Design007 Magazine

PCBD-Apr2016

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52 The PCB Design Magazine • April 2016 considerably reduced when exposed to thermal cycling conditions. If you need to under-fill a device, use one of the many under-fill formula- tions especially designed for this purpose. To summarize, this month's five top tips for design engineers are as follows: 1. Conformal coatings are not waterproof, but having said that, we have just developed a hydrophobic two-part coating system that is water-resistant. But as a rule of thumb, coatings are NOT waterproof. The housing around the assembly can often act as a water-trap due to condensation and poor drainage. Boards that are conformal-coated and subjected to long- term immersion in condensed water probably won't operate as you might expect. Airflow and drainage are important considerations during the design of the housing. 2. Speaking of airflow, coatings can be abrad- ed by particles drawn from the environment by cooling fans. Once the coating is abraded, unprotected parts of the circuit will be vulner- able to high humidity and condensation, which will lead ultimately to failure. If these operating conditions are expected, it is well worth con- sidering the use of a breathable membrane or particulate filter, or choosing a more abrasion resistant coating. 3. The interaction between solder paste, sol- der mask, fluxes and coatings is not easy to pre- dict and should always be tested for each design. The geometry and thermal profile of assemblies can significantly affect these interactions; just because it worked on the last assembly is no guarantee it will work on the next. 4. Returning to the subject of housings, many boards are qualified without their hous- ings as presumed worst case exposure. However, mechanical fasteners and fixtures in the hous- ing can significantly affect the behaviour of the board during thermal shock or cycling by causing additional stresses on certain areas and changing the expansion and contraction dy- namics of the system. Remember: condensation in the housing can lead to worse environmental conditions than testing outside of the housing. 5. Getting liquid coatings to cover sharp edg- es of components can be difficult. Poorly coated edges, leads etc., can be worse than not coating, due to the formation of "micro environmental hotspots" where the effect of contamination, corrosion and so on will be maximised. Try to avoid the use of high, sharp cut ceramic compo- nents in large arrays, since this will increase the difficulty of achieving good coverage through a combination of capillary flow, and the tendency of the coatings to pool. This is particularly true for ceramic capacitors, which are extremely sus- ceptible to corrosion due to the chemicals used in their manufacture. It is no easy task to choose the correct con- formal coating for your product, let alone have confidence that, in applying it, you will have achieved the ultimate goal of protecting your electronics. Conformal coatings are available in many generic types; each has its strengths and weaknesses. Choose the right coating for the intended use and operational environment, rather than one that is used by your subcon- tractor or qualified on another product line for a different end-use environment. Don't forget: Test your design to ensure that it has sufficient robustness for the intended application. Look for my next column in the May issue of The PCB Design Magazine. PCBDESIGN Phil Kinner joined electrolube in May 2014 as the technical director for the company's Conformal Coatings Division, which is represented in more than 55 countries across the globe. Design tips for easier conformaL coating " Boards that are conformal- coated and subjected to long-term immersion in condensed water probably won't operate as you might expect. "

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