1. Remove the oxide film. The essence is that the substances in the flux undergo a reduction reaction, thereby removing the oxide film. The reaction products become suspended slag, which floats on the surface of the solder.

2. Reducing surface tension and increasing the flowability of solder can help wet solder components.

3. Prevent oxidation. After melting, it floats on the surface of the solder, forming an isolation layer, thereby preventing oxidation of the welding surface.