonded for optimum performance. Consider a typical power line filter like that shown in Figure 12-1. If the return side of the filter (usually the housing) is inadequately bonded to the ground reference plane (typically the equipment case or rack), the bond impedance ZB may be high enough to impair the filter’s performance. The filter as shown is a low pass filter intended to remove high frequency interference components from the power lines of equipment. The filter achieves its goal in part by the fact that the reactance, Xc, of the shunt capacitors is low at the frequency of the interference. Interfering signals present on the ac line are shunted to ground along Path 1 and thus do not reach the load. If ZB is high relative to Xc, however, interference currents will follow Path 2 to the load and the effectiveness of the filter is compromised.

Figure 12-1 Effects of Poor Bonding on the Performance of a Power Line Filter If a joint in a current path is not securely made or works loose through vibration, it can behave like a set of intermittent contacts. Even if the current through the joint is at dc or at the ac power frequency, the sparking which occurs may generate interference signals with frequency components up to several hundred megahertz. Poor bonds in the presence of high level rf fields, such as those in the immediate vicinity of high powered transmitters, can produce a particularly troublesome type of interference.

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Poorly bonded joints have been shown to generate cross modulation and other mix products when irradiated by two or more high level signals. Some metal oxides are semiconductors and behave as nonlinear devices to provide the mixing action between the incident signals. Interference thus generated can couple into nearby susceptible equipment. 12.11.2 Resistance Criteria A primary requirement for effective bonding is that a low resistance path be established between the two joined objects. The resistance of this path must remain low with use and with time. The limiting value of resistance at a particular junction is a function of the current (actual or anticipated) through the path. For example, where the bond serves only to prevent static charge buildup, a very high resistance, i.e., 50 kilohms or higher, is acceptable. Where lightning discharge or heavy fault currents are involved, the path resistance must be very low to minimize heating effects. Noise minimization requires that path resistances of less than 50 milliohms be achieved. However, noise control rarely ever requires resistances as low as those necessary for fault and lightning currents. Bond resistance based strictly on noise minimization requires information on what magnitude of voltage constitutes an interference threat and the magnitude of the current through the junction. These two factors will be different for every situation. A bonding resistance of 1 milliohm is considered to indicate that a high quality junction has been achieved. Experience shows that 1 milliohm can be reasonably achieved if surfaces are properly cleaned and adequate pressure is maintained between the mating surfaces. A much lower resistance could provide greater protection against very high currents but could be more difficult to achieve at many common types of bonds such as at connector shells, between pipe sections, etc. However, there is little need to strive for a junction resistance that is appreciably less than the intrinsic resistance of the conductors being joined. Higher values of resistance tend to relax the bond preparation and assembly requirements. These requirements should be adhered to in the interest of long term reliability. Thus, the imposition of an achievable, yet low, value of 1 milliohm bond resistance ensures that impurities are removed and that sufficient surface contact area is provided to minimize future degradation due to corrosion. A similarly low value of resistance between widely separated points on a ground reference plane or network ensures that all junctions are well made and that reasonably adequate quantities of conductors are provided throughout the plane or network. In this way, resistive voltage drops are minimized which helps with noise control. In addition, the low value of resistance tends to force the use of reasonably sized conductors which helps minimize path inductance. It should be recognized that a low dc bond resistance is not a reliable indicator of the performance of the bond at higher frequencies. Inherent conductor inductance and stray capacitance, along with the associated standing wave effects and path resonances, will

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determine the impedance of the bond. Thus, in rf bonds these factors should be considered along with the dc resistance. 12.11.3 Direct Bonds Direct bonding is the establishment of the desired electrical path between the interconnected members without the use of an auxiliary conductor. Specific portions of the surface areas of the members are placed in direct contact. Electrical continuity is obtained by establishing a fused metal bridge across the junction by welding, brazing, or soldering or by maintaining a high pressure contact between the mating surfaces with bolts, rivets, or clamps. Examples of direct bonds are the splices between bus bar sections, the connections between lightning down conductors and the earth electrode subsystem, the mating of equipment front panels to equipment racks, and the mounting of connector shells to equipment panels. Properly constructed direct bonds exhibit a low dc resistance and provide an rf impedance as low as the configuration of the bond members will permit. Direct bonding is always preferred; however, it can be used only when the two members can be connected together and can remain so without relative movement. The establishment of electrical continuity across joints, seams, hinges, or fixed objects that must be spatially separated requires indirect bonding with straps, jumpers, or other auxiliary conductors. Current flow through two configurations of a direct bond is illustrated in Figure 12-2. The resistance, Rc, of the path through the conductors on either side of the bond is given by Rc = r l/A Where r is the resistivity of the conductor materials, l is the total path length of the current through conductors, and A is the cross-sectional area of the conductors (assumed equal). Any bond resistance at the junction will increase the total path resistance. Therefore, the objective in bonding is to reduce the bond resistance to a value negligible in comparison to the conductor resistance so that the total path resistance is primarily determined by the resistance of the conductors. Metal flow processes such as welding, brazing, and silver soldering provide the lowest values of bond resistance. With such processes, the resistance of the joint is determined by the resistivity of the weld or filler metal which can approach that of the metals being joined. The bond members are raised to temperatures sufficient to form a continuous metal bridge across the junction. For reasons of economy, future accessibility, or functional requirements, metal flow processes are not always the most appropriate bonding techniques. It may then be more appropriate to bring the mating surfaces together under high pressure. Auxiliary fasteners such as bolts, screws, rivets or clamps are employed to apply and maintain the pressure on the surfaces. The resistance of these bonds is determined by the kinds of metals involved, the surface conditions within the bond area, the contact pressure at the surfaces, and the cross-sectional area of the mating surfaces.

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IPC ACTION TO CREATE FIGURE 12-2 FROM MIL-HDBK-419A, FIGURE 7-2,

PAGE 7-5 AND INSERT IT HERE 12.11.3.1 Contact Resistance No metallic surface is perfectly smooth. In fact, surfaces consist of many peaks and valleys. Even the smoothest commercial surfaces exhibit an RMS roughness of 0.5 to 1 millionth of an inch; the roughness of most electrical bonding surfaces will be several orders of magnitude greater. When two such surfaces are placed in contact, they touch only at the tips of the peaks - so called asperities. Thus the actual area of contact for current flow is much smaller than the apparent area of metallic contact. An exaggerated side view of the actual contact surfaces at a bond interface is shown in Figure 12-3. Theoretically, two infinitely hard surfaces would touch at only three asperities. Typically, however, under pressure, elastic deformation and plasticity allows other asperities to come into contact. Current passes between the surfaces only at those points where the asperities have been crushed and deformed to establish true metal contact. The actual area of electrical contact is equal to the sum of the individual areas of contacting asperities. This actual area of contact can be as little as one millionth of the apparent (gross surface) contact area. IPC ACTION TO CREATE FIGURE 12-3 FROM MIL-HDBK-419A, FIGURE 7-3,

PAGE 7-6 AND INSERT IT HERE 12.11.3.1.1 Surface Contaminants Surface films will be present on practically every bond surface. The more active metals such as iron and aluminum readily oxidize to form surface films while the noble metals such as gold, silver, and nickel are less affected by oxide films. Of all metals, gold is the least affected by oxide films. Although silver does not oxidize severely, silver sulfide forms readily in the presence of sulfur compounds. If the surface films are much softer than the contact material, they can be squeezed from between the asperities to establish a quasi-metallic contact. Harder films, however, may support all or part of the applied load, thus reducing or eliminating the conductive contact area. If such films are present on the bond surfaces, they should be removed through some thermal, mechanical, or chemical means before joining the bond members. Even when metal flow processes are used in bonding, these surface films should be removed or penetrated to permit a homogeneous metal path to be established. Foreign particulate matter on the bond surfaces will further impair bonding. Dirt and other solid matter such as high resistance metal particles or residue from abrasives can act as stops to prevent metallic contact. Therefore, all such materials must be thoroughly removed from the surfaces prior to joining the bond members. 12.11.3.1.2 Surface Hardness The hardness of the bond surfaces also affects the contact resistance. Under a given load, the asperities of softer metals will undergo greater plastic deformation and establish greater metallic contact. Likewise, at a junction between a soft

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and a hard material, the softer material will tend to conform to the surface contours of the harder material and will provide a lower resistance contact than would be afforded by two hard materials. Table 12-5 shows how the resistance of 6.45 square cm (1 square inch) bonds varies with the type of metals being joined.

Table 12-5 DC Resistance of Direct Bonds Between Selected Metals Bond Composition Resistance (Micro-ohms)

Brass-Brass 6 Aluminum-Aluminum 25

Brass-Aluminum 50 Brass-Steel 150

Aluminum-Steel 300 Steel-Steel 1500

Notes: Bond Area: 1 in2(6.45 cm2) Fastener Torque: 100 in-lb 12.11.3.1.3 Contact Pressure The influence of mechanical load on bond resistance is illustrated by Figure 12-4. This figure shows the resistance variation of a 6.45 square cm (1 square inch) bond held in place with a 1/4-20 steel bolt as a function of the torque applied to the bolt. The resistance variation for brass is lowest due to its relative softness and the absence of insulating oxide films. Even though aluminum is relatively soft, the insulating properties of aluminum oxide cause the bond resistance to be highly dependent upon fastener torque up to approximately 40 in. -lb torque (which corresponds to a contact pressure of about 1200 psi). Steel, being harder and also susceptible to oxide formations, exhibits a resistance that is dependent upon load below 80 in.-lb or about 1500 psi (for mild steel). Above these pressures, no significant improvement in contact resistance can be expected. IPC ACTION TO CREATE FIGURE 12-4 FROM MIL-HDBK-419A, FIGURE 7-4,

PAGE 7-9 AND INSERT IT HERE 12.11.3.1.4 Bond Area Smaller bond areas with the same loadings would produce higher contact pressure which would decrease the resistance. However, as shown in Figure 12-4, an increase in pressure over 1500 psi for steel and 1200 psi for aluminum produces relatively slight changes in bond resistance. Further, the improvement in resistance due to increased pressure is offset by the smaller overall bond area. In a similar fashion, a larger bond area (with no change in fastener size) under the same torque results in a lowered pressure at the bond surfaces. The reduced pressure would be counterbalanced to some extent by the increased bond area, but the net effect can be expected to be an increase in bond resistance. Thus, when larger bond areas are used, larger bolts at correspondingly higher torques should be used for fastening. Bond mating surfaces with areas as large as practical are desirable for several reasons. Large surface areas maximize the cross-sectional area of the path for current and correspondingly maximizes the total number of true metallic contacts between the surfaces. In addition to the obvious advantage of decreased bond resistance, the current crowding which can occur during power fault conditions or under a severe lightning

