Design007 Magazine

PCBD-July2016

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62 The PCB Design Magazine • July 2016 This month, I consider some of the more common, and often very frustrating, problems that may be encountered when coating elec- tronic circuit boards and components. I also discuss some practical solutions. As we all know, nothing in life is straight- forward. In any engineering discipline, if there is the slightest chance that something might go wrong, you can bet your bottom dollar that it will. The secret is to be prepared for it. For the purposes of this column, I'm going to concen- trate on the use of conformal coatings for the protection of electronic assemblies, highlight- ing some of the potential pitfalls associated with the choice of coating and the method of application. In each case, I will suggest an ap- proach that should mitigate the majority of problems you are likely to encounter. Problem: The quality and performance of a conformal coating material could be compromised according to the method of application. This issue is commonly encountered when a product is transferred from one circuit manu- facturer to another; for example, a product may be dip-coated in one country but selectively coated in another, with the specification requir- ing that the same material be used at both sites. The problem that arises here, however, is that using a material formulated for dip-coating in selective coating equipment can result in poor yield due to excessively fast drying and bubble entrapment. One of my customers spent six months try- ing to solve a bubble issue internally, without re- alising that the root cause of this problem lay in the material formulation. After working with the customer, we found that by changing the solvent blend, the bubble entrapment issue could eas- ily be resolved. Moreover, this solution simpli- fied the process and reduced the cycle time. And since the non-volatile formulation remained the same, there was no need to re-qualify. Problem: Achieving incorrect coating thickness, especially with acrylics. The IPC specification allows a dry film thickness of between 30 and 130 microns, with the greater thickness being achieved by the ap- plication of multiple coating layers. Trying to achieve a 130-micron dry film thickness from a by Phil Kinner ELECTROLUBE CONFORMAL COATINGS DIVISION When Coatings Go Wrong SENSIBLE DESIGN

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