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52 SMT Magazine • April 2017 VAPOR DEGREASING CHEMISTRIES dues, however, the high surface tension of water still restricts rinsing capability. If the surface ten- sion of the mixture manages to allow for clean- ing under the low-standoff circuitry, it is unlike- ly that the deionized water will penetrate the same areas to remove the residing surfactants. Other factors to improve cleaning include op- erating temperature, chemistry concentrations, rinse cycles, water purity and spray/wash mech- anisms. With all of these different elements, it is easy to be overwhelmed with numerous options that provide less than ideal cleaning. Electron- ics manufacturers who have considered solvent cleaners have also been met with shortcomings; ionic removal is a difficult task for many hydro- fluorocarbon-based solvents due to their lack of polarity. However, new solvent and co-solvent formulations coming to the market have prov- en capabilities at removing ionic contamina- tion and cutting through burned-on residues. Most importantly, these advanced solvent for- mulations offer new benefits to solvent-cleaning without the need for new equipment. Manufacturers who are currently using a vapor degreasing process but looking for new solvents to improve cleaning will be able to do so without additional capital investment in equipment. Vapor Degreasing The original concept of vapor degreasing revolved around vapor-only cleaning; howev- er, modern vapor degreasers have been mod- ified to allow for liquid immersion in addi- tion to vapor cleaning. This has further im- proved the ability for solvent to penetrate in- tricate geometries and solubilize difficult soils. Many modern machines are equipped with two immersion tanks for cleaning: the "boil sump," which contains the heating ele- ments to produce the vapor zone, and the "rinse sump," which collects the clean distil- late. These machines function, essentially, as industrial stills; the liquid is boiled in the boil sump, condensed in the vapor zone, and then collected in the rinse sump as pure solvent. This means that even as contamination is introduced into the machine during the clean- ing process, clean solvent is continuously dis- tilled into the rinse sump, allowing for the contamination to stay trapped in the boil sump. Modern equipment also benefits from improved cold traps, which restrict solvent emissions and improve the distillation pro- cess. Figure 3 illustrates the design of a mod- ern two-sump vapor degreaser with two sets of cooling coils. Figure 3: Modern two-sump vapor degreaser.