Filler metal refers to the filler added in or near the gap for the purpose of combining two materials (or parts). Filler metal refers to the filler used to form a weld when brazing. The melting point of the filler must be lower than that of the welded material. Suitable for welding of precise, complex, multi - collar joints and heterogeneous materials. According to the melting point, the solder can be divided into soft solder (melting point less than 450℃) and hard solder (melting point more than 450℃) high-temperature solder (melting point more than 950℃). According to the composition of the solder solder solder tin base, lead base, zinc base and other solder. The brazing metals include aluminum base, silver base, copper base and nickel base. In welding, the filler metal wets the base material and forms a structure such as solid solution with the base material to realize the tight bonding of the welding material. Melting point, wettability, joint strength and other factors must be considered when choosing brazing metals, which have a direct impact on brazing quality. Due to the fact that the properties of brazing alloys not only affect the technological properties of brazing but also determine the joint properties of brazing to a large extent, the selection of brazing alloys among various brazing alloys should be determined from the perspectives of joint use requirements, mutual matching of brazing alloys and base materials, brazing heating technology and economic cost. Starting from the joint use requirements, for the brazing joint strength requirements are not high, can be brazing with soft solder; for the brazing joint strength requirements are relatively high, should choose the brazing solder brazing. For solders brazing at low temperatures, use a solder with a low tin content or a filler that prevents the occurrence of cold brittle elements such as antimony. For joints working at high temperature, solder with good high-temperature strength and oxidation resistance should be selected, such as nickel-based solder. The lead base solder containing a small amount of Ag and Sn has good flooding property and filling capacity, high melting point, high temperature strength and working temperature up to 250℃, suitable for soldering iron brazing and flame brazing of copper, copper alloy and steel. Welding temperature 798℃-850℃. Depending on the welding temperature, the ratio of tin to copper is also different Physical: Tin (Sn99.95) : melting point 232℃, density 7.29g/cm3. Silver: density 10.5g/cm3, melting point 960.8℃.