The principle of welding is based on solid bonding between metals, and the specific formation process involves several key aspects. First, the metal at the connection site is heated to a molten state, which is called melting core. When the temperature is high enough, the surface tension of the material being welded is reduced, allowing the molten metal (solder) to wet the substrate and solidify to form a strong contact as the temperature decreases.

The formation process of solder joints is affected by many factors, including welding time, welding current and welding pressure. Appropriate welding time and welding current can ensure the size of the molten core and the strength of the solder joint. At the same time, with the increase of welding pressure, the solder joint strength will decrease accordingly.

Reflow welding is a common welding method, which relies on the action of hot gas on the solder joint to make the flux physically react at a certain high temperature, so as to realize the welding of surface mounted devices (SMD). The lead-free solder paste used in the reflow process, such as SN96.5AG3.0Cu0.5tin-silver-copper alloy, needs to be carried out under a specific temperature curve to ensure welding results.

In short, the formation of the solder joint depends on the melting, wetting and subsequent curing of the metal during the welding process, which is affected by a combination of factors such as time, current, pressure and temperature