Views: 2 Author: Site Editor Publish Time: 2023-09-20 Origin: Site
Before the emergence of PCBs, circuits were composed of point-to-point wiring. The reliability of this method is very low because as the circuit ages, the rupture of the circuit can lead to an open circuit or short circuit at the circuit nodes. Wire winding technology is a significant advancement in circuit technology, which improves the durability and replaceability of circuits by wrapping small diameter wires around pillars at connection points.
As the electronics industry evolves from vacuum tubes and relays to silicon semiconductors and integrated circuits, the size and price of electronic components are also decreasing. Electronic products are increasingly appearing in the consumer field, prompting manufacturers to search for smaller and more cost-effective solutions. So, PCB was born.
Electronic products all require the use of PCBs, and the market trend of PCBs is almost a barometer of the electronics industry. But with the development of high-end and miniaturized electronic products such as mobile phones, laptops, and PDAs, the demand for flexible PCBs (FPCs) is increasing. PCB manufacturers are accelerating the development of thinner, lighter, and denser FPCs.
1、 Single layer FPC
It has a layer of chemically etched conductive patterns, and the conductive pattern layer on the flexible insulation substrate surface is rolled copper foil. The insulation substrate can be polyimide, polyethylene terephthalate, aramid fiber ester, and polyvinyl chloride. Single layer FPCs can be further divided into the following four subcategories:
1. Single sided connection without covering layer
The wire pattern is on an insulating substrate, with no covering layer on the surface of the wire. Its interconnection is achieved by soldering, fusion soldering, or pressure soldering, commonly used in early telephone sets.
2. Single sided connection with covering layer
Compared to the previous class, there is only an additional layer of coverage on the surface of the wire. When covering, the solder pad needs to be exposed, which can be easily left uncovered in the end area. It is the most widely used and widely used type of single sided soft PCB in automotive instruments and electronic instruments.
3. Double sided connection without covering layer
The connection plate interface can be connected on both the front and back of the wire, and a passage hole can be opened on the insulation substrate at the solder pad. This passage hole can be made by punching, etching, or other mechanical methods at the desired position of the insulation substrate.
4. Double sided connection with covering layer
The first type is different in that there is a layer of covering on the surface, which has access holes that allow both sides to be terminated while still maintaining the covering layer. It is made of two layers of insulation material and one layer of metal conductor.
2、 Double sided FPC
The double-sided FPC has a conductive pattern etched on both sides of the insulation substrate film, increasing the wiring density per unit area. Metallized holes connect the patterns on both sides of the insulation material to form conductive pathways to meet the design and functional requirements of flexibility. And the covering film can protect single and double sided wires and indicate the position of the component placement. According to demand, metallized holes and covering layers are optional, and this type of FPC is rarely used.
3、 Multilayer FPC
Multilayer FPC is the process of pressing three or more layers of single or double-sided flexible circuits together, forming metallized holes through drilling and electroplating, and forming conductive paths between different layers. This way, there is no need to use complex welding processes. Multilayer circuits have significant functional differences in terms of higher reliability, better thermal conductivity, and more convenient assembly performance.
Its advantage is that the substrate film is lightweight and has excellent electrical properties, such as low dielectric constant. The multi-layer flexible PCB board made of polyimide film as the substrate is about one-third lighter in weight than the rigid epoxy glass cloth multi-layer PCB board, but it loses the excellent flexibility of single-sided and double-sided flexible PCBs, and most of these products do not require flexibility. Multilayer FPCs can be further divided into the following types:
1. Flexible insulation substrate finished products
This type is made on a flexible insulation substrate, and its finished product is specified to be flexible. This structure usually bonds the two ends of many single or double-sided microstrip flexible PCBs together, but the central part is not bonded together, resulting in high flexibility. In order to have a high degree of flexibility, a thin and suitable coating, such as polyimide, can be used on the wire layer to replace a thicker laminated covering layer.
2. Soft insulation substrate finished products
This type is made on soft insulation substrates, and the finished product is not specified to be flexible. This type of multi-layer FPC is made by laminating soft insulation materials, such as polyimide films, into multi-layer boards, which lose their inherent flexibility after lamination.