This document discusses various advanced rigid-flex circuit board constructions that offer benefits over standard constructions. It describes options for odd layer counts, asymmetrical constructions, varying flex layer counts, integrated ZIF connections, blind and buried vias, air gap flex layers, multiple rigid area thicknesses, and shielded flex layers. These advanced constructions allow for higher levels of design integration, tighter bend radii, impedance control, and EMI shielding compared to standard constructions.

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Advanced Rigid-Flex
Circuit Constructions
06.29.17

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Introduction
 Todays Rigid-Flex materials and construction methods allow for a
wide variety of advanced design configurations.
 These options create the opportunity to achieve higher levels of
design integration and packing density.

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Introduction
 Options Include:
– Higher Flex Layer Counts
– Blind & Buried Via Structures
– ZIF Connections
– Flex Area Components
– Asymmetrical Constructions
– Shielded Flex Areas
– Multiple Rigid Area Thicknesses
 In this webinar we will introduce and discuss the benefits and applications of
some of the more common advanced rigid-flex circuit board constructions.
 Many additional configurations and combinations are available.

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Agenda
 Standard Construction Review
 Advanced Rigid-Flex Construction Options:
– Odd Number Layer Counts
– Asymmetrical Constructions
– Varying Flex Layer Count Areas
– Integrated ZIF Connections
– Blind & Buried Vias
– Air Gap Flex Layer Constructions
– Multiple Rigid Area Thicknesses
– Shielded Flex Layers

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Standard Rigid-Flex Construction
 Symmetrical construction
 Even layer counts in both rigid & flex areas
 Impedance Control available

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Standard Rigid-Flex Construction
 Most common construction (approx. 60%)
Soldmask (LPI)
Layer 1 Base Copper um + Plated 20 um min.
FR-4 (170 tg min.)
Layer 2 Copper
Coverlay:
Coverlay Adhesive:
Layer 3 Copper
Polyimide Core (Adhesiveless) Flex Thickness Rigid
Layer 4 Copper Thickness
Coverlay Adhesive:
Coverlay:
Layer 5 Copper
FR-4 (170 tg min.)
Layer 6 Base Copper um + Plated 20 um min.
Soldmask (LPI)
Prepreg (2 x 1080, no flow)
Prepreg (2 x 1080, no flow)

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Odd Layer Count Constructions
 7 layer Rigid area with 3 Flex layers
Soldmask (LPI)
Layer 1 Base Copper um + Plated 20 um min.
FR-4 (170 tg min.)
Layer 2 Copper
Coverlay:
Coverlay Adhesive:
Layer 3 Copper
Polyimide Core (Adhesiveless)
Adhesive Rigid
Layer 4 Copper Thickness
Polyimide Flex Thickness
Layer 5 Copper
Coverlay Adhesive:
Coverlay: 25 um
Layer 6 Copper
FR-4 (170 tg min.)
Layer 7 Base Copper um + Plated 20 um min.
Soldmask (LPI)
Prepreg (2 x 1080, no flow)
Prepreg (2 x 1080, no flow)

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Odd Layer Count Constructions
 5 layer Rigid area with 1 Flex layer
Soldmask (LPI)
Layer 1 Base Copper um + Plated 20 um min.
FR-4 (170 tg min.)
Layer 2 Copper
Coverlay:
Coverlay Adhesive:
Layer 3 Copper Flex Thickness Rigid
Polyimide Core (Adhesiveless) Thickness
Layer 4 Copper
FR-4 (170 tg min.)
Layer 5 Base Copper um + Plated 20 um min.
Soldmask (LPI)
Prepreg (2 x 1080, no flow)
Prepreg (2 x 1080, no flow)

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Odd Layer Count Constructions
 3 layer Flex area configuration allows for two sided shielding requirements in
flex areas.
– Stripline Impedance Control
– RF & EMI Requirements
 Odd layer counts can apply to both flex & rigid areas independent of each
other.
 Minimizes flex area thickness for improved flexibility and mechanical bend
capability / reliability.
 IPC 2223C compliant.
 Reduces cost by minimizing total number of flex layers required.

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Asymmetrical Constructions
 Applications:
– Convoluted impedance requirements
– Blind via constructions
• Minimizes blind via aspect ratio for improved manufacturability reliability.
 May result in warp & twist in assembly array
which may require a hold down fixture
for transport through assembly.

