REFGLOSSARYTOCQUIT Section 1 Design Considerations IPC Designer Certification Study Guide

REF GLOSSARYTOC QUIT

Section 1

Design Considerations

IPC Designer Certification Study Guide


Section 1.1

Interrelated Considerations for Design



Interrelated Considerations for Design - 1.1

The end product requirements are the characteristics of an individual part or assembly in its final completed state. To ensure the part or assembly will work as intended, the environment in which it will operate must be known at the time of the design.



Equipment environmental conditions such as ambient temperature, heat generated by components, ventilation, shock, vibration, etc., necessitate different materials, tolerances, and final product configurations.

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SM7823.5.23.5.3

Table 3-6



To facilitate communication between the designer and manufacturers, different performance classes have been developed to reflect progressive increases in sophistication, functional performance, and frequency and/or intensity of inspection or stress testing.



These are identified by a class designation where:

• Class 1 is defined as General ElectronicProducts

• Class 2 is defined as Dedicated ServiceElectronic Products

• Class 3 is defined as High Reliability Electronic Products



Each has its definition which can be related to end use environments such as computers, telecommunications, aerospace, or automotive applications. The user has the responsibility to determine the class to which their product belongs.

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22221.5.1

Extra1.1



Producibility levels have also been established to help communicate the design complexity to the manufacturer. These levels reflect progressive increases in sophistication of tooling, materials or processing and, therefore progressive increases in fabrication costs.



There are three levels of producibility:

Level A: General Design Complexity -- Preferred

Level B: Moderate Design Complexity -- Standard

Level C: High Design Complexity -- Reduced Producibility


Most printed board manufacturers can produce Level A products at a very high yield and, therefore, at a reasonable cost. The number of available manufacturers that have the precision capability to manufacture product at the C level drops to approximately 20%.


22211.6.3


The number of manufacturers able to accommodate designs that are state-of-the-art and need even greater sophistication of tooling, materials, and processing is around 1%. State-of-the-art technology cannot be standardized due to the fact that every few years, the levels shift causing that which was moderate to become general, and so on.



Since the design is intended to meet all the requirements of the product including performance, cost, reliability, etc., all of the issues must be discussed at the beginning of the design process.

These discussions should include manufacturing engineering for both the board assembly and test.




If a company wishes to stay competitive with the products they sell to the customer, the days of tossing the design “over the wall” are gone. The designer must be aware if “backward compatibility” is required. This means the design must be able to be used in any past installation without modifications.

Extra1.2



The physical constraints of the installation interface are the primary consideration when redesigning a board for a product already in the field. Thus, design for excellence is design for producibility and involves all the disciplines needed to manufacture and maintain the product.

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SM7823.4

3.4.4


Section 1.2

Copper Clad Laminates



Copper Clad Laminates - 1.2

Copper clad laminates used to produce printed boards consist of three parts:

• the resin - which is a natural or synthetic resinous material

• the reinforcement - such as different forms of paper, matte glass, or woven glass

• the copper foil

22214.1



The resin and reinforcement make up the base material which is the insulating material upon which a conductive pattern may be formed. Base material may be rigid or flexible or both. It may be a dielectric or insulated metal sheet.



Product safety may require that the base materials withstand tests performed by a product safety agency, such as Underwriters Laboratories. Tests include flame retardance (UL 94), printed board construction (UL 796), and base material (UL746).



Copper forms the cladding and is available in two types; rolled annealed, or electrodeposited (ED). Unclad base material can also be used when producing printed boards using additive technology, where copper is deposited only where required.

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4.2.1.2

4.3

22224.3

Table4-1



The most popular laminate resin system is epoxy. The most common thicknesses for laminate for rigid base material are 0.75mm (.030 inches), 1.5mm (.060 inches), and 02.4mm (.090 inches); however, minimum thicknesses for rigid base material is 0.05mm (.002 inches).



There are various improvements that have been made to epoxy resins over time. These include difunctional epoxy, multifunctional epoxy, BT epoxy and others. All of the improvements are intended to provide better and more consistent dimensional stability and minimal thermal expansion characteristics.

