Common Flexible PCB Issues and Solution

1. Air 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 mask cannot be printed on the substrate, resulting in the presence of air or moisture between the solder mask and the substrate. During curing and exposure, the gas is heated to expand and cause a single line to cause a single line. The line is 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 on the root of the line. There is a gas between the side of the line root and the solder mask, and bubbles are generated after heating.

Solution: During screen printing, visually check whether the screen printing material is completely printed on the substrate and the sidewall 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 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 for plate making; often check the cleanliness of the photographic plate during exposure.

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 by 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.

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

The main reason: the dirt on the surface is caused by flying feathers 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 an uneven surface.

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

5. FPC flexible circuit boards have ghosting and cracking

The main reason: the ghosting is due to the weak positioning of the FPC flexible circuit board during the screen printing and the residual ink on the screen plate not being removed in time and accumulating on the printed circuit board, resulting in the existence of regular ink dots next to the entire FPC pad. The crack is Due to insufficient exposure in the FPC exposure process, there are small cracks on the board surface.

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 reaches between 9-11 exposure levels. No cracks will appear.