With the diversification of uses and compactness, FPCs used in electronic devices require high-density circuits as well as high performance in a qualitative sense. Recent changes in FPC circuit density. The subtractive method (etching method) can be used to form a single-sided circuit with a conductor pitch of 30um or less, and a double-sided PCB circuit with a conductor pitch of 50um or less has also been put into practical use. The via diameters between conductor layers connecting double-sided circuits or multi-layer circuits are getting smaller and smaller, and now holes with via diameters below 100um have reached mass production.

Based on the standpoint of manufacturing technology, the possible manufacturing range of high-density circuits. According to circuit pitch and via hole diameter, high-density circuits are roughly divided into three types: (1) traditional FPC; (2) high-density FPC; (3) ultra-high-density FPC.

In the traditional subtractive method, FPC with a pitch of 150um and a via hole diameter of 15um has been mass-produced. Due to the improvement of materials or processing equipment, a circuit pitch of 30um can be processed even in the subtractive method. In addition, due to the introduction of processes such as CO2 laser or chemical etching, mass production and processing of via holes with a diameter of 50um can be realized, and most of the high-density FPCs currently mass-produced are processed by these technologies.

However, if the pitch is less than 25um and the via hole diameter is less than 50um, even if the traditional technology is improved, it is difficult to increase the yield, and new processes or new materials must be introduced. There are various processing methods for the proposed process, but the semi-additive method using electroforming (sputtering) technology is the most suitable method. Not only the basic process is different, but the materials and auxiliary materials used are also different.

On the other hand, the advancement of FPC joining technology requires FPC to have higher reliability performance. With the high density of circuits, the performance of FPC puts forward diversified and high-performance requirements, and these performance requirements largely depend on the circuit processing technology or the materials used.