There must be a welding problem in the welding area. This kind of black slag is generally caused by the lack of argon protection during welding. If there is oil stain, rust and oxide layer on the weld, it will lead to too much welding slag, and a small amount is normal.
Precautions for welding:
If the welding time is too short, the solder joint temperature is too low, the solder joint is not completely melted, and the solder joint is rough, which will easily lead to virtual welding. On the contrary, the welding time is too long, the solder is easy to flow, and the components are overheated and damaged. The amount of tin on the solder joint should not be too small. If it is too small, the welding is not firm and the mechanical strength is too poor.