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76 SMT007 MAGAZINE I OCTOBER 2019 Circuit Protection Using Conformal Coatings Conformal coatings are thin, protective poly- meric coatings that are often applied to elec- tronic circuit boards or assemblies to provide the required environmental protection without excessive cost or weight penalties. For the auto- motive industry, conformal coating applications are either in-cabin (electronics systems located within the passenger compartment) or under- hood (electronics in close proximity to the engine). Such distinct categories make it conve - nient to discuss the main requirements of each; however, with increased sophistication and the move towards multi-function assemblies, these traditional environments continue to merge while the drive to higher power electronics in electric cars blurs the lines still further. Electronic sensors and systems situated in the passenger cabin essentially occupy the same space as the vehicle occupants. In the winter, that can mean extreme cold and the tendency towards a condensing atmosphere, while in the summer, the tendency is towards a warm and humid atmosphere. Both condensation and high humidity challenge the reliability of electronics as they promote corrosion. In addition, electron - ics may be exposed to atmospheric pollutants, such as cleaning solutions, liquid splashes, etc.; any one of those factors may pose a potential reliability risk, especially in association with humidity and condensation (Figure 1). Corrosion is a complicated electrochemical process with a variety of potential mechan- isms and causes, and any detailed description is beyond the scope of this article. However, in the vast majority of cases, there are three con- ditions that must be met for corrosion to take place: 1. Intrinsically, electrochemically dissimi- lar metals (e.g., gold/silver and nickel/tin) are present, or an anode and cathode are created by application of applied bias. 2. The presence of an ionic species (usually salts, halides, hydroxides, etc.). 3. Mono-layers of condensed water are pres- ent that dissolve the ionic species, result- ing in an electrolyte solution. To prevent the possibility of corrosion, it is necessary to remove one of the prerequisite conditions. The choice of metals is limited to those used in the solder and solder finish chemistries, which are dissimilar. And there will always be areas of potential difference due to the nature of an electronic assembly. Clean- ing can help remove ionic species but cannot prevent the redeposition of ionic species from the operating environment. Conformal coatings help prevent the forma- tion of electrolytic solutions by acting as mois- ture barriers. The coating needs to be a good barrier against moisture and must have good adhesion to the substrate to prevent delamina- tion. Once the coating is delaminated, mois- ture can eventually collect in this pocket and form an electrolytic solution with any pre-exist- ing ionic contamination. This is why cleaning before conformal coating is recommended to provide a powerful synergistic elimination of two of the three prerequisite conditions for corrosion. Given the relatively benign operating envi- ronments experienced by in-cabin electron- ics, acrylic conformal coatings have histori- cally dominated this segment, offering good all-round properties, especially against high humidity and spills and splashes. Under-hood electronics, by dint of their location, face greater environmental challenges than in-cabin elec- Figure 1: Potential reliability risks.