flex printed circuit (FPC): An In-Depth Exploration

Introduction

flex printed circuits (FPC) are crucial in modern electronics due to their flexibility, lightweight nature, and ability to accommodate complex designs. This document provides a comprehensive overview of FPCs, including their concept, primary materials, types, advantages, and manufacturing processes, emphasizing the importance of each aspect in professional engineering applications.

Understanding FPCs

FPC Definition

Flex-printed circuits are known as flexible circuit boards. They are created by using imaging techniques and etching processes on flexible substrates to form conductor patterns. In double-sided and multilayer boards, electrical connections between outer and inner layers are made through metalized holes, with the circuit patterns insulated by PI and adhesive layers. FPCs are renowned for their high wiring density, lightweight nature, and thin profile, making them ideal for use in smartphones, laptops, PDAs, digital cameras, and LCMs.

Key Materials Used in FPCs

Substrates

Substrates are the foundational materials in FPCs, divided into two categories: Adhesive-Based Substrates: Comprising copper foil, adhesive, and PI, these substrates come in single-sided and double-sided forms. Single-sided substrates have copper foil on one side, while double-sided substrates feature copper foil on both sides.

Adhesive-Free Substrates: These substrates lack an adhesive layer, consisting only of copper foil and PI. They are thinner and offer superior dimensional stability, heat resistance, bendability, and chemical resistance compared to adhesive-based substrates. Adhesive-free substrates are widely used, particularly with the emergence of ultra-thin copper foils, such as 1/4OZ, which are ideal for producing extremely fine lines (0.05mm and below).

Coverlay  

The overlay is composed of release paper, adhesive, and PI. During manufacturing, only the adhesive and PI remain on the product, with the release paper removed. This layer protects the circuitry and provides insulation.

Stiffeners  

Stiffeners are specific materials used in FPCs to enhance support strength, compensating for the “softness” of FPCs. Common stiffeners include:

FR4 Stiffeners: Made of fiberglass cloth and epoxy resin, similar to the FR4 material used in PCBs.

Steel Stiffeners: Provide high hardness and support strength.

PI Stiffeners: Similar to the overlay, composed of thicker PI layers, ranging from 2mil to 9mil.

Other Auxiliary Materials  

Adhesive Film: This heat-curable acrylic adhesive film, consisting of protective paper/release film and adhesive, is mainly used for bonding in multilayer boards, rigid-flex boards, and FR-4/steel stiffener boards.

Electromagnetic Shielding Film: Applied to the board surface for shielding purposes.

Pure Copper Foil: Comprised solely of copper foil, used in the production of hollowed boards.

Advantages and Disadvantages of FPCs

Advantages  

Flexibility: FPCs can be freely bent, twisted, and folded, allowing for versatile space arrangements in three-dimensional designs, integrating component assembly and wire connections.

Size and Weight Reduction: FPCs significantly reduce the size and weight of electronic products, supporting the trend towards higher density and miniaturization, making them widely used in aerospace, military, mobile communications, laptops, computer peripherals, PDAs, digital cameras, and more.

Thermal and Solderability Benefits: FPCs offer excellent heat dissipation, solderability, and ease of assembly, with a lower overall cost. The combination of flexible and rigid design compensates for the slight lack of component-bearing capacity inherent in flexible substrates.

Disadvantages  

High Initial Cost: The design, layout, and photomask expenses for FPCs are high, making them less cost-effective for small-scale applications unless there is a specific need for flexible circuits.

Difficulty in Modification and Repair: Once manufactured, FPCs are challenging to modify, as changes require starting from the photomask or program. The protective film layer makes repairs difficult as it needs to be removed and then restored after the repair.

Size Limitations: FPCs are typically manufactured using intermittent processing, limiting their size due to equipment constraints.

Fragility: Improper handling can easily damage flexible circuits, requiring skilled personnel for soldering and rework.

Types of FPCs

Common Types

Single-Sided Flex (IPC-6013 Type 1): An overlay (PI + adhesive) is bonded to an adhesive-free single-sided FPC core, with or without stiffeners.

Double-Sided Flex (IPC-6013 Type 2): An overlay is bonded to both sides of an adhesive-free double-sided FPC core (two conductive layers) with plated through holes, with or without stiffeners.

Multilayer Flex (IPC-6013 Type 3): An overlay is bonded to both sides of an adhesive-free structure containing three or more conductive layers plated through holes, with or without stiffeners.

Other Types  

Hollow Boards: Also known as window boards, used for applications requiring open windows in the circuit.

Layered Boards: Two-sided circuits that are separated by insulation.

Rigid-Flex Boards: A combination of flexible and rigid board technologies.

Calculating the Bend Radius of FPCs

The bending radius of FPCs is critical to their performance. When bending, the inner side of the curve is under compression while the outer side is under tension. Excessive stress can lead to delamination, copper foil breakage, and other issues. The minimum bend radius depends on the FPC thickness and bend radius, and it is essential to design the FPC layers symmetrically to reduce stress.

Bend Radius Calculation for Single-Sided Flex Circuits  

The minimum bend radius can be calculated using the formula:

R=(c/2)[(100-Eb)/Eb]-D

Where:

R = Minimum bend radius (µm)

C = Copper thickness (µm)

D = Coverlay thickness (µm)

EB = Copper allowable deformation (percentage)

Different types of copper foil have varying deformation limits, with rolled copper allowing up to 16% and electrolytic copper up to 11%.

Bend Radius for Double-Sided Flex Circuits  

The calculation for double-sided flex circuits follows the same principles, with additional considerations for interlayer dielectric thickness.

Typical Bend Radius Guidelines

Single-Sided Boards: 3-6 times the conductor thickness.

Double-Sided Boards: 6-10 times the conductor thickness.

Multilayer Boards: 10-15 times the conductor thickness.

Dynamic Single-Sided Boards: 20-40 times the conductor thickness.

FPC Manufacturing Process

Single-Sided Board Manufacturing Process  

Material Cutting → Baking → Dry Film Lamination → Exposure → Development → Etching → Stripping → Pre-treatment → Coverlay Application → Lamination → Curing → Surface Treatment → Electrical Testing → Assembly → Final Lamination → Curing → Marking → Profiling → Final Inspection → Packaging

Double-Sided Board Manufacturing Process  

Material Cutting → Baking → Drilling → Black Hole → VCP → Pre-treatment → Dry Film Lamination → Exposure → Development → Etching → Stripping → Pre-treatment → Coverlay Application → Lamination → Curing → Surface Treatment → Electrical Testing → Assembly → Final Lamination → Curing → Marking → Profiling → Final Inspection → Packaging

Conclusion

FPCs are indispensable in modern electronics, offering unparalleled flexibility, lightweight, and the ability to support complex designs. Understanding their materials, types, advantages, and manufacturing processes is crucial for engineers looking to leverage FPCs in various applications, from consumer electronics to aerospace.