The document discusses the technical challenges in designing flex and rigid-flex printed circuit boards (PCBs), highlighting key issues like IPC 6013 requirements, mechanical specifications, and material callouts. It emphasizes the importance of accurate specifications to avoid delays or redesigns in manufacturing, and addresses common errors that can hinder production, such as incorrect callouts and design practices. Furthermore, it covers the use of combination coverlay and soldermask for high-density components and the implications of brand-specific material requirements on supply chains.

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Flex and Rigid-Flex PCBs:
Technical Issues In Data Sets
11.09.2023

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Introduction
 The Challenge:
– Rigid-flex and flex circuits have numerous
design requirements that either differ or do not
exist in rigid PCB designs.
 The Biggest Technical Issues:
– IPC 6013 requirements, board & panel profiles,
and coverlay/soldermask capabilities cover
majority of the technical issues that we see.
– Technical issues can create significant delays,
or even worse, can result in complete
redesigns.

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Agenda
 IPC 6013 Items:
– Correct Callouts
• Material callouts & specifications
• IPC QC & manufacturing specification
• Epoxy strain relief specifications
– Plated Through Holes & Vias
• Plating requirements
• PTH & via to flex transition minimum spacing
requirements

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Agenda
 Mechanical Requirements:
– Board & Panel Profile
• Part outline(s)
• Inside corners in part outline(s)
• Flex and rigid area array breakaway tabs
– Stiffener requirements and minimum sizes
 Coverlay/Soldermask:
– Combination coverlay/soldermask usage
– Minimum soldermask & coverlay web capabilities
– Coverlay shape restrictions

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IPC 6013 Items

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IPC 6013 Items – Material Callouts
 Material Callouts & Specifications:
– Preferred Methods:
• Use common names of base
materials
– I.E.: FR4, polyimide, etc.
• Use IPC-4204 specification sheet
codes
– Ensure up-to-date to IPC
codes used
– E.G.: adhesiveless polyimide
= IPC 4202/11

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IPC 6013 Items – Material Callouts
 Common Errors:
– Specific Manufacturer or Trade Names:
• E.g.: Kapton, DuPont, Felios, etc.
• Can hinder or stop
quotation/production due to
material stock/availability/UL
certification, etc.
• Not all materials brands available
at all geographic manufacturing
locations
• Many “other” equivalent materials
available outside of North
America

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IPC 6013 Items – Specifications
 IPC QC & Manufacturing Specifications:
– IPC-6012 vs. IPC-6013
• IPC-6012 applies to rigid PCBs only
• IPC-6013 applies to flex and rigid-flex PCBs
• Key differences:
– Different plating requirements
– Soldermask vs. flexible soldermask/coverlay
– IPC-6013 details many additional flex-specific features
» Stiffeners, PSA etc.
– Most often due to rigid PCB drawing template being adapted to flex/rigid-flex design

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IPC 6013 Items – Specifications

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IPC 6013 Items – Flexible Epoxy Strain Reliefs
 Epoxy Strain Relief Specifications:
– Dimensions:
• Minimum width: 1.5mm
• Minimum flex to rigid height delta in
stackup 0.010”
– Epoxy Strain Relief Formula:
• Available in stiff, semi-flex, and flexible
formulations
• Flexible is the only formulation used
– Common to see no specifications defined in
fab drawing

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IPC 6013 Items – Plated Through Holes & Vias
 Plating Requirements
– Per IPC-6013 for Flex & Rigid-Flex
• 2-layer flex
– Average 12um [472uin], Minimum 10um [394uin]
• 3+layer rigid-flex and flex boards
– Average 25um [984uin], Minimum 20um [787uin]
– Common Errors:
• Copper thickness callout for incorrect layer count
• Callouts for IPC-6012 plating

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Mechanical Requirements

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Mechanical Requirements – Board & Profile
 Part Outline:
– Best practice to include in Gerber format
• For flex designs, just the outline
• For rigid-flex designs, an outline of the rigid
and flex sections
– If not in Gerber format, DXF can suffice
– Common Errors:
• Not including the part outline or rigid outline
(for rigid-flex)
• Including in PDF format only

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Mechanical Requirements – Board Outline
 Inside Corners in Board Outline(s):
– Best design practice for board outlines is
to radius all corners (flex & rigid areas)
• Preferred: 0.020” min. radius
– Corners are rounded to:
• Allow for manufacturability
• Min./eliminate mechanical stress
concentrator(s)
– Common errors:
• Sharp inside corners, which can’t be
done properly in typical laser cutting
• Too tight of radii for corners