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discharge is lessened. Such current crowding produces a higher effective bond resistance than is present during low current flow. The increased bond resistance raises the voltage drop across the junction to even higher values and adds to the heat generated at the junction by the heavy current flow. Large bond areas not only lessen the factors which contribute to heat generation, they also distribute the heat over a larger metallic area which facilitates its removal. A further advantage of a large bond is that it will probably provide greater mechanical strength and will be less susceptible to long term erosion by corrosive products because only a small portion of the total bond area is exposed to the environment. 12.11.4 Direct Bonding Techniques Direct bonds may be either permanent or semi-permanent in nature. Permanent bonds may be defined as those intended to remain in place for the expected life of the installation and not required to be disassembled for inspection, maintenance, or system modifications. Joints which are inaccessible by virtue of their location should be permanently bonded and appropriate steps taken to protect the bond against deterioration. Many bonded junctions must retain the capability of being disconnected without destroying or significantly altering the bonded members. Junctions which should not be permanently bonded include those which may be broken for system modifications, for network noise measurements, for resistance measurements, and for other related reasons. In addition, many joints cannot be permanently bonded for cost reasons. Not permanently joined bonds are defined as semi-permanent bonds. Semi-permanent bonds include bolts, screws, rivets, clamps and other auxiliary devices for fasteners. 12.11.4.1 Welding In terms of electrical performance, welding is the ideal method of bonding. The intense heat (in excess of 4000° F) involved is sufficient to boil away contaminating films and foreign substances. A continuous metallic bridge is formed across the joint: the conductivity of this bridge typically approximates that of the bond members. The net resistance of the bond is essentially zero because the bridge is very short relative to the length of the bond members. The mechanical strength of the bond is high: the strength of a welded bond can approach or exceed the strength of the bond members themselves. Since no moisture or contaminants can penetrate the weld, bond corrosion is minimized. The erosion rate of the metallic bridge should be comparable to that of the base members; therefore, the lifetime of the bond should be as great as that of the bond members. Welds should be utilized whenever practical for permanently joined bonds. Although welding may be a more expensive method of bonding, the reliability of the joint makes it very attractive for bonds which will be inaccessible once construction is completed. Most metals which will be encountered in normal construction can be welded with one of the standard welding techniques such as gas, electric are, Heliare and exothermic. Conventional welding should be performed only by appropriately trained and qualified personnel. Consequently, increased labor costs can be expected. In many instances, also, the welding of bonds can be much slower than the installation of fasteners such as bolts

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or rivets. In such cases, the added costs of welding may force the use of alternate bonding techniques. An effective welding technique for many bonding applications is the exothermic mixture of aluminum, copper oxide, and other powders is held in place around the joint with a graphite mold. The mixture is ignited and the bent generated (in excess of 4000° F) reduces the copper oxide to provide a homogeneous copper blanket around the junction. Because of the high temperatures involved, copper materials can be bonded to steel or iron as well as to other copper materials. This process is advantageous for welding cables together, for welding cables to rods, or for welding cables to I-beams and other structural members. It is particularly attractive for the bonding of interconnecting cables to ground rods where the use of conventional welding techniques might be awkward or where experienced welders are not available. Because of the cost of the molds (a separate mold is necessary for each different bond configuration), this process is most economical when there are several bonds of the same configuration to be made. When using this process, the manufacturer’s directions should be followed closely. The mold should be dried or baked out as specified, particularly when the mold has not been used for several hours and may have absorbed moisture. The metals to be bonded should be cleaned of dirt and debris and should have the excess water dried off. Water, dirt and other foreign materials cause voids in the weld which may weaken it or may prevent a low resistance joint from being achieved. The mold size should match the cable or conductor cross sections; otherwise, the molten metal will not be confined to the bond region. 12.11.4.2 Brazing Brazing, to include silver soldering, is another metal flow process for permanent bonding. In brazing, surfaces are heated to a temperature above 800° F but below the melting point of the bond members. Metal with an appropriate flux is applied to the heated members which wets the bond surfaces to provide intimate contact between the brazing solder and the bond surfaces. As with higher temperature welds, the resistance of the brazed joint is essentially zero. However, since brazing frequently involves the use of metal different from the primary bond members, additional precautions must be taken to protect the bond from deterioration through corrosion. 12.11.4.3 Soft Solder Soft soldering is an attractive metal flow bonding process because of the ease with which it can be applied. Relatively low temperatures are involved and it can be readily employed with several of the high conductivity metals such as copper, tin and cadmium. With appropriate fluxes, aluminum and other metals can be soldered. Properly applied to compatible materials, the bond provided by solder is nearly as low in resistance as one formed by welding or brazing. Because of its low melting point, however, soft solder should not be used as the primary bonding material where high currents may be present. For this reason, soldered connections are not permitted by the National Electrical Code, and some military specifications, in grounding circuits for fault protection. Similarly, soft solder is not permitted for interconnections between elements

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of lightning protection networks by either the Military Standard, the National Fire Protection Association’s Lightning Protection Code or the Underwriter's Master Labeled System. In addition to its temperature limitation, soft solder exhibits low mechanical strength and tends to crystallize if the bond members move while the solder is cooling. Therefore, soft solder should not be used if the joint must withstand mechanical loading. The tendency toward crystallization should also be recognized and proper precautions observed when applying soft solder. Soft solder can be used effectively in a number of ways, however. For example, it can be used to tin surfaces prior to assembly to assist in corrosion control. Soft solder can be used effectively for the bonding of seams in shields and for the joining of circuit components together and to the signal reference subsystem associated with the circuit. Soft solder is often combined with mechanical fasteners in sweated joints. By heating the joint hot enough to melt the solder, a low resistance filler metal is provided which augments the path established by the other fasteners; in addition, the solder provides a barrier to keep moisture and contaminants from reaching the mating surfaces. 12.11.4.4 Bolts In many applications, permanent bonds are not desired. For example, equipment that must be removed from enclosures or moved to other locations which require that ground leads and other connections be broken. Often, equipment covers must be removable to facilitate adjustments and repairs. Under such circumstances, a permanently joined connection could be highly inconvenient to break and would limit the operational flexibility of the system. Besides offering greater flexibility, less permanent bonds may be easier to implement, require less operator training, and require less specialized tools. The most common semi-permanent bond is the bolted connection (or one held in place with machine screws, lag bolts, or other threaded fasteners) because this type bond provides the flexibility and accessibility that is frequently required. The bolt (or screw) should serve only as a fastener to provide the necessary force to maintain the 1200-1500 psi pressure required between the contact surfaces for satisfactory bonding. Except for the fact that metals are generally necessary to provide tensile strength, the fastener does not have to be conductive. Although the bolt or screw threads may provide an auxiliary current path through the bond, the primary current path should be established across the metallic interface. Because of the poor reliability of screw thread bonds, self-tapping screws should not be used for bonding purposes. Likewise, Tinnernman nuts (a.k.a., spring type nut-plates), because of their tendency to vibrate loose, should not be used for securing screws or bolts intended to perform a bonding function. The size, number and spacing of the fasteners should be sufficient to establish the required bonding pressure over the entire joint area. The pressure exerted by a bolt is concentrated in the immediate vicinity of the bolt head. However, large, stiff washers can be placed under the bolt head to increase the effective contact area. Because the load is distributed over a larger area, the tensile load on the bolt should be raised by increasing the torque. The monograph of Figure 12-5 may be used to calculate the necessary torque for the size bolts to be used. Where the area of the mating surfaces is so large that

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unreasonably high bolt torques are required, more than one bolt should be used. For very large mating areas, rigid backing plates should be used to distribute the force of the bolts over the entire area. IPC ACTION TO CREATE FIGURE 12-5 FROM MIL-HDBK-419A, FIGURE 7-7,

PAGE 7-15 AND INSERT IT HERE 12.11.4.5 Rivets Riveted bonds are less desirable than bolted connections or joints bridged by metal flow processes. Rivets lack the flexibility of bolts without offering the degree of protection against corrosion of the bond surface that is achieved by welding, brazing or soldering. The chief advantage of rivets is that they can be rapidly and uniformly installed with automatic tools. The bonding path established by a rivet is illustrated in Figure 12-6. The current path through a rivet is theorized to be through the interface between the bond members and the rivet body. This theory is justified by experience which shows that the fit between the rivet and the bond members is more important than the state of the mating surfaces between the bond members. Therefore, the hole for the rivet must be a size that provides a close fit to the rivet after installation. The sides of the hole through the bond members must be free of paint, corrosion products, or other non-conducting material. For riveted joints in shields, the maximum spacing between rivets is recommended to be approximately 2 cm (3/4 inch) or less. In relatively thin sheet metal, rivets can cause bowing of the stock between the rivets as shown by Figure 12-7. In the bowed or warped regions, metal-to-metal contact may be slight or nonexistent. These open regions allow rf energy to leak through and can be a major cause of poor rf shield performance. By spacing the rivets close together, warping and bowing are minimized. For maximum rf shielding, the seam should be provided with a gasket with some form of wire mesh or conductive epoxy to supplement the bond path of the rivets. IPC ACTION TO CREATE FIGURE 12-6 FROM MIL-HDBK-419A, FIGURE 7-8,

PAGE 7-17 AND INSERT IT HERE IPC ACTION TO CREATE FIGURE 12-7 FROM MIL-HDBK-419A, FIGURE 7-9,

PAGE 7-17 AND INSERT IT HERE 12.11.4.6 Conductive Adhesive Conductive adhesive is a silver-filled, two-component, thermosetting epoxy resin which when cured produces an electrically conductive material. It can be used between mating surfaces to provide low resistance bonds. It offers the advantage of providing a direct bond without the application of heat as is required by metal flow processes. In many locations, the heat necessary for metal flow bonding may pose a fire or explosion threat. When used in conjunction with bolts, conductive adhesive provides an effective metal-like bridge with high corrosion resistance along with high mechanical strength. In its cured state, the resistance of the adhesive may increase through time. It also tends to adhere tightly to the mating surfaces and thus an epoxy-bolt bond is less convenient to disassemble than a simple bolted bond.

