Summary of common problems in PCB dry area (circuit, solder mask, text) (3)

21. What is the cause of excessive solder mask development?

Answer: 1. The exposure energy is too low, 2. The developing speed is too slow, the pressure is too high, the concentration of the medicine is too high or the temperature of the developing cylinder is too high.

22. Why do we need to put the board with the green oil bridge facing down when solder mask is developed?

Answer: Because the width of the solder mask green oil bridge is generally small (minimum 0.08mm), it is relatively poor in resistance to the attack of developing potions. When the green oil bridge faces down, it sprays from the nozzle onto the board. The potion only has one impact force. If the potion sprayed upward from the nozzle onto the board will bounce and hit the upper wall of the developing cylinder, it will form another impact on the board, which will easily cause a broken green oil bridge. Therefore, the solder mask development green oil bridge should be placed downward.

23. What are the reasons for green oil foaming?

Answer: Yes: 1. Poor pre-treatment of the solder mask (too fast, too low temperature) caused the moisture in the hole to not be completely dried; 2. There is gas in the bad hole of the plug hole, 3. The solder mask oil is too thin, 4. Before the character, the plate is not pre-baked in segments or the post-curing time is too long, causing the green oil to become brittle. 5. The temperature of the processing equipment in the subsequent process is too high.

24. What are the conventional items for testing solder mask inks?

Answer: 1. Hardness test (6H pencil), 2. Acid, alkali, solvent (10%) resistance for 30 minutes at room temperature, 3. Adhesion (3M-600# glue), 4. Resistance to gold, tin, spray Tin, 5. Thermal shock resistance (288°C ± 5°C for 10 seconds ± 1 second), etc.

25. What are the different effects of the solder mask diagonal shift screen printing machine and the front and back, left and right shift screen printing machines in production?

Answer: When the screen printing machine that moves forward and backward and that moves left and right are in production, if the holes on the board to be printed are in a row or a row, it is easy to cause green oil to enter the holes, and at the same time, it is easy to pollute The left and right sides of the board are extremely inconvenient for the printer to hold the board, and the basic performance of the diagonally moving silk screen is superior to the first two. If the holes on the board to be printed are in a row or a row. The rows are horizontal, and the diagonal shift will not cause the holes to overlap and cause oil to enter the holes.

26. Why should PCB dry film be developed?

Answer: Adjust the speed of developing parameters by making developing points. If you don't do the development point, it is impossible to determine the different development conditions of each dry film, and at the same time do not know the best development ability of the developing machine used under certain parameter conditions. Usually the development point is controlled at 40%-60%.

27. Why use alkaline washing and acid washing when maintaining the developing machine?

Answer: The first alkali washing is to clean the residual dirt and green oil in each tank, and the acid washing is to absorb and neutralize the residual alkaline substances on the tank wall and nozzles and further clean.

28. How to improve the accuracy of solder mask alignment?

Answer: 1. Strengthen the training of the staff's practical operation skills, 2. Use a tenfold mirror to assist in checking the accuracy of the alignment during alignment, 3. Control the service life of the film, 4. Control the temperature in the darkroom within the required range To prevent film deformation, 4. Design alignment accuracy marks on the four corners of the board.

29. Why should the board be pre-baked before the PCB alignment exposure? And is it low temperature (75±5℃) baking?

Answer: The pre-baking of the board before the alignment exposure: it is to more fully evaporate the solvent in the ink, and it is also to prevent the ink on the board surface from sticking to the film without initial curing during the alignment, causing the board surface to drop oil and film pollution , Drop point. Solder mask liquid photosensitive ink is easy to be cured when baked at a temperature of >85°C. At the same time, due to its viscosity and fluidity during processing and printing, some pads and SMT are higher than the part of the substrate and the ink in some component holes It will be thinner. If it is baked at a temperature higher than 80°C, it will be completely cured. In addition, the high-energy ultraviolet light during the alignment exposure will cause the ink in this part to be completely cross-linked. It is not easy to be dissolved by the sodium carbonate solution, resulting in unclean development.

30. How to improve the film print on the board surface after solder mask registration exposure?

Answer: 1. Properly lengthen the pre-bake time of the board before aligning (not increase the temperature), 2. Appropriately reduce the vacuum degree of the exposure machine (10-15%), 3. Reduce a certain exposure energy, 4. Control The cleaning quality and service life of the film surface. 5. Control the frequency of scrubbing should not be too much.

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