Before the advent of PCBs, circuits were constructed using point-to-point wiring. This method had low reliability, as aging circuits often experienced broken wires, leading to open circuits or short circuits at connection points. The wire-wrapping technique was a significant advancement in circuit technology, improving durability and allowing easier replacement by wrapping small-gauge wires around connection posts.
As the electronics industry evolved from vacuum tubes and relays to silicon semiconductors and integrated circuits, the size and cost of electronic components decreased. Electronics began to appear more frequently in consumer products, prompting manufacturers to seek smaller and more cost-effective solutions. Thus, the PCB was born.
All electronic products require PCBs, and the PCB market trends are almost always an indicator of the electronics industry. However, with the development of high-end, miniaturized electronic products such as mobile phones, laptops, and PDAs, the demand for flexible PCBs (FPCs) has grown significantly. PCB manufacturers are accelerating the development of thinner, lighter, and higher-density FPCs.
What are the three main types of FPC?
1. Single-layer FPC
Single-layer FPCs have one layer of chemically etched conductive patterns on a flexible insulating substrate. The conductive pattern layer is typically made from rolled copper foil. The insulating substrate can be materials like polyimide, polyester (PET), aramid fiber ester, or polyvinyl chloride (PVC). Single-layer FPCs can be divided into four subcategories:
Single-sided connection without a cover layerThe conductive pattern is on the insulating substrate without a cover layer on the wire surface. Connections are made through soldering, welding, or pressure welding and were commonly used in early telephones.
Single-sided connection with a cover layerSimilar to the previous type, but with a cover layer on the wire surface. A pad is exposed for soldering, and sometimes the cover layer is omitted in terminal areas. This is the most widely used single-layer flexible PCB, often found in automotive instruments and electronic devices.
Double-sided connection without a cover layerThe connection pads are accessible on both the front and back of the wire. An opening in the insulating substrate allows access to the pads, which can be created using punching, etching, or other mechanical methods.
Double-sided connection with a cover layerUnlike the previous type, this version has a cover layer with vias that allow connections on both sides while maintaining the cover. It consists of two layers of insulating material and one layer of metal conductor.
2. Double-layer FPC
Double-layer FPCs have etched conductive patterns on both sides of the insulating substrate, increasing the wiring density per unit area. Metallized vias connect the patterns on both sides to form conductive pathways, enabling the design and functionality required for flexible circuits. Cover films protect the conductors and indicate the placement of components. Metallized vias and cover layers may or may not be present depending on the application, but this type of FPC is less commonly used.
3. Multilayer FPC
Multilayer FPCs consist of three or more layers of single-sided or double-sided flexible circuits laminated together. Metallized vias formed by drilling and electroplating create conductive pathways between layers, eliminating the need for complex soldering techniques. Multilayer circuits offer significant advantages in terms of higher reliability, better heat dissipation, and easier assembly. They provide excellent electrical characteristics, such as low dielectric constants, and are much lighter compared to rigid epoxy glass cloth multilayer PCBs—about one-third the weight. However, they lack the flexibility of single- or double-sided flexible PCBs, and most products in this category do not require flexibility. Multilayer FPCs can be further divided into the following types:
Flexible Insulation Substrate ProductThis type is manufactured on flexible insulation substrates, with the final product designed to be flexible. Typically, several layers of single- or double-sided microstrip flexible PCBs are bonded together at the ends but not in the central area, providing high flexibility. To maintain flexibility, a thin, suitable coating such as polyimide may be used on the conductor layers instead of a thicker laminated cover layer.
Soft Insulation Substrate ProductThis type is manufactured on soft insulation substrates but is not designed for flexibility. These multilayer FPCs are made by laminating soft insulation materials like polyimide films. After lamination, they lose their inherent flexibility.
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