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PCBD-Dec2016

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64 The PCB Design Magazine • December 2016 Over the past few months, I've covered quite a bit of ground regarding the choice of encapsu- lation and potting resins and the problems you are likely to encounter when using them. I hope these Sensible Design columns have provided plenty of food for thought as well as giving you a basic understanding of the principal resin ma- terials, their benefits and limitations. This month, I'm going to focus on the practi- cal aspects of applying and curing resins—what you should look out for in terms of material condition and the environment in which you are mixing and applying the resin, deciding on which mixing and application techniques are appropriate to your production circumstances, and paying due attention to achieving a satis- factory cure. It is important to check the state of your material stocks before you proceed to use them. Just as you might give a cursory glance to the "use-by" date of packaged food in your refriger- ator, before you even consider mixing together part A (the resin) and B (the hardener), always check that these materials are in date and that, visually at least, their condition is good. For example, part B of conventional two- part polyurethane resin formulations (the iso- cyanate) reacts strongly with moisture in the air, so should the packaging have been inadver- tently opened or compromised in any way and air has entered it, then a foam layer can form on top of the isocyanate and carbon dioxide gas will be released, pressurising the container. This provides evidence of a despoiled product, which should not be used and instead, be dis- posed of responsibly. If you purchase your resins and hardeners in bulk quantities and use just a fraction of the contents for each production run, then repeat- ed opening and closing of the containers will allow moist air to enter the air space above the liquids, with water being absorbed into the ma- terials as a result. Containers should be opened and closed as quickly as possible, or given a quick flush with dry nitrogen before closing to help prevent these problems. If this is not a practical proposition, a better solution might be to purchase the material in smaller container sizes, if your supplier stocks them (we supply resins in quantities ranging by Alistair Little ELECTROLUBE Understanding the Practicalities of Resin Application and Curing SENSIBLE DESIGN

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