Flux (metallurgy)
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.
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:
| (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 â†' 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).
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 or
hard soldering are much higher temperature processes than soft soldering even over 850
o
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]. Traditionally
borax was used for a flux for brazing, but there are now many different fluxes available, often using active chemicals such as
flourides [
2] as well as wetting agents. Many of these chemicals are toxic and due care should be taken during their use.
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.
*
Flux-cored arc welding
