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68 DESIGN007 MAGAZINE I DECEMBER 2019 areas where the coating does not fully cure. Similarly, it can cause the coating to crack as it cures or as the result of changes in temper- ature, or due to mechanical shock and vibra- tion. As a rule of thumb, conformal coatings should not be applied in thicknesses greater than necessary or exceed the thickness they were designed to be applied at. Coverage is of- ten more important than thickness, except in the harshest environments; however, if you are in any doubt about how to achieve the best coating thickness, we strongly advise discuss- ing this with your coating vendor. Know Your Components Knowing your components is central to achieving a successful coating process. Simply by being aware of what kinds of components should be coated or cannot be coated will pro- vide more flexibility to the coater in assembly operations. When a component must not be coated, it becomes a keep-out zone. The engi- neering drawing should not only identify com- ponents that must not be coated but identify the tolerance on that keep-out zone. It is important to be very specific on the tol- erance. The manufacturer needs to know ex- actly where you want coating and where you don't want coating. It is best practice to speci- fy the areas that must be coated and the areas that must not be coated as well as the "don't care" areas to help the coating process run as smoothly as possible. Avoid vague statements at all costs in an engineering drawing. This is especially true when specifying coating around connectors. It is also worth noting that design- ers should not rely on a conformal coating to compensate for poor part selection. Some are more moisture-sensitive than others, and if the component fails, the failure may have no re- lation to the coating. The selection process of the coating, in this instance, will be absolutely critical to achieving a successful outcome. Vapour-deposited Coating The designer should be aware, during coat- ing selection, that there is a vast difference be- tween solvent-borne or liquid coatings, and those coatings applied by vacuum deposi- tion or chemical vapour deposition process- es. Vapour-deposited coating is an expensive process, and conventional coatings can't be used in this process. The most common vac- uum-applied coating is polyparaxylylene (e.g., Parylene). Such processes require different masking, surface preparation, etc. If a design- er chooses a vapour-deposited coating, it's im- portant to be aware of the board's design as not all circuits are suitable for vacuum deposi- tion. If in doubt, designers should work with the coating vendor or manufacturer who will do the coating, to identify the special rules that go with such materials. Communication Is Key Finally, the biggest gift that a designer can offer to the production team is to invest a few days on the manufacturing floor. I cannot em- phasise enough how valuable this time will be in ensuring that the design flows through the coating process smoothly. It provides the op- portunity to observe the process and talk with the process professionals who do this every day. Such individuals will be able to give the designer key elements as well as identify bad practices. When design and production work together, the outcome will nearly always be successful. In my next column, I will look at more coat- ing-related considerations. DESIGN007 Phil Kinner is the global business and technical director of conformal coatings at Electrolube. To read past columns or contact Kinner, click here. Kinner is also the author of The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments. Visit I-007eBooks.com to download this and other free, educational titles. Knowing your components is central to achieving a successful coating process.