1. The surface of the welding area is contaminated, coated with flux, or metal compounds are generated on the surface of the SMT component. It can cause poor wetting. Sulfides on the surface of silver and oxides on the surface of tin can cause poor wetting.

2. When the residual metal in the solder exceeds 0.005%, the activity of the solder decreases and poor wetting occurs.

3. During wave soldering, there is gas on the surface of the substrate, which can also lead to poor wetting.

The methods to solve poor wetting include:

1. Strictly implement the corresponding welding process.

2. PCB circuit boards and component surfaces should be cleaned properly.

3. Select the appropriate solder and set a reasonable welding temperature and time.