In the appearance quality control of circuit board solder mask, the color of solder mask is a more important control point. For a production board using the same type of solder mask ink, the board surface solder mask has obvious color difference, which is generally not accepted by customers. So what causes the color difference of solder mask? Are there any preventive measures? The following editor will talk about it in detail.
Causes of color difference in solder mask
1. Part of the batch board is overprinted during the production process. The overprinted board has a difference in the thickness of the solder mask, resulting in a color difference with the normal printed board, and the phenomenon of color depth appears.
2. Improper placement of the board in the oven, resulting in uneven partial heating of the board and color difference.
3. When the solder mask is printed, the baking time is too long, which causes the solder mask to be overheated and the color changes.
4. For boards with green ink and yellow ink, after spraying tin, part of the boards are reworked and re-sprayed tin. Due to the impact of high temperature, the color of the solder mask will change, so the solder mask color of the re-sprayed tin board is different from the normal board resistance. The welding color produces chromatic aberration.
5. Some of the boards to the final inspection were repaired and refueled, and the final inspection took too long to bake the boards again, resulting in color difference between the solder mask of the board and the solder mask of the normal production board.
6. The customer has no special requirements for the color of the solder mask. Due to the characteristics of the green and yellow solder masks, the control is relatively large in the process, and color differences are prone to occur. Therefore, the final inspection is not controlled during the inspection, and the normal board is output. goods.
Preventive measures for solder mask color difference
1. If the board needs to be reworked and solder mask, the original solder mask must be removed in the process, and then the solder mask printing should be performed again according to the normal board production process. It is not allowed to directly perform solder mask overprinting on the solder mask printed board.
2. When placing the boards, employees must strictly follow the drying board placement requirements stipulated by the process to make the boards evenly heated and avoid local color difference.
3. When baking the board, each model must record the baking time and the time of the board. When the baking time is reached, the board must be taken out and cooled immediately. Process QA conducts online audit.
4. It is stipulated that only one re-spray tin is allowed for rework. After spraying tin, the solder mask color must be compared with the original board. If the solder mask color is significantly different, it will be scrapped.
5. It is stipulated that the baking time must be recorded when the final inspection repairs the oil replenishment board and the board is baked again to avoid long-time baking of the board, which may cause the solder mask color to change and cause the solder mask color difference.
6. If the customer has special requirements for the solder mask color, it is recommended that the customer make a note in the production instructions.
7. Due to the characteristics of the green and yellow inks, it is difficult to control in the manufacturing process. It is recommended that customers provide standards for acceptance of solder mask color differences, and the final inspection is carried out in accordance with the standards at the time of shipment.