Flux (metallurgy)
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- For other uses of this term, see Flux (disambiguation).
In metallurgy, flux is a substance which removes passivating oxides from the surface of a metal or alloy. Fluxes are routinely used to facilitate 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 or braze-welding ferrous metals.
In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to remove oxides from the metal surface and prevent oxidation of the base and filler materials. Tin-lead solder (e.g.) easily wets copper, but does not wet oxides of copper, which form quickly at soldering temperatures. Fluxes dissolve metal oxides and often provide a reducing atmosphere at elevated temperatures, preventing the further oxidation of the base material. The clean, activated metal surface is easily wetted by the molten solder or brazing alloy.
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[edit] 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 principal 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 temperature and humidity with a DC bias applied).
[edit] 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.
[edit] Brazing and Silver Soldering
Brazing (sometimes known as silver soldering or hard soldering) requires a much higher temperature than soft soldering, sometimes over 850 °C. As well as removing existing oxides, rapid oxidation of the metal at the elevated temperatures has to be avoided. This means that fluxes need to be more aggressive and to provide a physical barrier[1][2]. Traditionally borax was used for a flux for brazing, but there are now many different fluxes available, often using active chemicals such as fluorides[3] as well as wetting agents. Many of these chemicals are toxic and due care should be taken during their use.
[edit] 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.
[edit] Metal Salts as flux in Hot Corrosion
Hot Corrosion can affect gas turbines operating in high salt envrionments, (i.e. near the ocean). Salts, including chlorides and sulfates are ingested by the turbines and deposited in the hot sections of the engine. The heat from the engine melts the salts which flux passivating oxides on the metal componets of the engine, allowing corrosion to occur at an accelerated rate.
