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Flexible circuits aren’t just thin rigid boards. The materials, construction methods, and failure modes are fundamentally different, which is why IPC-6013 exists as a separate specification from IPC-6012. I’ve seen engineers specify “IPC-6012 Class 2” on rigid-flex designs and wonder why their fabricator pushes back—the answer is that flex and rigid-flex boards require their own qualification standard.
IPC-6013, the Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards, defines the requirements that separate a reliable flex circuit from one that cracks at the first bend. From coverlay adhesion to bend radius limitations, this specification addresses the unique challenges of flexible substrates. This guide breaks down what you need to know to properly specify, manufacture, and inspect flex and rigid-flex PCBs.
What Is IPC-6013?
IPC-6013 is the industry standard that establishes qualification and performance requirements specifically for flexible and rigid-flex printed circuit boards. While IPC-6012 covers rigid boards, IPC-6013 addresses the unique characteristics of circuits built on flexible polyimide substrates.
IPC-6013 Standard Overview
Attribute
Details
Full title
Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards
Current revision
IPC-6013E (September 2021)
Previous revisions
IPC-6013D (2017), C (2013), B (2009), A (1998)
Pages
84 pages
Parent document
IPC-6011 (Generic Performance Specification)
Design companion
IPC-2223 (Sectional Design Standard for Flexible Printed Boards)
Visual reference
IPC-A-600 (Acceptability of Printed Boards)
Addendum
IPC-6013EM (Medical Applications)
The specification works alongside IPC-2223 for design requirements and IPC-A-600 for visual acceptance criteria. Unlike rigid boards, flex circuits require additional consideration for dynamic flexing, coverlay integrity, and transition zone reliability.
IPC-6013E: What Changed in the 2021 Revision
IPC released Revision E in September 2021, incorporating lessons learned from HDI flex manufacturing and addressing gaps in rigid-flex transition requirements.
Key Updates in IPC-6013E
Area
Update
Final finishes
Updated requirements for ENIG, immersion silver, OSP on flex
Rigid-to-flex transition
New acceptance criteria for transition zones
Foreign inclusions
Clarified requirements by class
Surface mount lands
Added anomaly acceptance criteria
Plated internal layers
New requirements for buried structures
Dielectric removal
Wicking and etchback specifications
Copper-filled vias
Requirements for filled structures in flex
Microvia structures
HDI-specific acceptance criteria
The rigid-to-flex transition zone updates are particularly significant for designers working on complex rigid-flex assemblies where reliability at the junction is critical.
Understanding Flex PCB Types (Type 1-5)
IPC-6013 classifies flexible and rigid-flex boards into five types based on construction complexity and layer count.
IPC-6013 Board Type Classifications
Type
Description
Construction
Common Applications
Type 1
Single-sided flexible
One conductive layer, with or without stiffeners
Simple interconnects, LED strips, membrane switches
Type 2
Double-sided flexible
Two conductive layers with PTHs, with or without stiffeners
Consumer electronics, cameras, displays
Type 3
Multilayer flexible
Three or more flex layers with PTHs, with or without stiffeners
Medical devices, high-density applications
Type 4
Rigid-flex multilayer
Rigid and flexible sections laminated together with PTHs
Aerospace, military, complex assemblies
Type 5
Flex or rigid-flex without PTHs
Two or more layers without plated-through holes
Specialized interconnects, jumper applications
Type Selection Guidelines
Application Need
Recommended Type
Reasoning
Simple point-to-point connection
Type 1
Lowest cost, sufficient for basic routing
Component mounting on flex
Type 2
Two-sided allows SMT on flex areas
High-density flex routing
Type 3
Multiple layers for complex designs
Mixed rigid and flex requirements
Type 4
Connectors on rigid, flex for movement
Cost-sensitive, no vias needed
Type 5
Eliminates plating costs
Type 4 rigid-flex boards are the most complex, combining rigid FR-4 or polyimide sections with flexible polyimide areas. The transition between rigid and flex requires careful design and manufacturing attention.
Performance Classes for Flex and Rigid-Flex PCBs
IPC-6013 applies the same three-class system defined in IPC-6011, but with acceptance criteria tailored to flexible substrates.