22224.3

Table4-1



The most popular reinforcement is woven glass. It provides structural strength to the resin and comes in various thicknesses to accommodate the various thicknesses of the sheets used to produce the laminate. A sheet of woven glass that has been coated with resin is referred to as prepreg or preimpregnated reinforcement.



This material is also known as “B” stage since it is at a partial stage of cure of the resin.

B stage material can be handled, combined with other sheets, and then laminated under heat and pressure to form the base material.

4.3

22214.2.1.2

Extra1.3



Copper foil thickness is defined in ounces. The origin of the practice comes from the days when a copper foil was used to cover roofs. Therefore, half ounce copper is defined as the weight of a square foot of copper foil that is 17 micrometers thick (0.0007 inches).



Subtractive boards can be made by the process that starts with copper foil which is then plated and etched where the unwanted copper is removed.

Printed boards can also be additively produced. This is where the copper is patterned on a bare laminate in an electroless process (no electricity involved in moving the atoms of copper to the surface of the board).

Table 4-5

22214.4.9.1



New resins are appearing on the scene. These include such products as cyanate ester, polyimide, or PTFE (teflon); however, epoxy resin is still the most popular resin used in the United States.



Another polymer material used with the printed board is an epoxy permanent polymer coating known as solder mask. The fact that the polymers are very similar to the laminate permits good adhesion of the two systems to one another.

Extra1.4

22224.3.1

22224.3.2

2222Fig 4.1




Section 1.3

Thermal Management Techniques for Printed Boards



Thermal Management Techniques for Printed Boards - 1.3

Material selection for the printed board is an essential element of the design to accomplish the structural strength properties needed to:

• support the electronic components

•handle any vibrational requirements

•dissipate the heat from the conductors and the components



The characteristics that must be understood by the designer are the safe continuous operating temperature of the materials and the coefficients of thermal expansion (CTE). Some of these properties are provided by the reinforcement.

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4.1.1

7.3.1

Fig7-2

Table7-4

2222Table

4-1

-1,1,-1,1,

Extra1.5



Copper is a relatively good conductor of heat as well as being the main material used to conduct electricity. This capability permits the use of large planes of copper to perform the heat sinking function necessary to keep the board and the board assembly cool.



Depending on the amount of current being drawn through the circuit conductors, they can also become a heat generator. Therefore, proper conductor width and thickness are characteristics that must be checked to ensure that the wattage being passed through the conductor does not raise the temperature of the copper above a safe temperature, which may increase the failure rate of a printed board.



Charts are usually used to determine the original heat management goals for the conductors. These are later verified when the whole assembly comes together and the total heat generated by the components and the conductors are assessed.

2221Fig 6.4



The resin system responds to heat and temperature cycling by expanding. The higher the resin content, the greater the expansion rate. This expansion causes a strain on the barrels of the plated through-holes. Insufficient copper in the hole, or having copper that is not ductile are factors that influence the ability to withstand the thermal strain.



The hole size also is a contributor to the equation since the circumference of the barrel provides more copper volume in the larger holes. The combination of reinforcement and resin together provide the expansion model for the thickness or Z axis of the board. The industry defines the expansion rate in parts per million (ppm/ C or a percentage of the thickness.



All the laminate constructions have a very uniform rate of Z axis expansion until they reach a particular point.

At that temperature the expansion rate increases dramatically and the most damage can occur. This point is known as the glass transition temperature, or Tg. Many laminate structures are sold by their Tg capability.

22217.3.1

Fig7-2

Table7-4

2222Table

4-1



The reinforcement accounts for the dimensional characteristics of expansion in the X and Y axis. Some designers look for product that has high dimensional stability characteristics. In this regard, they are trying to achieve a low CTE of the material in the X and Y axis to reduce the thermal mismatch between the board and the components.



Another way to accomplish the same properties is to include a special plane in the board. These constraining core planes are also very effective in removing the heat from hot spots or hot components.



Heat is transferred to the plane through plated through-holes that are filled with a conductive material. Known as thermal vias they conduct heat away from the component, or other hot areas, and move the heat to the cooler planes. Planes are then connected to the frame of the housing containing the board. Thus heat is moved by conduction to the cooler surface.

