Sensible Design: Coatings—Five Essentials for Designers


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As I sit down to write my fourth column on the subject of conformal coatings, I’m reminded—as we in the UK continue to experience almost sub-arctic weather conditions this Spring, including snow in April—that it is always best policy to be prepared for those unexpected environmental elements. And where electronic circuit board protection is concerned, that means being prepared for all eventualities. So, as the central heating is fired up a notch or two and the barbecue stove is wheeled back into the garage, let’s consider what we’ve covered so far on the subject of conformal coatings.

True to form, I’ll be providing five PCB design pointers each month to help you avoid some common pitfalls when applying conformal coatings. We have so far looked at tricky production related issues that could so easily have been resolved at that all-important design stage, examined some of the more common dos and don’ts; those are issues that come up time and time again, despite meticulous attention to design detail, and which must be taken into consideration when coatings are finally applied. We’ve explored the implications of housing design on conformal coating performance, particularly the influence that fixtures and fittings can have in terms of thermal shock loading and the adverse effects of forced air cooling when abrasive particles are present. So, let’s cover how the physical shape of board components and the interaction with solder paste, solder masks and fluxes can affect conformal coating integrity.

The focus for this month will be my absolute essential facts—my “Never leave home without them” list, so to speak!

Fact 1: In an ideal world, PCB designs would not have an inherent weak point for corrosion; unfortunately, in the real world, they do. A really neat way of determining this weak point is to use stress tests such as powered condensation tests. When a weak point is revealed, you are better equipped to deal with it. Often the spacing of components, board finish and distance to ground planes can be optimised for corrosion resistance.

Once the design is optimised for corrosion resistance, the conformal coating will often be even more effective in ensuring corrosion free operation. Stress tests (thermal shock, for example) are not intended to be used on final assemblies for product validation. Failure mechanisms may well be different, and may be missed by stress tests. Product validation testing should mimic end use environments as closely as possible. (Here at Electrolube, our testing is designed to push products even further than in actual usage scenarios.)

To read this entire article, which appeared in the May 2016 issue of The PCB Design Magazine, click here.

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