Flux (metallurgy)

Overview
In metallurgy, a flux is a chemical cleaning agent which facilitates soldering, brazing, and welding by removing oxidation from 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. 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.

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:

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 → 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-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. 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. 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. Traditionally borax was used for a flux for brazing, but there are now many different fluxes available, often using active chemicals such as fluorides as well as wetting agents. 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. 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.

Metal salts as flux in hot corrosion
Hot corrosion can affect gas turbines operating in high salt environments, (e.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.

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.