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In some applications, the advantages of conductive adhesive may outweigh this inconvenience. 12.11.5 Indirect Bonds The preferred method of bonding is to connect the objects together with no intervening conductor. Unfortunately, operational requirements or equipment locations often preclude direct bonding. When physical separation is necessary between the elements of an equipment complex or between the complex and its reference plane, auxiliary conductors should be incorporated as bonding straps or jumpers. Such straps are commonly used for the bonding of shock mounted equipment to the structural ground reference. They are also used for by-passing structural elements, such as the hinges on distribution box covers or on equipment covers, to eliminate the wideband noise generated by these elements when illuminated by intense radiated fields or when carrying high level currents. Bond straps or cables are also used to prevent static charge buildup and to connect metal objects to lightning down conductors to prevent flashover. 12.11.5.1 Resistance. The resistance of an indirect bond is equal to the sum of the intrinsic resistance of the bonding conductor and the resistances of the metal-to-metal contacts at each end. The resistance of the strap is determined by the resistivity of the material used and the dimensions of the strap. With typical straps, the dc bond resistance is small. For example with a resistivity of 1.72 x 10-6 ohm-cm, (6.77 x 10-7 ohm-inches), a copper conductor 2.5 cm, (1 inch) wide, 40 mils thick, and 0.3 meters (1 foot) long has a resistance of 0.2 milliohms. To this resistance will be added the sum of the dc resistances of the direct bonds at the ends of the strap. With aluminum, copper, or brass straps, these resistances should be less than 0.1 milliohm with properly made connections. If long straps are required, however, the resistance of the conductor can be significant. 12.11.5.2 Frequency Effects 12.11.5.2.1 Skin Effect Because high conductivity materials attenuate radio frequencies rapidly, high frequency currents do not penetrate into conductors very far, i.e., they tend to stay near the surface. At frequencies where this effect becomes significant the ac resistance of the bond strap can differ significantly from its dc value. 12.11.5.2.2 Bond Reactance The geometrical configuration of the bonding conductor and the physical relationship between objects being bonded introduce reactive components into the impedance of the bond. The strap itself exhibits an inductance that is related to its dimensions. For a straight, flat strap of nonmagnetic metal, the inductance in micro henries is given by L = 0.002l 2.303 log 2l/ (b+c) + 0.5 + 0.2235 (b+c)/l µH where l= length in cm, b = width of the strap in cm c = thickness of the strap in cm

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Even at relatively low frequencies, the reactance of the inductive component of the bond impedance becomes much larger than the resistance. Thus, in the application of bonding straps, the inductive properties as well as the resistance of the strap must be considered. The physical size of the bonding strap is important because of its effect on the rf impedance. As the length of the strap is increased its impedance increases nonlinearly for a given width; however, as the width increases, there is a nonlinear decrease in strap impedance. Because of this reduction in reactance, bonding straps which are expected to provide a path for rf currents are frequently recommended to maintain a length-to-width ratio of 5 to 1 or less, with a ratio of 3 to 1 preferred. In many applications, braided straps are preferred over solid straps because they offer greater flexibility. Because the strands are exposed they are more susceptible to corrosion; braided straps may be undesirable for use in some locations for these reasons. Fine braided straps also are generally not recommended because of higher impedances at the higher frequencies as well as lower current carrying capacities. 12.11.5.2.3 Stray Capacitance A certain amount of stray capacitance is inherently present between the bonding jumper and the objects being bonded as well as between the bonded objects themselves. The combination of the inductance and capacitance can result in a resonant circuit condition at certain frequencies. These resonances can occur at surprisingly low frequencies -- as low as 10 to 15 MHz in typical configurations. In the vicinity of these resonances, bonding path impedances of several hundred ohms are common. Because of such high impedances, the strap is not effective. In fact, in these high impedance regions, the bonded system may act as an effective antenna system which increases the pickup of the same signals which bond straps are intended to reduce. The bond effectiveness indicates the amount of voltage reduction achieved by the addition of the bonding strap. Positive values of bonding effectiveness indicate a lowering of the induced voltage. At frequencies near the network resonances, the induced voltages are higher with the bonding straps than without the straps. At low frequencies where the reactance of the strap is low, bonding straps will provide effective bonding; At frequencies where parallel resonances exist in the bonding network, straps may severely enhance the pickup of unwanted signals. Above the parallel resonant frequency, bonding straps do not contribute to the pickup of radiated signals either positively or negatively. In conclusion, bonding straps should be designed and used with care with special note taken to ensure that unexpected interference conditions are not generated by the use of such straps.

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12.11.6 Surface Preparation To achieve an effective and reliable bond, the mating surfaces should be free of any foreign materials, e.g., dirt, filings, preservatives, etc., and non-conducting films such as paint, anodizing, and oxides and other metallic films. Various mechanical and chemical means can be used to remove the different substances which may be present on the bond surfaces. After cleaning, the bond should be assembled or joined as soon as possible to minimize recontamination of the surfaces. After completion of the joining process the bond region should be sealed with appropriate protective agents to prevent bond deterioration through corrosion of the mating surfaces. 12.11.6.1 Solid Materials Solid material such as dust, dirt, filings, lint, sawdust and packing materials impede metallic contact by providing mechanical stops between the surfaces. They can affect the reliability of the connection by fostering corrosion. Dust, dirt, and lint will absorb moisture and will tend to retain it on the surface. They may even promote the growth of molds, fungi, and bacteriological organisms which give off corrosive products. Filings of foreign metals can establish tiny electrolytic cells which will greatly accelerate the deterioration of the surfaces. The bond surface should be cleaned of all such solid materials. Mechanical means such as brushing or wiping are generally sufficient. Care should be exercised to see that all materials in grooves or crevices are removed. If a source of compressed air is available, air blasting is an effective technique for removing solid particles if they are dry enough to be dislodged. 12.11.6.2 Organic Compounds Paints, varnishes, lacquers, and other protective compounds along with oils, greases and other lubricants are non-conductive and in general, should be removed. Commercial paint removers can be used effectively. Lacquer thinner works well with oil-based paints, varnish, and lacquer. If chemical solvents cannot be used effectively, mechanical removal with scrapers, wire brushes, power sanders, sandpaper, or blasters should be employed. When using mechanical techniques, care should be exercised to avoid removing excess material from the surfaces. Final cleaning should be done with a fine, such as 400-grit, sandpaper or steel wool. After all of the organic material is removed, abrasive grit or steel wool filaments should be brushed or blown away. A final wipe down with denatured alcohol, dry cleaning fluid or lacquer thinner should be accomplished to remove any remaining oil or moisture films.

WARNING Many paint solvents such as lacquer thinner and acetone are highly flammable and toxic in nature. They should never be used around open flames and adequate ventilation should be present. Inhalation of the fumes must be prevented. Oils, greases, and other petroleum compounds should be wiped with a clean cloth or scraped off. Residual films should be dissolved away with an appropriate solvent. Hot soapy water can be used effectively for removing any remaining oil or grease. If water is used, however, the surfaces must be thoroughly dried before completing the bond. For small or intricate parts, vapor degreasing is an effective cleaning method. Parts to be cleaned are exposed to vapors of cleaning solvents until the surfaces reach the

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temperature of the vapor. In extreme cases, further cleaning by agitation in a bath of dry chromic acid, 2 lbs per gallon of water, and sulfuric acid, 4 oz per gallon of water, may be necessary. The average dip time should be restricted to less than 30 seconds because prolonged submersion of parts in this bath may produce severe etching and cause loss of dimension. This bath should be followed by a thorough rinse with cold water and then a hot water rinse to facilitate drying. 12.11.6.3 Plating and Inorganic Finish Many metals are plated or coated with other metals or are treated to produce surface films to achieve improved wear ability or provide corrosion resistance. Metal plating such as gold, silver, nickel, cadmium, tin, and rhodium should have all foreign materials removed by brushing or scraping and all organic materials removed with an appropriate solvent. Since such plating are usually very thin, acids and other strong etchants should not be used. Once the foreign substances are removed, the bond surfaces should be burnished to a bright shiny condition with fine steel wool or fine grit sandpaper. Care should be exercised to see that excessive metal is not removed. Finally, the surfaces should be wiped with a cloth dampened in a denatured alcohol or other appropriate solvent and allowed to dry before completing the bond. Chromate coatings such as iridite 14, iridite 18P, oadkite 36, and alodine 1000 offer low resistance as well as provide corrosion resistance. These coatings should not be removed. In general, any chromate coatings that have been applied in conformance with the applicable coating specification should be left in place. Many aluminum products are anodized for appearance and corrosion resistance. Since these anodic films are excellent insulators, they should be removed prior to bonding. Those aluminum parts to be electrically bonded either should not be anodized or the anodic coating should be removed from the bond area. 12.11.6.4 Corrosion By-Products Oxides, sulfides, sulfates, and other corrosion by-products should be removed because they restrict or prevent metallic contact. Soft products such as iron oxide and copper sulfate can be removed with a stiff wire brush, steel wool, or other abrasives. Removal down to a bright metal finish is generally adequate. When pitting has occurred, refinishing of the surface by grinding or milling may be necessary to achieve a smooth, even contact surface. Some sulfides are difficult to remove mechanically and chemical cleaning and polishing may be necessary. Oxides of aluminum are clear and thus the appearance of the surface cannot be relied upon as an indication of the need for cleaning. Although the oxides are hard, they are brittle and roughening of the surface with a file or coarse abrasive is an effective way to prepare aluminum surfaces for bonding. 12.11.7 Completion of the Bond After cleaning of the mating surfaces, the bond members should be assembled or attached as soon as possible. Assembly should be completed within 30 minutes if at all possible. If more than 2 hours is required between cleaning and assembly, a temporary protective coating should be applied. Of course, this coating must also be removed before completing the bond.

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The bond surfaces should be kept free of moisture before assembly and the completed bond should be sealed against the entrance of moisture into the mating region. Acceptable sealants are paint, silicone rubber, grease, and polysulfates. Where paint has been removed prior to bonding, the completed bond should be repainted to match the original finish. Excessively thinned paint should be avoided; otherwise, the paint may seep under the edges of the bonded components and impair the quality of the connection. Compression bonds between copper conductors or between compatible aluminum alloys located in readily accessible areas not subject to weather exposure, corrosive fumes, or excessive dust do not require sealing. This is subject to user agreement. 12.11.8 Bond Corrosion Corrosion is the deterioration of a substance (usually a metal) because of a reaction with its environment. Most environments are corrosive to some degree. Those containing salt sprays and industrial contaminants are particularly destructive. Bonds exposed to these and other environments should be protected to prevent deterioration of the bonding surfaces to the point where the required low resistance connection is destroyed. 12.11.8.1 Chemical Basis of Corrosion The basic diagram of the corrosion process for metals is shown in Figure 12-8. The requirements for this process to take place are that (1) an anode and a cathode are present to form an electrochemical cell and (2) a complete path for the flow of direct current exists. These conditions occur readily in many environments. On the surface of a single piece of metal anodic and cathodic regions are present because of impurities, grain boundaries and grain orientations, or localized stresses. These anodic and cathodic regions are in electrical contact through the body of metal. The presence of an electrolyte or conducting fluid completes the circuit and allows the current to flow from the anode to the cathode of the cell. IPC ACTION TO CREATE FIGURE 12-8 FROM MIL-HDBK-419A, FIGURE 7-

17, PAGE 7-30 AND INSERT IT HERE Anything that prevents the existence of either of the above conditions will prevent corrosion. For example, in pure water, hydrogen gas will accumulate on the cathode to provide an insulating blanket to stop current flow. Most water, however, contains dissolved oxygen which combines with the hydrogen to form additional molecules of water. The removal of the hydrogen permits corrosion to proceed. This principle of insulation is employed in the use of paint as a corrosion preventive. Paint prevents moisture from reaching the metal and thus prevents the necessary electrolytic path from being established. 12.11.8.1.1 Electrochemical Series The oxidation of metal involves the transfer of electrons from the metal to the oxidizing agent. In this process of oxidation, an electromotive force (EMF) is established between the metal and the solution containing the oxidizing agent. A metal in contact with an oxidizing solution containing its own metal ions establishes a fixed potential difference with respect to every other metal in the same condition. The set of potentials determined under a standardized set of conditions,

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including temperature and ion concentration in the solution, is known as the EMF (or electrochemical) series. The EMF series (with hydrogen as the referenced potential of 0 volts) for the more common metals is given in Table 12-6. The importance of the EMF series is that it shows the relative tendencies of metals to corrode. Metals high in the series react more readily and are thus more prone to corrosion. The series also indicates the magnitude of the potential established when two metals are coupled to form a cell. The farther apart the metals are in the series, the higher the voltage between them. The metal higher in the series will act as the anode and the one lower will act as the cathode. When the two metals are in contact, loss of metal at the anode will occur through oxidation to supply the electrons to support current flow. This type of corrosion is defined as galvanic corrosion. The greater the potential difference of the cell, i.e., the greater the dissimilarity of the metals the greater the rate of corrosion of the anode.

