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Dip tin effect
When the hot liquid solder dissolves and penetrates into the surface of the metal to be welded, it is called metal dip tin or metal dip tin. The mixture of solder and copper forms a new part of copper, partly a solder alloy, which is called dip tin, which forms intermolecular bonds between the various parts to form a metal alloy copolymer. The formation of good intermolecular bonds is the core of the welding process, which determines the strength and quality of the solder joints. Only the surface of the copper is not contaminated, not exposed to the oxide film formed in the air to tin, and the solder and the working surface need to reach the appropriate temperature.
Everyone is familiar with the surface tension of water, which forces the cold water droplets on the coated metal plate to remain spherical, since in this case the adhesion of the liquid on the solid surface tends to diffuse less than its cohesion. Wash with warm water and detergent to reduce its surface tension, the water will be coated with grease coated metal plate to the outflow to form a thin layer, if the adhesion is greater than cohesion will happen this situation.
Tin-lead solder has a greater cohesive force than water, making the solder spherical to minimize its surface area (the same volume case has the smallest surface area compared to other geometric shapes to meet the minimum energy state requirements) The The effect of the flux is similar to that of the cleaning agent on the grease-coated metal plate. In addition, the surface tension is highly dependent on the degree of cleaning and the temperature of the surface. Only when the adhesion energy is much larger than the surface energy (cohesion) tin.
When the temperature of the eutectic point of the solder is higher than about 35 ° C, when a drop of solder is placed on the surface of the hot coated flux, a meniscus is formed. To a certain extent, the ability of the metal surface to dip tin Can be assessed by the shape of the meniscus. If the solder meniscus has a clear bottom cut, shaped like a grease on a metal plate coated with grease, or even tends to be spherical, the metal is not weldable. Only the meniscus stretched into a less than 30. Of the small angle has a good weldability.