Solder flux categories

Dispensing. In surface-mount technology, three classes of materials must be dispensed on to the circuit board adhesives, fluxes, and solder pastes. Each of these three categories, which utilize very similar equipment options for the actual dispensing action, is discussed in the following subsections. 

A new category of low solids fluxes was introduced in the early 1990s to meet the needs of localities where volatile organic compounds (VOCs) are regulated. These fluxes are marketed as VOC-free or low-VOC fluxes. The solvent in this case is 100 percent water or at least greater than 50 percent water. Use of these fluxes requires special care in the preheat step where the water (solvent) must evaporate before the assembly reaches the solder wave. Failure to do this will result in excessive solder ball formation. 

Soldering processes associated with Pb-Sn solders have been performed with fluxes from each of three categories, the (natural) rosin-based fluxes, resin-based fluxes, and the organic acid (water-soluble) fluxes. ... 

As was noted with rosin-based fluxes, the activators added typically contain halides such as chlorides and bromides, which are fairly corrosive. There are, however, several categories of milder, halide-free activators such as organic acids and amine compounds. Because these fluxes are less aggressive, they are slower-acting and do not have the ability to remove thicker tarnish layers. For these fluxes to be effective, it is necessary to provide pristine solderable surfaces. 

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