IPC-6013 Class Definitions
Class
Name
Description
Typical Applications
Class 1
General Electronic Products
Function is primary requirement; cosmetic imperfections acceptable
Consumer electronics, toys, disposables
Class 2
Dedicated Service Electronic Products
Extended life required; uninterrupted service desired but not critical
Industrial equipment, automotive, communications
Class 3
High Reliability Electronic Products
Continued performance critical; equipment downtime unacceptable
Medical implants, aerospace, military, life support
Many flex fabricators don’t offer Class 1 production because the cost difference between Class 1 and Class 2 is minimal for flex circuits, and most customers require at least Class 2 reliability.
Installation Usage Classes
Beyond performance classes, IPC-6013 also defines installation usage categories:
Usage Class
Description
Flex Cycles
A
Flex-to-install
One-time bend during installation, static thereafter
B
Limited flex
Occasional flexing during service life
C
Dynamic flex
Continuous or frequent flexing in operation
D
Extreme dynamic
High-cycle flexing in demanding environments
Usage class affects design requirements like minimum bend radius, copper type (rolled annealed vs electrodeposited), and coverlay material selection.
IPC-6013 Class 2 vs Class 3: Key Differences
The Class 2 vs Class 3 comparison is critical for flex circuits because the differences impact both cost and reliability in dynamic applications.
Plating Requirements Comparison
Location
Class 2 Minimum
Class 3 Minimum
PTH wall (average)
20 µm (0.8 mil)
25 µm (1.0 mil)
Surface copper
Per design
Per design
Wrap plating
Per Table 3-6
Per Table 3-6 (stricter)
Flex circuits typically use “button plating” or “pad plating” rather than panel plating, focusing copper deposition on pads and holes rather than the entire surface. This reduces stiffness in flex areas.
Annular Ring Requirements
Condition
Class 2
Class 3
External breakout
90° acceptable
No breakout allowed
Internal breakout
90° acceptable
90° acceptable
Minimum ring (external)
50 µm (2 mil)
50 µm (2 mil)
Conductor Requirements
Requirement
Class 2
Class 3
Width reduction (max)
30%
20%
Thickness reduction
20%
10%
Edge definition
Minor roughness OK
Smooth required
Complete Class 2 vs Class 3 Comparison for Flex
Requirement
Class 2
Class 3
PTH plating
20 µm minimum
25 µm minimum
Annular ring breakout
90° external allowed
No external breakout
Conductor reduction
30% max
20% max
Coverlay adhesion
Standard
Enhanced testing
Soda strawing
Limited acceptable
More restricted
Foreign material
Per spacing rules
Stricter limits
Inspection level
Standard
Enhanced
Documentation
Standard
Full traceability
Cost impact
Baseline
20-40% higher
Coverlay and Flexible Dielectric Requirements
Coverlay is to flex circuits what soldermask is to rigid boards, but with critical differences. Coverlay is a pre-cut polyimide film with adhesive, while soldermask is liquid applied.
Coverlay vs Soldermask Comparison
Attribute
Coverlay
Soldermask
Material
Polyimide film + adhesive
Liquid photoimageable
Application
Laminated (heat + pressure)
Screen printed or sprayed
Flexibility
Excellent
Poor (cracks when bent)
Registration
Mechanical (pre-routed)
Photographic
Minimum opening
Limited by routing
Fine features possible
Cost
Higher
Lower
Coverlay Acceptance Criteria
Condition
Class 2
Class 3
Adhesive squeeze-out
Acceptable if not bridging
Minimal acceptable
Voids under coverlay
Limited acceptable
Not acceptable in critical areas
Coverlay lifting
Minor acceptable
Not acceptable
Registration tolerance
±0.15 mm typical
±0.10 mm typical
For dynamic flex applications, coverlay integrity is critical. Adhesive voids can propagate under repeated flexing, leading to delamination failure.
Flex-Specific Defects and Acceptance
Flexible circuits have unique defect types that don’t exist in rigid boards. IPC-6013 provides specific acceptance criteria for these conditions.
Soda Strawing
Soda strawing occurs when the coverlay adhesive wicks up between the polyimide base and copper conductor, creating a visible ring around features. The name comes from the appearance—like liquid being drawn up a straw.
