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Introduction to common FPC board surface defects

  1. Bubble

There are bubbles between lines of FPC printed board or on the side of single line after development. Main reasons: Bubbles between two or more lines are mainly caused by too narrow line spacing and too high line. When screen printing, solder mask cannot be printed on the base material, resulting in air or moisture between solder mask and the base material. When curing and exposure, the gas expands due to heating, resulting in a single line, which is mainly caused by too high line. When the scraper contacts the line, the line is too high, and the angle between the scraper and the line increases, The solder mask cannot be printed to the root of the line, so that there is gas between the side of the root of the line and the solder mask layer, and bubbles will be generated after heating. The solution is as follows: during screen printing, visually check whether the screen print material is completely printed to the base material and the side wall of the line, and strictly control the current during electroplating.

  1. Hole resistance welding

The reason is that the paper was not printed in time during screen printing, which caused the screen plate to accumulate too much residual ink, and the residual ink was printed into the hole under the pressure of the scraper. The solution is to print the paper in time. Also, the number of screen mesh is too low, which will also cause resistance to welding in the hole. The high mesh screen should be used for plate making. The viscosity of the printing material is too low. The printing material with high viscosity should be used instead. The angle of the scraper screen printing should be appropriately increased, and the blade of the scraper should be rounded, Sharpen the blade of the scraper.

  1. Jump printing is also called Feibai

It is mainly because the electroplating current is too large and the coating is thick, which causes the graphic lines to be too high. When screen printing FPC printed boards, the scraper knife and the screen printing frame form a certain angle, so the lines on both sides of the lines are too high, so the ink will not be applied. This causes skip printing. Another reason is that the scraper knife has a gap, and the gap does not apply ink, which causes skip printing. The solution is mainly to control the electroplating current and check whether the scraper knife has a gap.

  1. Oxidation

There are signs of blackening on the copper foil line under the solder mask layer of FPC printed circuit board. The causes are that the water is not dried after wiping the board, and the surface of the printed circuit board is splashed with liquid or hand mold before solder mask printing. The solution is to visually check whether the copper foil on both sides of the printed circuit board is oxidized during screen printing.

  1. The pattern has pinholes

The reason is that there is dirt on the photographic plate, so that the part of the printed board that should see light does not see light during the exposure process, resulting in pinholes in the graphics. The solution is to regularly check the cleanliness of the photographic plate during the exposure process.

  1. Dirt on the surface

Because the FPC printed board screen printing room belongs to a clean room, there should be an electrostatic wire at the screen printing air outlet to adsorb impurities such as flying hairs in the air. Therefore, in order to reduce surface contaminants, it is necessary to fully ensure the cleanliness of the clean room and properly implement some specific measures: for example, when entering the clean room, it is necessary to fully ensure the cleanliness of the operator, prevent irrelevant personnel from walking through the clean room, and regularly clean the clean room.

  1. Uneven surface

During screen printing, the paper was not printed in time, and the residual ink of the screen was removed, resulting in uneven surface. The solution is to remove the residual ink of the screen in time.

  1. Ghosting

There are regular ink spots near the bonding pad on the entire FPC printed board. The reason is that the FPC printed board is not firmly positioned during screen printing and the residual ink on the screen plate is not removed and piled up on the printed board in time. The solution is to fix it firmly with the positioning pin and remove the residual ink on the screen plate in time.

  1. Inconsistent colors on both sides

The reason may be that the number of blades on both sides of the screen printing is very different, and there is a mixture of new and old ink. It is possible that one side of the screen printing is the new ink that has been stirred and the other side is the old ink that has been used for a long time. The solution is to avoid the above two situations as far as possible.

  1. Craze

In the FPC exposure process, the exposure is insufficient, causing small cracks on the board surface. The solution is to measure the exposure so that the comprehensive values of the exposure lamp energy, exposure time and other parameters reach 9-11 exposure levels, and no cracks will occur within this range.



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