When hot liquid solder dissolves and penetrates the surface of the metal to be welded, it is called tin-stained or tin-stained metal. The molecules of the mixture of solder and copper form a new kind of alloy that is part copper and part solder. This solvent action is called tin-dipping. It forms intermolecular bonds between the various parts and produces a metal alloy co-compound. The formation of good intermolecular bonds is the core of welding technology, which determines the strength and quality of welding points. Only the surface of the copper is not contaminated, no oxide film formed by exposure to the air can be tin, and the solder and the working surface need to reach the appropriate temperature.
Everyone is familiar with the surface tension of water, the force that keeps a cold drop of water on a greasy metal plate spherical because, in this case, the adhesion that makes the liquid tend to diffuse over a solid surface is less than its cohesion. Clean with warm water and detergent to reduce the surface tension. The water will soak the greased metal plate and flow outward to form a thin layer. This can occur if the adhesion is greater than the cohesion.
Tin-lead solder has an even greater cohesion than water, making the solder a sphere with a minimum surface area (for the same volume, the sphere has a minimum surface area relative to other geometry to satisfy the minimum energy state). The effect of flux is similar to that of detergent on a greased metal plate. In addition, the surface tension is highly dependent on the cleanliness and temperature of the surface. Only when the adhesion energy is much greater than the surface energy (cohesion), can ideal tin-staining occur.
The metal bond between copper and tin forms grains whose shape and size depending on the duration and strength of the temperature at which they are welded. With less heat, fine crystal structure can be formed and excellent welding points with the best strength can be formed. Too long reaction time, whether due to too long welding time, too high temperature or both, will lead to a rough crystal structure, which is gravelly and brittle with little shear strength.
Copper is used as the metal base material, and tin-lead is used as the solder alloy. Lead and copper will not form any metal alloy compounds, but Tin can permeate into copper. The intermolecular bonds of tin and copper form metal alloy compounds Cu3Sn and Cu6Sn5 on the connection surface of solder and metal.
The metal alloy layer (Cu3Sn + Cu6Sn5) must be very thin. In laser welding, the thickness of the metal alloy layer is of the order of 0.1mm. In wave soldering and manual soldering, the thickness of the metal-to-metal bond at the best welding points is more than 0.5 m. Because the shear strength of the weld point decreases with increasing thickness of the metal alloy layer, it is often attempted to keep the thickness of the metal alloy layer below 1 m by making the welding time as short as possible.
The thickness of the metal alloy co-compound layer is dependent on the temperature and time at which the welding point is formed. Ideally, the welding should be completed within about 220’t 2second. Under this condition, the chemical diffusion reaction of copper and tin will produce an appropriate amount of the metal alloy binding materials Cu3Sn and Cu6Sn5 with a thickness of about 0.5 m. Inadequate intermetallic bonding is common at cold solder joints or welding points that do not rise to the appropriate temperature and may cause the weld surface to be cut off. Conversely, a layer of metal alloy that is too thick, which is common at points that are overheated or welded for too long, will result in a very weak tensile strength at the point.
When a drop of solder is placed on a hot, flux-coated surface at a temperature about 35 ° C higher than the eutectic point of solder, a meniscus is formed. The ability of the metal surface to stick tin can be assessed in part by the shape of the meniscus. If the surface of the solder crescent has a distinct bottom cut, shaped like a bead on a greased metal plate, or even tends to be spherical, the metal is not weldable. Only the meniscus stretches into less than 30. A small Angle is required for good weldability.
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