Welding is a complex physical and chemical process. When using solder copper, with the heating of the iron head and the help of flux, the solder first produces wetting on the welding surface, and gradually diffuses to the metal copper. The adhesion layer is formed on the contact surface of solder and metal copper, and the solid and reliable welding point is formed after cooling. The process can be divided into the following three steps:

The first step is wetting. The wetting process refers to the process in which the molten solder flows around the surface of the metal to be welded by capillary force along the fine convex and crystalline gap, so as to form an adhesion layer on the surface of the metal to be welded, so that the solder atoms are close to each other, to reach the distance of atomic gravity. The environmental conditions that cause wetting are: the surface of the circuit board must be clean and free of oxides or contaminants.

The second step is diffusion. With the wetting process, the diffusion between the solder and the atoms welded by the circuit board begins to occur. Usually, the atoms are in a state of thermal vibration in the lattice lattice. Once the temperature rises, the atomic activity is intensified, which will make the melted solder and the atoms welded by the circuit board cross the contact surface into each other's lattice lattice, and the moving speed and quantity of the atoms are determined by the heating temperature and time.

The third step, metallurgical combination. Since the solder and the metal to be soldered are diffused with each other, an intermediate layer -- a metal compound -- is formed between the contact surfaces. It can be seen that in order to obtain a good solder joint, metal compounds must be formed between the metal to be welded and the solder, so that the solder joint to achieve a strong metallurgical bonding state.

To sum up, whether a certain metal can be welded and whether it is easy to weld depends on two factors: one is whether the solder used can form a compound with the weldment; The second is to remove the joint rust on the flux. When welding, solder can react with most metals (such as gold, silver, copper, iron, zinc, etc.) to produce a rather hard and brittle metal compound. This compound is the binder of solder and weldment, but there are mainly tin lead solder, using tin wire as flux and solder, saving the waste of raw materials and simplifying the process of solder.