In the printed circuit board, the solder mask process is a printed board with solder mask after screen printing. Cover the pads on the printed board with a photo master so that it will not be exposed to ultraviolet light during exposure, and the solder resist protection layer is more firmly attached to the printed board surface after ultraviolet light, and the pads are not exposed to ultraviolet light. Light irradiation can expose the copper pads so that lead and tin can be applied during hot air leveling.
The solder mask process can be roughly divided into three operating procedures:
The first procedure is exposure. First, check whether the polyester film and glass frame of the exposure frame is clean before starting the exposure. If they are not clean, wipe them with an anti-static cloth in time. Then, turn on the power switch of the exposure machine, and then turn on the vacuum button to select the exposure program and shake the Exposure shutter. Before starting the formal exposure, let the exposure machine “empty exposure” five times. The function of “empty exposure” is to make the machine enter a saturated working state, and the most important thing is to make the energy of the ultraviolet exposure lamp enter the normal range. If you do not “empty exposure”, the energy of the exposure lamp may not enter the best working state. It will cause problems with the printed board during exposure. After five times of “empty exposure”, the exposure machine has entered the best working condition. Before using the photographic plate for alignment, check whether the quality of the plate is qualified. Check whether there are pinholes and exposed parts on the film surface of the base plate and whether it is consistent with the graphics of the printed board, because this will check the photo base to avoid rework or scrapping of the printed board due to unnecessary reasons.
The solar solder mask usually adopts visual positioning, using a silver salt master, aligning the pad of the master with the pad hole of the printed board, and fixing it with tape for exposure. There are many problems encountered in alignment. For example, because the master plate is related to factors such as temperature and humidity, if the temperature and humidity are not well controlled, the photographic master plate may be reduced or enlarged and deformed. In this way, the photographic plate and the printed circuit board pads are not completely consistent during the solder mask. When the bottom plate is reduced, see how much the difference between the bottom plate pad and the printed board pad is. If the difference is small, lead and tin can be applied during hot air leveling, then there is no big problem for selenium resist soldering. If there is a big difference, only re-pirate, try to make the bottom plate land overlap. Before aligning, you should also pay attention to whether the medicated film surface of the base plate is turned upside down. Make sure that the medicated film surface is facing down when it is aligned. If the film surface is facing upwards when it is aligned, the film surface will be scratched, resulting in exposure of the bottom plate, so that the printed board does not need to be exposed to solder resist, which will seriously cause the printed board to be scrapped.
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