In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder, for example, attaches very well to copper, but poorly to copper oxides (which form quickly at soldering temperatures). Flux is nearly inert at room temperature, yet becomes strongly reductive when heated. This helps remove oxidation from the metals to be joined, and inhibits oxidation of the base and filler materials. Secondarily, flux acts as a wetting agent in the soldering process, reducing the surface tension of the molten solder and causing it to better wet out the parts to be joined.
Fluxes currently available in three basic formulations. Water-soluble fluxes (no VOC's required for removal) are higher activity fluxes designed to be removed with water after soldering. No-clean fluxes which are mild enough to not require removal at all due to the non-conductive and non-corrosive residue. Performance of the flux needs to be carefully evaluated; a very mild 'no-clean' flux might be perfectly acceptable for production equipment, but not give adequate performance for a poorly-controlled hand-soldering operation.
Traditional rosin fluxes are available in non-activated (R), mildly activated (RMA) and activated (RA) formulations. RA and RMA fluxes contain rosin combined with an activating agent, typically an acid, which increases the wettability of metals to which it is applied by removing existing oxides. The residue resulting from the use of RA flux is corrosive and must be cleaned off the piece being soldered. RMA flux is formulated to result in a residue which is not significantly corrosive, with cleaning being preferred but optional.