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March 2017 • SMT Magazine 77 MINIATURE COMPONENTS ON PCBS REQUIRES FLEXIBLE CLEANING METHODS (particulate). "Super heat" and external dis- tillation are also money-saving, performance- enhancing choices. These systems are easily programmable and allow for excellent repeat- ability. Companies can pick and choose to tailor their equipment and processes to their PCBs and budgets. Vapor Degreasing Advantages In comparison to aqueous systems, the ad- vantages of vapor degreasing are clear. The sys- tems are very easy to use, highly efficient, small- er in footprint, and lower in overall capital and operating costs. Let's look and see how these benefits are delivered. First, solvent cleaning leverages the chemi- cal properties of the cleaning fluid, rather than fighting against those characteristics. Low-boil- ing solvents have a much lower surface ten- sion and viscosity than water, so they easily clean even under the smallest of parts. Most va- por degreasing fluids are also very heavy and dense, typically 20–40% heavier than water, which aids in dislodging particulate from com- ponents. Because the solvent is tailored for the application, delicate parts are easily cleaned and dried. It is possible to combine all these factors into a "wetting index" that permits apple-to-ap- ple comparisons (Chart 1). Another advantage is the cleaning fluid boils at a low temperature (usually slightly above room temperature), so heating the cleaning flu- id uses very little electricity. In comparison to an aqueous cleaner, a vapor degreaser of com- parable capacity will use about 1/6th the elec- trical power. Since burning fossil fuels to gen- erate electricity is one of the primary sources of carbon in the atmosphere, an energy efficient- cleaning system is highly attractive. An unexpected cost savings stems from the fact the solvents, unlike aqueous saponifi- ers and surfactants, can be re-used indefinite- ly. This is because a vapor degreaser is a recy- cling system. While aqueous cleaning dump the cleaning fluids after a single use, the re-usability of solvent cleaning makes the choice extremely cost-effective. These machines, when properly designed, equipped and configured, outperform the clean- ing efficiency of any other cleaning technology. Water—Not as Green as You Think Aqueous cleaning is the most common alter- native to solvent cleaning. Experience has prov- en that it can work well in non-critical appli- cations because water cleaning uses the kinet- ic energy of pumps and sprays to "power-wash" contamination from surfaces. But as parts be- come smaller and the stand-offs tighter, the per- formance of aqueous systems suffer. There are a number of reasons why the os- tensible advantages of water cleaning do not al- ways materialize as expected. First, clean water does not come straight from the tap. Water needs to be pre-treated to be acceptable to ensure there are no trace ma- terials, minerals or pre-existing bioburden that would compromise the cleaning process. Next, expensive cleaning fluids need to be added to the water to reduce the surface tension so the water can clean effectively. Lastly, substantial expertise is required to monitor the cleaning fluid (such as the pH) so extra people, consum- ables and services are required. In contrast, sol- vents come ready-to-use, require no mixing and can be used indefinitely. Aqueous cleaning processes operate hori- zontally and contain many moving parts, in- cluding conveyors, pumps, air knives and heat- ers. These machines and their support systems have an enormously larger footprint than a va- por degreaser, requiring more overall space in Chart 1: Wetting index comparison between different solvent cleaning solutions.