How can defects in SMT stencil soldering be solved?

 The presence of SMT stencil indicates that the process is not completely correct and that there is a risk of short-circuiting in the electronics and therefore needs to be eliminated. The internationally accepted standard for the presence of solder balls is that printed circuit assemblies should not have more than five solder balls in a range of 600. There are various reasons for the generation of tin balls and the root cause of the problem needs to be found. Tin balls in wave soldering Tin balls often appear in wave soldering for two main reasons.
First, due to welding the printed board, the moisture near the through-hole on the printed board is heated and turned into steam. If the hole wall metal plating is thin or void, water vapor will be excluded through the whole wall, if there is solder in the hole, when the solder solidifies water vapor will produce voids in the solder (pinhole), or extrusion of solder in the front of the printed board to produce tin balls.

Second, in the printed SMT stencil the opposite side (i.e., the side in contact with the wave crest) generated by the tin ball is due to improper setting of some process parameters in wave soldering. If the amount of flux coating or preheating temperature is set too low, it may affect the evaporation of the components within the flux, when the printed board into the wave crest, the excess flux by high-temperature evaporation, the solder from the tin bath splash out, in the printed board surface to produce irregular solder balls.

For both sides of the above reasons, we take the following corresponding solution measures.

First, the appropriate thickness of metal plating in the through-hole is very critical, the minimum copper plating on the whole wall should be 25um, and no gaps.

Second, the use of spray or foaming type coating flux. The foaming method, in adjusting the air content of the flux, should be maintained as far as possible to produce the smallest possible bubbles, foam and PCB contact surface is relatively small.

Third, the wave soldering machine preheats zone temperature should be set so that the temperature of the top surface of the circuit board to at least 100 ° C. Proper preheat temperature not only eliminates solder balls but also avoids deformation of the circuit board by thermal shock.