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Mechanical Requirements – Assembly Array
 Flex & Rigid Area Array Breakaway Tabs:
– Tabs are used in arrays for ease of separation
of boards after assembly
– Typically, breakaway tabs consist of:
• Breakaway holes or “mouse bites” for rigid
area tabs
• Solid tabs for pure flex areas
– Common errors:
• Insufficient spacing to copper features to
allow tabs to be recessed into the part
outline
• Breakaway holes added into flex tabs which
severely weaken the tabs

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Mechanical Requirements – Assembly Array

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Mechanical Requirements – Stiffener Requirements
 Stiffener Materials:
– FR4, polyimide, stainless steel, aluminum
– Most common: FR4 and polyimide
 Size and Thickness:
– FR4 & Aluminum:
• Minimum size approx. 0.5”
• Common thickness: 0.5mm & 1.0mm
– Polyimide
• Minimum size is design dependent
• Max. thickness is 0.010” (.25mm)
– Stainless Steel:
• Thickness: 0.005” – 0.008”

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Mechanical Requirements –Stiffener Requirements
 Common Errors:
– Stiffener dimensions and locations not specified
– Incorrect material specified for application
• I.E.: FR4 for ZIF applications
– Undersized stiffeners

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Coverlay & Soldermask

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Coverlay/Soldermask – Minimum Web Capabilities
 Coverlay:
– Solid sheet polyimide material
– Available colors:
• Natural amber, white, black
– Feature openings are machined
• Laser cut, drilling, punch & die
• Min. web thickness = 0.020”
• Webs below 0.020”
– Excessively fragile easily damaged
– May distort and prevent proper
alignment

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Coverlay/Soldermask – Minimum Web Capabilities
 Soldermask:
– Liquid epoxy with added flex agent
– Available colors:
• Green, red, blue, yellow, black, white, etc.
– Features are photo-imaged and chemically
formed (same as rigid PCBs)
– Min. web thickness
• 0.004” - Green only
• 0.005” - All other colors

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Coverlay/Soldermask – Minimum Web Capabilities
 Common Errors:
– 1:1 Opening to Pad Size
• Min. oversize required:
– Coverlay = +0.005” per pad side
– Soldermask = +0.002” per pad
side
– Thin Coverlay or Soldermask Webs:
• Resolved by either
– Decreasing opening sizes
– Gang opening features

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Coverlay/Soldermask
– Combination Coverlay/Soldermask
 Applied to Flex Designs with High-Density
SMT Components
– Webs between adjacent pads below min. 0.020”
manufacturable limit of coverlay material
– Flexible soldermask applied in high-density SMT
area(s)
– Coverlay used throughout balance of design
– Recommendation:
• Supply one file for coverlay
• Allow flex supplier to create separate
coverlay & soldermask data files

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Coverlay/Soldermask – Coverlay Shape Restrictions
 Coverlay Must be One Continuous Piece
Throughout Design
– Applied and laminated to production panel as
one piece
– Issues:
• Protruding/extended tabs:
– Easily damaged or misaligned
• Isolated “island” areas:
– Difficult to manufacture if at all
– Can be resolved by use of
combination coverlay/soldermask
• Excessively long thin webs
– Easily damaged or misaligned

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Summary
 Many unique elements and specifications for flex & rigid-flex designs
not included or not correctly defined in supplied data sets
– Leads to “conditional” quotes pending further customer review and
approvals
 Brand-specific material callouts, if imposed, can limit supply chain and
add excessive cost
– Not all materials supported by all suppliers due to availability and UL
certifications
 Use of combination coverlay & soldermask solution may be required
due to high-density SMT components

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Our Products
Battery Packs Flex & Rigid-Flex PCBs Cable Assemblies Printed Circuit Boards
CNC Machining User Interfaces Flexible Heaters EC Fans & Motors

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 You can quote and order rigid circuit boards, as well as flexible circuit
boards online with InstantPCBQuote™.
– Standard and Expedited Lead Times
– Various Material and Surface Finish Options
– Stiffeners Available
 Go to www.epectec.com/instantpcbquote/ to get started.
Did You Know?

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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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Thank You
Check out our website at www.epectec.com.
For more information email sales@epectec.com.
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