The functional part of the Welding Flux can be divided into three: 1, removal of the welding surface of the oxide, reducing the solder melting point and surface tension as soon as possible to reach the brazing temperature. To protect the weld metal in the liquid from the impact of harmful gases in the ambient atmosphere. 3, so that the liquid solder the appropriate flow rate to fill the brazing seam. Flux melting point filler metal should be lower than the melting point of 10-30 ° C, special circumstances will the melting point of the flux is higher than brazing. The melting point of the flux too small solder premature melting of the flux composition of flux loss of activity due to evaporation, and the role of parent material so that the solder melts. The flux of choice usually depending on the nature of the oxide film. Alkaline oxide film such as: the flux of Fe, Ni and Cu oxides often acidic boron anhydride (B2O3), the acidic oxide film such as to deal with cast iron oxide film with high SiO2 commonly used with alkaline Na2CO3 of the flux makes it into the slag generated fusible Na2SiO3. Fluoride gas used as a flux, their response is uniform, no residue after welding. The BF3 often N2 mix at a high temperature brazing of stainless steel. Below 450 ℃ brazing flux as soft flux. Soft flux is divided into two, first, the water-soluble hydrochloride and phosphate single or cable Lattice salt solution constitutes a high activity, corrosion resistance, and need to be cleaned after Welding Electrodes . The other is a water-insoluble organic matter flux, usually rosin or artificial resin, by adding organic acids, organic amines or HCL or HBr salt in order to enhance the capacity and activity to film.