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Resist Coating - Analysis of Double-sided FPC Fabrication Process

Author: Date:5/21/2018 10:01:51 AM
 Now, the coating method of the resist is classified into the following three methods depending on the precision and the yield of the circuit pattern: a screen printing method, a dry film/lighting method, and a liquid resist light sensing method.
Anti-corrosion inks are printed directly on the surface of the copper foil using the screen printing method. This is the most commonly used technology and is suitable for mass production with low cost. The accuracy of the formed line pattern can reach the line width/spacing 0.2~O. 3mm, but not for more precise graphics. With the refinement of this method can not gradually adapt. Compared with the dry film method described below, there is a need for a certain technical operator, and the operator must be trained 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. Most of the precision patterns below 0.3mm can be formed by the dry film method. Using a dry film, the thickness is 15 ~ 25μm, conditions permitting, batch level can produce 30 ~ 40μm line width graphics.

When the dry film is selected, it must be determined according to the matching with the copper foil board and the process and by experiments. Even if the level of experimentation has good resolution, it does not necessarily have a high pass rate when used in mass production. The flexible printed circuit board is thin and easy to bend. If a hard dry film is selected, it is brittle and has poor followability, so that cracks or peeling may also occur to lower the etching yield.



The dry film is roll-shaped and the production equipment and operations are relatively simple. The dry film consists of a three-layer structure consisting of a thin polyester protective film, a photoresist film, and a thick polyester release film. Before the film is peeled off, the release film (also called the diaphragm) is first peeled off, and then it is pressed on the surface of the copper foil with a heat roller, and then the upper protective film (also called the carrier film or cover film) is torn off before development. Generally, there are guide positioning holes on both sides of the flexible printed circuit board, and the dry film can be slightly narrower than the flexible copper foil to be filmed. The automatic film sticking device for rigid printed circuit boards is not suitable for the application of flexible printed circuit boards, and some design changes must be made. Due to the high linear velocity of the dry film film compared to other processes, many plants do not use automated film but use manual film.

After the dry film is attached, exposure should be performed after 15 to 20 minutes in order to stabilize it.

If the line width of the circuit pattern is less than 30μm, patterning with a dry film will result in a significant decrease in the yield. Dry film is generally not used in mass production but liquid photoresist is used. Different coating conditions, the thickness of the coating will be changed, if you apply a liquid photoresist with a thickness of 5 ~ 15μm on a 5μm thick copper foil, the laboratory level can etch the line width of 10μm or less.

The liquid photoresist must be dried and baked after coating. Since this heat treatment will greatly affect the performance of the film, the drying conditions must be strictly controlled.

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