Flux facilitates the soldering process. One of the obstacles to a successful solder joint is an impurity at the site of the joint, for example, dirt, oil or oxidation. The impurities can be removed by mechanical cleaning or by chemical means, but the elevated temperatures required to melt the filler metal (the solder) encourages the work piece (and the solder) to re-oxidize. This effect is accelerated as the soldering temperatures increase and can completely prevent the solder from joining to the work piece.
The most common type of flux used in electronics (soft soldering) was rosin-based, using the rosin drawn from pine trees. It was ideal in that it was non-corrosive and non-conductive at normal temperatures but became mildly reactive (corrosive) at the elevated soldering temperatures. Plumbing and automotive applications, among others, typically use an acid-based flux which provides cleaning of the joint. These fluxes cannot be used in electronics because they are conductive and because they will eventually dissolve the small diameter wires. Many fluxes also act as a wetting agent in the soldering process, reducing the surface tension of the molten solder and causing it to flow and wet the work pieces more easily.
Advantages to Using Flux Coated Preforms
Ability to use precise amount of solder on application
Increase production yield.
Elimination of residue.
No need for manual soldering
Can be applied to most preforms
Compliant to IPC standard J-STD-004B