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7.3.1

Fig5-2

Fig5-3ab

Fig7-2

Extra1.6


Section 1.4

Thermal Management Techniques for Assemblies



Thermal Management Techniques for Assemblies - 1.4

Temperature management is one of the most important aspects of printed board assembly design. As electrons pass through components and board circuitry, heat is generated. The component manufacturer usually specifies the amount of heat that each component generates based on the way it is used in the circuit.



Heat is generated mostly by complex integrated circuits, power transistors, transformers, and any component that draws a great amount of current; however, even resistors or capacitors can create heat if they are not compatible with the rated current that they must manage.



Most of these conditions relate to the number of watts that a component needs to manage. Thus passive components for the same value will be manufactured to handle different current capabilities. They are then specified as 1/2 watt, 1 watt, 2 watt, etc.

22217.0



Heat and thermal cycling is the enemy of board performance and long term reliability. Thus, the components should be distributed as evenly as possible across the board and oriented in a position which allows the best airflow over the components.



Components that run too hot affect their neighbors; change value or even fail. They can also damage the printed board mounting substrate, and cause problems in the solder joints that attach the components to the circuitry. Part of this problem is created by the difference in the coefficient of thermal expansion (CTE).



The definition of CTE is that it represents the linear dimensional change of a material per unit change in temperature. The variation can create a thermal expansion mismatch between the component and the printed board, which places a mechanical stress on the solder joint. The problem is not too severe with through-hole components, however, it can be very detrimental with surface mounted parts.

22217.3.27.3.3

Fig7-2



Solder in sufficient volume provides a certain amount of relief. Solder, when heated, becomes plastic in nature, thus the mechanical stress is taken up by the solder providing a certain amount of compliancy. Now the enemy becomes thermal cycling. If the assembly is continually turned on and off, the components become hot and then cool.



These conditions cause the solder to change states many times and eventually can cause a crack in the solder due to the solder joint fatiguing under the strain of the continual change from plastic to brittle. The greater the difference between the high and low ends of the temperature excursion and the greater the number of cycles, the sooner the problem can become noticeable. The first indication is an intermittent signal; the final is an open joint.

Extra1.5a



For all of the obvious reasons it is important to keep the assembly as cool as possible. Components can be individually provided with a heat sink to transfer or dissipate the excess temperature to the air or to another solid member that may be cooler.



When the heat is transferred to the printed board, the design must be able to accommodate the distribution adequately. Organic resins used to manufacture laminate for printed board manufacture or the reinforcements used in such laminate are not good conductors of heat, therefore the function must be transferred to the metal (usually copper) planes.

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7.2.1

8.1.10

Fig8-8


There are many techniques for heat sinking components. Included in these are the simple method of contact with a heat sinking plane to the more exotic of mounting the part in a specially designed finned metal bracket used to allow the ambient air to cool the part. Additionally, with some components, thermally conductive adhesive or paste can help transfer the heat effectively.


Extra1.6b




Section 1.5

Testing Techniques and Procedures



Testing Techniques and

Procedures - 1.5

Developing a test strategy should be done at the beginning of any design. It is important to involve everyone from the manufacturing cycle in the decision process that determines what will be tested and when it will be tested. At a minimum, the testability review team should include:• the board manufacturer• the assembly process engineer• testing experts from both disciplines• the circuit designer

Extra1.7



Procedures - 1.5

The circuit engineer knows the robustness and maturity of their circuit; s/he determines whether in-circuit testing is necessary or whether functional testing will suffice. In-circuit testing consists of applying test signal directly to a device’s input terminals and sensing the results directly from the device’s output terminals. In this manner it can be determined if the part is functioning as intended, or in its simplest form, electrical continuity.

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Procedures - 1.5

Functional testing analyzes the unit under test. This is usually done from the connector(s) of the board assembly and circuitry is considered a complete functional testing entity. The circuit is stimulated through the connector by applying specified inputs to exercise the circuit. By sensing the circuit outputs the operator can determine if the circuit is operating correctly.



