The cohesive force of tin lead solder is even greater than that of water, which makes the solder sphere to minimize its surface area. The surface tension is also highly dependent on the cleanliness and temperature of the surface. Only when the adhesion energy of the circuit board is far greater than the surface energy (cohesion), can the ideal tin adhesion occur.
- Production of metal alloy compounds
The intermetallic bond between copper and tin forms grains. The shape and size of grains depend on the duration and strength of the temperature during welding. Less heat during welding can form fine crystalline structure, forming excellent welding points with the best strength.
- Tinned corner
When the eutectic point temperature of solder is about 35 ℃ higher than that of solder, a meniscus is formed when a drop of solder is placed on the hot surface coated with flux. To some extent, the ability of metal surface to stick tin can be evaluated by the shape of the meniscus.
When the hot liquid solder dissolves and penetrates into the metal surface to be welded, it is called metal tin or metal tin. The molecules of the mixture of soldering tin and copper form a new kind of alloy that is partly copper and partly solder. This solvent action is called dipping tin, which forms intermolecular bonds between various parts to form a metal alloy compound. The formation of good intermolecular bond is the core of welding process, which determines the strength and quality of welding points. Only the copper surface is free from pollution, and no oxide film formed due to exposure to air can be stained with tin, and the soldering tin and working surface need to reach appropriate temperature.
- Copper is used as the metal substrate, and tin lead is used as the solder alloy. Lead and copper will not form any metal alloy complexes. However, tin can penetrate into copper. The intermolecular bond between tin and copper forms metal alloy complexes Cu3Sn and Cu6Sn at the junction surface of solder and metal.
As the shear strength of the welding point decreases with the increase of the thickness of the metal alloy layer, it is often tried to keep the thickness of the metal alloy layer at 1 μ M below, which can be achieved by making the welding time as short as possible.