Guide to Flexible Circuit Boards and COF Module Technology

Guide to the Structure and Manufacturing Methods of Flexible Circuit Boards

Abstract

This guide aims to help you review and consolidate your understanding of Flexible Printed Circuit Boards (FPCB), Chip on Film (COF) modules, and their manufacturing methods. The main content covers the components of flexible circuit boards, the characteristics of bent and non-bent areas, the arrangement and functions of various layers, and an overview of different manufacturing processes.

Main Concepts

• Flexible Circuit Board (FPCB): A type of circuit board made using a flexible substrate (such as polyimide) that can be bent or folded.
• Chip on Film (COF): A technology that directly mounts semiconductor chips (such as IC chips) onto flexible circuit boards. COF modules are typically used to connect display panels and printed circuit boards.
• Substrate: The base layer of a flexible circuit board, usually made of flexible insulating material, used to support the wiring pattern layer, plating layer, and protection layer.
• Wiring Pattern Layer: The conductive paths formed on the substrate for transmitting electrical signals, typically made of conductive metals such as copper.
• Plating Layer: A metal layer set on top of the wiring pattern layer to prevent oxidation, improve solderability, or provide other functions. Common materials include tin, nickel/gold alloys, etc.
• Protection Layer: An insulating layer set on top of the wiring pattern layer and/or plating layer to protect the circuit from environmental impacts, mechanical stress, and to increase reliability.
• Bent Area: The part of the flexible circuit board designed to be bent or folded.
• Non-bent Area: The part of the flexible circuit board that remains relatively flat, typically used for chip mounting or connecting to other circuit boards.
• Open Portion: The area in the plating or protection layer that does not cover the wiring pattern layer, usually located in the bent area or used for connecting external components.
• Overlapping Area: The area where the plating and protection layers contact and overlap, which helps to improve adhesion strength and reliability.
• Sacrificial Layer: A temporary layer used in certain manufacturing processes to define patterns or protect specific areas, which will be removed afterward.
• Through Hole/Via: Holes that pass through the substrate, filled with conductive material to connect the wiring pattern layers on different surfaces of the substrate.
• Lead Pattern Portion: The area in the conductive pattern used to connect external components (such as chips, display panels, printed circuit boards).
• Test Pattern Portion: The area in the conductive pattern used for product testing and defect inspection.
• Dummy Pattern: Additional conductive patterns set in non-critical areas during the etching process to improve the width and spacing uniformity of the main conductive patterns.

Structure of Flexible Circuit Boards (Exemplary Implementation)

• Basic Structure: A substrate with a wiring pattern layer, plating layer, and protection layer.
• Area Division: The substrate includes bent and non-bent areas. The wiring pattern layer is set on both the bent and non-bent areas of the substrate.
• Plating Distribution: The plating layer is set on the wiring pattern layer and includes open portions in the areas corresponding to the bent areas.
• Protection Layer Function and Position: The protection layer directly contacts one surface of the wiring pattern layer exposed at the open portion and the side of the plating layer. The thickness of the protection layer is usually greater than that of the plating layer.
• Multi-layer Plating and Protection Layers: May include two or more layers of plating (such as first plating and second plating) and two or more layers of protection (such as lower protection layer and upper protection layer). Multi-layer protection layers can disperse bending stress and improve reliability.
• Protection Layer Overlap: The protection layer forms overlapping areas with the plating layer on both sides of the open portion, and the width of these overlapping areas is usually greater than the thickness of the plating layer, which helps enhance adhesion strength.
• Double-sided Flexible Circuit Board: Both sides of the substrate are equipped with wiring pattern layers, plating layers, and protection layers. The structures on both sides can be symmetrical or asymmetrical.

COF Module and Electronic Device Applications

• COF Module Composition: A flexible circuit board and the chip (such as a driver chip) mounted on it.
• Connection Method: One end of the COF module can connect to a display panel, while the other end connects to a printed circuit board. Connections can be made through direct contact or via anisotropic conductive film (ACF).
• Role of Bent Area in Applications: The COF module typically includes a bent area between the display panel and the printed circuit board to accommodate the shape and spatial constraints of the device.
• Composition of Electronic Devices: Typically includes a flexible circuit board, display panel, and printed circuit board. The flexible circuit board connects the display panel and printed circuit board to achieve signal transmission.
• Diverse Applications: Flexible circuit boards in COF modules can be applied in various electronic devices, such as flexible touch windows, wearable devices, TVs, monitors, and laptops.