Procedures - 1.5

Both in-circuit and functional testing are performed on the entire assembly, therefore, the decision as to what approach to take many times is predicated on the complexity of the assembly. In theory, functional testing as a go/no go condition would be all that is necessary to ensure that a circuit works properly.



Procedures - 1.5

Some customers require that this test is performed while the assembly is at an elevated temperature. Burn in is the process of electrically stressing a device at an elevated temperature for the purpose of sorting out infant mortality.

3.5.3.1

3.5.4

3.5.4.1


Testing Techniques and Procedures - 1.5

Functional testing saves cost, however if the circuit fails the test, either at functional test or burn in, the fault must be located. This usually requires special test points on the critical nets at specified node points so that the fault can be isolated. These points are the same points that may be used to perform in-circuit testing using a bed-of-nails tester.

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Extra1.7b



Thus from a layout point of view, the critical circuit elements must still be fanned-out to locations away from the part to probe the portion of the circuit being evaluated. Probe points are spring loaded pins that are positioned in a bed-of-nails tester. They are usually on some standard grid to simplify the probe point fixture design.



The recommended land size on the board to accommodate probe points is 1.0mm [.040”] for a round land, and 0.9mm [.036”] for a square land.

The land may be a via or just a plain circuit feature, or even a wide exposed conductor.

3.5.6.2

3.5.6.5

Fig3-1

Fig3-3



Assembly testing can require as many as one point for every node or net in the circuit (full nodal access), or only for those nodes or nets that are critical components that are hard to establish as being at fault in circuit performance (partial nodal access).



A strategy is developed at the start of the layout to provide the direction as to where to leave room for the test points. Manufacturing must be involved to make sure that the tooling hole concept matches the fixture of the tester, and if the board is tested while still in a panel format, how the inputs are applied. Boards are usually separated from the panel for functional testing.


Bare board testing is done to prove that the printed board has full continuity and that there are no short circuits. In order to facilitate communication of problem areas, and accurately locate probe sites, it is essential that grid locations and net names be specified.


22213.5.6.1


The probe points are long solid pins that make contact with the conductors on the outer surfaces of the board. Probes must make contact with the end of every net. A voltage is applied to each conductor set to ensure electrical continuity.


Apdx.A

3.5.1


Section 1.6

Reliability Terms and Issues



Reliability Terms and Design Issues - 1.6

Designing for reliability means considering all the factors necessary to establish the probability that a device or assembly will function properly for a defined period of time under the influence of specific environmental and operational conditions. This is the expected product life or the printed board failure rate.



This is usually identified in terms of Mean Time Between Failures (MTBF); the higher the number of hours, the more reliable a part is. However, some products only function once in their life cycle, therefore a different measure to determine reliability is established.



Stress conditioning is applied to the product in terms of temperature cycling, vibration, and/or shock to determine that the product will survive these Highly Accelerated Stress Test (HAST) conditions.

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Extra1.7c



The main failure mechanism in a printed board assembly, besides a component failure, is a cracked plated through-hole or a cracked solder joint. Both conditions can occur due to the thermal exposure of the assembly to temperature variation. The larger the range between the upper and lower temperature working conditions, the greater the stress and potential for a failure.



The number of thermal cycles that a part will experience during its service life is also a significant factor in determining reliability. Accelerated test conditions will many times indicate early failures or infant mortality conditions, however to prove true reliability often requires long exposure to conditions intended to simulate field exposure.



As an example:

automotive under-the-hood products must survive both extreme cold and hot temperatures.

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Table7-4



In developing a new board structure, many companies find that it is beneficial to incorporate a test coupon into the board production panel. The coupon can be specially designed to provide information on the reliability of the final product. The test coupon allows destructive testing of the board structure, without sacrificing a good board.



Plated through-hole structures can be destructively conditioned and then examined in order to determine the robustness of the design and the manufacturing processes. A high quality manufacturing process will not improve on the reliability of the design; however, a poor quality manufacturing process can have a detrimental effect on the reliability.



Balanced construction of the printed board about the center of the board and on individual layers can improve the performance of the plated through-hole survival. Hole diameter, aspect ratio, plating thickness, as well as copper plating ductility are some of the factors related to hole failures.



For the assembly the common failure is the solder joint.

