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There are several reasons why the circuit board is connected with tin. You need to check the specific reasons yourself:

by:Rigao      2020-05-08
1. There is no design solder mask between the circuit board pads, which are connected after being printed with solder paste; The solution is to change the design and add a solder mask dam/bridge; 2, solder paste printing deviation, and then tin after reflow soldering; 3. The circuit board itself is designed with a solder resist dam/bridge, but some or all of it is lost when the finished product is made, so it is easy to connect tin; 4. No flux is used or the effect of flux is not good. The surface tension of tin in molten state is not released, resulting in easy tin connection. When we use wave soldering to weld circuit boards, there will be tin connection. What is the reason? Let's talk about the reasons. If this situation has not been used when wave soldering has been used before, then the impurity of solder should be analyzed first. If the impurity is high, the melting point of solder will become high, therefore, it seems that we can solve the problem by raising the solder temperature. If similar continuous welding can be avoided, it is basically certain that the impurities exceed the standard. Generally speaking, the main reason is that the copper impurity content is too high. If there is such a problem with this type of plate at the beginning, the flux or the direction of the plate should be checked. Flux is mainly a flux with high solid content and stronger activation performance, which will be better. Seeing the rosin residue on the surface of the flux board, it is speculated that the flux used may be a cleaning-free flux, while the board itself is a flux precoated with rosin, which leads to uneven Rosin residue after wave soldering.
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