Overview of Manufacturing Methods

• General Steps: Prepare the substrate, form the wiring pattern layer, form the plating layer, and set the protection layer.
• Plating Formation: Plating (such as tin plating) can be performed on the wiring pattern layer. A mask layer may need to be set to define open areas before forming the plating layer.
• Protection Layer Setting: The protection layer material can be coated onto designated areas using screen printing or other methods, followed by curing. The protection layer can be set before, between, or after the formation of the plating layer.
• Double-sided Circuit Board Manufacturing: The process of forming wiring pattern layers, plating layers, and protection layers needs to be repeated on both sides of the substrate.
• Patterning Techniques: Techniques such as dry film mask method, photoresist (PR) printing method, PET mask method, or gravure printing method may be used to form patterns.
• Spurt Jar Electroplating: A plating method used to form a plating layer in specific areas, which may be used to form a first plating layer with open portions in the corresponding areas of the bent area.
• Etching: Used to remove unwanted conductive materials to form wiring patterns. In some cases, the surface of the substrate may be etched to prevent short circuits.

Key Review

• Understand the functions and interrelationships of the various layers of flexible circuit boards (substrate, wiring pattern layer, plating layer, protection layer).
• Distinguish and understand the importance of bent and non-bent areas in the design and function of flexible circuit boards.
• Master how the design of plating layers (especially multi-layer plating) and protection layers (especially multi-layer protection and their overlapping structures) enhances the reliability and bending resistance of flexible circuit boards.
• Understand the role of COF modules in electronic devices and how they connect to display panels and printed circuit boards.
• Familiarize yourself with the key steps in the manufacturing process of flexible circuit boards and the different patterning methods.

Flexible Circuit Board Understanding Ability Test

Instructions: Please briefly answer the following questions in 2-3 sentences.

1. What are the main advantages of Flexible Circuit Boards (FPCB)?
2. What is the role of flexible circuit boards in Chip on Film (COF) modules?
3. What role does the substrate play in flexible circuit boards? What materials is it typically made of?
4. What is the main function of the wiring pattern layer? What materials is it typically made of?
5. What is the main purpose of the plating layer set on the wiring pattern layer?
6. Why does the plating layer in the bent area of flexible circuit boards typically have open portions?
7. What is the role of the protection layer in flexible circuit boards? How does its thickness typically relate to the thickness of the plating layer?
8. Why are two or more layers of protection layers used in some flexible circuit board designs?
9. What is the significance of the overlapping area between the protection layer and the plating layer near the open portion?
10. How are flexible circuit boards connected to display panels or printed circuit boards?

Answer Key

1. The main advantage of flexible circuit boards is their flexibility, allowing them to bend, fold, or twist to fit the shapes and spatial constraints of various electronic devices. This flexibility enables devices to be thinner, lighter, and more compact.
2. In Chip on Film (COF) modules, flexible circuit boards serve as carriers and interconnection platforms for semiconductor chips. They are used to mount driver chips and transmit the electrical signals from the chips to the display panel or printed circuit board through their wiring pattern layers.
3. The substrate is the foundational support layer of the flexible circuit board, used to carry and support the wiring pattern layer, plating layer, and protection layer above it. It is typically made of flexible insulating materials such as polyimide (PI), but may also be made from polymers like PET or PEN.
4. The main function of the wiring pattern layer is to form conductive paths for transmitting electrical signals across the circuit board. It is typically made from metals with excellent conductivity, most commonly copper (Cu), but may also include aluminum, chromium, nickel, silver, molybdenum, gold, titanium, and their alloys.
5. The main purpose of the plating layer set on the wiring pattern layer is to protect the wiring pattern layer from oxidation and corrosion, improve its solderability, and potentially provide other surface treatment functions. For example, tin plating can provide good solderability and corrosion resistance, while nickel/gold alloys can be used in connection areas.
6. In the bent area of flexible circuit boards, the plating layer typically has open portions (areas not covered by the wiring pattern layer) to reduce stress concentration and cracking that may occur when bending in that area. This maintains flexibility in the bent area and prevents the plating layer from breaking, thereby improving bending reliability.
7. The protection layer in flexible circuit boards serves to provide physical protection for the wiring pattern layer and plating layer from environmental impacts such as moisture, foreign objects, scratches, and mechanical stress. As described, the thickness of the protection layer is typically set to be greater than that of the plating layer to provide stronger protection and support.
8. Using two or more layers of protection layers (such as lower protection layer and upper protection layer) in some flexible circuit board designs is to further disperse the stress generated during bending, especially in multi-layer structures. Layered protection can alleviate stress concentration, preventing the wiring pattern layer and plating layer from cracking, thus significantly enhancing the reliability and lifespan of flexible circuit boards.
9. The overlapping area between the protection layer and the plating layer near the open portion is very important because it increases the contact area between the protection layer and the plating layer as well as the wiring pattern layer. This larger contact area enhances the adhesion strength of the protection layer, preventing it from delaminating, especially during bending or stress, thereby improving the reliability of the flexible circuit board.
10. Flexible circuit boards are typically connected to display panels or printed circuit boards at one or both ends of their non-bent areas. The connection can be made through direct contact or using anisotropic conductive film (ACF). ACF contains conductive particles that can achieve both mechanical bonding and electrical connection simultaneously.

