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 (e.g.) attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Secondarily, flux acts as a wetting agent in soldering and brazing processes.
A flux must be properly selected for the process; a soldering flux will vaporize and have no effect at the high temperatures used for welding. For hand soldering operations, 'flux-core' solder is often used. The solder is formed into a small tube, with a liquid flux in the centre. Flux is automatically delivered in the appropriate quantity as the solder is used. Similarly, brazing rods are usually available with a coating of solid flux.
Traditionally, soldering fluxes have required post-process removal due to their chemical activity, which would eventually erode the base material and result in an unreliable connection. This required the use of alcohol or other volatile solvents, which typically resulted in pollution of the air and/or the environment. Fluxes currently available include water-soluble fluxes (no VOC's required for removal) and 'no-clean' fluxes which are mild enough to not require removal at all. 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.
In some modern processes, an inert gaseous environment is used which doesn't support oxidation. These processes can obviate the need for flux.
