Exploring Flexible Printed Circuit Boards

Rigid PCBs work perfectly—until they don't fit. When your design needs to bend around corners, flex during use, or squeeze into impossibly tight spaces, flexible printed circuit boards become essential. These remarkable circuits enable products that would be impossible with traditional rigid boards.

This guide explores flexible PCB technology: what makes them different, when to use them, and how to design them successfully.

What is a Flexible PCB?

A flexible printed circuit board (flex PCB, FPC, or flex circuit) uses thin, bendable substrate materials instead of rigid fiberglass. The same copper conductors carry signals, but the base material allows the circuit to bend, fold, or twist.

Flex PCBs range from simple single-layer ribbons to complex multilayer designs. Some flex continuously during product operation; others bend once during assembly and remain static afterward.

The flexible PCB market reached $21.8 billion in 2023, representing 30% of the total PCB market. That growth reflects increasing demand for compact, lightweight, and dynamic electronic assemblies across industries.

Rigid PCB vs. Flexible PCB

Characteristic	Rigid PCB	Flexible PCB
Substrate	FR4 (fiberglass/epoxy)	Polyimide or polyester film
Thickness	1.6mm typical	0.1-0.5mm typical
Bending	Cracks under stress	Designed to flex
Weight	Heavier	Significantly lighter
3D Shapes	Cannot conform	Bends to fit enclosures
Connector needs	Requires cables between boards	Can replace cables
Initial cost	Lower	Higher
Assembly cost	Standard	Often lower

Neither type is universally better. Each suits different applications.

Materials Used in Flex PCBs

Base Substrate Materials

Polyimide (PI)

The dominant flex PCB substrate, polyimide offers:

• Excellent thermal stability (withstands soldering temperatures)
• Chemical resistance
• Dimensional stability
• Long flex life

DuPont's Kapton is the most recognized polyimide brand, though many manufacturers offer equivalent materials. Most flex PCBs intended for demanding applications use polyimide.

Polyester (PET)

A cost-effective alternative with limitations:

• Lower heat resistance (not suitable for soldering without special processing)
• Good flexibility and moisture resistance
• Lower cost than polyimide
• Suitable for less demanding applications

Common in consumer electronics where cost pressure is extreme and thermal requirements are modest.

Liquid Crystal Polymer (LCP)

Specialized material for high-frequency applications:

• Excellent high-frequency performance
• Low moisture absorption
• Dimensional stability
• Higher cost, justified for RF applications

Used in 5G antennas, mmWave circuits, and high-speed data applications.

Polyethylene Naphthalate (PEN)

Emerging alternative balancing properties:

• Better heat resistance than PET
• Lower cost than polyimide
• Good dimensional stability
• Suitable for moderate-temperature applications

Conductor Materials

Copper

Standard conductor for flex PCBs, available in:

• Rolled annealed (RA) copper: Better flex life, preferred for dynamic applications
• Electrodeposited (ED) copper: Less expensive, suitable for static applications

Rolled annealed copper's grain structure handles repeated bending better than electrodeposited copper.

Coverlay

Flexible circuits use coverlay instead of rigid board solder mask. Coverlay consists of polyimide film with adhesive, laminated over the circuit to:

• Protect traces from damage
• Provide electrical insulation
• Enable selective exposure for component pads

Photoimageable coverlay alternatives offer finer feature resolution for high-density designs.

Types of Flexible PCBs

Single-Sided Flex

One conductive layer on one side of the substrate. The simplest and lowest-cost flex PCB type.

Applications: Ribbon cables, simple interconnects, LED strips, keypad membranes

Advantages: Lowest cost, maximum flexibility, simplest design

Limitations: Single layer limits routing complexity

Double-Sided Flex

Conductive layers on both substrate sides, connected by plated through-holes.

Applications: More complex interconnects, moderate-density circuits, fold-flat designs

Advantages: More routing capability, still highly flexible

Limitations: Higher cost than single-sided, via placement affects bend areas

Multilayer Flex

Three or more conductive layers with insulating layers between them. Can include buried and blind vias.

Applications: Complex circuits requiring high interconnect density, HDI designs

Advantages: Maximum circuit density, complete routing freedom

Limitations: Reduced flexibility, higher cost, longer lead times

Rigid-Flex PCBs

Combines rigid sections (standard FR4 areas) with flexible sections in a single integrated assembly. Components mount on rigid sections; flex sections provide connections.

Applications: Folding electronics, devices with hinged sections, replacing cables between boards

Advantages: Eliminates connectors, reduces assembly, improves reliability

Limitations: Complex fabrication, higher cost, design requires careful planning

Advantages of Flexible PCBs

Space and Weight Savings

Flex PCBs are thin—typically 0.1-0.5mm versus 1.6mm for rigid boards. In weight-critical applications like aerospace, wearables, and portable devices, this reduction matters significantly.

The ability to bend into three-dimensional shapes also enables designs that simply can't use rigid boards. A flex circuit wrapping around battery packs or conforming to curved housings eliminates the need for complex mechanical structures.

