With the development of 5G technology and the introduction of flexible mobile phones by many brands, FCCL and related films have become the focus of attention, especially the localization of LCP (liquid crystal polymer) films and high-end PI (polyimide) films. Most of the insulating base membranes of FCCL are polyimide (PI) and polyester (PET) membranes, as well as polynaphthalene ester (PEN) membranes. However, the heat resistance of PET and PEN films is not good, and the moisture absorption of PI films is too large. This shortcoming may lead to the reliability of FPC at high temperature, including the oxidation and peeling strength reduction of copper foil caused by evaporation of water vapor at high temperature. In addition, the curling phenomena caused by moisture absorption of PI membranes will cause problems in use. Therefore, the industry is continually striving to develop new high-performance polymer materials to replace PI membranes, among which liquid crystal polymers (LCP) and polyether ether ketone (PEEK) are the most influential ones. From the basic properties of the films, these two kinds of macromolecule materials can effectively remedy the defects of PI films, including low hygroscopicity, low thermal expansion coefficient, low dielectric constant and high dimensional stability.
Characteristics of LCP Films
At present, the synthesis of LCP is mainly polycondensation reaction. The synthesized LCP mainly includes polyamides, polyesters, polyethers, polythiazoles, polyimidazoles, etc. of main chain type, polyisocyanates, polyazo, polydimethylsiloxane and polyacrylates of side chain type. In addition, there are some special mechanisms such as polymer liquid crystals. Compared with other organic macromolecule materials, LCP has unique molecular structure and thermal behavior. Its molecule is composed of rigid rod-like macromolecule chains which are heated, melted or dissolved by solvents. It no longer has most properties of solid substances, but forms a transitional mesophase with some properties of solid and liquid, liquid crystals. The molecular arrangement of the state is between the ideal liquid and crystal, showing a one-to-one or long-range order. The molecular arrangement shows disorder in the position, but there is still some orderliness in the upward direction of the molecule, showing good anisotropy. The anisotropy of LCP makes it have high strength, high modulus and self-reinforcing properties, outstanding heat resistance, excellent cold and hot alternating properties, excellent corrosion resistance, flame retardancy, electrical properties and processing properties. Its linear expansion coefficient and friction coefficient are very small, and it has excellent radiation resistance and good transparency to microwave. It has been widely used in electronic devices, precision instrument parts, household appliances, medical devices, automobile parts and chemical equipment parts and other fields.
Moreover, the dielectric properties of LCP are much better than those of PI, which has a strong advantage in the application of FPC in the high-end field.
Despite the obvious advantages of LCP film performance, because the manufacturing technology of LCP resin and film is mastered by foreign enterprises, there is no localization at present. The following are the related performance indicators of a brand LCP film.
Application of LCP Film in FCCL
Because of the structural defects of PI film, more and more research on LCP has been done in the industry. The following figure illustrates the basic performance comparison between typical LCP-based FCCL and PI-based FCCL. The disadvantage of LCP-based FCCL is its low peeling strength.
The greatest advantage of LCP is its low moisture absorption and excellent dimensional stability. It can still maintain good dimensional stability and peeling strength in extremely humid environment, and the boundary loss of LCP is relatively small, almost at the same level as PTFE. It is suitable for high frequency lines. It is mainly used for LCP, PDP driver, IC package, t-BGA, wireless LAN, communication network equipment and high-speed digital connector.
Problems in LCP Films
Although LCP has excellent properties, it also has some disadvantages, such as difficult to control the processing technology, anisotropic physical properties of products, poor mechanical properties in the direction of right angle with shear flow, and easy to original fibrosis, so it needs to be modified to improve the mechanical properties of LCP films. On the other hand, the high price of LCP raw materials leads to the high price of LCP, which is the most important problem affecting the development of LCP. Therefore, cost reduction has become the research focus of manufacturers.
