Soldering is a science. The principle of electric iron welding is to heat and melt the solid solder wire through a heated soldering iron, and then use the action of flux to make it flow between the metals to be soldered, and form a firm and reliable solder joint after cooling. .
When the solder is a tin-lead alloy and the soldering surface is copper, the solder first wets the soldering surface. With the occurrence of the wetting phenomenon, the solder slowly diffuses to the metal copper, forming an adhesion layer on the contact surface between the solder and the metal copper. The two are firmly combined. So soldering is accomplished through the three physical and chemical processes of wetting, diffusion and metallurgy.
1. Wetting: The wetting process refers to the fact that the molten solder flows around the surface of the base metal with the help of capillary force along the subtle unevenness and crystalline gaps on the surface of the base metal, thereby forming an adhesion layer on the surface of the base metal to be welded. The atoms of the material and metal are close to each other, reaching the distance at which the atomic gravitational force acts.
Environmental conditions that cause wetting: The surface of the base metal to be welded must be clean and free of oxides or contamination.
Image metaphor: drop water on the lotus leaves to form water droplets, that is, water cannot wet the lotus. When water is dropped on cotton, the water penetrates into the cotton, that is, water can wet the cotton.
2. Diffusion: With the progress of wetting, the mutual diffusion between the solder and the metal atoms of the base metal begins to occur. Usually atoms are in a thermal vibrational state in a lattice lattice, once the temperature is raised. The intensified atomic activity causes the atoms in the molten solder and the base metal to cross the contact surface and enter the lattice lattice of each other. The moving speed and number of atoms are determined by the heating temperature and time.
3. Metallurgical bonding: Due to the mutual diffusion of the solder and the base metal, an intermediate layer---metal compound is formed between the two metals. To obtain a good solder joint, a metal compound must be formed between the base metal to be welded and the solder. Thereby, the base metal can reach a firm metallurgical bonding state.
