In the manufacturing process of printed circuit boards, pattern transfer is a key process. In the past, dry film technology was commonly used to transfer printed circuit patterns. Now, wet film is mainly used for the production of inner layer circuit patterns of multilayer printed boards and the production of outer layer circuit patterns of double-sided and multi-layer boards.
1. Crafting Process
Pretreatment→screen printing→ baking→ exposure→ development→ anti-plating or anti-corrosion→ film removal→ next process
2. Key Process Analysis
(1) Selection of coating method
Wet film coating methods include screen printing, roll coating, curtain coating, and dip coating.
Among these methods, the wet film surface film layer produced by the roller coating method is not uniform, which is not suitable for the production of high-precision printed boards. The wet film surface film layer produced by the curtain coating method is uniform and the thickness can be precisely controlled, but curtain coating equipment is expensive and suitable for mass production. The wet film surface film produced by the dip coating method is thin and has poor electroplating resistance. According to the current PCB production requirements, the screen printing method is generally used for coating.
The bonding of the wet film and the printed board is accomplished by chemical bonding, usually the wet film is an acrylate-based polymer, which is formed by freely moving unpolymerized acrylate groups and copper combination. This process adopts the method of first chemical cleaning and then mechanical cleaning to ensure the above-mentioned bonding effect, so that the surface is free of oxidation, oil and water.
(3) Control of Viscosity And Thickness
At the point of 5%, the dryness of the wet film is 150PS, which is lower than the thickness of this viscosity printing and cannot meet the requirements. In principle, no diluent is added for wet film printing. If it is to be added, it should be controlled within 5%.
The thickness of the wet film is calculated by the following formula:
In the formula, hw is the wet film thickness; hs is the screen thickness; S is the filling area; P is the ink solid content.
Take 100 mesh wire mesh as an example:
Screen thickness: 60 μm; Aperture area: 30%; Solid content of ink: 50%.
Thickness of wet film=[60-(60×70%)]×50%=9μm
When the wet film is used for anti-corrosion, its film thickness is generally required to be 15-20 μm; when it is used for anti-electroplating, its film thickness is generally required to be 20-30 μm. Therefore, when the wet film is used for anti-corrosion, it should be printed twice, and the thickness is about 18 μm, which meets the requirements of anti-corrosion; when it is used for anti-plating, it should be printed three times, and the thickness is about 27 μm, which is in line with the thickness of the anti-plating film. Require. When the wet film is too thick, it is easy to produce shortcomings such as insufficient exposure, poor development, and poor etching resistance. When resisting electroplating, it will be eroded by liquid medicine, resulting in film release, and the pressure sensitivity is high. ; When the film is too thin, it is prone to overexposure, poor electroplating insulation, film release and the phenomenon of electroplating metal on the film layer. In addition, when the film is overexposed, the film removal speed is also slow.
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