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110 SMT Magazine • January 2017 exposure to elevated or high heat can be rela- tively long in some cases. These fluxes can po- tentially be depleted by the time they are sol- dered in the selective soldering process. Leaving a little rosin in the flux helps combat the heat issue as well as encapsulate remaining ionic res- idues. For this reason, most low-solids fluxes have some small amount of rosin, or a synthet- ic equivalent. These fluxes work best if you have a fairly homogeneous product type and typically run short programs, or don't require a lot of pre- heat—and require minimal residues. You can dial the process in to get very good results. VOC-free, Low-Solids/No-Clean Fluxes VOC-free low-solids fluxes are similar, in that they do not have rosin, but because they are water based they do handle more expo- sure to heat and can tend to be a little more ac- tive. However, the issue with fluxes can be that they are water based—they require more heat to evaporate the water, and can force you into a longer preheat cycle than you would not oth- erwise need, increasing your overall cycle time. If you primarily run heavy boards and a high heat programs/processes, VOC-free may work best, as the water takes longer to evapo- rate and they can be a little more active. Table 4. Table 3. CHOOSING THE CORRECT FLUX —ADVANTAGES/DISADVANTAGES