Flux Coated Preforms
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 workpiece.
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
Types of Flux
Rosin based fluxes are available in non-activated ®, mildly activated (RMA) and activated RA formulas. RMA and RA contain rosin combined with an activating agent, typically an acid, which increases the wetability of metals to which it is applied by removing existing oxides. The residue resulting from the use of RA flux is corrosive and must be cleaned. RMA is formulated to result in a residue which is not significantly corrosive with cleaning being considered optional.
No Clean flux is mild enough to not “require” removal due to their non-conductive and non-corrosive residue. These fluxes are called “no-clean” because the residue left after the solder operation is non-conductive and won’t cause electrical shorts; nevertheless they leave a plainly visible white residue. No-clean flux residue is acceptable on all 3 classes of PCBs as defined by IPC-610 provided it does not inhibit visual inspection, access to test points, or have a wet, tacky or excessive residue that may spread onto other areas. Connector mating surfaces must also be free of flux residue.
Water-soluble fluxes are higher activity fluxes designed to be removed with water after soldering.