PCB Tech - What are the common defects of FPC board

1. Bubbles are generated between the lines of the FPC flexible circuit board or the side of a single line

The main reason: the bubbles between two or more lines are mainly due to the narrow line spacing and high lines. During screen printing, the solder resist cannot be printed on the substrate, resulting in the presence of air or moisture between the solder resist and the substrate. During curing and exposure, the gas is heated to expand and cause a single line to cause the line to be too high. When the squeegee is in contact with the line, the line is too high, and the angle between the squeegee and the line increases, so that the solder resist cannot be printed to the root of the line. There is gas between the side of the line root and the solder mask, and bubbles will be generated after heating.

Solution: During screen printing, visually check whether the screen printing material is completely printed on the substrate and the side wall of the line, and strictly control the current during electroplating.

2. There are solder masks and pinholes in the holes of the FPC flexible circuit board

The main reason: The FPC flexible circuit board did not print paper in time during the screen printing, which caused the screen to accumulate too much residual ink. The residual ink was printed into the hole under the pressure of the squeegee, and the screen mesh number was too low, which would also cause holes In the solder mask. Dirt on the photographic plate causes the part of the FPC flexible circuit board that should be exposed to light during the exposure process to not see light, resulting in pinholes in the pattern.

Solution: Print paper in time and select high-mesh screen to make a plate; often check the cleanliness of the photographic plate during the exposure process.

3. There are signs of blackening on the copper foil lines under the solder mask of the FPC flexible circuit board

The main reason: the FPC flexible circuit board is not dried in water after the board is wiped, and the surface of the printed board is splashed by liquid or hand-molded before the solder mask is printed.

Solution: Visually inspect the copper foil on both sides of the printed board for oxidation during screen printing.

What are the common defects of FPC board

4. There is dirt and unevenness on the surface of the FPC flexible circuit board

The main reason: the dirt on the surface is caused by flying hairs and other debris in the air. The uneven surface is caused by not paying attention to printing paper in time during screen printing and removing the residual ink on the screen, resulting in uneven surface.

Solution: The clean room must fully ensure the cleanliness of the operator, avoid irrelevant personnel from passing through the clean room, clean the clean room regularly, and print paper in time to remove the residual ink on the screen during screen printing.

5. FPC flexible circuit board has ghosting and cracking phenomenon

The main reason: ghosting is due to the weak positioning of the FPC flexible circuit board during screen printing and the inability to remove the residual ink on the screen plate in time, which causes regular ink dots to exist next to the entire FPC pad. The crack is Due to insufficient exposure in the FPC exposure process, there are small cracks on the surface of the board.

Solution: Fix the paper firmly with positioning pins and remove the residual ink on the screen in time; measure the exposure so that the comprehensive value of the parameters such as the energy of the exposure lamp and the exposure time can reach between 9-11 exposure levels. No cracks will appear.

6. The color on both sides of the FPC flexible circuit board is inconsistent and the problem of skipping and flying white

The main reason: the number of knives in the screen printing on both sides is very different, and there is a mixture of new and old inks. It is possible that one side is using new ink that has been stirred, while the other side is using old ink that has been left for a long time. Skip printing is caused by excessive electroplating current and thick plating, which causes the pattern lines to be too high. When the FPC flexible circuit board is screen printed, the squeegee blade and the screen printing frame are screen printed at a certain angle, so the lines are too high on both sides of the line., There will be no ink and skip printing. Another reason is that the squeegee has a gap, and no ink can be placed at the gap, causing skip printing.

Solution: Try to keep the number of knives in the screen printing consistent, and avoid the mixing of new and old inks; mainly control the electroplating current and check whether the squeegee knife has gaps.