Table 12-6 Standard Electromotive Series Metal Electrode Potential (Volts) (Note: 1)

Magnesium 2.37 Aluminum 1.66

Zinc 0.763 Iron 0.440

Cacmium 0.403 Nickel 0.250

Tin 0.136 Lead 0.126

Copper -0.337 Silver -0.799

Palladium -0.987 Gold -1.50

Note: 1 Signs of potential are those employed by the American Chemical Society 12.11.8.1.2 Galvanic Series The EMF series is based on metals in their pure state -- free of oxides and other films -- in contact with a standardized solution. Of greater interest in practice, however, is the relative ranking of metals in a typical environment with the effects of surface films included. This ranking is referred to as the galvanic series. The most commonly referenced galvanic series is listed in Table 12-7. This series is based on tests performed in sea water and should be used only as an indicator where other environments are of concern. Galvanic corrosion in the atmosphere is dependent largely on the type and amount of moisture present. For example, corrosion will be more severe near the seashore and in polluted industrial environments than in dry rural settings. Condensate near the seashore or in industrial environments is more conductive even under equal humidity and temperature conditions due to increased concentration of sulfur and chlorine compounds, The higher conductivity means that the rate of corrosion is increased.

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Table 12-7 Galvanic Series of Common Metals and Alloys in Seawater

(ANODIC OR ACTIVE END) Magnesium

Magnesium Alloys Zinc

Galvanized Steel or iron 1100 Aluminum

Cadmium 2024 Aluminum

Mild Steel or Wrought Iron Cast Iron

Chromium Steel (active) Ni-Resist (high-Ni cast iron) 18-8 Stainless Steel (active)

18-8 Mo Stainless Steel (active) Lead-Tin Solders

Lead Tin

Nickel(active) Inconel (active)

Hastelloy B Manganese Bronze

Brasses Aluminum Bronze

Copper Silicon Bronze

Monel Silver Solder

Nickel Inconel

Chromium Steel 18-8 Stainless Steel

18-8 Mo Stainless Steel Hastelloy C Chlorimet 3

Silver Titanium Graphite

Gold Platinum

(CATHODIC OR MOST NOBLE END)

12.11.8.1.3 Relative Area of Anodic Member When joints between dissimilar metals are unavoidable, the anodic member of the pair should be the larger of the two. For a given current flow in a galvanic cell, the current density is greater for a small electrode than for a larger one. The greater the current density of the current leaving an anode, the greater is the rate of corrosion. As an example, if a copper strap or cable is bonded to a steel column, the rate of corrosion of the steel will be low because of the large anodic area. On the other hand, a steel strap or bolt fastener in contact with a copper plate will corrode rapidly because of the relatively small area of the anode of the cell. 12.11.8.1.4 Protective Coatings Paint or metallic plating used for the purpose of excluding moisture or to provide a third metal compatible with both bond members should be applied with caution. When they are used, both members should be covered.

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Covering the anode alone should be avoided. If only the anode is covered then at imperfections and breaks in the coating, corrosion will be severe because of the relatively small anode area. All such coatings should be maintained in good condition. 12.11.9 Workmanship Whichever bonding method is determined to be the best for a given situation, the mating surfaces should be cleaned of all foreign material and substances which would preclude the establishment of a low resistance connection. Next, the bond members must be carefully joined employing techniques appropriate to the specific method of bonding. Finally, the joint should be finished with a protective coating to ensure continued integrity of the bond. The quality of the junction depends upon the thoroughness and care with which these three steps are performed. In other words, the effectiveness of the bond is influenced greatly by the skill and conscientiousness of the individual making the connection. Therefore, this individual should be aware of the importance of electrical bonds and should have the necessary expertise to correctly implement the method of bonding chosen for the job. Those individuals charged with making bonds should be carefully trained in the techniques and procedures required. Where bonds are to be welded, for example, work should be performed only by qualified welders. No additional training should be necessary because standard welding techniques appropriate for construction purposes are generally sufficient for establishing electrical bonds. Qualified personnel should also be used where brazed connections are to be made. Exothermic welding can be effectively accomplished by personnel not specifically trained as welders. Every individual doing exothermic welding should become familiar with the procedural details and with the precautions required with these processes. Contact the manufacturers of the materials for such processes for assistance in their use. By taking reasonable care to see that the bond areas are clean and free of water and that the molds are dry and properly positioned reliable low resistance connections can be readily achieved. Pressure bonds utilizing bolts, screws, or clamps should be given special attention. Usual construction practices do not require the surface preparation and bolt tightening necessary for an effective and reliable electrical bond. Therefore, emphasis beyond what would be required for strictly mechanical strength is necessary. Bonds of this type should be checked rigorously to see that the mating surfaces are carefully cleaned, that the bond members are properly joined, and that the completed bond is adequately protected against corrosion. 12.12 Electrostatic Discharge Control (ESD) Program Wire and cable harness assemblies are not subject to damage due to electrostatic fields unless they are attached to assemblies containing ESD sensitive components such as microcircuits and semiconductors, or otherwise to ESD sensitive components. However, when the assemblies are installed into their next higher assembly, they may be attached to printed wiring assemblies or other assemblies that contain static sensitive components. In such instances, the manufacturer should establish, implement and maintain an ESD Control Program conforming to ANSI/ESD S20.20.

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If cable and wire harness connectors are connected to ESD sensitive components or assemblies, the connectors or terminals should be covered with an ESD protective cap or ESD protective material until they are installed for use to protect against ESD damage. HDBK_620_Section_12_GLB.doc

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13 Verification and/or Validation of Cable and Wire Harness Assemblies 13.1 Scope IPC/WHMA-A-620 provides accept/reject criteria and process information for manufacture of various types of cable and wire harness assemblies. Manufacturers who implement the manufacturing and process criteria specified in IPC/WHMA-A-620, by practicing due diligence, and with an effective process control program in place, as supplemented by sound verification and validation practices, should be able to consistently produce quality cable and wire harness assemblies conforming to IPC/WHMA-A-620, and end-item performance requirements. It is recognized that people sometimes make unintended mistakes, some of which could have a minimal or otherwise significant impact on the cable and wire harnesses produced. To ensure that the workforce is capable of detecting, and subsequently correcting such mistakes prior to product shipment, it is imperative that a means of verification and validation of cable and wire harness acceptability be implemented, and that the results of verification and validation be continuously analyzed for individual defects that can have an impact on the end-item system performance requirements, and for repetitive defects that are indicative of an adverse product quality trend. Verification and validation is one tool that is available to identify the need to drive corrective action and continuous process improvements. Verification and validation as referred to herein is the series of in-process inspections that may be referred to within the individual sections of IPC/WHMA-A-620, and the specific electrical and mechanical testing methodologies included in Section 19 of IPC/WHMA-A-620. Normally, the user (the ultimate customer) should have identified in the contract documentation, the extent of verification and validation required to ensure that cable and wire harness assemblies will perform their intended function over the expected life-time of the end item in its specified worst case environmental conditions. However, if the contract does not specifically identify the verification and validation methods, then Section 19 of IPC/WHMA-A-620 may be used for verification and validation methods, to the extent, as agreed to between the user and the supplier (AABUS). Supplier as referred to herein means the organization that ultimately delivers the completed cable and wire harness assemblies to the customer, and this supplier is responsible for proper flow-down of all applicable user defined contract requirements to lower-tier suppliers and/or manufacturers. Section 19 of IPC/WHMA-A-620 provides a verification and validation methodology that may be implemented, AABUS. However, nothing in Section 19 is considered mandatory for implementation unless, AABUS. For example, if the user did not specifically require performance of a dielectric strength test, then the fact that this test methodology is included within IPC/WHMA-A-620 does not mandate performance of

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this testing, AABUS. Additionally, if the user specified the methodology and accept/reject criteria for the testing, as example, dielectric strength testing, then the user specified criteria applies in lieu of the criteria of IPC/WHMA-A-620, unless, AABUS. Data obtained from the results of verification and validation methods, if properly analyzed and used to drive corrective actions and/or preventive actions when warranted, is an effective tool that should be used by the manufacturer to provide a quality product on a continuing basis. Management should periodically review the data and provide for the tools and resources to enable continuous process improvement. Although verification and validation, if effectively implemented, is capable of detecting and correcting defects prior to product shipment, it is not intended to replace the need for an effective process control program. One cannot inspect and/or test the quality into completed cable and wire harness assemblies. Quality can only be assured by implementing an effective process control program. There are many processes involved in the manufacture of cable and wire harness assemblies (e.g., stripping, crimping, soldering, etc.). Each of these processes has important process parameters that, if not properly controlled, can produce defects. For example, failure to have a crimp tool properly calibrated and/or verified prior to use can result in an improper crimp connection which could cause an open or intermittent circuit in the end-item. It is imperative that an effective process control methodology be implemented, and that Management establishes process performance metrics which are periodically reviewed and the results of these reviews are used to drive continuous improvement. The use of process metrics to drive continuous improvement is a mandatory requirement for organizations that intend to conform to the quality program requirements such as, but not limited to, the requirements of ISO-9001. This section of the Handbook provides a high level overview of the various verification and validation methods that are available as one of the many tools used to ensure a quality wire and cable harness assembly is delivered to the ultimate customer. Verification and validation as addressed herein means the methodology applied to production cable and wire harness assemblies. It does not include the numerous verification and validation methods that may have been used for qualification of cable and wire harness assemblies for the harsh environments that the assemblies may be subjected to when installed in the end item. For example, the end item may need to survive in a shipboard or aerospace environment where shock, vibration or high humidity conditions apply. Verification and validation methods to qualify cable and wire harnesses to survive in the end item environment such as, but not limited to a high humidity, shock and vibration environment are not included herein or within IPC/WHMA-A-620. Users of this Handbook and IPC/WHMA-A-620 are cautioned that any verification and validation method that may be employed in conjunction with the manufacture of cable and wire harness must be carefully chosen to preclude damage to the completed cable and

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wire harness assemblies. For example, selection of a test voltage for dielectric strength test must not be high enough to cause permanent damage to the assemblies. Some of the information herein is based on common test methodologies used for military applications (e.g., MIL-STD-202; however, it also has relevance for non-military applications as well). 13.2 Visual Inspection. Performing visual inspection of cable and wire harness assemblies is one of the easiest forms for verification that contract requirements have been met. The inspection frequency (100 percent or sample inspection) specified in the contract applies. If defects or nonconforming conditions are discovered during sample inspection, the balance of the items submitted for inspection may need to be 100 percent inspected for the noted condition. If inspection results reflect a condition that may have gone undetected in shipped equipment, this should be made known to all affected customers. Magnification aids should be used, as appropriate, depending on the physical size of the area being inspected. Automated Optical Inspection (AOI) may be used as appropriate; however, this is normally used for inspection of printed circuit boards. Although visual inspection alone cannot determine whether the item is functionally acceptable (e.g., meets DC resistance limits, insulation resistance limits, will not break down under dielectric strength testing, etc.), visual inspection can detect the types of inspectable characteristics noted below. • Parts used conform to the contract drawing parts list or bill of materials (BOM). • No evidence of damage (insulation, conductor, parts, part plating, etc.). • Workmanship (e.g., soldering, crimping, lacing, taping, etc.) conforms to

applicable requirements. • Overall length and length/location of breakouts conform to contract document

requirements. • Sleeving or other protective material provided (when required). • No evidence of unauthorized repairs (e.g., wire splicing). • Wire markers contain correct information and are installed in the correct location. • Other markings (when such marking is required) (e.g., part number, serial

number, national stock number, drawing revision level, reference designations, etc.) are correct.

