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Materials of the High-Density Flexible Printed Circuit Boards

by: Jan 16,2019 3601 Views 0 Comments Posted in Flexible PCB


The basic materials of high-density flexible PCB are substrates, conductors and cover layers (cladding). Substrates and conductors are usually laminated together to form copper clad laminates.

Materials of Conductors:

One of the main ways to achieve HDI flexible printed circuit boards is to use the thin, fine grained and low-profile copper foils. The standard copper foil used in conventional flexible printed circuit boards has always been RA foil, usually with a thickness of 1OZ (35um). 1OZ RA copper foil can provide appropriate performance for graphics of ordinary density. However, for HDI applications, most manufacturers use 1/2 OZ of RA foil, or even 1/3 OZ of RA foil. The cost of RA foil increases with its thinness, and the cost of thin and fast etching copper foil is high. There is no doubt that 1/2 OZ copper foil laminate will be the standard material for HDI applications and 1/3 OZ copper foil will be added. In the future, the thickness of RA foil can be less than 10um, and it can be made into carrier copper foil. However, the cost of technology and materials will be greatly increased.

Another alternative is the use of fine grained low profile ED foil. The new ED foil provided at present is usually called dynamic flexural ED copper foil (DFED). This kind of copper foil is the same as RA copper foil in flexural performance, and some even exceed RA copper foil. Coupled with the low cost, ED copper foil is quite popular in the market.

Materials of Substrates:

PI film is the base material and core material of traditional FPC. Here are three materials for HDI flexible circuit boards.


1.      High Performance PI Film:

PI film has been widely used in flexible circuit boards, such as soldering and wire bonding, because of its high temperature resistance. Among these films, the Kapton H film and the Apical AV film have been used for a long time because of their good performance, which are the main materials of traditional flexible PCB. However, the Kapton H film and the Apical AV film are limited in the application of HDI flexible PCB due to the lack of performance which cannot meeting the requirements. Among these, the dimension stability of them is the most vital problems. What is impossible to be realized for HDI flex-PCB is that the CTE of the Kapton H film and the Apical AV film is over 30ppm. The high hygroscopicity of the Kapton H film and the Apical AV film is another big problem which will rise many problems.


2.      Liquid PI Film

3.      Non-Adhesive Materials

At present, flexible substrate materials can be divided into adhesive and non-adhesive forms. The main difference between the two kinds of substrate materials is that the non-adhesive substrate material has the characteristics of high dimensional stability, high deflection and thin, which is suitable for the production of high-density flexible circuit board.

Materials of Cover Layer (Cladding):

The cladding is an insulating protective layer covering the surface of flexible circuit board. Its function is to protect the flexible circuit from dust, moisture, chemical corrosion and reduce the stress in the bending process. In particular, the flexible circuit board has a high toughness during a long bending period, so as to give full play to its important advantages. However, in the selection of cladding, we usually choose the same adhesive as the substrate as the cladding, such as PET, PI film. This type of material has good physical balance characteristics and high reliability, especially for the application of dynamic flexible circuit boards requiring high bending. However, the manufacturing process of this material is a bit complex and it’s almost impossible to realize the automated production. When manufacturing the high-density flexible PCB, laminating the cladding always meets many difficulties. For example, it would be limited, if only machining by machines, which also cannot guarantee dimensional accuracy or meet the technical requirements of bar spacing less than 0.50mm. In addition, the cost of this process is very high. Some electrical products use the solder mask instead of the cladding in order to save money, although solder mask can play a role of protecting the wires but it does not have the good mechanical characteristics or be suitable for flexible circuit board applications, which makes its application be very limited.

Therefore, in order to solve this problem, liquid photosensitive cladding has emerged. This cladding can not only solve the problem of machining accuracy smoothly, but also reduce the cost and simplify the whole production process. As long as the general standard UV exposure machine for exposure, and then use water-soluble developer for development, and then it will be heated and solidified, so as to save the conventional lamination process.


HDI trend of flexible circuit board is imperative and has been applied in various products. Therefore, PCBWay will keep up with this development trend and provide customers with the most advanced, convenient and fast FCB manufacturing services!

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