IPC-2223 Explained: Flex & Rigid-Flex PCB Design Standard

Flex circuits aren’t just thin rigid boards that happen to bend. The moment you start designing flexible or rigid-flex PCBs, you enter a world where bend radius calculations, material selection, and conductor routing rules completely change. Ignore these differences, and you’ll end up with cracked traces, delaminated layers, and field failures that could have been prevented.

IPC-2223 is the sectional design standard that addresses these unique challenges. Officially titled “Sectional Design Standard for Flexible/Rigid-Flexible Printed Boards,” this document provides the specific requirements for designing circuits that bend—whether once during installation or millions of times during operation. If you’re working on wearables, medical devices, aerospace systems, or any application requiring flexible interconnects, IPC-2223 is essential reading.

What Is IPC-2223?

IPC-2223 establishes the design requirements for flexible printed boards and rigid-flex combinations. It covers everything from material selection to bend radius calculations, conductor routing in flex areas, coverlay design, stiffener requirements, and the critical transition zones where rigid meets flex.

Like other sectional standards in the IPC-2220 family, IPC-2223 works in conjunction with IPC-2221 (the generic design standard). For rigid-flex designs, you’ll also reference IPC-2222 for the rigid portions of your board.

IPC-2223 Version History

The current version, IPC-2223E, reflects modern flex circuit manufacturing capabilities and addresses design challenges in high-density rigid-flex applications.

IPC-2223 Standard Overview

How IPC-2223 Works with Other Standards

Designing flex and rigid-flex boards requires juggling multiple IPC standards. Understanding which standard applies where prevents confusion and ensures complete coverage of design requirements.

Standards Relationship for Rigid-Flex Design

For a rigid-flex board, you need all three design standards: IPC-2221 for generic requirements, IPC-2222 for rigid section specifics, and IPC-2223 for flexible sections and transition zones.

IPC-2223 Flex Board Classifications

IPC-2223 classifies flexible boards by layer count and construction method. The classification determines applicable design rules and manufacturing processes.

Flex Board Types per IPC-2223

Rigid-Flex Configurations

The bookbinder construction—where flex layers remain unbonded in bend areas—allows multilayer flex to achieve bend radii that would otherwise cause layer separation.

Bend Radius Requirements in IPC-2223

Bend radius is the most critical design parameter for flexible circuits. Get it wrong, and copper traces crack, coverlays delaminate, and circuits fail. IPC-2223 defines minimum bend radii based on application type and layer count.

Static vs Dynamic Flex Applications

The distinction matters enormously—dynamic applications require bend radii 10-20 times larger than static applications.

IPC-2223 Minimum Bend Radius Ratios

Where r = minimum bend radius and h = total thickness of flexible portion.

Bend Radius Calculation Example

For a double-layer dynamic flex circuit with total flex thickness of 0.15mm:

Minimum bend radius = ratio × thickness Minimum bend radius = 150 × 0.15mm = 22.5mm

For the same construction in a static application: Minimum bend radius = 12 × 0.15mm = 1.8mm

This twelve-fold difference explains why understanding your application type is critical before starting the design.

Bend Radius Best Practices

Material Selection per IPC-2223

Flexible circuit materials differ fundamentally from rigid board materials. IPC-2223 provides guidance on selecting appropriate dielectrics, adhesives, and copper types.

Flexible Dielectric Materials

Polyimide dominates the flex market due to its balance of flexibility, thermal stability, and cost.

Adhesive vs Adhesiveless Construction

For dynamic flex applications, adhesiveless construction typically provides better reliability because there’s no adhesive layer to crack during repeated bending.

Copper Type Selection

This is non-negotiable for dynamic applications: specify RA copper. ED copper will crack under repeated flexing due to its grain structure.

Conductor Routing in Bend Areas

How you route traces through bend zones dramatically affects flex circuit reliability. IPC-2223 provides specific guidance for conductor design in areas that will flex.

Bend Area Routing Rules

Conductor Width Guidelines for Flex

Traces on the outside of a bend experience tension; traces on the inside experience compression. Narrow traces tolerate compression better than tension, which is why IPC-2223 recommends placing small traces on the inside of the bend.

Coverlay Design Requirements

Coverlay (cover layer) protects flex circuit conductors and provides the flexible equivalent of solder mask. IPC-2223 addresses coverlay material selection and access hole design.

Coverlay Material Options

Access Hole Requirements

Coverlay access holes expose pads for soldering while protecting surrounding conductors. Undersized access holes cause soldering defects; oversized holes expose conductors to damage.

