1 - tin staining
When hot liquid solder dissolves and penetrates into the surface of the metal to be welded, it is called metal tin or metal tin. The molecules of the mixture of solder and copper form a new kind of alloy with copper as the part and solder as the part. This kind of solvent action is called tin dipping. It forms intermolecular bond between various parts and forms a kind of metal alloy copolyester. The formation of good intermolecular bond is the core of welding process, which determines the strength and quality of welding point. Only when the surface of copper is free of pollution, and there is no oxide film formed due to exposure to air, can tin be stained, and the soldering tin and working surface need to reach appropriate temperature.
2 - surface tension
Everyone is familiar with the surface tension of water, which keeps the cold water drop on the greased metal plate spherical. This is because in this case, the adhesive force that makes the liquid on the solid surface tend to diffuse is less than its cohesion. Use warm water and cleaning agent to reduce the surface tension. Water will infiltrate the greased metal plate and flow out to form a thin layer. If the adhesion is greater than the cohesion, this will happen.
The cohesion of tin lead solder is even greater than that of water, which makes the solder appear as a sphere to minimize its surface area (in the same volume, the sphere has the smallest surface area compared with other geometric shapes to meet the needs of the lowest energy state). The effect of flux is similar to that of cleaner on metal plate coated with grease. In addition, the surface tension is highly dependent on the cleanliness and temperature of the surface. Only when the adhesion energy is far greater than the surface energy (cohesion), can the ideal tin staining occur.
3 - formation of metal alloy eutectic
The intergranular bond between copper and tin forms grains, whose shape and size depend on the duration and strength of temperature during welding. When welding, less heat can form fine crystal structure and excellent welding point with the best strength. If the reaction time is too long, whether it is due to too long welding time, too high temperature or both, it will lead to rough crystal structure, which is Sandy and brittle, with small shear strength.
Copper is used as the metal substrate, tin lead is used as the solder alloy, lead and copper will not form any metal alloy eutectic, however, tin can penetrate into copper, and the intermolecular bond between tin and copper forms metal alloy eutectic cu3sn and Cu6Sn5 on the connection surface of solder and metal, as shown in the figure.
The metal alloy layer (N phase + ε phase) must be very thin. In laser welding, the thickness of the metal alloy layer is 0.1M m. In wave soldering and manual soldering, the thickness of the inter metal bond of the excellent welding points is more than 0.5 μ M. 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 below 1 μ m, which can be achieved by making the welding time as short as possible.
The thickness of the eutectic layer of metal alloy depends on the temperature and time of forming the welding point. Ideally, the welding should be completed within about 2 s of 220 't. under this condition, the chemical diffusion reaction of copper and tin will produce an appropriate thickness of metal alloy binding materials cu3sn and Cu6Sn5 of about 0.5 μ M. Inadequate metal to metal bonding is common in cold welds or welds that do not rise to the proper temperature during welding, which may cause cutting of the welding surface. On the contrary, too thick metal alloy layer, which is common in over heated or welded joints for too long time, will result in very weak tensile strength of welded joints, as shown in the figure.
4 - tin angle
When a drop of solder is placed on a hot surface coated with flux, a meniscus will be formed. To some extent, the ability of metal surface to touch tin can be evaluated by the shape of meniscus. If there is an obvious undercut on the meniscus of the solder, such as water drops on the greased metal plate, or even tends to be spherical, the metal is not weldable. Only the meniscus is stretched to less than 30. The small angle of the welding has good weldability.
Sep 10, 2019
