Flux

Flux is a chemical agent which helps for soldering, brazing and welding of metals by removing the oxide layer from the metal parts. Different fluxes, mostly based on sodium chloride, potassium chloride, and a fluoride such as sodium fluoride, are used in foundries for removing impurities from molten nonferrous metals such as aluminum, or for adding desirable trace elements such as titanium.

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. Additionally, flux allows solder to flow easily on the working piece rather than forming beads as it would otherwise.

In soldering of metals, flux serves a threefold purpose: it removes oxidation from the surfaces to be soldered, it seals out air thus preventing further oxidation, and by facilitating amalgamation improves wetting characteristics of the liquid solder. Flux is corrosive, so the parts have to be cleaned with a damp sponge or other absorbent material after soldering to prevent damage.

There are following types of Flux:

R ( Rosin)

RA (Rosing Activated)

RMA (Rosin Mildly Activated)

WS ( Water Soluble)

Any of these categories (except WS) may be no-clean, or not, depending on the chemistry selected and the standard that the manufacturer requires.

The function of flux is primarily to remove oxide, with the general formula being:

Metal oxide + Acid → Salt + Water