Fluxes have several important properties:
Activity - the ability to dissolve existing oxides on the metal surface and promote wetting with solder. Highly active fluxes are often of acidic and/or corrosive nature.
Corrosivity - the promotion of corrosion by the flux and its residues. More active fluxes tend to be corrosive at room temperatures and require careful removal. As activity and corrosivity are linked, the preparation of surfaces
to be joined should allow use of milder fluxes. Some water-soluble flux residues are hygroscopic, which causes problems with electrical resistance and contributes to corrosion. Fluxes containing halides and mineral acids are highly corrosive and require thorough removal. Some fluxes, especially borax-based brazing ones, form very hard glass-like coatings that are difficult to remove.
Cleanability - the difficulty of removal of flux and its residues after the soldering operation. Fluxes with higher content of solids tend to leave larger amount of residues; thermal decomposition of some vehicles also leads to formation of difficult-to-clean, polymerized and possibly even charred deposits (a problem especially for hand soldering). Some flux residues are soluble in organic solvents, others in water, some in both. Some fluxes are no-clean, as they are sufficiently volatile or undergoing thermal decomposition to volatile products that they do not require the cleaning step. Other fluxes leave non-corrosive residues that can be left in place. However, flux residues can interfere with subsequent operations; they can impair adhesion of conformal coatings, or act as undesired insulation on connectors and contact pads for test equipment.
Residue tack - the stickiness of the surface of the flux residue. When not removed, the flux residue should have smooth, hard surface. Tacky surfaces tend to accumulate dust and particulates, which causes issues with electrical resistance; the particles themselves can be conductive or they can be hygroscopic or corrosive.
Volatility - this property has to be balanced to facilitate easy removal of solvents during the preheating phase but to not require too frequent replenishing of solvent in the process equipment.
Viscosity - especially important for solder pastes, which have to be easy to apply but also thick enough to stay in place without spreading to undesired locations. Solder pastes may also function as a temporary adhesive for keeping electronic parts in place before and during soldering. Fluxes applied by e.g. foam require low viscosity.
Flammability - relevant especially for glycol-based vehicles and for organic solvents. Flux vapors tend to have low autoignition temperature and present a risk of a flash fire when the flux comes in contact with a hot surface.
Solids - the percentage of solid material in the flux. Fluxes with low solids, sometimes as little as 1-2%, are called low solids flux, low-residue flux, or no clean flux. They are often composed of weak organic acids, with addition of small amount of rosin or other resins.
The surface of the tin-based solder is coated predominantly with tin oxides; even in alloys the surface layer tends to become relatively enriched by tin. Fluxes for indium and zinc based solders have different compositions than fluxes for ordinary tin-lead and tin-based solders, due to different soldering temperatures and different chemistry of the oxides involved.
The composition of fluxes is tailored for the required properties - the base metals and their surface preparation (which determine the composition and thickness of surface oxides), the solder (which determines the wetting properties and the soldering temperature), the corrosion resistance and ease of removal, and others.
Organic fluxes are unsuitable for flame soldering and flame brazing, as they tend to char and impair solder flow.
Some metals are classified as "unsolderable" in air and have to be either coated with another metal before soldering or special fluxes and/or protective atmospheres have to be used. Such metals are beryllium, chromium, magnesium, titanium, and some aluminium alloys.
Fluxes for high-temperature soldering differ from the fluxes for use at lower temperatures. At higher temperatures even relatively mild chemicals have sufficient oxide-disrupting activity, but the metal oxidation rates become fairly high; the barrier function of the vehicle therefore becomes more important than the fluxing activity. High molecular weight hydrocarbons are often used for this application; a diluent with a lower molecular weight, boiling off during the preheat phase, is usually used to aid application.
Information above from http://en.wikipedia.org/wiki/Flux_(metallurgy) under the Creative Commons Attribution-ShareAlike License
