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January 2017 • SMT Magazine 109 benefit of acting as protective barrier during sol- dering and after soldering can entrap ionic res- idues, preventing them from being mobile and reacting in a destructive manner. However, rosin leaves a residue on the board and can contaminate manufacturing equip- ment—which serves as the main motivator to cleaning this kind of flux from circuit boards, more so than the reliability issues of water solu- ble fluxes. Although, with the high demands of today's electronics, rosin residues can also lead to failures as well in harsh environments. The cleaning process for rosin can also be an expensive process to operate, but also usually re- quires a solvent that brings its own complica- tions. However, the benefits of rosin were enough to carry it over into low-solids/no-clean fluxes. Low-Solids/No-Clean Fluxes As most realize, these types of fluxes were employed so that cleaning could be eliminat- ed. As the name implies, these fluxes have less chemistry and activity, and make soldering with the same results more challenging. Alcohol-based, Rosin Low-Solids/ No-Clean Fluxes Initially, low-solids/no-clean fluxes were es- sentially rosin-like fluxes, just less of it. Today, these fluxes are much more sophisticated. But, the principle is that with less chemistry, there will be little to no active chemistry left on the board after the soldering process thus eliminat- ing the need to clean the board. So rather than the 35% solids in a full rosin flux, the low-sol- ids fluxes range from 1.5–8%. With less chem- istry, the issue now becomes having enough ac- tive chemistry to be an effective flux. NOTE: Low-solids fluxes are not necessarily no-clean for everyone or every application. The same flux and residues in one application may be perfectly safe, while in another application they may be harmful. It is up to the custom- er, the product designers, etc., anyone who un- derstands the demands of the end-use environ- ment for that product, to dictate whether a low- solids flux is actually a no-clean flux for them. Even in small amounts, rosin in effect, is used as a way to avoid simply adding active chemistry to improve performance. It is a key ingredient in many of these low-solids fluxes, as it helps protect the cleaned metal and the little chemistry in these fluxes during the soldering process. It allows the flux to withstand a longer, hotter exposure to heat than without it. With rosin in the flux, you may see slightly more res- idue, but the idea is again that any remaining chemistry will be contained by the rosin. Even so, some customers find any residue visibly un- desirable or potentially more unsafe to leave on the board. In these cases, many end up clean- ing, or are asked by their customer to clean the low-solids flux residues from the boards. Not only are these fluxes good in long heat/ preheat exposures or higher heat processes, they work well in low heat applications, or short processes, as the alcohol evaporates very quick- ly allowing the soldering to begin almost im- mediately. Alcohol-Based, Non-Rosin, Low-Solids/ No-Clean Fluxes To partially address this issue, most flux manufacturers offer low-solids fluxes with no rosin at all, which tends to allow the active flux to volatilize more completely during soldering and leave the least amount of residue. However, as a result, these fluxes tend to not last as long because they are not shrouded by rosin. For se- lective soldering this can be a problem, as time CHOOSING THE CORRECT FLUX —ADVANTAGES/DISADVANTAGES Figure 5: Flux residues after soldering. (Source: www.lo-tech.co.uk)