Overview

Flexible Printed Circuits, or FPCs, have gained prominence due to their lightweight, thin profile, and ability to bend and fold. These attributes are particularly valuable in the rapidly evolving electronics industry, where precision and density are paramount. Developed in the 1970s by the United States for aerospace applications, FPCs are built from polyester or polyimide films. They allow for the integration of numerous precision components in confined spaces, making them ideal for modern electronic products that require compact and reliable interconnections.

Introduction

Flexible Printed Circuits (FPCs) are advanced circuit boards made from flexible substrates like polyimide or polyester films. Known for their high reliability and exceptional flexibility, FPCs are characterized by high wiring density, lightweight construction, thin profiles, and excellent flexibility. As electronic devices become more compact and complex, FPCs offer a vital solution for achieving high-density designs in limited spaces.

Materials Used in FPCs

Insulating Films

Insulating films form the foundational layer of FPCs. These films are adhered to copper foils with adhesives. In multi-layer designs, insulating films are bonded to inner layers and also serve as protective covers to shield the circuit from dust and moisture. Common insulating materials include polyimide and polyester films. Polyimide, known for its non-flammability, dimensional stability, and high tear strength, is used by approximately 80% of U.S. FPC manufacturers. Polyester, also used for its lower dielectric constant and moisture resistance, is suitable for less demanding applications where high temperatures are not a factor.

Conductors

Copper foil is the preferred conductor material for FPCs. It can be deposited through electroplating (ED) or electroplating processes. ED copper foil features a glossy side and a matte side, with the latter often treated to enhance adhesive bonding. This material is flexible, available in various thicknesses and widths, and is ideal for dynamic applications requiring both flexibility and rigidity.

Adhesives

Adhesives play a crucial role in FPCs by bonding insulating films to conductive materials and serving as protective or cover layers. The primary distinction lies in the application methods. Cover layers and adhesive layers both contribute to the circuit’s lamination structure. Adhesive-free lamination structures can offer thinner circuits and better thermal conductivity, making them suitable for environments where adhesive-based circuits might fail.

FPC Manufacturing Process

Double-Sided Boards

- Cutting → Drilling → PTH → Plating → Pre-treatment → Dry Film Application → Alignment → Exposure → Development → Pattern Plating → Stripping → Pre-treatment → Dry Film Application → Alignment Exposure → Development → Etching → Stripping → Surface Treatment → Overlay Film Application → Pressing → Curing → Nickel-Gold Plating → Marking → Cutting → Electrical Testing → Punching → Final Inspection → Packaging → Shipping

Single-Sided Boards

- Cutting → Drilling → Dry Film Application → Alignment → Exposure → Development → Etching → Stripping → Surface Treatment → Overlay Film → Pressing → Curing → Surface Treatment → Nickel-Gold Plating → Marking → Cutting → Electrical Testing → Punching → Cutting → Final Inspection → Packaging → Shipping

Advantages and Disadvantages of FPCs

Advantages

1. Flexibility and Space Efficiency: FPCs can be bent, twisted, and folded, allowing for versatile arrangement in three-dimensional space, which is crucial for integrating components and wire connections.  
2. Reduced Size and Weight: FPCs significantly reduce the volume and weight of electronic products, making them ideal for applications in aerospace, military, mobile communications, laptops, peripheral devices, PDAs, and digital cameras.
3. Thermal and Assembly Benefits: FPCs offer excellent thermal dissipation and ease of assembly, and the soft-hard combination design can mitigate minor deficiencies in component support.

Disadvantages

1. High Initial Costs: Due to the specialized design and manufacturing requirements, the initial cost of FPCs is high, making them less suitable for low-volume applications.
2. Difficult Repairs and Replacements: Once manufactured, FPCs are challenging to modify. Repairing involves removing and reapplying protective films, which is labor-intensive.
3. Size Limitations: FPCs are often produced in bulk due to equipment size constraints, limiting their dimensions.
4. Fragility: Improper handling during installation and connection can damage the circuits, requiring trained personnel for soldering and rework.

Soldering Procedures for FPCs

1. Pre-Soldering Preparation: Apply flux to the pads and use a soldering iron to prevent poor soldering due to oxidation or tinning issues. Chips generally do not need processing.
2. Chip Placement and Soldering: Carefully position PQFP chips on the PCB, aligning them with the pads. Use a soldering iron set above 300°C to solder diagonally opposite pins first, ensuring chip stability.
3. Soldering All Pins: Apply solder to all pins, keeping them moist with flux. Touch each pin’s end with the soldering iron until the solder flows. Ensure the iron's tip remains parallel to the pins to prevent overlap.
4. Cleaning and Inspection: After soldering, clean excess solder and inspect for errors. Use alcohol and a stiff brush to remove solder residues.
5. SMD Component Soldering:  Place SMD resistors and capacitors on the solder points, solder one end, align, and then solder the other end.

Conclusion

Flexible Printed Circuits (FPCs) represent a crucial advancement in electronics, offering flexibility, space efficiency, and reduced weight. Despite their higher initial costs and challenges in repair, their benefits in high-density, compact designs make them indispensable in modern electronic applications. Companies like Gekun are at the forefront of FPC technology, driving innovations that continue to enhance the performance and versatility of these essential components.