Flux (metallurgy)
Some of the earliest known fluxes were sodium carbonate, potash, charcoal, coke, borax,[1] lime,[2] lead sulfide[3] and certain minerals containing phosphorus.
[4] Fluxes are also used in foundries for removing impurities from molten nonferrous metals such as aluminium, or for adding desirable trace elements such as titanium.
As reducing agents, fluxes facilitate soldering, brazing, and welding by removing oxidation from the metals to be joined.
In some applications molten flux also serves as a heat-transfer medium, facilitating heating of the joint by the soldering tool.
[6] Some fluxes are corrosive, so the parts have to be cleaned with a damp sponge or other absorbent material after soldering to prevent damage.
A mass of hot sticky flux can transfer more heat to skin and cause more serious burns than a comparable particle of non-adhering molten metal, which can be quickly shaken off.
Flux residues also tend to outgas in vacuum and space applications, and traces of water, ions and organic compounds may adversely affect long-term reliability of non-hermetic packages.
Removal of the native oxide layer is more troublesome; physical or chemical cleaning methods have to be employed and the surfaces can be protected by e.g. gold plating.
The gold layer has to be sufficiently thick and non-porous to provide protection for reasonable storage time.
[citation needed] The self-dissolved oxide degrades the solder's properties and increases its viscosity in molten state, however, so this approach is not optimal.
Pastes have to contain smooth spherical particles, preforms are ideally made of round wire.
Problems with preforms can be also sidestepped by depositing the solder alloy directly on the surfaces of the parts or substrates, by chemical or electrochemical means for example.
In contact with surface oxides it forms hydroxides, water, or hydrogenated complexes, which are volatile at soldering temperatures.
Argon-hydrogen gas compositions with hydrogen concentration below the low flammable limit can be used, eliminating the safety issues.
[citation needed] Active atmospheres are relatively common in furnace brazing; due to the high process temperatures the reactions are reasonably fast.
[citation needed] Bombardment with atomic particle beams can remove surface layers at a rate of tens of nanometers per minute.
The oxide disruption and removal involves cavitation effects between the molten solder and the base metal surface.
[16] Common fluxes are ammonium chloride or resin acids (contained in rosin) for soldering copper and tin; hydrochloric acid and zinc chloride for soldering galvanized iron (and other zinc surfaces); and borax for brazing, braze-welding ferrous metals, and forge welding.
The chemicals used often simultaneously act as both vehicles and activators; typical examples are borax, borates, fluoroborates, fluorides and chlorides.
Halogenides are active at lower temperatures than borates, and are therefore used for brazing of aluminium and magnesium alloys; they are however highly corrosive.
The general reaction of oxide removal is: Salts are ionic in nature and can cause problems from metallic leaching or dendrite growth, with possible product failure.
An example is the group of fluxes containing zinc, tin or cadmium compounds, usually chlorides, sometimes fluorides or fluoroborates.
In Europe, rosin for fluxes is usually obtained from a specific type of Portuguese pine; in America a North Carolina variant is used.
A more aggressive early composition was a mixture of saturated solution of zinc chloride, alcohol, and glycerol.
[citation needed] Stainless steel is a material which is difficult to solder because of its stable, self-healing surface oxide layer and its low thermal conductivity.
A solution of zinc chloride in hydrochloric acid is a common flux for stainless steels; it has however to be thoroughly removed afterwards as it would cause pitting corrosion.
Another highly effective flux is phosphoric acid; its tendency to polymerize at higher temperatures however limits its applications.
Hot corrosion can affect gas turbines operating in high salt environments (e.g., near the ocean).
J-STD-004 characterizes the flux by reliability of residue from a surface insulation resistance (SIR) and electromigration standpoint.
It 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).