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Asymmetrical Constructions
Soldmask (LPI)
Layer 1 Base Copper 18 um + Plated 35 um min.
FR-4 (170 tg min.)
Layer 2 Copper
Prepreg (1 x 1080, no flow)
Layer 3 Copper
FR-4 (170 tg min.)
Layer 4 Copper Rigid
Coverlay: Thickness
Coverlay Adhesive:
Layer 5 Copper
Polyimide Core (Adhesiveless) Flex Thickness
Layer 6 Copper
Coverlay Adhesive:
Coverlay:
Layer 7 Copper
FR-4 (170 tg min.)
Layer 8 Base Copper 18 um + Plated 35 um min.
Prepreg (2 x 1080, no flow)
Prepreg (1 x 1080, no flow)

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Varying Flex Layer Count Constructions
 Allows for tighter bend capabilities in reduced flex layer count area.
 Multiple configurations available.
 Requires use of “Air Gap” flex layer construction.

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Varying Flex Layer Count Constructions
Soldmask
Layer 1 Base Copper um + Plated 20 um min.
FR-4 (170 tg min.)
Coverlay:
Coverlay Adhesive:
Layer 2 Copper
Polyimide Core (Adhesiveless) Flex
Layer 3 Copper Thickness
Coverlay Adhesive:
Coverlay: Rigid
Thickness
Coverlay:
Coverlay Adhesive:
Layer 4 Copper
Polyimide Core (Adhesiveless) Flex
Layer 5 Copper Thickness
Coverlay Adhesive:
Coverlay:
FR-4 (170 tg min.)
Layer 6 Base Copper um + Plated 20 um min.
Soldmask
AIR GAP
Prepreg (2 x 1080, no flow)
Prepreg (3 x 1080, no flow)
Prepreg (2 x 1080, no flow)

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Integrated ZIF Tail Constructions
 Eliminates the need for a ZIF
connector and separate flex circuit.
 Reduces real-estate requirements in
rigid area(s).
 Improves reliability through
elimination of points of interconnect.
– ZIF connector solder joints, ZIF
finger contacts
 Available in a wide variety of
configurations.

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Integrated ZIF Tail Constructions
Soldmask (LPI)
Layer 1 Base Copper 35 um + Plated 20 um min.
ZIF
Adhesive Thickness
Polyimide Core (Adhesiveless) Rigid Thickness Flex Thickness
Layer 2 Copper
Coverlay Adhesive:
Coverlay:
Layer 3 Base Copper 35 um + Plated 20 um min.
Soldmask (LPI)
Prepreg (2 x 1080, no flow)
Prepreg (2 x 1080, no flow)
FR-4 (170 tg min.)
Polyimde Stiffener
FR-4 (170 tg min.)

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Blind & Buried Constructions
 Applications: Similar to Rigid PCB Blind & Buried Constructions
– High density BGA applications with via in pad etc.
 May require an asymmetrical construction if blind vias interconnect to
flex layers.
 Configurations may be limited due to number of sequential lamination
cycles required. Less than Rigid PCBs due to material dimensional
tolerances and manufacturing methods.
 Via fill and cap available.

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Blind & Buried Constructions
Soldmask (LPI)
Layer 1 Base Copper 18 um + Plated 35 um min.
FR-4 (170 tg min.)
Layer 2 Copper
Prepreg (1 x 1080, no flow)
Layer 3 Copper
FR-4 (170 tg min.)
Layer 4 Copper
Coverlay:
Coverlay Adhesive:
Layer 5 Copper
Polyimide Core (Adhesiveless) Flex Thickness Rigid
Layer 6 Copper Thickness
Coverlay Adhesive:
Coverlay:
Layer 7 Copper
FR-4 (170 tg min.)
Layer 8 Base Copper 18 um + Plated 35 um min.
Prepreg (2 x 1080, no flow)
Prepreg (1 x 1080, no flow)

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Flex Layer Air Gap Constructions
 Flex layers configured as independent pairs of 2.
 Allows design to meet IPC 2223C design guidelines.
– Construction results in no flex adhesives in rigid areas.
– Preferred configuration for 4 or more flex layers.
 Provides greater flexibility due to flex layers bending
independent of each other.