• Marking is legible and permanent (simple test for permanency is to lightly rub the marking several times with your finger. If it smudges, it is not permanent).

• Any items to be shipped with the item (e.g., separately bagged and tagged) are correct and are provided with the assembly.

• Manufacturing, inspection and test records for the item reflect completion of all required manufacturing operations, inspections and tests. Any identified non-

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conforming conditions have been cleared, including customer approval of any “use-as-is” conditions (when required).

• Connectors with removable contacts have the correct number of contacts installed at the correct position in the connector insert, and are fully seated within the insert.

• The shipping paperwork reflects the item being shipped (correct part number, quantity, and drawing revision level – when required) and contains all required information, including proper completion of Government shipment forms (DD250 and/or DD1149, when applicable.

• Packaging, packing and marking for shipment conforms to applicable contract requirements, including any special markings (see IPC J-STD-609, etc).

13.3 Electrical Testing. Electrical testing of the types noted below can determine the functional acceptability of cable and wire harness assemblies. It is important that all testing be performed verbatim compliance to applicable test procedures (approved by the user when required). Test instruments requiring periodic calibration should be verified as being within current calibration. Test fixtures, calibrated when required, should be as specified in the test procedure. Data should be formally documented in the test records as the test results are obtained. It is not good practice to document results after the test is completed. Test data should be subjected to independent review when required, and should confirm that all test data entries have been completed, and that the data reflects acceptable test results. Test voltages/current can cause damage to the item being testing in event the limits identified in the test procedure are not adhered to. Some testing requires the application of voltages/currents that may be lethal to personnel; therefore implement appropriate safety precautions as required by the organization doing the testing. 13.3.1 Continuity Test. Continuity testing verifies that the point-to-point electrical connections conform to the assembly drawing, wire list, wiring diagram, or electrical schematic. Normally, continuity of an electrical connection assures that the DC resistance is low (e.g., 2 ohms or less – not counting the resistance of the wire). However, in some cases, a maximum resistance value in ohms is specified. Continuity testing is performed using various types of test equipment, from a simple “light bulb/LED indicator” type test; a standard digital ohmmeter, or an automated continuity tester dedicated to performing continuity testing. If a specific DC resistance value is required, then the simple “light bulb/LED indicator type testing is not appropriate unless a test circuit is designed to only illuminate when the correct resistance exists. Continuity testing can be performed as an in-process test, or otherwise as part of final assembly acceptance testing. Testing is sometimes performed while the assembly is still mounted to a “harness board”. It is good practice to combine the continuity test with the visual examination for correct wire marking since point-to-point connections are being verified and should align with the appropriate wire markers.

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The continuity test usually precedes the higher voltage tests such as the dielectric withstanding voltage test and the insulation resistance test. 13.3.2 Shorts Test. The shorts test is similar in concept to the continuity test, except that in lieu of looking for low DC resistance to verify a point-to-point connection, the shorts test verifies that there is no unwanted connection (i.e., an open circuit). Low voltage is used for this test; in some cases, limits pertaining to voltage, current and ohms may apply. Failure to adhere to the low voltage/current specified in the test procedure may cause product damage. Unless otherwise specified by the user, the shorts test is not required when the insulation resistance or dielectric strength test is performed since these tests normally verify proper isolation between circuits/conductors that are intended to be isolated (i.e., no unintended connections). 13.3.3 Dielectric Withstanding Voltage (DWV) Test. 13.3.3.1 Purpose. The dielectric withstanding voltage test (also called high-potential, over potential, voltage break down, or dielectric-strength test) consists of the application of a voltage higher than rated voltage for a specific time between mutually insulated portions of a component part or between insulated portions and ground. This is used to prove that the component part can operate safely at its rated voltage and withstand momentary over potentials due to switching, surges, and other similar phenomena. Although this test is often called a voltage breakdown or dielectric-strength test, it is not intended that this test cause insulation breakdown or that it be used for detecting corona; rather, it serves to determine whether insulating materials and spacing in the component part are adequate. When a component part is faulty in these respects, application of the test voltage will result in either disruptive discharge or deterioration. Disruptive discharge is evidenced by flashover (surface discharge), spark over (air discharge), or breakdown (puncture discharge). Deterioration due to excessive leakage currents may change electrical parameters or physical characteristics. 13.3.3.2 Precautions. The dielectric withstanding voltage test should be used with caution, as even an over potential less than the breakdown voltage may injure the insulation and thereby reduce its safety factor. Therefore, repeated application of the test voltage on the same specimen is not recommended. In cases when subsequent application of the test potential is specified in the test routine, it is recommended that the succeeding tests be made at reduced potential. When either alternating-current (ac) or direct current (dc) test voltages are used, care should be taken to be certain that the test voltage is free of recurring transients or high peaks. Direct potentials are considered less damaging than alternating potentials which are equivalent in ability to detect flaws in design and construction. However, the latter are usually specified because high alternating potentials are more readily obtainable. Suitable precautions must be taken to protect test personnel and apparatus because of the high potentials used.

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13.3.3.3 Factors affecting use. Dielectric behavior of gases, oils, and solids is affected in various degrees by many factors, such as atmospheric temperature, moisture, and pressure; condition and form of electrodes; frequency, waveform, rate of application, and duration of test voltage; geometry of the specimen; position of the specimen (particularly oil-filled components); mechanical stresses; and previous test history. Unless these factors are properly selected as required by the type of dielectric, or suitable correction factors can be applied, comparison of the results of individual dielectric withstanding voltage tests may be extremely difficult. 13.3.3.4 High voltage test source. The nature of the potential (ac or dc) used for the test is identified in the test procedure. When an alternating potential is specified, the test voltage provided by the high voltage source is nominally 60 hertz in frequency and approximates, as closely as possible, a true sine wave in form. Other commercial power frequencies may be used when specified. All alternating potentials shall be expressed as root-mean square (RMS) values, unless otherwise specified. The kilovolt-ampere (KVA) rating and impedance of the source are selected such as to permit operation at all testing loads without serious distortion of the waveform and without serious change in voltage for any setting. When the test specimen demands substantial test source power capacity, the regulation of the source is specified. The test procedure may specify a minimum KVA rating is required. When a direct potential is specified, the ripple content is normally controlled so as not to exceed 5 percent RMS of the test potential. If high surge currents can damage the item under test a suitable current-limiting device is used to limit current surges to the value specified. 13.3.3.5 Voltage and current measuring devices. In some cases, a maximum leakage current requirement may be specified. It is important to select test instruments that have the requisite accuracy to measure applied test voltage and leakage current (when required). A five percent tolerance on voltage and/or current is recommended unless otherwise specified. 13.3.3.6 Fault indicator. Suitable means to indicate the occurrence of disruptive discharge and leakage current is recommended in case it is not visually evident in the test specimen. The voltage and current instruments noted above, or an appropriate indicator light or an overload protective device may be used for this purpose. 13.3.3.7 Test performance using the approved (when required) test procedure. 13.3.3.7.1 Preparation. The test procedure may specify special preparations or conditions such as special test fixtures, reconnections, grounding, isolation, or immersion in water. 13.3.3.7.2 Test voltage and points of application. Specimens are subjected to a test voltage of the magnitude and nature (ac or dc) specified in the test procedure and the test voltage is applied between mutually insulated portions of the specimen or between insulated portions and ground as specified. The method of connection of the test voltage to the specimen may need to be specified when it is a significant factor.

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13.3.3.7.3 Rate and duration of application of test voltage. The test voltage is normally raised from zero to the specified value as uniformly as possible, at a rate of approximately 500 volts (RMS or DC) per second, unless otherwise specified, or it may be applied instantaneously. Unless otherwise specified in the test procedure, the test voltage is maintained at the specified value for a period of 60 seconds for qualification testing, or at reduced voltage for production testing. Upon completion of the test, the test voltage is gradually reduced to avoid surges, or may be removed instantaneously in event surges are not a concern. During the dielectric withstanding voltage test, the fault indicator is monitored for evidence of disruptive discharge and leakage current, followed by a visual examination to determine the effect of the dielectric withstanding voltage test on specific operating characteristics, when specified. 13.3.3.7.4 Examination and measurement of specimen. During the dielectric withstanding voltage test, the fault indicator is monitored for evidence of disruptive discharge and leakage current. Following this, the specimen is examined and measurements performed to determine the effect of the dielectric withstanding voltage test on specific operating characteristics, when specified. Normally, the insulation resistance test follows the dielectric withstanding voltage test, while using the same isolated circuit connections used for the dielectric withstanding voltage test. The operating test, if required, normally follows the insulation resistance test. Normally the test procedure includes criteria for the following; a. Special preparations or conditions, if required. b. Magnitude and type (AC or DC) of test voltage and duration of voltage application. c. Points of application of test voltage. d. Method of connection of test voltage to specimen, if significant. g. Test power source voltage/current regulation, when applicable. 13.3.4 Insulation resistance (IR) test. 13.3.4.1 Purpose. This test measures the resistance offered by the insulating members of a component part to an impressed direct voltage tending to produce a leakage of current through or on the surface of these members. Knowledge of insulation resistance is important, even when the values are comparatively high, as these values may be limiting factors in the design of high-impedance circuits. Low insulation resistances, by permitting the flow of large leakage currents, can disturb the operation of circuits intended to be isolated, for example, by forming feedback loops. Excessive leakage currents can eventually lead to deterioration of the insulation by heating or by direct current electrolysis. Insulation resistance measurements should not be considered the equivalent of dielectric withstanding voltage or electric breakdown tests. A clean, dry insulation may have a high insulation resistance, and yet possess a mechanical fault that would cause failure in the dielectric withstanding voltage test. Conversely, a dirty,

