Solder Mask Process in the PCB Board Production Process: Repairing Board

The following is the third operation procedure of solder mask process in the PCB board production process: repairing the board.

Repairing the board includes two aspects, to repair the defects of the image, and to remove the defects that are not related to the required image. In the process of repairing, you should pay attention to wearing gauze gloves to prevent hand sweat from contaminating the board.
Common board defects are:
1. Jumpyin. The main reason is that the electroplating current is too large and the plating layer is thick, which causes the graphic lines to be too high. In the screen printing of the printed board, because the squeegee blade and the screen printing frame are at a certain angle, the lines are too high on both sides of the line, there is no ink, causing skip printing. Another reason is that the squeegee blade is notched, and the ink does not fill in the gap, causing skipping. The main solutions are to control the electroplating current and check whether the squeegee blade has a gap.

2. Oxidation. There are signs of blackening on the copper foil lines under the solder mask of the printed board. The cause is that the water is not dried 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. The method is to visually inspect the copper foil on both sides of the printed board for oxidation during screen printing.

3. The surface is uneven, and the paper is not printed in time during the screen printing, and the residual ink on the screen is removed, which causes the surface to be uneven. The solution is to print the paper in time to remove the residual ink on the screen.

4. Solder mask in the hole. The cause is that the paper was not printed 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. The solution was to print the paper in time and the number of screen meshes was too low. It will also cause solder resistance in the hole. It is necessary to select a high mesh screen to make a plate. The viscosity of the printing material is too low. Change to a printing material with a high viscosity. The edge of the blade becomes rounded, and the blade edge of the scraper is sharpened.

5. The graphics have pinholes. The reason is that there is dirt on the photographic plate, so that the part of the printed board that should be exposed to light during the exposure process is not visible, causing the pattern to have pinholes. The solution is to frequently check the cleanliness of the photographic plate during the exposure process.

6. Dirt on the surface. Because the printed board screen printing room is a clean room, there should be an electrostatic wire at the screen printing air outlet to absorb the fly and other debris in the air. Therefore, in order to reduce the surface dirt, it is necessary to fully ensure the cleanliness of the clean room and implement some specific measures appropriately: such as entering the clean room to fully ensure the cleanliness of the operator, avoiding irrelevant personnel from passing through the clean room, and cleaning the clean room regularly.

7. The colors on both sides are inconsistent. The cause may be that the number of knives printed on the two sides is very different, and there is also a mixture of new and old inks. It may be 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. The solution is to try to avoid the above two situations.

8. Cracked. Due to insufficient exposure during the exposure process, resulting in small cracks on the board surface, the solution is to 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, within this range, there will be no cracks.

9. Bubble. Bubbles are generated between the lines of the printed board or the side of a single line after development. 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 be too high. When the squeegee is in contact with the line, the angle between the squeegee and the line increases, so that the solder mask cannot be printed on the root of the line, and the side of the line root and the solder mask layer exists gas, and bubbles are generated after heating. The solutions are: visually check whether the screen printing material is completely printed on the substrate and the sidewall of the line during screen printing, and strictly control the current during electroplating.

10. Ghosting: There are regular ink dots next to the pads on the entire printed circuit board. The reason for this is that the printed board is not firmly positioned during screen printing and the residual ink on the screen is not removed in time and accumulated on the printed board. The method is to fix the paper firmly with positioning pins and remove the residual ink on the screen in time.

In the process of revision, because some printed boards have serious defects that cannot be repaired, use sodium hydroxide aqueous solution to dissolve the original solder resist, and then rework after re-screen printing and exposure. If the printed board defect is small, such as a small copper dew point, which can be carefully repaired with a fine brush moistened and adjusted solder mask.