FPC: The full English term is Flexible Printed Circuit, which translates to “柔性印刷电路板” in Chinese, commonly known as a soft board. It is created by forming conductive circuit patterns through light imaging pattern transfer and etching processes on a flexible substrate. The surface and inner layers of double-sided and multi-layer circuit boards are electrically linked to the inner and outer layers via metallized holes. The circuit pattern’s surface is safeguarded and insulated by a layer of polyimide (PI) and adhesive.
FPCs are primarily categorized into single-sided boards, hollow boards, double-sided boards, multi-layer boards, and rigid-flex boards.
PCB: The English term is Printed Circuit Board, which means “刚性印刷电路板” in Chinese, commonly referred to as a hard board.
Features of Flexible Circuit Boards:
1. **Short**: Reduced assembly time, with all lines configured to eliminate redundant cable connections.
2. **Small**: Smaller volume compared to PCBs (hard boards), effectively reducing product size and enhancing portability.
3. **Light**: Lighter than PCBs (hard boards), contributing to a decrease in the overall weight of the final product.
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**4Thin:** The thickness is less than that of traditional PCBs (hard boards), enhancing flexibility and facilitating assembly within three-dimensional spaces constrained by limited environments.
**Advantages of Flexible Circuit Boards**
Flexible printed circuit boards (FPCs) are circuits constructed from flexible insulating substrates and offer numerous benefits that rigid printed circuit boards do not:
1. They can be bent, wound, and folded freely, allowing for arrangement according to spatial layout requirements. This enables movement and expansion in three-dimensional spaces, facilitating the integration of component assembly and wire connections.
2. Utilizing FPC can significantly reduce the size and weight of electronic products, making them suitable for developing high-density, miniaturized, and highly reliable electronic products. As a result, FPCs are extensively used in aerospace, military applications, mobile communications, laptops, computer peripherals, PDAs, digital cameras, and other fields.
3. FPCs also provide excellent heat dissipation and solderability, easy assembly, and lower overall costs. The design that combines soft and hard elements compensates for the minor shortcomings of the flexible substrate in terms of component carrying capacity.
**Main Raw Materials of FPC**
The primary raw materials are listed below:
1. Base material
2. Cover film
3. Reinforcement
4. Other auxiliary materials
**1. Substrate**
1.1 **Adhesive Substrate**
Adhesive substrates consist of three components: copper foil, adhesive, and polyimide (PI). They are available as single-sided and double-sided substrates. A single-sided substrate has copper foil on one side, while a double-sided substrate features copper foil on both sides.
1.2 **Non-Adhesive Substrate**
Non-adhesive substrates lack the adhesive layer found in traditional substrates. Comprising only copper foil and PI, they are thinner, offer better dimensional stability, higher heat and bending resistance, improved chemical resistance, and other advantages. These substrates are now widely adopted.
**Copper Foil:** Commonly used copper foil thicknesses include 1OZ, 1/2OZ, and 1/3OZ, with a new thinner option at 1/4OZ being introduced. This material is already in use in China, enabling the production of ultra-fine circuits (with line widths and spacings of 0.05mm or less). As customer demands increase, materials of this specification are expected to gain wider application in the future.
**2. Cover Film**
This component consists of three parts: release paper, adhesive, and PI. At the end of the production process, only the adhesive and PI remain, while the release paper is discarded (its purpose is to protect the adhesive from contamination).
**3. Reinforcement**
Reinforcement materials are specifically designed for FPCs and are used in certain areas to enhance support strength, addressing the “soft” nature of FPCs. Common reinforcement materials include:
– **FR4 Reinforcement:** Comprised of glass fiber cloth and epoxy resin, identical to the FR4 material used in PCBs.
– **Steel Sheet Reinforcement:** Made from steel, providing strong hardness and support.
– **PI Reinforcement:** Similar to the cover film, consisting of PI and release paper, but with a thicker PI layer, produced from 2MIL to 9MIL.
**4. Other Auxiliary Materials**
– **Pure Adhesive:** This heat-curing acrylic adhesive film features protective paper/release film and a layer of glue, primarily used for layered boards and flexible/rigid boards, bonding them effectively.
– **Electromagnetic Shielding Film:** Applied to the board surface for protection.
– **Pure Copper Foil:** Comprising solely copper foil, mainly used in hollow board production.
**5. Types of FPC**
FPCs can be categorized into six distinct types:
A. **Single Panel:** Wiring is present only on one side.
B. **Double Panel:** Wiring is present on both sides.
C. **Hollow Board:** Also referred to as a window board (with openings on the surface).
D. **Layered Board:** Features a two-sided circuit (separate).
E. **Multilayer Boards:** Contains circuits with more than two layers.
F. **Rigid-Flex Board:** A combination of soft and hard boards.
**Future Innovations in FPC**
FPCs will continue to evolve in three main areas:
1. **Thickness:** Future FPCs must be more flexible and thinner.
2. **Folding Resistance:** Enhanced bending capabilities are an inherent requirement for future FPCs, ensuring greater resistance to folding.
3. **Process Level:** To meet diverse requirements, FPC processes must be upgraded to accommodate smaller apertures and meet stricter standards for line width and spacing.
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