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62 DESIGN007 MAGAZINE I JULY 2018 available for design engineers to consider. In recent years, such alternatives have progressed considerably in terms of their performance as protectors of electronic circuits while comply- ing with increasingly onerous environmen- tal constraints and internationally recognised standards. Water-based coatings are a case in point. For instance, all of us have now become used to using water-based paint products that have very little odour when applied and thankfully make brush cleaning much easier and less messy. Indeed, water-based paints now dominate the market for internal decorative applications. We also have had water-based conformal coatings for more than a decade, but in an electronics context, and particularly where they are used as a corrosion prevention medium, they have two major limitations. The first one is obvious: They contain water. Water is electrically conductive and water- based coatings can remain sufficiently conduc- tive for many, many months after application, creating signal integrity or crosstalk issues on sensitive board designs. Secondly, to prepare the emulsions or dispersions, some form of compatibility with water is necessary, being either a modification of the polymer (disper- sion) or surfactant (emulsion). The problem with either approach is that both types remain somewhat sensitive to water throughout their lifetimes, thus limiting the protection they can afford in high humidity environments. Overall, the performance of current water- based materials is not comparable with that of solvent-based resins and is very far from the performance of solvent-less materials. How- ever, with the current levels of R&D activity being conducted into the development of via- ble water-based systems, I am confident that these limitations will eventually be overcome. Key Benefits of Solvent-Free Solvent-free materials are generally lower- odour, lower-hazard materials and are there- fore much safer for users to apply at the manu- facturing stage. They are usually non-flamma- ble and consequently pose a lower insurance risk both at the production stage as well as in use. Significantly, they help minimise levels of VOC emissions, ensuring compliance with environmental standards. Where 2K (two- part) conformal coatings are concerned, it is possible to apply thicker coatings and achieve better protection without compromising other performance requirements such as tolerance of thermal shock. Clearly, some coatings must be able to per- form in exceptionally harsh environments, so what might be a typical combination of ele- ments that represents the harshest environ- ment that a coating must protect against? Let's take corrosion as an example; for a metal sur- face to corrode, a potential difference, electro- lyte and ionic impurities must all be present. If a coating is well applied, it will prevent cor- rosion by inhibiting external ionic species and liquids such as water from reaching the metal surface. Any environment that can cause degradation of the coating and provide the required condi- tions for corrosion to occur can potentially be tough on the coating. For example, an environ- ment in which there is cycling between high and low temperatures places the coating under a lot of stress, while an abnormally high tem- perature can speed up polymer degradation; these conditions can combine to cause crack- ing of the coating, exposing parts of the circuit to impurities which will ultimately lead to cir- cuit failure. The cyclical nature of the thermal environment will create differences between the board and its local ambient conditions, leading to the formation of damaging conden- sation. Should the assembly also be exposed Let's take corrosion as an example; for a metal surface to corrode, a potential difference, electrolyte and ionic impurities must all be present.

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