1. It may be because the soldering iron is dirty. Wipe the welding head with a wet sponge before welding;

2. It may be that the flux is used too much (the general solder wire is hollow and filled with flux). After welding, it is best to wipe with alcohol cotton balls, which will increase the smoothness of solder joints;

3. Another possibility is that the soldering iron temperature is too high and the welding time is too long. If the temperature is around 370 degrees, the time is preferably within 3 seconds.

I hope my answer is helpful! ! ! !