Chapter 1 the advantage of wet film on PCB process

Pre-face

Wet film was used as the pattern transfer material in the earliest PCB production process. With the continuous use of wet film and the improvement of PCB technical requirements, the shortcomings of wet film have also been revealed. There are too many pinholes and debris on the back surface of the membrane, and it is difficult to develop in the holes. This leads to many problems in the follow-up production of wet film, and it is difficult to control the process. Finally, the wet film is replaced by the dry film.

With the development of PCB assembly technology, as well as the smaller and smaller PCB lines and line spacing, and the increased precision and density requirements, the traditional dry film has problems in such a PCB pattern transfer process, which is mainly reflected in the fact that the resolution of the dry film cannot be Further improvement (mainly caused by the polyester cover film of dry film) can no longer meet the needs of special PCB production. Additionally, the needle holes, pits and scratches of the substrate affect the effect of the film, resulting in trachoma, gaps, Short line etc. Advanced wet film offers a way to solve these problems

How the issue come

The dry film has been used during the graphic transfer (which also includes the graphic transfer of the inner layer), and the effect is also good without any problems. However, in April and May of this year, the lines and line spacing of the inner and outer layers of the circuit boards we produced reached 0.15mm,

When the dry film pattern transfer is used, the inner layer is scrapped in large quantities, the outer layer is reworked due to trachoma, gaps, and broken lines after developing, and the inner layer get more serious after black-oxidation; even there is a serious single-panel pad  Falling off, lead to big scrap, which seriously affected normal production. In response to these problems, my colleagues and I made corresponding tests to find out the reasons, and tracked the effects of cleaning and brushing. Finally, before film laminated the boards achieved the best statement; we also changed the method of laminate and Optimized the parameters of the film laminate machine, changed the parameters of the develop chemical, and tracked the scrubbing performance for single panels. In the end, the problem of the single panel was solved, but the problem of the senoir inner and outer layers was not solved. According to the needs of the market, in order to improve the technical level of our company, in order to solve this problem, we use advanced wet film to transfer graphics

 feature of wet film

1. Good adhesion and coverage

The wet film itself is composed of photosensitive resin, a blue viscous liquid with photosensitive agent', colorant, seasoning and solvent added. The pits and scratches on the substrate are in good contact with the wet film, and the wet film is mainly bonded to the substrate through the action of chemical bonds, so that the wet film and the copper foil of the substrate have excellent adhesion. Using screen printing can get good coverage, which provides conditions for the processing of high-density fine-line PCBs.

The touch btw Wet film and substrate contact is god. The coverage is good, and the film contact-type exposure is adopted, which shortens the optical path, reduces the energy loss of light and the error caused by light scattering. This makes the resolution of the wet film generally below 25μm, which improves the precision of graphic production, while the resolution of the dry film in actual production is difficult to reach 50μm.

2、low cost

The thickness of the wet film is controllable. Generally speaking, it is thinner than the dry film, and the packaging cost is also lower. Relatively speaking, the cost of the wet film is lower. The qualified rate of wet film is greatly improved in the inner layer production process of fine lines, which saves 20% of material cost compared with dry film. The wet film developing speed is 30% faster, the etching speed can also be increased by 10-20%, and the film stripping speed can also be increased, thereby saving energy, improving equipment utilization, and ultimately reducing total costs。

3 Eliminate hair on the edge of the board

  The edge of the board with dry film is easy to be hairy and prone to film breakage, which will affect the pass rate of the board during the production process. The edge of the board with wet film is free of film breakage and hairiness.

 Process flow of wet film

According to its own conditions, a variety of wet films were used, and finally the wet film of Beijing Lituoda was selected. Its inner layer process operation flow is as follows：

Inner layer of multiple layer PCB :

brushing（ccl pre-treatment）---screen print---bake----expose—develop---plating— film strip — etch