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Flex Layer Air Gap Constructions
Soldmask
Layer 1 Base Copper um + Plated 20 um min.
FR-4 (170 tg min.)
Coverlay:
Coverlay Adhesive:
Layer 2 Copper
Polyimide Core (Adhesiveless)
Layer 3 Copper
Coverlay Adhesive:
Coverlay:
Coverlay:
Coverlay Adhesive:
Layer 4 Copper Rigid
Polyimide Core (Adhesiveless) Flex Thickness Thickness
Layer 5 Copper
Coverlay Adhesive:
Coverlay:
Coverlay:
Coverlay Adhesive:
Layer 6 Copper
Polyimide Core (Adhesiveless)
Layer 7 Copper
Coverlay Adhesive:
Coverlay:
FR-4 (170 tg min.)
Layer 8 Base Copper um + Plated 20 um min.
Prepreg (3 x 1080, no flow) AIR GAP
AIR GAP
Prepreg (2 x 1080, no flow)
Prepreg (3 x 1080, no flow)
Prepreg (2 x 1080, no flow)

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Multiple Rigid Area Thickness Construction
 Rigid area thickness vary between sections.
 Practically limited to 2 rigid thicknesses maximum.
 May have limits in rigid area thicknesses due to required materials.
 Very costly construction
– Manufacturing process is equivalent to making 2 boards to get 1.

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Multiple Rigid Area Thickness Construction
Soldmask (LPI)
Layer 1 Base Copper 18 um + Plated 30 um min.
Soldmask (LPI)
Layer 2 Copper + Plating
Coverlay:
Coverlay Adhesive:
Layer 3 Copper
Polyimide Core (Adhesiveless) Rigid Thickness 1 Rigid
Layer 4 Copper Thickness 2
Coverlay Adhesive:
Coverlay:
Layer 5 Copper + Plating
Soldmask (LPI)
Layer 6 Base Copper 18 um + Plated 30 um min.
Prepreg
Prepreg
FR-4 (170 tg min.)
FR-4 (170 tg min.)
Prepreg (1 x 106, + 1 x 1080, no flow)
Prepreg (1 x 106, + 1 x 1080, no flow)
Prepreg (2 x 1060, no flow)
Prepreg (2 x 1060, no flow)

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Shielded Flex Layers Construction
 Allows for effective EMI & RF shielding without additional copper
layers.
– Reduced cost
– Thinner flex area construction for improved flexibility
 Uses specialized shielding films (i.e.: Tatsuta, APlus etc.)
 Shielding films are laminated to Flex area
Coverlays and interconnect to ground
through selective Coverlay openings and
electrically conductive adhesive.

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Shielded Flex Layers Construction
Soldmask (LPI)
Layer 1 Base Copper um + Plated 25 um min.
Shield Layer
Coverlay:
Coverlay Adhesive:
Layer 2 Copper
Polyimide Core (Adhesiveless) Flex Thickness Rigid
Layer 3 Copper Thickness
Coverlay Adhesive:
Coverlay:
Shield Layer
Layer 4 Base Copper um + Plated 25 um min.
FR-4 (170 tg min.)
FR-4 (170 tg min.)
Prepreg (2 x 1080 no flow)
Prepreg (2 x 1080 no flow)

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Summary
 The combination of todays Rigid PCB and Flex circuit technology
allows for a very wide variety of Rigid-Flex constructions that can add
a significant amount of functionality, integration and overall packaging
reduction to a design.
 Majority of the specific constructions can also be combined together to
create an almost endless number of configurations.

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Our Products
Custom Battery Packs Flex & Rigid-Flex PCB’s High Reliability SMART HMIs
RF Product Solutions Cable Assemblies Printed Circuit Boards

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Design Centers & Technical Support
 Battery Pack & Power Management – Denver, CO
 SMART User Interfaces – Largo, FL
 Flex & Rigid Flex – Toronto, Canada
 RF Products – New Bedford, MA & Largo, FL
 Cable Assemblies – Largo, FL
 Printed Circuit Boards – New Bedford, MA & Shenzhen, China
 Our Engineering and Design teams are ready to help our
customers create world class and cost effective product
solutions.

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Q&A
 Questions?
– Enter any questions you may have
in the Control Panel
– If we don’t have time to get to it, we
will reply via email

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