How to Design Flexible PCBs

Introduction

Flexible printed circuit boards (PCBs) offer unique capabilities for electronics by enabling dynamic bending, twisting and folding in applications where rigid boards are impractical. However, designing reliable flex circuits requires following specialized layout practices.

This article provides a step-by-step guide on key considerations when designing a flex PCB:

• Selecting flexible substrate materials
• Determining layer stackup
• Trace routing guidelines
• Component placement tips
• Managing layer transitions
• Minimizing flex stresses
• Material and construction selection
• Manufacturing and assembly processes

Following flex-specific design guidelines will help harness the benefits of flexible PCB technology while avoiding common failures.

Flexible PCB Design Process Overview

Designing a flex PCB shares similarities with conventional rigid board layouts but also involves additional specialized steps:

• Define flex circuit performance requirements
• Select flexible substrate materials
• Determine layer stackup arrangement
• Develop board outline and dynamic bend areas
• Place components distributing across rigid and flex areas
• Route traces following flex routing practices
• Define layer transitions including rigid-flex boundary
• Add stiffeners, shielding, strain relief features
• Verify electrical constraints are satisfied
• Validate mechanical stresses are within limits
• Review manufacturability DFM/DFA considerations

Thoughtful planning is necessary in each step to architect a flex PCB optimized for the product’s functionality, lifespan, and performance.

Selecting Flexible Substrate Materials

Choosing the right flexible dielectric material is the first key step:

Polyimide (Kapton) – Most common option provides high temperature durability and tear resistance with moderate cost.

Polyester (PET) – Inexpensive material with relatively good chemical resistance. Limited temperature range.

Polyamide – Extremely flexible even under cold temps but higher dielectric losses.

Fluoropolymers (PTFE) – Expensive but offers excellent chemical inertness and heat tolerance.

Liquid Crystal Polymer (LCP) – High frequency performance but costly and lower flex life than polyimide.

For most applications, polyimide films (Kapton) offer the best balance of electrical performance, mechanical properties, chemical resistance and cost-effectiveness.

Determining Layer Stackup Arrangement

Typical flex stackups use 1-6 conductive layers. Arrange layers considering:

• Number of routing channels needed
• Grouping signal layers to simplify layer transitions
• Assigning power and ground layers
• Separating noise-sensitive signals

A 4-layer stackup with flexible +/-45° angled power-ground planes enhances dynamic flexing reliability.

Minimize the number of layers to control thickness for bend radius.

Trace Routing Design Guidelines

Follow these key trace routing practices on the flex layers:

Trace Width and Spacing

• Use larger trace widths and spacings in dynamic flexing areas
• 2:1 spacing to width ratio minimum; 3:1 optimal

Bend Radius

• Route traces perpendicular to bend axis
• Maintain 3X-10X board thickness bend radius

Corners

• Use rounded corners on traces
• Avoid acute trace angles at rigid-flex junctions

Vias

• Implement thermal relief around vias
• Use tear-drop shapes to reduce stress concentration

Pads

• Employ rounded pad shapes without sharp corners
• Thermally relieve pads at board edges

These guidelines minimize mechanical stresses from dynamic bending and folding motions.

Component Placement Considerations

Position components by:

• Avoiding heavy components in highly dynamic areas
• Distributing weight evenly across rigid and flex zones
• Setting clearance from edges and rigid-flex transitions
• Allowing sufficient spacing for routing traces

Strategic component placement is key for performance and reliability.

Managing Layer Transitions

Carefully plan transitions between rigid and flex board sections:

• Position components away from layer junctions
• Implement pad thermal reliefs nearby transitions
• Avoid routing high speed signals over boundaries
• Add flexible “S” shaped links for signals crossing over

This reduces stresses and discontinuities between sections.

Minimizing Flexing Stresses

Use these strategies to minimize mechanical stresses:

• Add stiffeners on outer layers along flexing regions
• Incorporate shielding layers to reduce conductor stresses
• Design generous fillets and openings at points of flex
• Allow sufficient flex clearances in housing
• Implement strain relief features and flexible connectors

Material and Construction Selection

• Choose adhesive type suitable for flexing – acrylic or polyimide
• Specify dielectric films tested for number of flex cycles needed
• Define copper weight, finishes and tolerances
• Specify requirements for solder mask, silkscreen, etc.

Communicate specific material needs for optimal flex performance.

Manufacturing and Assembly Processes

• Outline board fabrication process and procedures
• Define soldering profile compatible with materials
• Ensure assembly process handles flexible substrate
• Specify handling and protection requirements

Careful process selection ensures a reliable finished flex circuit assembly.

Flex PCB Design Review Checklist

Once the flexible PCB layout is completed, review the key points:

• Bend radii meet minimum requirements
• Trace spacing and widths are within guidelines
• Layer transitions properly designed with pad relief
• Components placed to avoid rigid-flex junctions
• Strain relief provisions added at flex zones
• Conductors perpendicular to bend axes
• Materials meet mechanical and electrical specs

Flexible PCB Design Guidelines FAQs

Q: What are some best practices for routing traces on flex PCBs?

Some key trace routing guidelines are maintaining adequate spacing and widths for dynamic flex, routing perpendicular to bend axes, using rounded trace corners, implementing tear-drop vias, and thermally relieving pads.

Q: What are recommended minimum bend radii for flex PCBs?

For static bending, maintain a minimum bend radius of at least 3X the total board thickness. For dynamic flexing, recommended bend radius is 10X thickness.

Q: How many conductive layers can flex PCBs support?

Typical flex PCBs range from 1-6 metal layers. Highly complex designs are now reaching up to 12 layers but require very careful mechanical stress management.

Q: What are some ways to improve flex PCB reliability?

Reliability can be enhanced by generous spacing, stiffeners, shield layers, strain relief features, avoiding components in bend areas, tear-drop vias, rounded trace corners, and flex-compatible adhesives.

Q: What are the most commonly used materials for flexible PCB substrates?

The most popular flexible PCB dielectric materials are polyimide films like Kapton, followed by other polymers including polyester (PET), polyamide, and PTFE. Polyimide offers the best overall balance.

Conclusion

The key to a successful flex PCB design is following specialized guidelines to accommodate the mechanical bending stresses and avoid common failure modes. Leveraging rigid sections connected using flexible interconnects provides a robust implementation. With sound flex design practices, you can harness the many benefits of flex circuits.