Class
Soda Strawing Acceptance
Class 1
Acceptable if no functional impact
Class 2
Limited acceptable (per specification)
Class 3
Minimal acceptable, restricted locations
Creases and Wrinkles
Condition
Class 2
Class 3
Creases in flex area
Acceptable if no conductor damage
Not acceptable
Wrinkles in coverlay
Minor acceptable
Not acceptable
Kinks
Acceptable outside bend area
Not acceptable
Measling and Crazing
IPC conducted extensive studies showing that measling (white spots in laminate) and crazing (fine cracks in resin) don’t significantly affect flex circuit performance. However, acceptance varies by class:
Condition
Class 2
Class 3
Measling
Acceptable
Limited acceptable
Crazing
Acceptable
Limited in critical areas
Rigid-to-Flex Transition Zone Requirements
The transition zone where rigid and flex sections meet is the most failure-prone area in rigid-flex designs. IPC-6013E added specific requirements for this critical region.
Transition Zone Acceptance Criteria
Feature
Requirement
Adhesive bead
Required to prevent prepreg migration
Copper stress relief
Recommended at transition
Layer termination
Staggered preferred
Coverlay overlap
Minimum specified by class
Transition Zone Design Recommendations
Design Element
Guideline
Bend location
Minimum 1.5 mm from rigid edge
Via placement
No vias within 0.5 mm of transition
Copper pour
Avoid large copper at transition
Layer count change
Gradual reduction preferred
Bend Radius Requirements
Bend radius is critical for flex circuit reliability, especially in dynamic applications. IPC-2223 provides design guidelines that IPC-6013 references for acceptance.
Minimum Bend Radius Guidelines
Application
Minimum Bend Radius
Single-sided, flex-to-install
6× flex thickness
Double-sided, flex-to-install
12× flex thickness
Multilayer, flex-to-install
24× flex thickness
Dynamic flex (single-sided)
12× flex thickness minimum
Dynamic flex (double-sided)
24× flex thickness minimum
Bend Radius Calculation Example
Construction
Flex Thickness
Minimum Bend Radius (Static)
Single-sided 1 oz
0.1 mm
0.6 mm
Double-sided 1 oz
0.2 mm
2.4 mm
4-layer flex
0.3 mm
7.2 mm
Using rolled annealed (RA) copper instead of electrodeposited (ED) copper allows tighter bend radii because RA copper has superior fatigue resistance.
IPC-6013 vs IPC-6012: When to Use Each
Understanding when to apply IPC-6013 versus IPC-6012 is essential for proper specification.
Specification Selection Guide
Board Type
Correct Specification
Rigid FR-4
IPC-6012
Rigid polyimide
IPC-6012
Single-sided flex
IPC-6013
Double-sided flex
IPC-6013
Multilayer flex
IPC-6013
Rigid-flex
IPC-6013
Metal core (rigid)
IPC-6012
Key Differences Between Standards
Aspect
IPC-6012
IPC-6013
Substrate focus
Rigid laminates
Flexible polyimide
Bow and twist
Applicable
Only rigid sections
Coverlay requirements
Not addressed
Detailed requirements
Bend radius
Not applicable
Critical requirement
Transition zones
Not applicable
Detailed requirements
Soda strawing
Not addressed
Acceptance criteria defined
IPC-6013 vs MIL-P-50884: Comparison
Military and aerospace applications often reference MIL-P-50884 (now MIL-PRF-31032). Understanding the relationship helps when transitioning between specifications.
Specification Comparison
Aspect
IPC-6013
MIL-P-50884/MIL-PRF-31032
Performance levels
3 classes (1, 2, 3)
Single level (high reliability)
Document format
User-friendly index
Complex cross-references
Updates
Current (2021)
Less frequently updated
Transition zone
Detailed requirements
Limited guidance
Foreign material
Translucent acceptable
Stricter requirements
Solder wicking
Class-dependent limits
Not specifically addressed
OSP finish
Allowed
Not addressed
QPL requirement
No
Yes (certification required)
IPC-6013 Class 3 is accepted by many government agencies as a COTS (Commercial Off-The-Shelf) equivalent to MIL-PRF-31032 for non-critical applications.
Quality Assurance and Testing
IPC-6013 Section 4 defines quality assurance provisions specific to flex circuits.
Required Testing by Class
Test
Class 2
Class 3
Visual inspection
100%
100%
Dimensional
Sample
100% critical
Electrical (continuity/isolation)
100%
100%
Peel strength
Periodic
Per lot
Flexural endurance
If required
Required for dynamic
Thermal stress
Sample
Per lot
Microsection
Periodic
Per lot
Flexural Endurance Testing
For dynamic flex applications, IPC-TM-650 Method 2.4.3 defines the flex testing procedure:
Parameter
Typical Requirement
Bend radius
Per design minimum
Flex cycles
1,000 to 1,000,000+
Flex rate
30-60 cycles/minute
Pass criteria
No opens, shorts, or cracks
Frequently Asked Questions About IPC-6013
What is the difference between IPC-6012 and IPC-6013?