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deteriorated insulation with a low insulation resistance might not break down under a high potential. Since insulating members composed of different materials or combinations of materials may have inherently different insulation resistances, the numerical value of measured insulation resistance cannot properly be taken as a direct measure of the degree of cleanliness or absence of deterioration. The test is especially helpful in determining the extent to which insulating properties are affected by deteriorative influences, such as heat, moisture, dirt, oxidation, or loss of volatile materials. 13.3.4.2 Factors affecting use. Factors affecting insulation resistance measurements include temperature, humidity, residual charges, charging currents of time constant of instrument and measured circuit, test voltage, previous conditioning, and duration of uninterrupted test voltage application (electrification time). In connection with this last-named factor, it is characteristic of certain components (for example, capacitors and cables) for the current to usually fall from an instantaneous high value to a steady lower value at a rate of decay which depends on such factors as test voltage, temperature, insulating materials, capacitance, and external circuit resistance. Consequently, the measured insulation resistance will increase for an appreciable time as test voltage is applied uninterruptedly. Because of this phenomenon, it may take many minutes to approach maximum insulation resistance readings, but specifications usually require that readings be made after a specified time, such as 1 or 2 minutes. This shortens the testing time considerably while still permitting significant test results, provided the insulation resistance is reasonably close to steady-state value, the current versus time curve is known, or suitable correction factors are applied to these measurements. For certain components, a steady instrument reading may be obtained in a matter of seconds. When insulation resistance measurements are made before and after a test, both measurements should be made under the same conditions. 13.3.4.3. Test apparatus. Insulation resistance measurements are made on an apparatus suitable for the characteristics of the component to be measured such as a megohm bridge, megohm-meter, insulation resistance test set, or other suitable apparatus. The direct potential applied to the specimen depends on the circuit voltage and is as specified in the test procedure. Unless otherwise specified, the measurement error at the insulation resistance value required is within 10 percent. Proper guarding techniques may be required to prevent erroneous readings due to leakage along undesired paths. . 13.3.5 Voltage standing wave ratio (VSWR), insertion loss, and reflection coefficient tests1. 13.3.5.1 Overview. VSWR, Insertion Loss and Reflection are parameters that are interrelated with each other and are important parameters, in particular for cable and wire harness assemblies that must transmit signals/power at high frequencies (i.e., RF). Testing to verify the aforementioned parameters is performed using specialized test equipment. One such equipment is the network analyzer. In RF applications we are most

9

concerned with getting signals from one point to another with maximum efficiency and with minimum distortion. Testing using network analyzer instruments enables one to characterize the signal received at the load versus the known signal input at the source. There are two different types of network analyzers; a scalar type, based on a diode detection scheme, or a vector type, based on a tuned-receiver (narrowband) techniques. The scalar type is the least expensive; however, it is not as accurate as the vector type. The type of network analyzer used should by based on the desired accuracy required for the application. A network analyzer must provide a source for stimulus, signal-separation devices, receivers for signal detection, and display/processing circuitry for reviewing results. The source is usually a built-in-phase-locked-voltage-controlled oscillator. Signal-separation hardware allows measurements of any portion of the incident signal to provide a reference for ratio measurements, and it separates the incident (forward) and reflected (reverse) signals present at the input to the item being tested. 13.3.5.2 Signal propagation in DC circuits versus RF systems. It is important to understand the difference between how signals propagate in DC circuits versus RF systems. In DC circuits or circuits where the propagating signal has low frequencies, the voltage of the signal at different points on a cable in the signal path varies minimally. This is not so in the case of RF or high-frequency signals where wavelength of the signal is considerably small in comparison to the length of the cable allowing multiple cycles of the signal to propagate through the cable at the same time. Consider an example where two waves (signals) of different frequencies are made to propagate through a 1 m coaxial cable. The frequency of the first signal is 1 MHz while that of the second signal is 1 GHZ. To calculate their wavelengths, we will use the following formula:

λ = VF 3 X 108 m f

In the above formula, λ is the wavelength of the signal, f is its frequency, and VF is the velocity factor of the cable. Let us assume that the coaxial cable being used to route both signals is of type RG8, which is known to have a velocity factor of 0.66. Then for Signal 1 where f = 1MHz: λ1 = (0.66) 3 X 108 m = 198m

1 X 106

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In the case of Signal 1, the length of the coaxial cable is considerably small in comparison to the wavelength of the signal propagating through it. Therefore, as can be seen in the above Figure , the variation in potential of the signal at different points in the cable is negligible. For Signal 2 where f = 1GHZ: λ2 = (0.66) 3 X 108 m = 0.198m

1 X 109

In the case of Signal 2, the length of the coaxial cable is much larger (almost 5X) than the wavelength of the signal propagating through it. Therefore, at any given time, multiple cycles of the signal will travel through the cable simultaneously. Because of their small wavelengths, high-frequency signals travel through cables in waves. Such signals therefore suffer reflections and power loss when traveling between varying media (wave theory). In the case of electrical circuits, this variation in medium takes place when the signal (wave) is made to pass through system components that have varying characteristic impedances. Therefore, to minimize reflections and power loss, RF systems must be constructed using suitable components with matched impedances. As a rule of thumb, signal degradation due to power loss and reflections that occur in the transmission line become significant once the length of the cable exceeds 0.01 of the wavelength of the signal it is being used to route.

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13.3.5.3.1 Characteristic impedance. Characteristic impedance is a transmission line parameter that is determined by the physical structure of the line. It also helps determine how propagating signals are transmitted or reflected in the line. Impedance of RF components is not a DC resistance and, in the case of a transmission line, can be calculated using the following formula:

In the above formula: Z0 = Characteristic impedance L = Inductance per unit length of the RF transmission line caused due to magnetic fields that are formed around the wires when current flows through them. C = Capacitance per unit length of the RF transmission line. This is also the capacitance that exists between two conductors. R = DC resistance per unit length of the RF transmission line. G = The dielectric conductance per length. ω = Frequency (radians/s). Because an ideal cable has no resistance or dielectric leakage, its characteristic impedance can be calculated using the above formula as: Z0 = [L/C]1/2 Because all components in an RF system have to be impedance matched to minimize signal losses and reflections, component manufacturers specifically design their equipment to have a characteristic impedance of either, 50 or 75 Ω. 50 Ω systems make up the bulk of the RF market and include most communications systems. 75 Ω RF systems are smaller in number and are prevalent mainly in video RF systems. It is crucial that engineers ensure that parts such as cables and connectors, in addition to other instruments that may reside in the test system, are all impedance matched. 13.3.5.4 Insertion loss. Significant power loss in the signal occurs if the length of the transmission line it is made to propagate through is greater than 0.01 of its own wavelength. The “insertion loss” specification of a cable assembly is a measure of this power loss and signal attenuation. Insertion loss of a cable assembly at a particular

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frequency can be used to calculate the power loss or voltage attenuation caused by the cable assembly on a signal at that frequency. Formula for calculating power loss: Insertion Loss (dB) = 10 log10(Pout/Pin) Formula for calculating voltage attenuation: Insertion Loss (dB) = 20 log10(Vout/Vin) To understand the concept of insertion loss, think of a cable assembly as a low pass filter. Every cable assembly in the real world has some parasitic capacitance, resistance, and conductance. These parasitic components combine to attenuate and degrade the signal the cable assembly is being used to route. The power loss and voltage attenuation caused by these components varies with the frequency of the input signal and can be quantified by the insertion loss specification of the cable assembly at that frequency. It is therefore critical to ensure that the insertion loss of the cable assembly is acceptable at the bandwidth requirement of the application. 13.3.5.5 Voltage standing-wave ratio (VSWR). VSWR is the ratio of reflected-to-transmitted waves. As mentioned earlier, at higher frequencies, signals take the form and shape of waves when passing through a transmission line or cable. For this reason, just as in the case of sound and light waves, reflections occur when the signal traverses over different media (such as components with unmatched impedances). In a cable assembly, this mismatch can be between the characteristic impedance of the connector, the cable itself, and the connected load. Because VSWR is a measure of the power of the reflected wave, it can also be used to measure the amount of power loss in the transmission line. The reflected wave, when summed with the input signal either increases or decreases its net amplitude, depending on whether the reflection is in phase or out of phase with the input signal. The ratio of the maximum (when reflected wave is in phase) to minimum (when reflected wave is out of phase) voltages in the “standing wave” pattern is known as VSWR. To understand how to calculate VSWR and return loss in an RF system, let us consider the RF transmission in the figure below.

In the above circuit, the impedance of the load (40.5 Ω) is not equal to that of the source and the transmission line (50 Ω). For this reason, some portion of the signal propagating through the transmission line is reflected back from the load. We can measure this reflection using the following formula: Insertion Loss (dB) = 10 log10(Preflected/Pin).

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As you can see, return loss is a measure of the power of the reflected signal. It is also a subset of insertion loss. The higher the return loss (or reflections) in an RF system, the higher the insertion loss. VSWR is another way of measuring signal reflections. It can be calculated as:

VSWR = 1 + (Г) 1 – (Г) In the above formula, Г is the reflection coefficient and can be calculated using the following formula:

Г = ZL – Z0

ZL + Z0

From the circuit in the above figure, we calculate VSWR to be:

VSWR = 1 + ZL – Z0 ZL + Z0

_______________

1 - ZL – Z0

ZL + Z0

Inserting the numbers we get: 1 + 0.1/1-0.1 = 1.22. To visualize what is happening in this example, let us imagine that the signal being sourced in the RF system is a 1 Vpp sine wave. Because the reflection coefficient for the system is 0.1, we can determine that the magnitude of the reflected is 0.1 X 1 = 0.1 V or 100 mv. The following figures display the maximum and minimum amplitudes of the resultant signal which occurs when the reflected wave is in phase and 180 deg out of phase with the input signal, respectively.

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As stated earlier, VSWR is the ratio of maximum voltage to minimum voltage in the standing wave pattern. Using this definition, we can calculate VSWR from the above figures to be:

VSWR = 1.1/0.9 = 1.22

13.3.6 User Defined Electrical Tests. The specific tests listed in IPC/WHMA-A-620, Section 19 are recommended default tests for use in event the user (customer) has not invoked any test requirements in the contract. Normally, the user will invoke a standard test suite that, if not the same as specified in IPC/WHMA-A-620, is similar in scope. However, specific test equipment, test parameters, and accept/reject criteria may be different. Additionally, the user may specify different tests. In all instances, unless, AABUS, the user identified testing applies. 13.4 Mechanical Testing. Various types of mechanical testing are normally performed to verify the mechanical integrity of the cable assembly such as adequacy of a crimp connection, insertion of a contact into a connector, etc. The following types of mechanical tests are normally performed: 13.4.1 Crimp testing (Crimp height, pull force/tensile testing, crimp force

monitoring)2, 3. In order to better understand the various types of crimp testing that are used to verify the acceptability of crimp connections, it is important to understand some of the more important aspects of the crimping process/tooling. The following discussion provides an oversight of the crimping process and the crimp tooling used.

