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Coating of Resist-Analysis of Double sided FPC Manufacturing Process

Author:Luwis zeng Date:8/14/2018 9:52:28 AM
 Now, the coating method of the resist is classified into the following three methods according to the precision and yield of the circuit pattern: a screen printing method, a dry film/photosensitive method, and a liquid resist photosensitive method.
The resist ink is directly printed on the surface of the copper foil by screen printing, which is the most commonly used technology, and is suitable for mass production and low cost. The accuracy of the formed line pattern can reach line width / spacing 0.2 ~ O. 3mm, but not for more precise graphics. With the miniaturization, this method is gradually unable to adapt. Compared with the dry film method described below, it is necessary to have a certain technical operator, and the operator must be cultured for many years, which is an unfavorable factor.

The dry film method can produce a line width pattern of 70 to 80 μm as long as the equipment and conditions are complete. At present, most of the precision patterns of 0.3 mm or less can be formed into a resist pattern by a dry film method. A dry film is used, the thickness of which is 15 to 25 μm, and the conditions allow, and the pattern of 30 to 40 μm line width can be produced at the batch level.


When a dry film is selected, it must be determined according to the compatibility with the copper foil sheet, the process, and by experiment. Even if the level of the experiment has good resolving power, it does not necessarily have a high yield in mass production. The flexible printed circuit board is thin and easy to bend. If a hard dry film is used, it is brittle and has poor followability, so cracks or peeling are also generated to lower the pass rate of etching.



The dry film is roll-shaped, and the production equipment and operations are relatively simple. The dry film is composed of a three-layer structure such as a thin polyester protective film, a photoresist film, and a thick polyester release film. Before the film is applied, the release film (also called the separator) is first peeled off, and then pressed on the surface of the copper foil with a heat roller, and the upper protective film (also referred to as a carrier film or a cover film) is removed before development. Generally, the flexible printed circuit board has guiding positioning holes on both sides, and the dry film can be slightly narrower than the flexible copper foil to be filmed. The automatic filming device for rigid printed circuit boards is not suitable for the filming of flexible printed circuit boards, and some design changes must be made. Since the dry film is more linear than other processes, many factories do not use automatic filming, but use manual filming.

After the dry film is applied, in order to stabilize it, it should be placed for 15 to 20 minutes before exposure.

If the line width of the line pattern is below 30 μm, the pattern is formed by a dry film, and the pass rate is significantly lowered. In the case of mass production, dry film is generally not used, and liquid photoresist is used. The coating thickness varies depending on the coating conditions. If a liquid photoresist having a thickness of 5 to 15 μm is applied to a copper foil having a thickness of 5 μm, the line width of 10 μm or less can be etched at the laboratory level.

The liquid photoresist must be dried and baked after coating. Since this heat treatment has a great influence on the performance of the resist film, the drying conditions must be strictly controlled.

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