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. Metallurgy is a domain of materials science and of materials engineering that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. ... (De)soldering a contact from a wire. ... Brazing is a joining process whereby a non-ferrous filler metal and an alloy are heated to melting temperature (above 450°C / 800°F) and distributed between two or more close-fitting parts by capillary action. ... Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. ... Ammonium chloride or Sal Ammoniac (chemically ammonium chloride (NH4Cl); also nushadir salt, zalmiak, sal armagnac, sal armoniac, salmiakki, salmiak and salt armoniack) is, in its pure form, a clear white water-soluble crystalline salt with a biting taste. ... A 20 g cake of amber violin bow rosin. ... General Name, Symbol, Number tin, Sn, 50 Chemical series poor metals Group, Period, Block 14, 5, p Appearance silvery lustrous gray Atomic mass 118. ... The chemical compound hydrochloric acid is the aqueous (water-based) solution of hydrogen chloride (HCl). ... Zinc chloride (ZnCl2) is a colorless or white compound of zinc and chlorine that is extremely hygroscopic. ... Galvanization, named after the Italian scientist Luigi Galvani, was originally the administration of electric shocks (in the 19th century also termed Faradism, after Michael Faraday). ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... General Name, Symbol, Number zinc, Zn, 30 Chemical series transition metals Group, Period, Block 12, 4, d Appearance bluish pale gray Atomic mass 65. ... It has been suggested that Sodium boric acid be merged into this article or section. ... Brazing is a joining process whereby a non-ferrous filler metal and an alloy are heated to melting temperature (above 450°C / 800°F) and distributed between two or more close-fitting parts by capillary action. ... Ferrous in chemistry is a term used for the iron with an oxidation number +2. ...

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 materal. The clean, activated metal surface is easily wetted by the molten solder or brazing alloy.

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: An amalgam is any mixture or blending of mercury with another metal or with an alloy. ...

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:

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. see dendrite for the biological usage A dendrite in metallurgy is a characteristic tree-like structure of crystals growing as molten metal freezes, the shape produced by faster growth along energetically favourable crystallographic directions. ...

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).

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. Zinc chloride (ZnCl2) is a colorless or white compound of zinc and chlorine that is extremely hygroscopic. ... Ammonium chloride or Sal Ammoniac (chemically ammonium chloride (NH4Cl); also nushadir salt, zalmiak, sal armagnac, sal armoniac, salmiakki, salmiak and salt armoniack) is, in its pure form, a clear white water-soluble crystalline salt with a biting taste. ...

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. Brazing is a joining process whereby a non-ferrous filler metal and an alloy are heated to melting temperature (above 450°C / 800°F) and distributed between two or more close-fitting parts by capillary action. ... Brazing is a joining process whereby a non-ferrous filler metal or alloy is heated to melting temperature (above 450°C; 800°F) and distributed between two or more close-fitting parts by capillary action. ... Brazing is a joining process whereby a non-ferrous filler metal or alloy is heated to melting temperature (above 450°C; 800°F) and distributed between two or more close-fitting parts by capillary action. ... It has been suggested that Sodium boric acid be merged into this article or section. ... Fluoride is the ionic form of fluorine. ...

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. Electric phosphate smelting furnace in a TVA chemical plant (1942) Chemical reduction, or smelting, is a form of extractive metallurgy. ... Blast furnace in Sestao, Spain. ... Cupola is used for dome-shaped items, so that: a cupola is a dome-shaped architectural structure a cupola is the turret of a armoured fighting vehicle a proposed observation area on the ISS is called the cupola a cupola is a form of geometric solid a cupola is a... A path through a slag heap in Clarkdale, Arizona, showing the striations from the now rusting corrugated sheets used to hold it back. ... A liquid will usually assume the shape of its container. ... For other uses, see Steel (disambiguation). ... Limey shale overlaid by limestone. ... Fuel is any material that is capable of releasing energy when its chemical or physical structure is changed or converted. ... One of the components in the proximate analysis of biological materials, consisting mainly of carbonates and bicarbonates of metals. ...

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. This machine has a single-stage centrifugal compressor and turbine, a recuperator, and foil bearings. ...

References

See also

• Flux-cored arc welding