1, removal of the welding surface of the oxide, reducing the solder melting point and the 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 should be lower than the melting point of brazing 10-30 ° C, under special circumstances allows the  Welding Wire point higher than the brazing. The melting point of the flux too small solder premature melting 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  Welding Flux  makes it into the slag generated fusible Na2SiO3.