PCB Technical - PCB process Base material for flexible circuit boards

PCB process Base material for flexible circuit boards

1. Background description
Information and communication electronics, semiconductors and optoelectronics industries have become the mainstream of global industrial development. With the development of electronic products towards the trend and demand of portability, high density, high reliability, and low cost, organic polymer film materials can only be available. Become a major development trend. The high-performance films required by these industries are mainly high-temperature organic high-molecular polymers, because organic high-molecular polymers have the advantages of easy acquisition, good electrical insulation, and easy processing and molding. Among the organic polymers that meet the above characteristics, the main high-temperature stable materials include polyimide film (PI), polycarbonate film (PC), and polyetherimide film (referred to as PEI), polyether sulfone film (Polyester film, referred to as PES), and relatively poor temperature resistance polyester film (Polyester film, referred to as PET), etc. There are many other types of high-temperature resistant organic polymer films that can be used, and the main selection is based on the product's application characteristics and process requirements.

In the classification of organic polymer materials, it can generally be divided into two types: Amorphous materials and Semi-Crystalline materials. The semi-crystalline material has a neatly arranged molecular structure and a clear melting point. When the temperature rises, the semi-crystalline material will not gradually soften but maintain the hardness until it absorbs a certain amount of heat energy and then quickly changes to a low-viscosity liquid. These materials also have good chemical resistance. Although their load-bearing capacity is exceeded above the glass transition temperature (Tg), semi-crystalline materials can still maintain appropriate strength and rigidity. Therefore, semi-crystalline polymer materials have an irregularly arranged molecular structure and generally do not have a clear melting point. When the temperature rises, it will gradually soften. Generally, non-crystalline materials have worse temperature resistance than semi-crystalline materials, and are more susceptible to thermal deformation, but have lower shrinkage and less susceptible to warping. In terms of temperature resistance, we can further classify polymer materials. We can roughly distinguish the temperature resistance characteristics of materials from the glass transition temperature (Tg) or temperature resistance of various materials. High Performance Plastics (High Performance Plastics), which is also an important group in today's high-performance thin-film electronic materials, the uppermost polyamide material (Polyimide, PI) has a glass transition temperature (Tg) of up to 380°C, which surpasses temperature resistance. Of all the polymer materials, there is no other material in the polymer material film category. In addition, the classification of amorphous and semi-crystalline materials mentioned in the above description. A small part of the crystalline structure also exists in the amide molecular structure, but its proportion is less than 10%, which cannot be classified as a semi-crystalline material. therefore. Polyimide has the advantages of both non-crystalline and semi-crystalline materials. For example, polyimide has the characteristics of transparency and softness of non-crystalline materials in the film state, and it also has the characteristics of semi-crystalline materials and chemical resistance and size stability., And these characteristics are exactly what the soft board material needs. The characteristics developed by such a structure domain are rare in organic polymer materials. Polyimide film has stable and excellent physical, chemical, electrical and mechanical properties in a wide temperature range (-269～400 degree Celsius), which is unmatched by other organic polymer materials. It can be used in a short time at 450 degree Celsius To maintain its physical properties, the long-term use temperature is as high as 300°C. Not only that, the polyimide film has a very important position in the radiation resistance and the application of the information and communication industry.

2. The function of polyimide substrate
Polyimide resin has excellent heat resistance, chemical resistance, mechanical properties and electrical properties, so it is widely used in various industries such as aviation, electrical machinery, machinery, automobiles, and electronics. In the past year, Yauchi’s semiconductor, electronics, and communications industries have developed vigorously, driving the development of the domestic economy. The demand for electronic chemicals and materials has also increased. Polyimide resins also play an important role in electronic materials. The application type of polyimide resin in the electronics-related industries is mainly films and coatings. It is mainly used in IC semiconductor manufacturing, flexible circuit boards, liquid crystal displays, etc. In the applied products, the amount of polyimide in the form of film accounts for maximum. Polyimide molecule has imide group, which makes the main chain of polymer have high rigidity (Rigidity) and strong intermolecular force, so the treatment has the same as all kinds of engineering plastics. In addition to heat resistance and chemical resistance, it also has the following characteristics: a. Excellent heat resistance: It can be used for a long time at a temperature of 250 degree Celsius～300 degree Celsius, and the heat resistance temperature is higher than 400 degree Celsius. Some products even The temperature can reach 500°C, and the thermal stability of the film is excellent. b. The linear expansion coefficient is small: in the temperature range of -250 degree Celsius～+250 degree Celsius, the size change is very small. c. High freezing resistance. d. Resistant to chemical solvents and radiation, insoluble in general organic solvents. e. It does not melt and has excellent flame resistance. It does not drip or produce a lot of smoke during combustion. f. Good electrical performance and excellent insulation properties.

The substrate material used for the soft board is generally made of copper foil and thin film material (base material) to make a flexible copper foil substrate (FCCL), plus protective film (Coverlay), reinforcement board, anti-static layer and other materials to make The main function of the substrate is to be used as a support material for the flexible circuit, and it needs to have the characteristics of an insulating circuit. Generally, the thin film materials commonly used in the flexible substrate are mainly PEI and PI materials. In terms of practical application, the use of PI in flexible board substrates accounts for the vast majority of applications. At present, more than 90% of the flexible board substrates use PI film. The main reason is the poor temperature resistance of PET film ( Its Tg is less than 100%), and the dimensional change is too large at high temperature. This is the high temperature environment required in the soft board manufacturing process and the actual use environment, and PET cannot meet the requirements. The thickness of PI film can be divided into 0.5㏕ (half mil), 1mil, 2mil, 3mil, 5mil, 7mil, 9mil, and even more than 10mil products. Advanced or high-end soft boards need thinner thickness and more stable dimensional stability. PI film. The general protective film uses 1 mil and 0.5 mil PI film, and the thicker PI film is mainly used to reinforce the board for other purposes.
General FPC and flexible carrier boards that can be used as active and passive components are currently the two largest electronic application markets for polyimide films. The main application products include substrate materials (FCCL), protective films (Coverlay), and reinforcing plates. FPC applications include Juyong, automobiles, computers, laptops, cameras, communications, etc. Recently, the flexible substrates used in the driver IC assembly of LCD modules, such as Chip on Film (COF), have more and more value trends, mainly because COF's circuit refinement characteristics can effectively improve product miniaturization and reduce overall manufacturing cost. Generally, the PI used for FPC and flexible substrates has some differences in characteristics. Generally, because the application of flexible substrates requires dynamic and repeated flexing, the required PI substrate needs to be soft and its flexural characteristics must be sufficient. However, the PI substrate used in the soft carrier board must carry the active and passive components on it, so it is necessary to choose a PI film with better rigidity, and because of the structure of the component loading board, it is stable in size and hygroscopicity of the substrate. It needs to be higher than the PI standard for general FPC, that is to say, the PI film used for the flexible carrier must have a lower moisture absorption rate and better dimensional stability to meet the high reliability required for component assembly. sex.