The fluxes used for brazing copper joints are different in composition from soldering fluxes. The two types cannot be used interchangeably.
Unlike soldering fluxes, brazing fluxes are water based. Similar to soldering fluxes, brazing fluxes dissolve and remove residual oxides from the metal surface, protect the metal from reoxidation during heating and promote wetting of the surfaces to be joined by the brazing filler metal.
Brazing fluxes also provide the craftsman with an indication of temperature (Figure 14.6b). If the outside of the fitting and the heat-affected area of the tube are covered with flux (in addition to the end of the tube and the cup), oxidation will be minimized and the appearance of the joint will be greatly improved.
The fluxes best suited for brazing copper and copper alloy tube should meet AWS Standard A5.31, Type FB3-A or FB3-C.
Figure 14.7 illustrates the need for brazing flux with different types of copper and copper-alloy tube, fittings and filler metals when brazing.