Reliability Through Elimination

Every connector is a potential failure point. Flex PCBs can replace cable assemblies, eliminating:

• Connector contact resistance
• Mechanical connector failures
• Cable routing and strain relief
• Assembly labor for cable installation

A single flex circuit replacing multiple cables and connectors inherently improves system reliability.

Dynamic Flex Capability

Some applications require circuits that flex continuously during operation:

• Printer heads moving across carriages
• Hard drive actuator cables
• Robot joint wiring
• Folding phone hinges
• Camera stabilization mechanisms

Well-designed flex circuits survive millions of flex cycles. Rigid boards crack on the first bend.

Improved Thermal Performance

Thin polyimide substrates dissipate heat more effectively than thick FR4. For power-constrained designs, this improved thermal path can be significant.

Assembly Simplification

Flex circuits can reduce total part count dramatically. A rigid-flex design replacing three rigid boards, two cables, and four connectors with a single assembly saves:

• Part procurement complexity
• Inventory management
• Assembly steps
• Potential failure points

The total cost may be lower despite higher PCB cost.

Flex PCB Design Considerations

Bend Radius

The minimum bend radius depends on:

• Material thickness
• Number of layers
• Copper weight
• Static vs. dynamic flexing

General guidelines:

• Static bends: Minimum radius = 6× overall thickness
• Dynamic bends: Minimum radius = 12× overall thickness

Tighter bends stress copper and may cause failures. Always check with your fabricator for specific material capabilities.

Trace Routing in Bend Areas

Traces in bend zones need special attention:

• Route traces perpendicular to bend axis when possible
• Use curved traces, not 90-degree angles in bend zones
• Avoid vias in areas that will flex
• Stagger traces on opposite layers (don't stack them directly)
• Use wider traces and larger spacing in flex areas

Copper Distribution

Uneven copper distribution causes uneven stress during bending. Use hatched ground planes instead of solid copper in flex areas to distribute stress more evenly.

Stiffeners

Add rigid stiffeners to support:

• Component mounting areas
• Connector locations
• ZIF connector insertion zones
• Areas requiring flatness

Stiffeners can be FR4, polyimide, aluminum, or stainless steel depending on requirements.

Layer Stack-Up

In multilayer flex, layer arrangement affects flexibility:

• Odd layer counts help balance stress
• Neutral axis placement affects flex behavior
• Adhesiveless constructions improve flex life

Work with your fabricator to optimize layer stack for your specific flex requirements.

Applications by Industry

Consumer Electronics

• Smartphone internal connections
• Laptop display hinges
• Camera modules
• Wearable devices
• Earbuds and headphones

Smartphones alone consume vast quantities of flex PCBs, connecting displays, cameras, and batteries in compact packages.

Medical Devices

• Hearing aids
• Pacemakers and implants
• Endoscopes
• Wearable monitors
• Diagnostic equipment

Medical applications often require small form factors, biocompatibility, and extreme reliability—flex PCBs deliver all three.

Automotive

• Instrument clusters
• Airbag connections
• LED lighting assemblies
• Seat sensors
• Infotainment systems

Automotive flex circuits must survive temperature extremes, vibration, and decades of service life.

Aerospace and Defense

• Satellite systems
• Avionics interconnects
• Missile guidance
• Space-qualified assemblies

Weight savings in aerospace translate directly to fuel savings or increased payload capacity.

Industrial

• Robotics
• Motion control
• Industrial sensors
• Test equipment

Robotic applications particularly benefit from flex circuits that survive continuous motion.

Cost Considerations

Flex PCBs cost more per square inch than rigid boards. But total system cost often favors flex:

Flex PCB costs more:

• Material cost (polyimide vs. FR4)
• Specialized processing
• Coverlay instead of solder mask
• Potentially lower fabrication yields

Flex PCB saves money:

• Eliminates cables and connectors
• Reduces assembly labor
• Decreases part count
• Improves reliability (fewer warranty claims)

Run a total cost analysis including assembly and reliability, not just bare board pricing.

Working with Flex PCB Manufacturers

Design Review

More important for flex than rigid—submit designs for fabricator review before finalizing. Flex-specific issues that rigid board designers overlook:

• Bend radius violations
• Inadequate stiffener placement
• Copper stress concentrations
• Material compatibility

Panel Utilization

Flex PCB material is expensive. Panel layout significantly affects cost. Work with your fabricator to optimize part placement.

Prototype vs. Production

Flex PCB prototypes can be expensive due to tooling and setup costs. Consider:

• Consolidating prototype orders
• Using panel services for small quantities
• Accepting longer lead times for lower prototype pricing

Summary

Flexible PCBs solve problems that rigid boards cannot: fitting in curved spaces, surviving dynamic motion, and replacing cable assemblies. While more expensive per unit area, they often reduce total system cost through assembly simplification and improved reliability.

Successful flex design requires understanding material properties, bend constraints, and fabrication capabilities. The investment in learning these differences pays off in products that would be impossible any other way—and that's the real value of flexible circuits.