With the development of 5G technology and the introduction of flexible mobile phones by a number of brands, FCCL and related films have become the focus of attention, especially LCP(liquid crystal polymer) film and high-end PI(polyimide) film localization is also constantly refreshing people's vision. The insulating base film of FCCL mostly uses polyimide (PI) film and polyester (PET) film, as well as polynaphthyl ester film (PEN) and so on. However, the heat resistance of PET film and PEN film is poor, and the moisture absorption of PI film is too large, which may lead to the reliability of FPC under high temperature conditions, including the oxidation of copper foil and the reduction of peel strength caused by water vapor evaporation at high temperature. In addition, PI film moisture absorption caused by curling phenomenon, will cause problems in use, so the industry is continuing efforts to develop new high-performance Polymer materials, in the hope of replacing PI film, the most influential is Liquid Crystal polymer (Liquid Crystal Polymer, referred to as LCP) and polyether ether ketone (PEEK). From the basic performance of the film, these two categories of polymer materials can effectively make up for the shortcomings of PI film, mainly including low hydorimetric property, low thermal expansion coefficient, low dielectric constant and high dimensional stability, this paper focuses on LCP film.

LCP film characteristics

At present, the synthesis of LCP is mainly a polycondensation reaction, the synthesis of LCP are mainly: main chain polyamides, polyesters, polyethers, polythiazolidazole and so on; Side chain type of polyisocyanates, polyazo, polydimethylsiloxanes, polyacrylates and so on. In addition, there are some special institutions of polymer liquid crystal and so on. Compared with other organic polymer materials, LCP has a more unique molecular structure and thermal behavior. Its molecules are composed of rigid rod-like macromolecular chains. After being heated and melted or dissolved by solvents, LCP no longer has most of the properties of solid substances, but forms a transitional intermediate phase state with the properties of solid and liquid parts. — The liquid crystal state, whose molecules are arranged between the ideal liquid and crystal, is a remote order of one or, the molecular array shows disorder in position, but there is still a certain order in the molecular upward, showing good anisotropy. The anisotropy of LCP makes it have high strength, high modulus and self-reinforcing performance, outstanding heat resistance, excellent cold and thermal alternating performance, excellent corrosion resistance, flame retardancy, electrical properties and molding processing performance. Its linear expansion coefficient and friction coefficient are very small, but also has excellent radiation resistance and good transparency to microwave, it is widely used in electronic devices, precision instrument parts, household appliance accessories, medical devices, auto parts and chemical equipment parts and other fields.

And from the key indicators of the intermediary electrical properties of electronic products, the dielectric properties of LCP are far better than PI, which has a strong advantage in the high-end field of FPC applications.

Although the performance advantages of LCP film are obvious, but because the manufacturing process of LCP resin and film is mastered in foreign enterprises, it has not been realized at present. The following is the related performance indicators of a brand LCP film.

The application of LCP film in FCCL

Due to the structural defects of PI film, the industry has more and more research on LCP. The following figure lists the basic performance comparison between typical LCP-based FCCL and PI-based FCCL. The defect of LCP-based FCCL is low peel strength. The biggest advantage of

LCP is its very low hygroscopic property and excellent dimensional stability, which can still maintain good dimensional stability and peel strength in extremely humid environments, and the boundary loss of LCP is relatively small, almost at the same level as PTFE, which is suitable for high-frequency lines. Mainly used in LCP, PDP driver, IC package, t-BGA, wireless LAN, communication network equipment and high-speed digital connectors.

LCP film problems

LCP although the performance is excellent, but there are also molding processing technology is not easy to control, the physical properties of the product is anisotropic, and the molding shear flow direction of poor mechanical properties, easy to fibrate and other shortcomings, so it needs to be modified to improve the mechanical properties of LCP film. On the other hand, the price of LCP raw materials is expensive, which leads to the high price of LCP, which is the most important problem affecting the development of LCP, so reducing the cost has become the research focus of manufacturers.

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