Introduction to Flex and Rigid Flex Materials and Constructions

DELIVERING QUALITY SINCE 1952.

Introduction to Rigid-FlexMaterials & Constructions

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Scope

Rigid-Flex Design Evolution

Rigid-Flex Reliability

Copper Clad Flex Laminate Constructions

Coverlay Constructions

Design Review / Quoting Process

Summary

Case Studies

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Design Evolution

Significant Design Evolution In The Past Decades

“Older” Designs:– “Point to Point” interconnects & Simple Circuits

• PTH Connectors & Components• Low layer counts

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Design Evolution

Significant Design Evolution In The Past Decades

Current Designs:– Rigid areas serve as fully capable “Rigid” boards– Push the same limits of complexity and density

• Fine lines / spacing• High aspect ratio sub 0.010” vias• Blind & Buried Vias• High Layer Counts• Higher Operating Temperatures• RoHS Assembly

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Advancing designs identified a Via reliability issue.– Formation of Copper plating cracks within Via holes

Root Cause:– Use of Flex circuit adhesives within Rigid areas– High Co-Efficient of Thermal Expansion of

Flex Adhesives (10 -20X of FR-4) place viasunder Z-Axis stress during thermal cycles.

– Thermal Cycle Sources:• RoHS Assembly • Multiple Assembly cycles• Higher System & Component

operating temperatures– Confirmed through failed Interconnect

Stress Testing (IST) and Scanning ElectronMicroscope Imaging (SEM)

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Potential Sources of Adhesive:– Adhesive based Copper Clad Flex Laminates

– Full coverage Coverlay constructions

– Rigid to Flex layer bonding with adhesives

Worst Case Construction: 6 Adhesive Layers

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VIA Reliability Solution:– Manufacturing, Material Suppliers and Industry Standards

Organizations worked together to develop the following elements to resolve the issue:

• Adhesiveless Copper Clad Flex laminates

• Selective Coverlay Constructions

• Rigid to Flex layer bonding with No- Flow FR-4 prepreg

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IPC 2223C - Sectional Design Standard forFlexible Printed Boards– Eliminate / Minimize use of adhesives within rigid areas

– Use of adhesiveless based substrates

– Use of Selective / Partial Coverlay Construction

– Secs. 4.1, 5.2.2.2., A2

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Adhesive Based:– Copper bonded to Polyimide core with Acrylic or Modified Epoxy

adhesives

Adhesiveless:– Copper directly attached to Polyimide Core

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Benefits and Drawbacks:

– Adhesiveless Copper Clad Laminates:

• Thinner constructions:– Up to 33% thinner per Flex laminate core

• More flexible & reliable designs

• Higher Operating Temperature rating

• Higher Copper peel strengths

• Reduced thermal expansion stress on vias

• Slight material cost increase

– Adhesive Based Copper Clad Flex Laminates:

• Lower Cost

• Limited to Flex Only applications:– Point to Point Interconnects– Limited Thermal Excursions

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Full Coverage Coverlay Construction:– Coverlays extend throughout Rigid areas

• Vias & PTH holes drilled through Coverlay adhesive and exposed to excessive Z-Axis thermal expansion stress

Poor Construction Design: 2 Adhesive Layers

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Selective Coverlay Construction:– Coverlays restricted to exposed flex areas only

– Up to 0.050” interface with Rigid areas

– PTH holes restricted within Coverlay to Rigid interface area

– Rigid layers bonded with High Temp No – Flow FR-4 Prepreg

IPC Specified Construction Design: 0 Adhesive Layers

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Benefits and Drawbacks:

– Selective Coverlay Construction:

• Significantly reduced thermal expansion stress placed on vias

• Higher Operating Temperature rating

• Higher layer to layer bond strength

• Thinner Constructions

• Slight cost increase due to greater number of manufacturing process steps

– Full Coverage Coverlay Construction:

• Lower Cost

• Flex adhesive based Thermal stressreliability issues

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Design Review / Quoting Process

Design Review:– Specifications

– Materials

– Construction

– Gerber Data DFM

– Application (electrical & mechanical requirements)

– Identify areas of opportunity

• Improved reliability, Improved functionality, Reduced Cost

– Communicate and resolve all technical items and potential areas of opportunity

Quoting:– Accurate quote

– Based on manufacturable, reliable, cost effective design

– Eliminates / Minimizes technical issues after PO is placed

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Summary

Modern IPC specified Material and Construction methods have virtually eliminated Thermal Stress induced Via reliability issues

Many current Rigid-Flex designs do not adequately or correctly define modern IPC specified materials and constructions

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Summary

Incorrect / Inadequate Designs May:

– Allow supplier(s) to utilize less expensive materials and constructions to gain a competitive pricing advantage at the expensive of future reliability issues

– Limits customers recourse in the event of reliability issues:• Parts made to print

– Complicates the quoting process• Proper materials and constructions require deviations from the supplied

specifications to be identified

– Deviations from print may not be understood resulting in rejection of viable quotes

– Quoting delays due to added technical communications

– Supplier(s) may not identify deviations on quote• Parts not built to print• Potential order rejection at in-coming inspection

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Case Study: Rigid-Flex

Application:– Power Distribution Control Circuits

Customer Requirement:– Improve quality and supply chain reliability while maintaining existing price

targets Challenge:

– Supplied designs did not meet current IPC standards for materials and construction

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Solution:– Epec revised designs to meet current standards and to utilize more cost

effective materials– Performed technical sessions with both end user and CM (customer) to inform

them of the benefits Summary:

– Epec awarded both PNs– End User to implement design revisions in to current and new part numbers

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Application:– Medical Care Communication system

Customer Requirement:– Develop technical and supply chain partner to assist in product design and

production requirements Challenge:

– End Customer & Design House lacked experience in all areas of Rigid-Flex design (Gerber layout, material selection, Manu. Specs etc.)

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Solution:– Epec worked closely with Design Team and CM to develop design and ensure

manufacturability– Utilized in-house capabilities for Gerber layout

(impedance controlled), provided complete materialsand specifications package

Summary:– Successful introduction,

passed all tests / qualifications– In volume production

(2 additional PN’s won)

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Build To Print Electronics

We are a leading provider of printed circuit boards, flex and rigid-flex circuits, membrane switches, touch panels, silicone rubber keypads, graphic overlays, custom battery packs, and electronic fans & motors.

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Thank You

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Introduction to Flex and Rigid Flex Materials and Constructions