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28 SMT007 MAGAZINE I AUGUST 2019 According to a quick internet search, the process of conformally coating an electronic assembly goes all the way back to I don't know when; I couldn't find that information. Sorry. But I was able to contact an industry colleague that works for one of the major chemical man- ufacturers. Their company has strict restric- tions about being cited in publications, so they will remain nameless, but "the man" can't keep me from telling you what their thoughts are. Here's what they had to say. "Conformal coatings being used to protect printed circuits dates back to before the 1960s when the Department of Defense authored specification MIL-I-46058. A general industry standard IPC-CC-830, has more recently carried the industry forward. Conformal coatings can be used to protect printed circuits from current leakage/short circuits (arcing/corona), corro- sion, solder joint fatigue, mechanical stresses, Sealing Your Fate such as shock and vibration, along with pro- tection from dust and dirt debris." So, there's that bit of history that pretty much sums up what a good coating is used for. Now that we've cleared that up, let's talk about my favorite part—how CMs continue to do it wrong. Remember, I work for an analyti- cal lab. Four Main Types There are several types of coating material available, and it normally comes down to the end-use environment when deciding which one is best for your product. In general, the choices are acrylic, silicone, urethane, and Parylene. There are probably some exotics out there as well, but for most of what we see, those are the big four. Out of those four, the one we see the least amount of issues with is Parylene because it is applied with a vapor deposition process that requires the substrate to be extremely clean for proper adhesion. When that pro- cess is properly done, the residues that can normally facilitate electrical leakage or electrochemical migra- tion are removed. On top of that, Parylene creates a near hermetic seal when com- pared to other coating mate- rials. The drawback for Parylene is the cost and the time it takes to apply it. We don't see Parylene used in high- volume production, such as Quest for Reliability Feature Column by Eric Camden, FORESITE INC.

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