Flux (metallurgy)
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- For other uses of this term, see Flux (disambiguation).
In metallurgy, flux is a substance which facilitates soldering, brazing, and welding by chemically cleaning the metals to be joined. Common fluxes are: ammonium chloride or rosin for soldering tin; hydrochloric acid and zinc chloride for soldering galvanized iron (and other zinc surfaces); and borax for brazing, and welding ferrous metals.
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Soldering
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. Several types of flux are used in electronics:
A number of Standards exist to define the various flux types. The principle standard is J-STD-004.
J-STD-004 characterises the flux by type (e.g. Rosin (RO), Resin (RE), Organic (OR), Inorganic (IN)), its activity (strength of fluxing) and reliability of residue from a surface insulation resistance (SIR) and electromigration standpoint, and whether or not it contains halide activators.
This replaces the old MIL QQS standard which defined fluxes as:
| R | (Rosin) |
| RMA | (Rosin Mildly Activated) |
| RA | (Rosin Activated) |
| WS | (Water soluble) |
Any of these catergories (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 → Metal + Salt + Water
Salts are ionic in nature and can cause problems from metallic leeching or dendrite growth, with possible product failure. In some cases, particularly in high-reliabiliy applications, flux residues must be removed.
J-STD-004 includes tests for electromigration and surface insulation resistance (which must be greater than 100 MΩ after 168 hours at elevated temeprature and humidity with a DC bias applied).
Dangers
Acid flux types (not used in electronics) may contain zinc chloride or ammonium chloride, both of which are harmful to humans. Therefore, flux must be handled with gloves and goggles, and used with adequate ventilation.
Smelting
A related use of the term flux is to designate the material added to the contents of a smelting furnace or a cupola for the purpose of purging the metal of impurities, and of rendering the slag more liquid. The flux most commonly used in iron and steel furnaces is limestone, which is charged in the proper proportions with the iron and fuel. The slag is a liquid mixture of ash, flux, and other impurities.