Material properties of the substrate compared to the component’s coefficient of thermal expansion suggest keeping this mismatch as low as possible.



Leaded parts, or parts that have good solder volume between the part termination and the board land pattern, are more likely to be able to withstand thermal swings, thereby extending the fatigue life of the solder joint. The lead or solder joint compliancy helps to offset the stresses that occur when the board expands at a greater rate than the component body.



The choice of material reinforcement in many instances can help limit the expansion of the board in the X and Y axis, thus improving the conditions of CTE mismatch.



Balancing all the elements of the design to arrive at the optimum solution is the challenge of the printed board designer.

For the product that sees dramatic thermal stress during its life cycle, the evaluation is crucial; for a product that must function in a benign environment the issues are not as critical.

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Table7-4


Section 1.7

Purpose of Tooling Holes



Purpose of Tooling Holes - 1.7

A tooling hole is a physical feature in the form of a hole, or slot, on a printed board fabrication panel or assembly panel. Tooling features are used exclusively to position a printed board or assembly during fabrication processes, assembly, and test processes.



This includes:

• registration of phototooling

•positioning core layers during lamination

•panels during drilling

•boards at bare board testing

•panels or boards during automated assembly

• functional test



Layer-to-layer registration is the manner in which lands and holes are related to one another within a multilayer board. Tooling holes are used to assure the accuracy of the relationship.



The designer is responsible for indicating the tooling holes that stay with the board or panel.

The board manufacturer is responsible for determining the tooling holes needed for board fabrication.



Since it is preferred to assemble boards in panel format, many designers specify the assembly panel as the required purchase item with its inherent tooling holes or features needed by the assembly processes. The holes are often used to position the board mechanically on the conveyer and define the relationship between the tooling hole location and the location or position of the components.



If the tooling holes are not required after depanelization (for programming, test, etc.), it is preferred to place them in the borders of the panel.



Tooling holes are unsupported holes (i.e. a hole in a printed board that does not contain plating or other type of reinforcement), and are toleranced tightly in order to avoid movement between the tooling pin and the board.

This is especially important if the holes are being used for registration.



Pins are usually very precise with tolerances in the range of 0.025mm [.001”] or less. The holes also have very precise tolerances which are generally in the range of 0.05mm [.002”]. Maximum Material Condition (MMC) and Least Material Condition (LMC) are terms used to describe the relationship between the hole and the pin.

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9.2.1.2

Table9-2



Line to line conditions are considered as an interference fit, thus the MMC of the hole (when the hole is smallest) is usually considered with as small a clearance as possible with the MMC of the pin (when the pin is as large as possible). A 0.025mm [.001”] clearance is usually sufficient provided that the hole does not get too large or the pin too small.

Extra1.7d



The tolerances for tooling holes are tighter than those used for leads of components.

To assure accuracy of the tooling system, they are handled with special care by the manufacturer.



Since tooling systems generally have several holes involved in the system, various methods are used to avoid mechanical stress as the pins and holes are aligned.

A popular tooling system used by board manufacturers employs round pins positioned in slots. This reduces the surface contact of the pin with the board and avoids a stress build-up.



Often the slots are positioned at the centers (near the edge) or at the four corners of the panel.

Another technique for reducing the stress is to use pins that are diamond shaped in a round hole.


Section 1.8

Purpose of Stiffeners



Purpose of Stiffeners - 1.8

Stiffeners are mechanical parts that are added to a printed board in order to reduce the possibility of bow & twist, they also provide rigidity to the assembly. Stiffeners can come in a variety of configurations with the simplest form being a piece of sheet metal, or sheet metal bent into an “L” shaped bracket, to that of extruded metal shapes specifically designed to be attached to a printed board and provide the stiffness needed to resist bending.



Stiffeners may be made from a variety of different materials. They may be made of a dielectric (do not conduct electricity) or they may be made of metal (do conduct electricity). Steel and aluminum are the most popular metals with aluminum being the choice if the stiffener is an extruded shape. While metal stiffeners are popular, they must have an adequate finish to protect the metal from corrosion.

