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Design007-Apr2018

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66 DESIGN007 MAGAZINE I APRIL 2018 4. There is an unexpected interaction with some other process material used to prepare/ build the PCB. Flux residues are particularly illustrative of this type of problem. In a no- clean process, for example, these can inhibit the cure of some types of coating or lead to a loss of insulation of the system, greater than either material in isolation, which can result in current leakage and short circuit formation. 5. Other factors are at play indirectly related to the coating. Unless there has been meticu- lous attention to preparation or pre-coat clean- ing regimes, corrosive residues bridging the PCB's conducting tracks can cause failures over time. It is worth noting that whilst the coating may delay failure for many years, at some point failure will inevitably happen. What are your top tips for avoiding coating failure? As highlighted in the previous section, choose the right material for the protection required, apply and cure it well. Check for interactions with other process chemistries, and thoroughly clean the assembly prior to coating. When it comes to application, would a thicker coating be less likely to fail? It depends. Thicker can be better up to a point, but at some stage the coating material will be too thick and will either crack itself, or even cause cracking of the coated components themselves—during thermal shock or thermal cycling, for example. Depending on the type of coating material used, solvent entrapment (i.e., the solvent not having enough time to evaporate from the coating film before it hard- ens) can become an issue, leading to poor properties or the formation of bubbles, neither of which are good for protection. In addition to this, by adding unnecessary amounts of coating material you are in effect wasting it, adding to costs and, importantly for some applications, adding weight. Com- promised thermal management issues can also arise as it may prove difficult to dissipate heat away from thickly coated components. It is well to remember that above a certain thickness, which does vary according to the material being applied, any increase is likely to deliver diminishing returns. Get the advice of a reputable supplier: they have laboured hard to establish optimum coating thicknesses for their products in all kinds of operating environments. How important is the application method to the reliability of coatings? Good question! This is probably the num- ber one determinant of success. Often a poor material applied well can be just as good as or sometimes better than a material with great properties that is applied badly. At the end of the day, coating is about getting sufficient cov- erage of the sharp edges and metal surfaces without applying the material too thickly else- where. Of course, some materials apply better than others and make this process as easy and fool-proof as possible. But in the end, the per- formance of liquid applied coatings will always be determined by how well they were applied. Are some PCBs impossible to coat by virtue of their design? Nothing is impossible if you have enough time and money, but design is important in determining suitable application methodol- ogy and therefore the cycle times and the costs involved. Some simple things, like trying to keep connectors or other no-coat areas on the same edge of the assembly, can make a huge difference to the ease of coating an assem- bly, the cost of coating that assembly and, of course, the overall reliability of that assembly. Well, hopefully, this column has kicked off the series to a great start. Look out for my next column which will focus more closely on a dif- ferent area of conformal coatings. If you have any questions in the meantime, please send them to us by clicking here. We appreciate your feedback. DESIGN007 Phil Kinner is the global business/ technical director for the Coatings Division of Electrolube. He is also the author of The Printed Circuit Assembler's Guide to... Conformal Coatings for Harsh Environments.

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