Quality, cost, and throughput are associated with specific measurements and linked to process variables. Crimp height, pull test values, leads per hour, and crimp symmetry are some of the measures used to monitor production termination processes. Many variables affect the process such as wire and terminal quality, machine repeatability, setup parameters, and operator skill. Crimp tooling is a significant contributor to the overall crimp termination process. The condition of crimp tooling is constantly monitored in production by various means. These

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means are often indirect measures. Crimp quality monitors and crimp cross sections are methodologies that infer the condition of the crimp tooling. Visual inspection of the crimp tooling can be used to check for gross failures such as tool breakage or tooling deformation which occurred as a result of a machine crash. Continuous monitoring of production will help determine when the process needs to be adjusted and the replacement of crimp tooling can be one of the adjustments that are made. Crimp tooling can a have positive effect on the quality, cost, and throughput of the termination process. High quality crimp tooling can produce high quality crimps with less in-process variation over a greater number of terminations. It is difficult to distinguish critical tooling attributes with visual inspection only. Some attributes cannot be inspected even by running crimp samples. The following discussion provides the reader with information that identifies key crimp tooling attributes and the effect of those attributes on the crimping process. 13.4.1.1 Key crimp tooling characteristics. There are four key characteristics for crimp tooling. These are: • Geometry and associated tolerances • Materials • Surface condition • Surface treatment Each of these categories contributes to the overall performance of the production termination process. 13.4.1.2 Geometry and associated tolerances. Terminals are designed to perform to specification only when the final crimp form is within a narrow range of dimensions. Controlling critical crimp dimensions is influenced by many factors including: • Wire size and material variation • Terminal size and material variation • Equipment condition The final quality and consistency of a crimp can never be any better than the quality and consistency of the tooling that is used. If other variations could be eliminated, tooling can and should be able to produce crimp forms that are well within specified tolerances. In addition, variation from one tooling set to another should be held to a minimum. Crimp tooling features that are well controlled and exhibit excellent consistency from tooling set to tooling set can result in shorter setup time as well as more consistent production results. Some critical crimp characteristics are directly defined by the tooling form and are obvious. These include:

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• Crimp width • Crimp length Other critical crimp characteristics can be related to several tooling form features and/or other system factors. These may be less obvious and include: • Flash • Roll, twist, and side-to-side bend • Up/down bend • Crimp symmetry • Bellmouth The following discussion focuses on two characteristics, crimp width and flash, as examples of how tooling can affect crimp form. Similar arguments can be applied to the others. 13.4.1.2.1 Crimp width. Crimp width is a good example of a feature that should be consistent and in control between different crimpers of the same part number. The reason for this is quite straightforward. For a given terminal and wire combination, it is necessary to achieve an area index, AI, which is determined by the terminal designer for optimal mechanical and electrical performance. Crimp height, CH, and crimp width,CW, directly affect achieving proper AI. Area index, AI(as a percentage), is defined as:

AI = [At/(AW + AB)] X 100

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where At is the total area of the wire and barrel after crimping. AW and AB are, respectively, the initial cross-sectional areas of the wire and barrel before crimping. A typical design point for AI is 80%. In order to maintain the same AI, the crimp height, CH, needs to change inversely to the change of crimp width, CW, in approximately the same proportion. Thus, if the CW increases +2%, the CH needs to change approximately -2% in order to achieve the same AI design point. At first glance that may not seem significant, but in reality it can be very significant. Using another general industry design rule of the ratio of CH to CW of approximately 65%, a typical set of dimensions used as an example may be: CW = 0.110 in CH = 0.068 in Therefore, varying the CW by 2% would result in a CH variation of 2%, or 0.0014 in. At a CH tolerance of ± 0.002 in, 35% of the total CH tolerance would be used by a 2% variation in CW. Thus, the importance of crimp width control is obvious when tooling is changed during a production run.

Cross Sections Showing Minimum (a) and Maximum (b) Area Index per Terminal Specification – a variation of ± 3.5%

(a)

(b)

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13.4.1.3. Flash. Most crimp terminations have a requirement to limit flash. Flash is defined as the material which protrudes to the sides of the terminal down and along the anvil. Flash is normal in the crimping process but excessive flash is very undesirable. Controlling flash requires a balance of several geometric factors. Other factors influencing flash are related to surface finish and friction, which will be discussed later. A dominant factor in controlling flash is controlling the clearance between the crimper and anvil during the crimp process. Defining the ideal clearance could in itself be a simple matter were it not for two facts: • In order to minimize terminals’ sticking in the crimper, the sides of the terminal are tapered. Thus the clearance between the anvil and crimper varies throughout the stroke. • Crimper and anvil sets are typically designed to terminate two to four wire sizes. This creates multiple crimp heights. Since the sides of the crimper are tapered to minimize terminal sticking, the maximum clearance permitted without creating flash must be assigned to the maximum crimp height specified for the tooling set. In addition, a minimal clearance must be maintained for the smallest crimp height specified by the tooling set to prohibit contact between the anvil and crimper. Crimper to anvil clearance is thus a combination of crimp width, crimper leg taper, anvil width, and crimp height. The critical design point is at the largest crimp height. This contribution to the gap is directly dependent on dimensional control. The following is offered as an example: Nominal condition: CH = 0.073 in CW = 0.110 in

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Crimper leg taper = 3.0 degree Anvil Width = 0.109 in Nominal anvil to crimper total clearance = 0.005 in The clearance can grow rapidly with small changes to the nominal dimensions: CH remains unchanged = 0.073 in Increase in crimp width, CW, = 0.0008 in Increase in crimper leg taper = 0.8 degree Decrease in anvil width = 0.0008 in The total increase in total clearance is this case = 0.0026 in. This more than a 50% increase in the nominal design clearance, which can result in unacceptable flash (see below). Dimensional control is clearly critical. In the photographs below, significant flash can be generated with excessive anvil to crimper distance, as shown by the nominal design condition (a) and +0.003 in over nominal condition (b).

(a) (b) 13.4.1.4 Material system. The material selection for tooling is critical. The material must be able to meet the in-service demands placed on the tooling components. The two critical tooling components to be reviewed are the wire crimper and the anvil. The wire crimper and the anvil have different functional demands. Both have the need to withstand high loads and moderate shock. However, the wire crimper is in fact an aggressive forming tool. It must withstand high shear loading that is a result of frictional

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loads generated as the terminal barrel slides along the crimper surfaces in the forming process, and then as the barrel terminal is plastically deformed and extruded to complete the termination. The anvil experiences some of the same conditions but to a much lower level of severity. The wire crimper and the anvil can be likened to a punch and die in the world of metalworking. The materials used in punch and die applications have been well documented, along with the material selection process. The added severity of the aggressive forming and the terminal and wire extrusion during crimping add complexity to the material selection. The material selection process involves: • Strength of materials with emphasis on toughness needed to withstand the moderate shocks generated during crimping • Wear resistance to maintain form In addition to the above design considerations, there exists another phenomenon that occurs during crimping that can significantly shorten the useable life of a wire crimper. Material can be transferred from the terminal barrel to the wire crimper. This material buildup can result in unacceptable terminations. The crimped terminal surfaces can actually be deformed by the indentations of the deposited material. Crimp deformation may result due to increased friction. Tooling wear can be accelerated due to higher crimp forces. Surface treatments that minimize this material transfer are critical to extended tooling life. 13.4.1.4.1 Strength of materials. Crimpers and anvils are designed to be able to withstand stresses that are typically encountered during crimping. The basic design of tooling with reference to size and geometry has been well analyzed and generally stresses generated during crimping are able to be accommodated. However, there are always demanding applications that will tax the design to its stress limits. In those cases, geometry and material may depart from the standard design. These exceptions are dealt with on a one-by-one basis and will not be discussed here. It is the unique requirement of stress and shock that needs to be discussed. Peak crimp loads go from zero to maximum in less than 40 ms. Tooling needs to withstand this load cycle at a rate of greater than once per second. Several classes of tool steels are suitable and are well described in the material handbooks. It is the processing of these materials that can make a significant performance difference. In order to withstand the rapid loading to a high stress on a repeated basis, the surface of the material must minimize cracks and imperfections that may be generated during the machining and/or heat treat operations. It is important that grain structure be controlled in size and orientation to achieve maximum and consistent service life. Decarburization of the surface during heat treating must be controlled. Heat treating process controls are critical to reproducing the optimal surface. Machining processes

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must also be controlled to avoid surface cracking due to excessive heat generation during overly aggressive material removal. Likewise, localized tempering may occur, which can soften material beyond the effective range. These variations in final material and surface conditions are not readily detectable with a visual inspection. They can manifest themselves during service and result in unacceptable tooling performance. 13.4.1.4.2 Wear resistance. Wear is generally described as the gradual deterioration of a surface through use. Several types of wear exist and include adhesive, abrasive, and pitting. By design, the tooling is able to withstand normal surface loads. Thus, pitting is typically not an issue. Abrasion can occur depending on terminal surfaces. If a terminal is plated with an abrasive substance, the tooling could suffer from abrasive wear. This would be an atypical condition and would be handled by special design. The primary wear mode experienced by crimp tooling is adhesive wear. Adhesive wear occurs as two surfaces slide across each other. Under load, adhesion, sometimes referred to as cold welding, can occur. Wear takes place at the localized points of adhesion due to shear and deformation. Adhesion is highest at the peaks of surface finish because that is where the load is greatest. During crimping, the ideal conditions exist for adhesive wear. That is, • High loading due to crimp force • Sliding surfaces due to crimp formation, and terminal and wire extrusion Wear will generally manifest itself more significantly at edges of a surface. However, adhesive wear is often observed over substantial areas of the tooling. It is important to note here that the wire crimper is the component most susceptible to adhesive wear. Generally, adhesive wear will be directly related to load and to the amount of relative movement between the two materials. Although the anvil may have equal loading, the amount of relative movement between the terminal and tooling is many times more at the crimper than at the anvil. The insulation crimper typically experiences lower adhesive wear because the load is reduced compared to the wire crimp and the relative movement is less than that of the wire crimper, since there is no terminal and wire extrusion at the insulation crimp. Adhesive wear can be controlled in the selection of the material. Different alloys exhibit better or worse wear properties. These properties can be measured and are well documented. Adhesive wear is inversely proportional to the hardness of the material. Thus, the harder the material, the less adhesive wear. In crimp tooling, there is often a tradeoff that is made. In order to achieve higher wear resistance, the material often exhibits lower toughness by composition, hardness, or both. The final material selection is often based on years of experience. One material may have high wear characteristics and lower toughness, and be suitable for a small terminal since the margin of safety on stress is high. Another terminal may be large and the toughness could be of more importance due a lower stress design margin. The ability to design and manufacture

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crimpers from several materials will enable optimal material selection for a specific application. The final property that affects adhesive wear is surface finish. As stated earlier, adhesion is highest at the peaks of the surface. Thus, the smoother the finish, the less significant the peaks and the less significant the adhesion. Adhesive wear can be reduced with a lower surface finish. Surface finish affects other crimping performance parameters. These are discussed in the next section. 13.4.1.5 Surface condition. Surface condition can affect the performance of the crimp tooling as well as the longevity of service. As noted in the previous section, a hard, smooth surface has improved adhesive wear properties and, thus, longer service life. The other attribute that needs to be considered is friction. Friction is a contributing factor in determining the final crimp form and process characteristics. Low tooling friction results in lower crimping force and thus can influence crimp form as well as tooling life. Consistent frictional characteristics between tooling sets will result in reduced process variation.

