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48 The PCB Design Magazine • February 2016 and drawbacks. Conformal coatings are avail- able in solvent-based, water-based and 100% active materials, as well as vapour-deposited coatings in which monomeric gases are mixed together in a vacuum, where they are polymer- ized and deposited onto the surface of the PCB as a protective film. Most conformal coatings are either qualified to MIL-I-46058C or meet the requirements of the closely allied IPC-CC-830B specifications. Further, they can additionally be recognised as either a permanent coating, in which case the flammability of the coating is assessed to UL94V0, or as a conformal coating, where the electrical properties will be assessed as part of the UL746E standard. So, here are five essential facts to consider if you intend to conformally coat your electronics products: FACT 1. All solder resists are not created equal! The adhesion results with conformal coat- ings can be very varied when utilising what ap- pears to be the 'same' spec of solder resist from different board suppliers, which can create havoc during the conformal coating process. A quick and very effective solution to this can be to specify a surface energy of >40 dynes/cm on incoming bare boards and ensure that each incoming batch is religiously tested prior to as- sembly and rejected if they do not meet this minimum value. FACT 2. Always leave a buffer Conformal coatings are usually liquid when applied, and they will flow with a combina- tion of gravity and the capillary forces present. Whether you are masking or relying specifically on selective conformal coating, a production team will be greatly relieved if you leave a buf- fer of at least 3 mm clear between the areas to be coated. This small buffer will make the produc- tion process easier and prevent future issues in production. FACT 3. Simplify, simplify, simplify If possible, spend time simplifying the coat- ing process at the PCB design stage. Placing the connectors and components that must not be coated along one edge of the assembly will greatly simplify the conformal coating applica- tion process. This will also allow dip coating to be explored as a potential methodology, and the net result will be quicker application times and reduced costs. If it is not possible to locate all of these connectors and components along one edge, then keeping them to the edges will also optimize the coating process. FACT 4. Discrete components: the downside Large arrays of discrete components rep- resent a massive coating challenge due to the high levels of capillary forces present, and the result is often quite disastrous, with areas of no coverage/protection on the board and con- versely areas of excessive thickness prone to stress-cracking, delamination and other coating defects. Ultimately this will lead to premature failure of the assemblies. Try to avoid this if at all possible! FACT 5. Is bigger really better? Tall components present challenges of their own by creating shadowed or hard-to-reach ar- eas. Splashing, where the coating splashes into "keep-out areas," is another associated prob- lem. Try to avoid siting tall components next to "must-coat" or "must not coat" components/ areas to minimise this. Remember: Thoughtful design strategies will pay huge dividends down the line. And PCB designers will have friends for life among your production colleagues when you make their jobs just a little bit easier! See you next month. PCBDESIGN Phil Kinner is the global business/ technical director for the coatings division at electrolube. To contact him, click here. how to design-out production proBleMs

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