Formatting Issues Suggestions

1. Detail the structural differences between bent and non-bent areas in flexible circuit board design and how they respond to different mechanical stresses (tension and compression). Analyze how the structures of the plating and protection layers enhance the reliability of flexible circuit boards in bending applications.
2. Compare and contrast the applications, advantages, and disadvantages of the dry film mask method, PET mask method, photoresist printing method, and Spurt Jar electroplating method in flexible circuit board manufacturing, and their impact on the final product structure (such as the formation of open portions in the plating layer).
3. Analyze the role of multi-layer plating (such as first plating and second plating) in flexible circuit boards, particularly how it prevents whisker formation and improves electrochemical migration resistance. Discuss the potential advantages of different plating material combinations (such as Cu/Sn alloys and pure Sn).
4. Describe in detail the structural characteristics of multi-layer protection layers (such as lower protection layer and upper protection layer), their relative positions and width relationships, and how they work together to disperse stress, enhance adhesion strength, and prevent environmental penetration, thereby increasing the reliability of flexible circuit boards.
5. Discuss the specific application scenarios and advantages of flexible circuit boards in Chip on Film (COF) modules. Analyze how flexible circuit boards with specific structural features (such as multi-layer protection layers and open portions in the plating layer) meet the demands for high-performance displays and wearable devices in the trend of miniaturization and lightweighting of electronic devices.

Glossary of Terms

• Substrate: The base layer of a flexible circuit board, providing mechanical support and electrical insulation.
• Wiring Pattern Layer: The conductive layer that forms the electrical connection paths.
• Plating Layer: The metal layer covering the wiring pattern layer, used for protection and improving connectivity.
• Protection Layer: The insulating protective layer that safeguards the underlying circuit structure.
• Bent Area: The part of the flexible circuit board designed to withstand bending.
• Non-bent Area: The part of the flexible circuit board designed to remain flat.
• Open Portion: The area in the plating or protection layer that is uncovered.
• Overlapping Area: The area where the plating and protection layers overlap each other.
• Chip on Film (COF): A technology for mounting chips on flexible circuit boards.
• Driver Chip: Typically mounted on COF modules, used to control display panels, etc.
• Display Panel: The display part of electronic devices, connected by flexible circuit boards.
• Printed Circuit Board (PCB): The main circuit board in electronic devices, connected to flexible circuit boards.
• Anisotropic Conductive Film (ACF): A bonding material used to connect flexible circuit boards and other components, containing conductive particles.
• Sacrificial Layer: A temporary layer used during manufacturing processes.
• Through Hole/Via: Conductive connection holes that pass through the substrate.
• Lead Pattern Portion: The conductive pattern area used for external connections.
• Test Pattern Portion: The conductive pattern area used for testing.
• Dummy Pattern: Non-functional conductive patterns set during manufacturing to improve uniformity.
• Sprocket Hole: Holes located at the edges of the substrate for positioning and transport during roll-to-roll manufacturing.
• Inner Lead Pattern Portion: The lead pattern portion near the chip connection area.
• Outer Lead Pattern Portion: The lead pattern portion away from the chip connection area, typically connecting to the display panel or PCB.
• Electroless Plating: A chemical plating process that does not use external current.
• Electroplating: A process that deposits metal films on the substrate surface using external current.
• Sputtering: A physical vapor deposition (PVD) technique used to deposit films on substrate surfaces.
• Screen Printing: A method of printing materials onto substrates through a screen.
• Photoresist (PR): A light-sensitive material used in the patterning process.
• Developing: The process step of removing dissolved or undissolved portions of the photoresist.
• Etching: The process of removing unwanted materials to form patterns through chemical or physical methods.
• Whisker: Tiny, usually single-crystal metal filaments that grow on metal surfaces, potentially causing short circuits.
• Electrochemical Migration: The movement of metal ions on insulating surfaces under the influence of electric fields and humidity, leading to short circuits.

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