With the exception of the assembly of components in copper with copper-phosphorus alloys, all other brazing operations require the use of a brazing alloy in conjunction with a suitable flux.
The purpose of the flux is:
to attack the surface of the metals to be joined, reduce any oxides on the surface and unable the brazing alloy to wet the surface,
to protect both the brazing alloy and the surfaces to be joined from oxidation during the brazing process.
The active range of flux must extend over the melting range of the brazing alloy chosen. The choice of the flux will depend also upon the metals to be joined.
Fluxes are corrosive and strict attention must be paid to health and safety requirements.