Introduction to Flexible Printed Circuits (FPCB)

FPCBs are lightweight, thin, and flexible, making them easy to bend and fold. These characteristics constitute their main advantages and unique features. Therefore, FPCs are especially suitable for products with limited space, where a single rigid PCB cannot fulfill all design requirements.
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Flexible Printed Circuit Board (FPCB) Introduction

Flexible Printed Circuit Boards (FPCBs), also called flexible boards or simply FPCs, are a type of Flex-PCB. FPCs share similar functions and structures with rigid PCBs, which are usually referred to as Printed Circuit Boards (PCB). The main difference is that PCBs are rigid, whereas FPCs are flexible.

FPCBs are manufactured by forming conductive circuit patterns on a flexible substrate through photolithography and etching processes. For double-sided and multilayer boards, the inner and outer layers are electrically connected via plated through-holes, and the circuit patterns are protected and insulated with polyimide (PI) and adhesive layers.

FPCBs are mainly classified into single-layer boards, windowed boards, double-layer boards, multilayer boards, and rigid-flex boards.


1. Characteristics of FPCs

FPCs are lightweight, thin, and flexible, which makes them easy to bend and fold. These properties are their main advantages and unique features. Therefore, FPCs are particularly suitable for products with limited space where a single rigid PCB cannot meet all design requirements.

  • In some products, multiple PCBs may need to overlap to fully utilize internal space, requiring flexible FPCs for interconnection.

  • In others, soft boards may be used to connect distant PCBs to transmit signals efficiently, or to allow certain PCBs to cross vertically or at different angles.

Advantages include:

  1. Short assembly time: All circuits are preconfigured, eliminating the need for extra wiring connections.

  2. Compact size: Smaller than rigid PCBs, effectively reducing product volume and improving portability.

  3. Lightweight: Lighter than rigid PCBs, reducing overall product weight.

  4. Thin: Thinner than rigid PCBs, increasing flexibility and enabling three-dimensional assembly in limited spaces.


2. Advantages of Flexible Printed Circuit Boards

Flexible PCBs are made from flexible insulating substrates and offer many advantages that rigid PCBs do not:

  1. They can bend, fold, and twist freely and can be arranged according to spatial requirements, allowing three-dimensional movement and integration of components and wiring.

  2. Using FPCs can significantly reduce the size and weight of electronic products, meeting the trend toward high-density, miniaturized, and highly reliable electronics. FPCs are widely applied in aerospace, military, mobile communications, laptops, computer peripherals, PDAs, digital cameras, and more.

  3. FPCs also feature good heat dissipation, solderability, ease of assembly, and relatively low overall cost. Rigid-flex designs further compensate for the slightly lower component-bearing capacity of flexible substrates.


3. Main Raw Materials of FPCs

The main raw materials of FPCs include:

  1. Substrate

    • Adhesive substrate: Composed of copper foil, adhesive, and PI. Single-sided substrates have copper on one side; double-sided substrates have copper on both sides.

    • Adhesive-free substrate: No adhesive layer, composed of copper and PI only. Advantages include thinner thickness, better dimensional stability, higher heat resistance, better bendability, and improved chemical resistance. Increasingly used in fine-pitch applications.

  2. Coverlay
    Composed of release paper, adhesive, and PI. After production, only the adhesive and PI remain on the product; release paper is removed.

  3. Reinforcement
    Used in specific areas to increase support strength and compensate for the flexibility of FPCs. Common reinforcement materials:

    • FR4 reinforcement: Made from fiberglass cloth and epoxy resin, same as rigid PCBs.

    • Steel reinforcement: Provides high hardness and support.

    • PI reinforcement: Similar to coverlay, but with a thicker PI layer (2 MIL to 9 MIL).

  4. Other auxiliary materials

    • Adhesive film: Thermosetting acrylic adhesive with release liner, used for multilayer or rigid-flex boards.

    • EMI shielding film: Provides electromagnetic shielding.

    • Pure copper foil: Used for windowed board production.


4. Types of FPCs

  1. Single-layer board: Circuits only on one side.

  2. Double-layer board: Circuits on both sides.

  3. Windowed board: Also called “finger window board.”

  4. Stacked board: Double-sided but separated layers.

  5. Multilayer board: More than two layers of circuits.

  6. Rigid-flex board: Combination of flexible and rigid PCB sections.