Tin dipping
When the hot liquid solder dissolves and penetrates to the surface of the metal to be welded, it is called "tin" or "tin". A mixture of molecules with copper solder to form a new part is copper, part of solder alloy, the solvent effect is called tin, it forms intermolecular bonding between the various parts, a metal alloy is natural compounds. The formation of good intermolecular bonds is the core of the welding process, which determines the strength and quality of the welding points. Only the surface of copper is free from contamination and there is no tin film that can be exposed to the oxide film that is exposed to the air and the solder and the working surface need to be properly heated.

Surface tension
Everyone is familiar with the surface tension of water, the force of coating metal plate with grease on the water droplets to keep ball, this is because in this case, the solid surface tends to be less than the liquid diffusion adhesion cohesion. Wash with lukewarm water and detergent to reduce the surface tension. The water infiltrates the coated metal plate and forms a thin layer toward the outside. If the adhesion is greater than the cohesion, this will happen.
Tin lead solder cohesion and even greater than water, the solder is sphere, to minimize surface area (the same volume, the sphere and other geometric shape compared with minimum surface area, to meet the needs of the lowest energy state). The effect of flux is similar to the cleaning agent on the metal plate coated with grease effect, in addition, the surface tension is also highly depends on the cleanliness and surface temperature, only the adhesion energy is far greater than the surface energy (cohesion), can be the ideal tin.

The production of metal alloy compounds
The intermetallic bonds between copper and tin form grains, and the shape and size of the grains depend on the duration and strength of the welding. When welding, less heat can form a fine crystalline structure, forming a good welding point with the best strength. The reaction time is too long, no matter whether the welding time is too long or the temperature is too high or both of them will lead to rough crystalline structure, which is Sandy and brittle and has little shear strength.
Using copper as base metal, tin lead alloy as solder, lead and copper will not form any metal alloy co compounds, however tin can penetrate into copper, tin and copper bonding between molecules and metal in the solder connection surface forming metal alloy were Cu3Sn and Cu6Sn5.
A metal alloy layer (n + phase) must be very thin, laser welding, the number of metal alloy layer thickness is 0.1mm, wave crest welding and manual welding iron, excellent weld metal bonding between the thickness of the majority of more than 0.5 m. Due to the shear strength of the welding points decreases with the increase of the metal thickness of the alloy layer, so do try to keep the metal alloy layer thickness below 1 m, this can make the welding time as short as possible to achieve.
The metal alloy co substrate layer thickness depends on the formation of welding temperature and time, ideally, the welding should be completed in about 220't 2S, under this condition, the chemical diffusion reaction of copper and tin will have a combination of material Cu3Sn and Cu6Sn5 thickness is about 0.5 m the amount of metal alloy. Inadequate metal bonding is common at cold welding points or when welding does not rise to the point where the temperature is appropriate. It may cause welding surfaces to be cut off. In contrast, too thick metal alloy layers are common in over heating or welding for too long welding points, which will lead to very weak tensile strength of welding points.

wetting angle
Than the solder eutectic temperature higher than about 35 DEG C, when placed in a drop of solder surface coated with the heat flux, the formation of a meniscus, to some extent, can be evaluated by the meniscus shape metal surface stained tin. If the solder meniscus has an obvious undercut edge, such as water droplets on the coated metal plate or even tends to be spherical, the metal is not weldable. Only the meniscus stretched into less than 30. Small angle to have good weldability.