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Design007-Jan2019

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70 DESIGN007 MAGAZINE I JANUARY 2019 When I last broached the subject of potting and encapsulation resins, I went into some depth on the subject, explaining their chemistries and physical properties, how they behave when being mixed, applied and cured. For this column, I'm going to return to our tried-and-trusted Q&A format to offer four commonly asked questions about resins and their application, together with my responses, which I hope will help you achieve the best outcomes for all your potting and encapsulation jobs. So, setting material choice aside for the moment, let's start with a key aspect of potting: getting the resin in place. Q: What common problems are associated with the physical mechanics of potting? A: There are several problems that you will encounter when applying a resin to pot a component or an enclosed area on a PCB. Two problems that frequently crop up and must be addressed if you are to avoid some basic problems include: A c h i e v e m e n t of an even flow of resin within the volume to be potted, ensuring that all of the components within this volume are covered Ensuring that there is minimal air entrapment within the resin, which often occurs if the flow is turbulent; air bubbles formed in close proximity to the components are most undesirable as the heat transfer performance of the resin is compromised, and component overheating is likely to occur, which could also create an area for chemicals to attack the component Q: How do I avoid these problems? A: This will depend upon the size and geometry of the unit to be potted. For example, where medium to large size units are concerned, it is better to apply the resin in two or more locations to allow control of the flow of resin. If you are using a mobile dispensing head, I would suggest a layer of resin across the whole unit rather than just depositing the required amount of resin in one location and allowing it to flow naturally throughout the unit. The more components there are on a board (or in an assembly), and the more variable they are in terms of their geometry, the more complicated the flow patterns are likely to be. It may be possible to heat the resin to reduce its viscosity and improve the flow around a c o m p l i c a t e d c o m p o n e n t layout, but the downside of this will be a reduction in the resin's working and cure times. Where air entrapment is a concern, it might be better to review the layout of the components to reduce the incidence Getting the Best Performance from Encapsulation Resins Sensible Design by Alistair Little, ELECTROLUBE

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