Flux (metallurgy) - Citizendia

For other uses of this term, see Flux (disambiguation).

In metallurgy, a flux is a chemical cleaning agent which facilitates soldering, brazing, and welding by removing oxidation from the metals to be joined. Metallurgy is a domain of Materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their Soldering is a process in which two or more Metal items are joined together by melting and flowing a filler metal into the joint the filler metal having a relatively low Brazing is a joining process whereby a Filler metal or Alloy is heated to melting temperature above - or by the traditional definition in the United States Welding is a fabrication process that joins materials usually Metals or Thermoplastics by causing coalescence. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state 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. Ammonium chloride ( N[[Hydrogen H]]4 Cl) (also Sal Ammoniac, salmiac, nushadir salt, zalmiak, sal armagnac Rosin, formerly called colophony or Greek pitch ( Pix græca) is a solid form of Resin obtained from Pines and some other Plants Tin is a Chemical element with the symbol Sn (stannum and Atomic number 50 Hydrochloric acid is the Solution of Hydrogen chloride ( H[[Chlorine Cl]] in water Zinc chloride is the name of Chemical compound with the formula Zn[[chlorine Cl]]2 and its hydrates Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Zinc (ˈzɪŋk from Zink is a Metallic Chemical element with the symbol Zn and Atomic number 30 Borax (from Persian burah) also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important Boron Brazing is a joining process whereby a Filler metal or Alloy is heated to melting temperature above - or by the traditional definition in the United States Ferrous, in the chemical science realm indicates a bivalent iron compound (+2 oxidation state (as opposed to Ferric, which indicates a trivalent iron compound (+3 oxidation 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.

1 Soldering
- 1.1 Dangers
2 Brazing and silver soldering
3 Smelting
4 Metal salts as flux in hot corrosion
5 List of fluxes
6 References
7 See also

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. No-clean processing refers to the Printed circuit board assembly process that lets flux residues remain after reflow

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

Metal oxide + Acid → Salt + Water

Salts are ionic in nature and can cause problems from metallic leeching or dendrite growth, with possible product failure. 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 In some cases, particularly in high-reliability 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).

Dangers

Acid flux types (not used in electronics) may contain zinc chloride or ammonium chloride, both of which are harmful to humans. Zinc chloride is the name of Chemical compound with the formula Zn[[chlorine Cl]]2 and its hydrates Ammonium chloride ( N[[Hydrogen H]]4 Cl) (also Sal Ammoniac, salmiac, nushadir salt, zalmiak, sal armagnac Therefore, flux must be handled with gloves and goggles, and used with adequate ventilation.

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. Brazing is a joining process whereby a Filler metal or Alloy is heated to melting temperature above - or by the traditional definition in the United States Brazing is a joining process whereby a Filler metal or Alloy is heated to melting temperature above - or by the traditional definition in the United States Brazing is a joining process whereby a Filler metal or Alloy is heated to melting temperature above - or by the traditional definition in the United States 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. Borax (from Persian burah) also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important Boron Fluoride is the reduced form of Fluorine. Both organic and Inorganic compounds containing the element fluorine are considered fluorides Many of these chemicals are toxic and due care should be taken during their use.

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. Chemical reduction, or smelting, is a form of Extractive metallurgy. A blast furnace is a type of metallurgical Furnace used for Smelting to produce metals generally Iron. Slag is the By-product of Smelting Ore to purify Metals They can be considered to be a mixture of metal Oxides however The flux most commonly used in iron and steel furnaces is limestone, which is charged in the proper proportions with the iron and fuel. Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0 Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3 Fuel is any material that is burned or altered in order to obtain energy The slag is a liquid mixture of ash, flux, and other impurities. In Analytical chemistry, ash is the name given to all non- Aqueous residue that remains after a sample is Burned, and consist mostly of metal Oxides

Metal salts as flux in hot corrosion

Hot corrosion can affect gas turbines operating in high salt environments, (e. "Hot corrosion" is a mechnism of "high temperature corrosion" others inculde high temperature oxidation sulfidation and carbonization A gas turbine, also called a combustion turbine, is a rotary Engine that extracts energy from a flow of Combustion gas g. 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 components of the engine, allowing corrosion to occur at an accelerated rate. Passivation is the process of making a material "passive" in relation to another material prior to using the materials together

List of fluxes

borax - used for brazing
beeswax
tallow - used with lead
paraffin
palm oil
zinc chloride (aka killed spirits)
zinc chloride & sal ammoniac
olive oil & sal ammoniac, 50/50 - for iron
rosin, tallow, olive oil, zinc chloride - for aluminium
cryolite
cryolite & phosphoric acid
phosphoric acid & alcohol
cryolite & barium chloride
oleic acid
MgCl₂, NaCl, KCl.

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