Read more IPC Standards:

MIL-PRF-38534: Hybrid Microcircuits Military Specification Guide for Engineers

MIL-PRF-31032: The Definitive Guide to Military-Grade PCB Requirements

MIL-PRF-19500: Military Semiconductor Device Specification Explained

MIL-PRF-123: High-Reliability Ceramic Capacitor Military Standard

MIL-I-46058CMIL-I-46058C: Military Conformal Coating Specification Complete Guide

MIL-HDBK-263: ESD Control Handbook & Implementation Guide

J-STD-075 Guide: PSL Classification for Passive Components & Assembly Process Limits

J-STD-035 Guide: Acoustic Microscopy for IC Package Inspection & Defect Detection

J-STD-033 Guide: MSD Handling, Floor Life, Baking & Dry Storage Requirements

J-STD-032 Explained: Ball Grid Array Ball Construction & Performance Specs

J-STD-027 Guide: Mechanical Outline Standard for Flip Chip & CSP Packages

J-STD-020 Guide: MSL Classification, Reflow Profiles & Moisture Sensitivity Testing

IPC/WHMA-A-620 Explained: Complete Guide to Cable & Wire Harness Standards

IPC-SG-141: Complete Guide to S-Glass Fabric Specifications for PCB Laminates

IPC-QF-143: Complete Guide to Quartz Fabric Specifications for RF/Microwave PCB Laminates

Rigid-Flex Transition Zone Design

The transition zone—where rigid meets flex—is the highest-stress area in a rigid-flex board and the most common failure location. IPC-2223 dedicates significant attention to transition zone design.

Transition Zone Requirements

Strain Relief Methods

The bookbinder technique—leaving flex layers unbonded—allows each layer to slide independently during bending, dramatically reducing stress compared to bonded construction.

Stiffener Requirements

Stiffeners provide mechanical support for components mounted on flex circuits and protect flex areas from damage during assembly.

Stiffener Material Options

Stiffener Design Guidelines

Common IPC-2223 Design Violations

Understanding common mistakes helps you avoid them. These violations cause the majority of flex and rigid-flex failures.

Frequent Design Errors

Tools and Resources for IPC-2223

Official Documentation

Related IPC Standards

Frequently Asked Questions About IPC-2223

What’s the difference between static and dynamic flex?

Static flex (Use A per IPC-2223) bends fewer than 100 times in its lifetime—typically just during installation. Dynamic flex (Use B) bends repeatedly during operation, from hundreds to millions of cycles. The distinction dramatically affects design requirements: dynamic applications need bend radii 10-20 times larger and require rolled annealed copper instead of electrodeposited copper.

Can I place vias in the bend area of a flex circuit?

No—IPC-2223 strongly recommends excluding vias from bend areas. The plated barrel creates a stress concentration point that cracks under repeated bending. If vias must be near bend zones, use teardrop pads and additional plating, but relocating vias to stiffened areas or rigid sections is the preferred solution.

How do I calculate the minimum bend radius for my flex design?

Determine your application type (static or dynamic) and layer count, then find the corresponding ratio in IPC-2223. Multiply the ratio by your total flex thickness. For a double-layer dynamic flex at 0.2mm thickness: 150 × 0.2mm = 30mm minimum bend radius. Add 20-30% margin for manufacturing variation and always measure from the inside surface of the bend.

Do I need all three standards (IPC-2221, 2222, 2223) for rigid-flex?

Yes. IPC-2221 provides generic requirements applicable to all boards. IPC-2222 covers the rigid sections of your rigid-flex. IPC-2223 covers the flexible sections and the critical transition zones where rigid meets flex. Using all three ensures complete coverage with no gaps in your design specification.

What copper type should I specify for flex circuits?

For static flex applications, electrodeposited (ED) copper can work but rolled annealed (RA) is safer. For dynamic flex applications, always specify rolled annealed copper—it’s non-negotiable. RA copper’s grain structure handles repeated bending far better than ED copper, which will crack under dynamic flexing.

Designing Flex Circuits That Last

IPC-2223 distills decades of flex circuit experience into actionable design requirements. The standard exists because flexible circuits fail in predictable ways—ways that proper design prevents.

Start every flex design by classifying your application: static or dynamic. Calculate your minimum bend radius with appropriate margin. Select materials that match your thermal and flexibility requirements—RA copper for dynamic applications, adhesiveless construction for demanding bends. Route conductors perpendicular to bend axes with no vias in flex zones. Design transition zones with proper strain relief and layer staggering.

The combination of IPC-2221 for generic requirements, IPC-2222 for rigid sections, and IPC-2223 for flex sections gives you a complete framework for designing reliable flex and rigid-flex circuits. Follow these standards, and your flex circuits will survive the bending they’re designed for.