IPC-6012 covers rigid printed boards built on materials like FR-4, while IPC-6013 covers flexible and rigid-flex boards built on polyimide substrates. The key differences involve flex-specific requirements like coverlay adhesion, bend radius, soda strawing acceptance, and rigid-to-flex transition zones. If your design includes any flexible sections, you should specify IPC-6013, not IPC-6012. Even the rigid portions of a rigid-flex assembly fall under IPC-6013 because the overall construction and transition zones require flex-specific acceptance criteria.
Can I use soldermask instead of coverlay on flex circuits?
Technically possible but not recommended for most applications. Standard liquid photoimageable soldermask is rigid when cured and will crack when the flex circuit bends. Flexible soldermask formulations exist but still don’t match coverlay performance for dynamic flex applications. Coverlay (polyimide film with adhesive) remains the standard for flex circuits because it maintains flexibility and provides superior protection. For static flex-to-install applications with minimal bending, flexible soldermask may be acceptable, but always consult your fabricator.
What copper type should I specify for dynamic flex applications?
Rolled annealed (RA) copper is strongly recommended for dynamic flex applications. RA copper has an elongated grain structure that provides superior fatigue resistance compared to electrodeposited (ED) copper. ED copper can be used for flex-to-install (static) applications where the circuit bends once during installation and remains static thereafter. For any application involving repeated flexing, specify RA copper. The cost premium is minimal compared to the reliability improvement, especially in Class 3 applications.
How does IPC-6013 address rigid-flex transition zones?
IPC-6013E (2021) added specific requirements for rigid-to-flex transition zones, recognizing this as the most failure-prone area in rigid-flex designs. Requirements include adhesive bead application to prevent prepreg migration onto flex areas, acceptance criteria for layer terminations, and visual inspection requirements for transition integrity. Designers should keep bend locations at least 1.5 mm from the rigid edge and avoid placing vias within 0.5 mm of the transition. Proper design and manufacturing of transition zones is critical for reliable rigid-flex assemblies.
Is IPC-6013 Class 3 equivalent to military specifications?
IPC-6013 Class 3 is accepted by many government agencies as a Commercial Off-The-Shelf (COTS) equivalent to MIL-PRF-31032 for many applications. The performance requirements are comparable, and IPC-6013 Class 3 actually provides clearer guidance in some areas like transition zones and soda strawing acceptance. However, for programs that specifically require qualified products, MIL-PRF-31032 with QPL listing may still be mandatory. Always verify specification requirements with your program’s quality assurance authority before assuming equivalence.
IPC-6013 is the definitive specification for flexible and rigid-flex printed circuit board qualification and performance. Understanding this standard is essential whether you’re designing consumer wearables or aerospace flight hardware.
Key takeaways:
Use IPC-6013, not IPC-6012 – Any board with flex sections requires IPC-6013
Know your board type – Types 1-5 have different requirements and capabilities
Class matters for flex – Class 2 vs Class 3 differences are significant for reliability
Coverlay isn’t soldermask – Different materials, different requirements
Transition zones are critical – Most rigid-flex failures occur at rigid-to-flex junctions
Specify copper type – RA copper for dynamic, ED acceptable for static
Flexible circuits offer tremendous advantages in size, weight, and assembly simplification, but they require specialized knowledge to specify correctly. IPC-6013 provides the framework for ensuring your flex and rigid-flex designs meet their reliability requirements.
Inquire: Call 0086-755-23203480, or reach out via the form below/your sales contact to discuss our design, manufacturing, and assembly capabilities.
Quote: Email your PCB files to Sales@pcbsync.com (Preferred for large files) or submit online. We will contact you promptly. Please ensure your email is correct.
Notes: For PCB fabrication, we require PCB design file in Gerber RS-274X format (most preferred), *.PCB/DDB (Protel, inform your program version) format or *.BRD (Eagle) format. For PCB assembly, we require PCB design file in above mentioned format, drilling file and BOM. Click to download BOM template To avoid file missing, please include all files into one folder and compress it into .zip or .rar format.