The Figure above shows the typical effect of friction on crimp force. Friction of the crimp tooling surfaces is influenced by factors similar to those that influence adhesive wear—hardness and surface finish. Generally, harder materials exhibit lower coefficients for sliding friction. Friction coefficients have also been shown to be related to surface finish. Manufacturing processes need to produce consistent results

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such that when tooling sets need to be changed in production, minimum disruption in crimp quality is achieved. It has been found that maintaining surface hardness above Rc 55 as well as keeping surface finishes to 8 micro-inches or less is desirable to obtain consistent crimp results and minimize adhesive wear. 13.4.1.6 Surface treatment. Surface condition can affect the performance of the crimp tooling as well as the longevity of service. As noted in the previous section, a hard, smooth surface has improved adhesive wear properties and, thus, longer service life. The other attribute that needs to be considered is friction. A commonly accepted approach to improved crimp tooling performance and life has been to apply a surface treatment to the crimp area. The wire crimper has been defined in previous discussions as tooling component that is subjected to the severest duty cycle. Thus, applying an appropriate surface treatment to the wire crimper will have the most benefit to crimp performance and tooling life. These treatments can include hard metal plating or ceramic coating. An example of a treatment that has been successful in achieving significant level of performance and life improvements is hard chromium plating. There are several valid reasons for this success. First, chromium plating has a very low coefficient of friction. As noted, friction has a significant effect on crimp form. The static and sliding coefficients of friction for steel on steel are typically 0.30 and 0.20 respectively. Chromium plated steel on steel can reduce the static and sliding coefficients to 0.17 and 0.16. In addition, chromium plating can be highly polished so that there is no loss of surface finish characteristics due to the plating process. The second area that is greatly improved with chromium plating is wear resistance. Adhesive wear resistance is improved as surface hardness improves. Chromium plating typically exhibits hardness Rc 65+. This hardness level greatly enhances resistance to adhesive wear. Also, this now frees up the designer to consider more base metal options. A base material of reduced wear resistance but greater toughness can be selected and its wear resistance improved with chromium plating. Thus, chromium plating can enable a better tooling solution for the crimp production process. Third, and perhaps one of the most significant effects of chromium plating, is its resistance to adhesion and cold welding. A side effect of adhesive wear is the transfer of material from the terminal to the wire crimper. By definition, adhesive wear is caused by material adhering to localized points on the surface. Some of the adhesion results in the surface material being worn away and some in the transfer of material from the terminal to the crimper. As more cycles occur, more material is transferred. Thus there a resultant buildup of terminal material on the crimper. This buildup will result in two potentially catastrophic failures:

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• The built-up material will create deformations in the terminal surface, resulting in unacceptable crimps. • Crimps will be greatly distorted due to significant changes in the friction factor and result in the terminals not conforming to the desired form. Unacceptable crimp forms, such as unsymmetrical cross-section, excessive flash, and open barrels can result. Chromium plating has the ability to be applied uniformly and consistently and exhibits excellent adhesion to the base metals. The unique benefits of chromium plating, such as ease of application, consistency of plating, adhesion to base metal, extremely low coefficient of friction, very high hardness, and resistance to adhesion, make it truly difficult to match in crimp performance and durability. However many alternative coatings are being attempted, and some show excellent promise in specific applications. 13.4.1.7 Crimp height. When a connector design engineer begins to design a crimp, several things are considered, including the size and composition— solid/stranded/aluminum/copper, etc.—of the wires to be crimped, and the required electrical and mechanical properties. After optimizing to obtain the best results in both these properties, the engineer determines the best crimp profile and the appropriate crimp height range and width, to achieve the desired reduction in the area of the wire. Deviations from crimp height standards can result in degradation of either mechanical or electrical performance. A loose crimp will result in poor mechanical qualities, and likely, poor or noisy electrical conduction. Too tight a crimp may improve the electrical properties up to a point, but mechanical properties may suffer as a result. Individual wire strands may get cut or the wire may start to undergo excessive plastic flow, leading to a reduction in crimp tensile strength or vibration resistance. The Figure below illustrates the tradeoffs in optimizing crimp design and how crimp height is an accurate measure that combines both electrical and mechanical performance.

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13.4.1.7.1 Comparing crimp pull-out force, crimp force, and crimp height quality measurement technologies. It is common to measure pull-out force as a basic criterion for crimp quality. Indeed, it checks the mechanical properties of the crimp. However, as discussed above, too tight a crimp can be as bad as one that is not tight enough. Pull-out force may not be sufficiently sensitive to detect over-crimping in cases where the process has not gone so far as to break the wire or terminal. A crimped terminal that passes the pull-out test may nonetheless have a reduced lifetime or resistance to subsequent damage from handling, installation, or vibration. Of course, since it can be a destructive test, it may be performed on a sampling basis, only. This sampling still provides an auditable quality record. Crimp force has also been employed. Strain gauges or other force sensors appropriately mounted can acquire force data. Specifically, force during the crimp and the peak force are evaluated in relation to average and standard deviation specifications and, as soon as a crimp cycle occurs in which the value of either exceeds a preset multiple of the standard deviation, the termination is evaluated for faults. This test does allow 100%, non-destructive testing, but does not validate the crimp to the original terminal design intent. Crimp height, as illustrated in the above Figure, is also a common measurement. Crimp height can be used to validate crimp to original design intent but by itself, crimp height can not discern process variables such as wire and terminal variation. As a standard, this measurement can be performed with a vernier caliper fitted with appropriate jaw modifications. As such, of course, it is useful for standardizing but is too slow for 100% production inspection.

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The best method to validate crimp quality is to use a combination of crimp force and crimp height. This results in being able to confirm the achievement of the terminal design intent as well as discerning wire and terminal variables that can result in poor terminations.

13.4.1 Contact Retention. The purpose of this test is to impose axial forces on the connector contacts to determine the ability of the connector to withstand forces that tend to displace contacts from their proper location within the connector insert and resist contact pullout. These forces may be the result of:

a. Loads on wire connected to the contact.

b. Forces required to restrict contact “push-through” during assembly of

removable type contacts into connector inserts.

c. Forces produced by mating contacts during connector mating.

d. Dynamic forces produced by vibration and shock during normal use of the connectors.

e. Forces relating to bundling strains on the wire.

13.4.2.1 Test equipment. Test equipment required to perform this test normally consists of the following:

a. Force gages, of suitable range for the contact size under test, so that readings lie in the middle 50 percent of the scale, where practical, with a nominal full scale accuracy of ± 2 percent.

b. Dial indicator gages or other suitable instruments of such range for the

contacts under test that the readings lie in the middle 50percent of the scale, with a nominal full scale accuracy of ± 2 percent.

c. Contact removal and insertion tools, as required.

d. Suitable compression device.

e. Steel test probes, to adapt the force gage plunger to the particular contact (pin,

socket, etc.) front or wiring end under test.

f. The actual test sample – The plug or receptacle with suitable contacts in place. Normally 20 percent of the contact complement, but not less than 3 contacts of each size are tested unless otherwise specified.

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13.4.2.2 Preparation. All back shell hardware and compression rings, if any, are removed. If testing on the wire side of the connector is required (destructive test), the contacts have the wires cut off flush or the contacts are replaced. All contacts are in place in the connector. Simulated contacts which duplicate the retention feature geometry are sometimes used in lieu of actual contacts to facilitate testing. The unmated connector is mounted in a position of axial alignment of the contacts with the plunger of the test gage. Normally, a minimum space of ¼ inch is required on the opposite side under test to permit any “push-through” that may occur. 13.4.2.3 Test procedure. One method of performing the contact retention test consists of the following steps:

a. Determine the direction (axially) in which the test is conducted. Apply a sufficient seating load (“push” force) to take up any slack of the contact in its retention system. Avoid any sudden or excessive loads.

b. Establish the reference (zero displacement) position of the contact. The

contact may be lightly preloaded (3 pounds, maximum) to assure proper seating.

c. Apply an axial load to the contact at the rate of approximately one pound per

second, until the specified force has been reached. Maintain the specified force for five to 10 seconds during which measurement of displacement is made or the load is removed and displacement measured.

d. If the test is required in two directions, repeat the aforementioned steps.

13.4.2.4 Typical axial loads and acceptance criteria for contact retention test. The following example was extracted from the MIL-DTL-5015 connector specification:

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The maximum axial displacement of a contact under the load conditions in the above table is 0.025 inches. 13.4.3 Coaxial shield pull test. This test is performed to verify the mechanical integrity of the cable or wire shield to its termination (e.g., connector). It is similar to the pull force/tensile testing done for crimp connections. The following test methods are destructive and as such, the test specimen is not suitable for use after testing.

a. Pull and break – Increasing axial force is applied to the connection until either the connector and shield separate or the shield breaks.

b. Pull and return – A specified force is applied to the connection. Once the

specified force is achieved, the force is removed.

c. Pull and hold – A specified force is applied to the connection and held without maintaining that peak value for a specified period of time; then the force is decreased to zero.

d. Pull, hold and break – The connection is pulled to a specified force and held

for a specified period of time; then the connection is pulled until either the terminal or contact is separated from the wire or the wire breaks.

In performing the test, the user and/or the manufacturer specifies the required pull force, the pull rate, and the type of test required. 13.4.4 Torsion Test. The torsion (twist) test may be performed on un-terminated wire or on completed cable or wire harness assemblies. The test is normally performed as follows:

a. The item to be tested is clamped in position in a test apparatus using suitable clamping heads that allow the test item to be subjected to the applied load without causing damage to the test specimen.

b. Select a test sample about 8 inches long and straighten the sample. There

should be no nicks, scratches or other types of deformation in the test sample that could give false indications during testing. For example, a knick in the test sample may cause the sample to fail at the knick location; which would not represent the true strength of the test sample. A slight tension load may be applied to maintain the test item straight during application of the rotation cycles.

c. With the test specimen in the test machine, rotate one end of the specimen at a

constant speed until fracture occurs, while recording the total number of rotations until fracture. The recommended tensile forces to be applied and the minimum number of rotations to be applied should be as specified.

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d. There are many published standards for performing this test. One such standard is ASTM A 938, Standard Test Method for Torsion Testing of Wire.

13.4.5 User defined mechanical tests. The specific tests listed in IPC/WHMA-A-620, Section 19 are recommended default tests for use in event the user (customer) has not invoked any test requirements in the contract. Normally, the user will invoke a standard test suite that, if not the same as specified in IPC/WHMA-A-620, is similar in scope. However, specific test equipment, test parameters, and accept/reject criteria may be different. Additionally, the user may specify different tests. In all instances, unless otherwise agreed to between the user and the manufacturer, the user identified testing applies. References. This document contains some copyright material that is included with specific documented permission from the sources identified below. 1. National Instruments Tutorial Entitled Chapter 1: Understanding Key RF Switch

Specifications dated June 20, 2007 2. Tyco Electronics – AMP, Paper Entitled Crimp Tooling –Where Form Meets

Function 3. Tyco Electronics – AMP, Paper Entitled Crimp Height – Employing the Most

Effective Crimp Quality metric for Meeting Contemporary Quality Standards HDBK_620_Test_Nov_26_2007.doc

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