Introduction
A flexible printed circuit board (PCB) is a type of circuit board that can bend and flex. Flexible PCBs allow connections between electronics in situations where rigid boards would be difficult or impossible to use. They are commonly used in consumer electronics, medical devices, aerospace and defense systems, and other applications where flexibility is required.
Designing a flexible PCB requires special considerations compared to rigid PCBs. The flexible substrate material, conductive traces, components, and assembly process need to be selected appropriately to create a functional and reliable flex circuit. This article provides a step-by-step guide on how to design a flexible PCB.
Selecting the Flexible Substrate
The flexible substrate, also known as the flex dielectric, is the base material that the conductive traces are printed on. The substrate must be able to withstand bending and flexing without damage. The most common types of flexible substrate materials are:
- Polyimide (Kapton) – High temperature resistance, excellent chemical resistance, high cost
- Polyester (PET) – Low cost, moderate temperature resistance
- Polyamide – High flexibility and tear resistance
- Fluoropolymer (PTFE) – Excellent chemical resistance, high frequency applications
Polyimide films like Kapton are most widely used due to their combination of electrical, mechanical, and chemical properties. The thickness of the substrate is also an important factor, with common thicknesses ranging from 12.5 to 100 microns. Thinner substrates are more flexible but less durable.
Substrate Thickness Guidelines
| Application | Thickness Range |
|---|---|
| Consumer electronics | 25-50 microns |
| Medical devices | 50-75 microns |
| High flex applications | 12.5-25 microns |
| Rigid-flex boards | 50-100 microns |
Designing Circuit Traces
The conductive copper traces carry current through the circuit and transmit signals. Traces on flex boards need to be designed to endure continual bending without damage.
Trace Width and Spacing
- Minimum trace width and spacing is typically 125-150 microns
- Wider trace widths are needed for higher current carrying applications
- Traces should not neck down at bending regions
- Spacing between traces should be increased at fold lines
Trace Routing
- Traces should follow direction of bend where possible
- Avoid traces perpendicular to bend axis
- Use curved traces at bending zones instead of angular traces
- Stagger traces if crossing bend areas
Pads and Vias
- Use tear-drop shaped pads at trace ends to relieve stress
- Pads should be larger than on rigid boards with increased spacing
- Limit number of vias, use staggered or filled vias at bend areas
Flexible Circuit Components
Special consideration is needed when selecting and placing components on a flexible PCB.
Component Types
- Chip components: Resistors, capacitors, diodes work well
- Flat components: Ideal for flex circuits to avoid bulges
- Bare die components: Epoxy encapsulated dice, no packaging
- Stiffeners: Plates added under components to reduce flexing
Component Placement
- Place components in areas with minimal bending
- Use small components to increase flexibility
- Adhere components with flexible epoxy or solder
- Avoid placing components where traces change directions
Flex PCB Layer Stackup
Flex PCBs typically have a simple single or double sided layer stackup. Complex multilayer flex boards are also possible for dense interconnections.
Single Sided Flex
- One conductive layer on a flexible dielectric
- Low cost, simple layout
- Limited routing capabilities
Double Sided Flex
- Two conductive layers bonded together
- Provides additional routing capabilities
- Allows for crossover traces on two layers
Multilayer Flex
- Bond dielectric and conductive layers into a multilayer stackup
- Permits much higher connection density
- Improves impedance control and cross section
- Cost increases significantly
Flexible PCB Assembly and Connectors
Specialized assembly methods are required to mount and interconnect components on a flex board.
Assembly Methods
- Soldering: Requires precise process control to avoid damaging flex substrate
- Conductive epoxy: Adheres components without reflow or heating
- Mechanical attachment: Clips, clamps and screws can attach parts
Connectors and Terminations
- Flex-to-board: Aligns flex PCB traces to a rigid PCB for interconnection
- ZIF connectors: Zero-insertion force connectors with hinged flex slot
- Edge card connector: Flex board plugs into card slot on rigid PCB
- Flat cable: Ribbon cable extends or terminates flex board traces
Flex PCB Design Guidelines Summary
- Select flexible substrate to meet electrical and mechanical needs
- Use wide, curved traces spaced appropriately for flexing
- Carefully place components in least flexing areas
- Minimize vias, pads and conformal coatings where traces bend
- Consider special assembly methods like conductive epoxy
- Incorporate appropriate connectors and terminations
Careful attention during the design process is necessary to create a robust and reliable flexible printed circuit board.
FQA
What are the main advantages of flexible PCBs?
Some key advantages of flexible PCBs include:
- Flexibility – Can bend and flex to fit varied form factors and motions
- Durability – Withstand repeated bending and flexing without damage
- Weight savings – Thin, light flexible circuits vs rigid boards
- Reliability – Dynamic flexing also aids reliability through repeated movement
- Design freedom – Flexible circuits permit industrial design innovations
- Ease of assembly – Can integrate interconnections between components
What are some common applications of flexible PCBs?
Flexible PCBs are used in many products where bending, folding, or flexing is required including:
- Wearable electronics – Fitness bands, smart watches
- Medical devices – Hearing aids, imaging equipment
- Consumer devices – Cell phones, cameras
- Automotive – Sensors, lighting, engine systems
- Robotics – Arms and joints
- Aerospace – Satellites, flight systems
What types of substrate materials are used for flex PCBs?
Common flexible PCB substrate materials include:
- Polyimide (Kapton) – High temp, chemical resist, cost
- Polyester (PET) – Low cost, moderate temp resistance
- Polyamide – High flexibility, tear resistance
- Fluoropolymer (PTFE) – Chemical resist, RF apps
- Liquid Crystal Polymer (LCP) – High frequency, stiffness
Polyimide is the most widely used due to its superior electrical, mechanical and chemical properties.
How are components assembled on flexible PCBs?
Components can be assembled on flex PCBs using:
- Soldering – Precision reflow process to avoid damaging flex
- Conductive epoxy – Cures at lower temps to adhere parts
- Mechanical attachment – Clips, screws or clamps
- Socketed mounting – Place ICs in sockets rather than soldering
The assembly method depends on the components used and PCB construction.
What are considerations for flex PCB trace routing?
Guidelines for routing flex PCB traces include:
- Follow bend direction when possible
- Minimize traces perpendicular to bend axis
- Use curved traces at bending zones
- Increase trace spacing at fold lines
- Use tear-drop shaped pads at trace ends
- Stagger traces when crossing bend areas
These methods help relieve stresses from continual flexing and prevent conductor damage.
