Flexible PCB is a kind of soft printed board material which appeared in 1970s. The base material is made of polyimide or polyester film. Flexible PCB is good reliable and excellent flexible,and its output is close to the output of rigid pcb.FPC widely used in portable communication equipment, computers, printers and other fields.
In recent years, a two layer FCCL has been developed in order to reduce the thickness of flexible printed boards, improve flexibility, heat resistance and peel resistance to meet high performance and thin PCB. It does not require adhesives, and is directly composed of flexible insulating materials and copper foil. Due to the absence of acrylic binder, the thermal expansion coefficient of the substrate in the Z direction is small, and the dielectric loss of high-speed signal transmission is small. It is the preferred material for flexible substrates in rigid flex printed boards. However, at present, the supply of the substrate is not as large as that of the three layer substrate, and the cost is also high.
The main flexible sheet has copper clad foil polyester film, copper clad polyimide film, copper clad polyimide fluorocarbon film and thin epoxy glass coated copper sheet (thin FR – 4).
Copper-clad polyester film（PET）
The mechanical properties of copper-clad polyester film are better in tensile strength, dielectric constant and insulation resistance. It also has good anti-moisture and dimensional stability after moisture absorption. Disadvantages are poor heat resistance, large size change after heating, and lead bad welding.The working temperature is lower (below 105 degrees C). PET is only used in printed transmission lines without welding and flat cables in electronic machines.
Copper-clad polyimide film（PI）
The copper clad foil polyimide film has good electrical properties, mechanical properties, flame retardancy, chemical resistance and weather resistance. The most prominent feature is high heat resistance,the glass transition temperature of Tg is higher than 220. The disadvantage is that the hygroscopicity is high, and the shrinkage rate is high and the cost is high at high temperature or after moisture absorption, Pre bake to remove moisture before PCB welding. PI is suitable for flexible printed boards for high-speed circuit microstrip or strip line signal transmission, it’s also one of the most widely used substrates in flexible substrates.
Thin epoxy glass cloth copper clad plate
Thin epoxy glass cloth copper clad laminate is a flexible substrate developed in recent years. The thin FR-4 PCB is better than the traditional FCCL in moisture resistance performance of external insulation in addition to flexural performance, Dimension stability and other comprehensive properties. At present, it has been applied to banded IC package and the cost is lower than the PI board.
The other flexible PCB are different according to the demand. In recent years, a flexible copper clad plate with special materials has been developed, such as fluorocarbon film, aromatic polyamide paper, polysulfone film and liquid crystal polymer (LCP) film.
According to the manufacturing process, these flexible substrates are divided into two layers and three layers of flexible substrates. The difference between them is that the three layer method is made by traditional process method, that is compounded by copper foil, insulating film and binder. The two layer method is made up of insulating film and copper foil. It has several different manufacturing processes, but the common feature is no binder.
Compared with the three layer flexible substrate, the two layer method has the advantages of thin thickness, light weight, better flexibility and flame retardancy, easy impedance matching and good dimensional stability.
The two layer flexible plate is more suitable for the high speed circuit with impedance matching requirements for FPC, FPC with high density wiring, and the board of COF, TBA, CSP and other devices, and the flexible parts of the rigid flex of the printed board.
In the above two kinds of materials, it can be classified into general base material, high frequency and high speed PCB base material, high frequency circuit base material, high heat resistant base material, high dimensional stability and green environment type base material, etc.