8.2.7

22215.3.2

D3258.3



The proper spacing must be maintained between the stiffener and components mounted on the board and conductors. The component mounting sequence must be considered so that the stiffener is not in the way of the insertion or pick-and-place heads. Therefore, it is important for the designer to have a clear understanding of the assembly operation and the physical clearances required. Electrical clearance between the stiffener and the conductors can be achieved in a variety of ways.



If the electrical clearance conditions are tight, fiber or plastic insulators can be located between the stiffener and the conductors. It should be noted that moisture traps can occur between the board and the stiffener depending on how it is mounted or secured. Another way to provide isolation of the metal is to make the protective coating serve the dual function of protecting and insulating. A few examples of these are epoxy coated metal and porcelainized steel.

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8.2.7



Stiffeners may be screwed, glued, or riveted to the board. Riveting appears to be the preferred method because it makes a positive connection and will not come apart easily. With riveting, the stiffener becomes an integral part of the board.



The use of screws requires an extra assembly operation which takes additional time. And the use of an adhesive or glue requires that the adhesive is compatible with the surface conditions of the board and the stiffener. Depending on the method of attachment, the techniques for documentation of the stiffener requirements also vary.

22218.2.7



If the stiffener is glued to the board and is made of laminate, it could become part of the master drawing and be fabricated at the same time the board is laminated; if screwed on, it could be part of the assembly drawing; and if riveted, it could be the first assembly operation.

It should be noted that some companies have a separate drawing for riveting stiffeners.



No matter what the technique, stiffeners prevent flexing of the printed board thus reducing the incidence of solder or copper foil cracking during shock and vibration exposure.D325

8.3

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Extra1.7e


Quiz1

• Who assigns the class to which a printed board belongs?– the printed board user– the bare board fabricator– the assembly manufacturer– the government procuring agency

Answer: the printed board user


Quiz1

• The major function of a design team, if assembled prior to starting the layout, is to ensure which condition?– that the time-to-market schedules can be met– that customer requirements are properly

addressed– that all factors for design and production are

considered– that bill of material and electronic diagrams are

ready to release

Answer: that all factors for design and production are considered


Quiz1

• If a tooling pin is 2.85mm to 2.87mm [.112” to .113”], what is the size of the optimal tooling hole?– 2.85-2.90mm [.112-.114”]– 2.90-2.95mm [.114-.116”]– 2.90-3.00mm [.114-.118”]– 2.95-3.10mm [.116-.122”]

Answer: 2.90-2.95mm [.114-.116”]


Quiz1

• How is the location of tooling holes determined by the assembler?– the holes are visually inspected and are not plated

through– the holes are documented and indicated on the

master drawing– the holes are documented and indicated on the

assembly drawing– the holes are visually inspected and have no

electrical significance

Answer: the holes are documented and indicated on the assembly drawing


Quiz1

• Proper tooling hole systems consist of a certain number of holes which are positioned in certain locations on the board. What are their characteristics?– two tooling holes in opposite corners– three tooling holes near each of three corners– two tooling holes located on the break-away tabs– a specific number and location to facilitate the

assembly operation

Answer: a specific number and location to facilitate the assembly operation


Quiz1

• What are the 3 most common dielectric thicknesses of copper-clad laminate used to produce double-sided printed boards?– 0.50mm [.020”]– 0.75mm [.030”]– 1.00mm [.040”]– 1.50mm [.060”]– 2.00mm [.080”]– 2.40mm [.090”]

Answer: 0.75mm [.030”], 1.5mm [.060”], 2.4mm [.090”]


Quiz1

• Copper thickness is defined in ounces based on the weight of a square foot of foil material. What is the thickness of one ounce copper?– 0.009mm [0.0004”]– 0.017mm [0.0007”]– 0.025mm [0.0010”]– 0.035mm [0.0014”]– 0.050mm [0.0020”]– 0.070mm [0.0028”]– 0.075mm [0.0030”]

Answer: 0.035mm 0.0014”


Quiz1

• What is the most commonly used resin system for producing rigid printed boards?– PTFE– epoxy– polyimide– cyanate ester

Answer: epoxy

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REFGLOSSARYTOCQUIT Section 1 Design Considerations IPC Designer Certification Study Guide