At present, the coating method of resist is divided into the following three methods according to the precision and output of circuit graphics: screen missing printing method, dry film / photosensitive method and liquid resist photosensitive method.
It is the most low-cost method to resist leakage printing on the surface of copper foil, which is the most commonly used method for mass production of copper foil. The accuracy of the formed line pattern can reach the line width / spacing of 0.2 ~ o.3mm, but it is not suitable for more precise patterns. With the miniaturization, this method can not adapt gradually. Compared with the dry film method described below, operators with certain skills are required, and the operators must be trained for many years, which is a disadvantageous factor.
As long as the equipment and conditions are complete, 70 ~ 80 can be prepared by dry film method μ Line width graphics of M. At present, most of the precision patterns below 0.3mm can form anti-corrosion line patterns by dry film method. Dry film is adopted, and its thickness is 15 ~ 25 μ m. If conditions permit, the batch level can be 30 ~ 40 μ M line width.
When selecting dry film, it must be determined by test according to the matching with copper foil and process. Even if the experimental level has good resolution, it does not necessarily have a high qualified rate in mass production. The flexible printed board is thin and easy to bend. If a harder dry film is selected, it will be brittle and have poor follow-up performance, so it will also produce cracks or spalling, which will reduce the qualification rate of etching.
The dry film is rolled, and the production equipment and operation are relatively simple. The dry film is composed of a thin polyester protective film, a photoresist film and a thick polyester release film. Before applying the film, the release film (also known as diaphragm) should be stripped first, and then pasted on the surface of the copper foil with a hot roller. Before development, the upper protective film (also known as carrier film or covering film) should be torn off. Generally, there are guide positioning holes on both sides of the flexible printed board, and the dry film can be slightly narrower than the flexible copper foil board to be applied. The automatic film pasting device for rigid printed boards is not suitable for the film pasting of flexible printed boards, and some design changes must be made. Due to the high linear speed of dry film coating compared with other processes, many factories do not use automatic coating, but use manual coating.
After the dry film is pasted, in order to make it stable, it should be placed for 15 ~ 20min before exposure.
