Polyimide film performance requirements for flexible copper clad laminates

- Dec 21, 2019-

Polyimide film performance requirements for flexible copper clad laminates

   Summary of content: There are many application areas for electronic grade PI films, and their performance requirements are different in different application areas. The performance requirements of the largest flexible copper clad laminate (FCCL) in the application area exemplified here.

 

As the performance requirements of flexible circuit boards become higher and higher, how to properly select its flexible substrate material, FCCL's high-performance raw materials, can ensure that the manufactured product can still be better in the subsequent processing process. It is very important to maintain its original electrical, thermal and mechanical properties.

 

So far, FPC has used polyester resins, epoxy resins, polyester-epoxy, polyetherimide (PEI), polytetraethylene resin (PTFE), and polyether in the varieties of film substrates. Examples of thin film substrates such as ketoether (PEEK), liquid crystal polymer (LCD), and roll-shaped RF-4 copper-clad foils (thickness below 50 μm, produced by companies such as Sumitomo Bakelite, Panasonic, Kyocera Chemical, and Lichang Industries) , But still use polyimide resin as the vast majority. At present, there is no ideal new type of insulating base film to replace the polyimide film. The reason is that the heat resistance, rigidity, flexibility, and electrical characteristics of the polyimide resin film are superior to those of other resin films. However, its disadvantage is that the current price of the film is higher than the resin substrate (or other film) for high heat-resistant rigid CCL. In summary, polyimide film is still one of the most important film materials for manufacturing flexible copper clad laminates. Its use in the insulating base film used by FCCL accounts for more than 85% of the total use. Similar to the function of the insulating substrate material of the rigid copper-clad board, the most important function of the flexible insulating base film is to provide mechanical support and high dielectric insulation for electronic circuits. Choosing a flexible insulating base film rather than a rigid insulating substrate is the main characteristic of a flexible circuit board, and this characteristic is also the fundamental difference between a flexible circuit board and a rigid circuit board.

 

At present, the flexible insulating film used by FCCL can be either thermoplastic or thermosetting. Thermoplastic materials soften when heated and solidify (solidify) when cooled, but thermoset materials cannot be softened when heated. In the use of PI films, the vast majority are now using thermosetting base films.

 

FCCL requires that its flexible insulating base film must have excellent flexibility, dimensional stability, and heat resistance in addition to good mechanical properties and dielectric insulation properties. Flexibility is one of the important physical properties of the insulation film used for flexible copper clad laminates. Flexible dielectric substrates sometimes need to withstand tens of thousands of flexing movements, especially during dynamic flexion. important. Flexible dielectric substrates must have different degrees of expansion and contraction during processing, and these dimensional changes are usually greater than those found in rigid materials.

 

Because the polyimide film is a thermosetting polymer, there is no typical softening or melting point, so that the flexible circuit board made of the applied polyimide film is hot melted, and the film will not be degraded during soldering break down.

 

Thermal shrinkage is one of the important considerations for high-density flexible packaging substrates. Low shrinkage is the guarantee of the accuracy of producing fine patterns through multiple exposures in the TAB process, and it is also a guarantee of the mutual alignment of the vias of different layers in the process of making a multi-layer substrate. Similarly, the shrinkage rate is low when making large areas of fine patterns It seems particularly important.

 

Low thermal expansion coefficient is also an important index to be considered. The thermal expansion coefficient of the polyimide film is required to be as close as possible to the copper signal line to reduce the internal stress caused by the large difference in thermal expansion coefficient between the two. According to calculations, if the thermal expansion coefficient of the polyimide film is less than 18 ppm / ° C, the above-mentioned accumulation of internal stress can be effectively avoided.

 

For high-density flexible packaging substrate applications, the lower the moisture absorption of the polyphthalimide film, the better. In fact, polyimide films have ideal water absorption of less than 1.5% due to the presence of phthalimide groups. According to reports, when the moisture absorption rate is less than 2%, the flexible package substrate will not produce bubbles or peeling when it is subjected to a high temperature of 250-300 ° C during the pattern production process.

 

Polyimide film can be chemically etched with strong alkali solution such as NaOH, KOH, etc., to make various through holes. This is often used in two-layer TABs with only polyphthalimide and metal. The through hole is formed by etching the polyphthalimide film after covering the metal layer. If the polyphthalimide film cannot pass caustic.

List of main properties of polyimide film commonly used in flexible copper clad laminate manufacturing

 

Especially recently, in the high-frequency circuit flexible FPC higher than GHZ requirements for flexible substrate materials, more emphasis is placed on the substrate's low dielectric constant, low water absorption, and high reliability. The polyimide film has the dielectric constant that cannot be lowered (about ε = 3.0), the water absorption rate is too large, the peeling strength of the copper foil is greatly reduced during high temperature welding, and the dimensional change rate after the substrate is hygroscopic. Larger issues. None of these meet the requirements of current high-performance FPCs. In addition, the two-layer FPC made of polyimide film currently has problems of low bonding strength and low alkali resistance, which need to be further solved in the future. At the same time, in terms of environmental protection, polyimide film as a FPC substrate, it is difficult to achieve recyclability.

 

Rigorous and scientific research methods can ensure the accuracy and quality of research reports. The main research methods used in the "2011-2016 Polyimide (PI) Film Industry Market Analysis and Research Report" are: 1) General Survey: We conducted interviews with nearly 100 practitioners in the polyimide (PI) film industry. Interview or phone interview to get the best first-hand data. 2) Follow-up research: In order to ensure real-time understanding of the polyimide (PI) film industry, we have established a follow-up research mechanism in the polyimide (PI) film industry, and obtain polyimide ( PI) Development trends of the film industry. 3) Government agency data: We checked the industrial and commercial archives, statistics bureau archives, customs import and export data of key enterprises in the polyimide (PI) film industry, and obtained more authoritative information. 4) SOWT analysis: Using SWOT analysis, Porter's Five Forces analysis and other methods, we analyzed the competitive advantages and disadvantages of the polyimide (PI) film industry and enterprises, as well as potential threats and development opportunities. 5) Scientific prediction: We used regression analysis, time series analysis, factor analysis, combination analysis and other methods to make a full prediction of the development trend of polyimide (PI) films.

Previous:Why do PCB boards need test points? Next:Standards for printed board drilling covers and pads published