Welding is the process of connecting metal parts with various fusible alloys (solder). The melting point of solder is lower than that of the material to be welded, so that the parts will end the welding through intermolecular contact on their surface without being melted.

       Welding can be divided into soft welding and hard welding. The temperature of soft welding is lower than 450 ℃ and that of hard welding is higher than 450 ℃. Hard welding is usually used for silver, gold, steel, copper and other metals. Its welding point is much stronger than soft welding, and its shear strength is 20 ~ 30 times that of soft welding. The term welding is usually used for both of the above thermal connections, because in both cases, molten solder is written into a slender gap on the clean and close solid metal surface of the two devices to be installed.