Wetting

Wetting is the leading role in soldering, and its bonding is made by wetting substrates using liquid solders to achieve the bonding effect. This phenomenon is just like water being poured on the surfaces of solids. What is different is that after temperature drops, the solder will be condensed and forms welds. When solders wet the substrates, both are bonded by chemical bonds and form a kind of successive bonding. However, as the case may be, substrates are eroded by air and surrounding environments, hence a coat of oxide. The oxide coat will stop solders from achieving the wetting effect. The phenomenon is just like water being poured on a plate full of grease, and water is only concentrated on part of places and it is unable to be well distributed on the plate. If we are unable to remove the oxide coat, even it is reluctantly soldered, the bonding strength is very weak. When metal plates applied with grease are dipped in water, there is no wetting. No matter how thin the grease is applied to the surface, it will be completely invisible. But water will become sphere shaped water drops and will drop the time it is shaken. Therefore, water is not wetted or stuck to the thin metal plates. If metal plates are placed in cleaning agents to be rinsed and are made dry with care, and then dipped in water, the liquids will be completely spread on the surfaces of such metal plates forming a thin and well distributed film. It will not drop however you shake it, that is, it has already wetted such a metal plate. Therefore, when soldering surfaces and metal surfaces are also very clean, solders will also wet the surfaces of such metal. The requirement of the cleaning level is more than the water on metal, for there must be close bonding between solders and metals. The extremely thin oxide coat will also hinder the solder wetting effect on metal surfaces.

The soldering flux is just like the solvents to the metal plates applied with grease, in which the solvents remove grease and allow water to wet the metal surface and reduce the surface tension. The function of soldering flux is (1) to clear oxide on metal surfaces, 92) to reduce tension on the solder surface so as to enhance the diffusion capacity.

Wetted thermodynamic equilibrium

When solder wetting is wetted on the metal at the basic level, when it becomes still, that is the thermodynamic state. The following graph is the chart of the solder thermodynamic state.