Introduction
This article provides a comprehensive discussion of flexible printed circuits.
After reading this article, you should understand:
- What flexible printed circuits are
- How they differ from standard printed circuits
- Design guidelines and considerations for flexible printed circuits
- The manufacturing process of flexible printed circuits
- Different types of flexible printed circuits
- Applications and benefits of flexible printed circuits
And much more...
Chapter One: Understanding Flexible Circuits: Differences from Standard Printed Circuits, Key Guidelines, and Considerations for Flexible Printed Circuit Design
What are Flexible Printed Circuits?
Flexible printed circuits (FPCs) consist of conductive tracks on thin, flexible base materials. Unlike rigid PCBs, these circuits are designed to bend or twist. They may include surface-mounted components like resistors and LEDs, typically featuring connectors for system integration. The substrate choice is crucial, differing from standard PCBs in design, manufacturing, and functionality. Polyimide is commonly used for its unique properties in circuit construction. FPCs are made by laminating copper foil onto a resin substrate, with layers bonded through heat, pressure, or adhesive to form a unified board. The structure includes conductive, insulating, and adhesive layers for proper operation.
Flexible PCBs are ideal for applications requiring lightweight, slim, and high-density circuitry. They enable three-dimensional wiring and space-efficient designs while offering better heat dissipation than traditional PCBs, improving device longevity and performance.
How do Flexible Circuits Differ From Standard Printed Circuits?
While flexible and standard PCBs serve similar electrical functions, they differ significantly in design and structure. The key difference is flexibility - flexible circuits can bend or twist, requiring different materials and manufacturing approaches.
Standard PCBs focus on thermal and chemical adhesive properties unrelated to flexibility. Flexible circuits need stretchable adhesives with distinct thermal, mechanical, and chemical characteristics.
PCB base layers typically use glass-reinforced materials for strength, like flame-retardant epoxy resin composites. Flexible circuits use polyimide for flexibility, though it offers less mechanical support. Copper films also differ - rolled and annealed copper is preferred for flexible circuits due to its greater flexibility compared to electro-deposited copper used in standard PCBs.
Guidelines and Considerations for Flexible Printed Circuits
Designing flexible PCBs requires different considerations than rigid ones. Here are key guidelines:
Manufacturer Selection
Choose manufacturers with current technology to address material properties, movement, and thermal coefficients. Ensure they can produce high-quality, high-density designs with good conductivity and thermal dissipation. Key considerations include:
- Cost factors including materials, expertise, and design complexity
- Material selection impacts final product cost and should be discussed openly
Bend Radius Characteristics
Establish bend radius early in design to prevent copper trace damage. Avoid features like slots in bending areas that could weaken the circuit.
Cover Material
Select cover materials based on application requirements, considering performance, specifications, and operating environment. Quality is crucial for both personal and commercial use.
Adaptable Routing
Route traces through low-stress areas to enhance safety and prevent short circuits. Avoid discontinuities in bending zones that could cause fractures.
Stacking Flexible PCB Layers
Position the most flexible layers centrally when stacking to maintain flexibility and reduce movement.
Delamination Risk
Prevent layer detachment by using appropriate solder paste and designing for flexibility requirements.
Adhesive Thickness
Specify overall PCB width including dielectric materials affecting impedance. Prepare comprehensive drawings focusing on critical features.
Chapter Two: How Flexible Printed Circuits are Manufactured
The Manufacturing Process of Flexible Circuits
Flexible printed circuits (FPCs) use high-performance materials like copper for conductivity and polyimide for insulation. These materials enable bending and folding, making FPCs suitable for medical devices, consumer electronics, and automotive systems.
Manufacturing involves four main stages with strict quality control to meet IPC-6013 and RoHS standards:
First Phase: Flex PCB Construction and Board Preparation
This phase prepares polyimide substrates for flexibility and strength. Key steps include:
- Conductor Size: Using thin copper foil (often 1oz/ft² or less) for flexibility
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Etching: Defining circuit patterns with chemical etchants
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Drilling: Creating micro-vias with laser or mechanical systems
- Looping: Adding slack for mechanical movement
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Routing: Optimizing conductor paths for reliability
- Copper Coating: Reinforcing traces and holes for connectivity
Second Phase: Fabricating Flexible Printed Circuit Boards
This phase focuses on layer fabrication with attention to:
Edge Distance
Maintaining proper clearance between traces and board edges
Surface Manipulation
Improving surface properties through treatments like plasma deposition
Button Plating
Strengthening plated through-holes
Hole Size
Creating micro-vias as small as 30 micrometers
Trimming
Precisely cutting finished boards
Third Phase: Physical Restrictions
Applying protective cover layers from materials like:
- Screen-printable liquid overcoats
- Adhesive-backed films
Fourth Phase: Examination and Quality Control
Ensuring quality through:
- Visual inspections and automated optical checks
- Sampling testing (6%-10% of production)
- Electrical testing for continuity and insulation
These measures ensure compliance with industry standards for high-reliability applications.
