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Design007-Jan2019

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72 DESIGN007 MAGAZINE I JANUARY 2019 of turbulent flows and the potential for air entrapment. Alternatively, you might want to consider the use of vacuum potting, which will ensure that the risk of air entrapment is as low as possible and that the resin will penetrate all of the available spaces. Q: How do I select the most appropriate resin for the job? A: Beyond ensuring that your electronic assembly will meet its design criteria in use, choosing the correct resin for encapsulation purposes is possibly the second most critical aspect of the entire design process. It is important to understand where and how the finished unit is going to be utilised and what performance criteria are expected of it. It is best practice to draw up a list of the standard operating conditions that the unit will be exposed to in order of importance, and then a list of what the extremes of those operating conditions are likely to be. Also, the duration of time that the unit is exposed to the extremes of the expected operating conditions is critical. There is a world of difference between specifying a chemically resistant resin that can withstand fully permanent immersion in antifreeze, for example, and one that only requires resistance to occasional splashes of anti- freeze that are wiped clear after short periods of exposure. In a similar vein, if an application reaches a maximum temperature of 150°C, but this only occurs once a day for a couple of minutes, and the rest of the time the normal operating maximum is 90°C, then it is quite satisfactory to specify a resin with a normal operating temperature of 120°C, for instance. Most modern materials will tolerate quite wide excursions for short periods. Other application requirements to be considered might include flame retardancy, should UL certification be necessary; optical clarity and UV resistance, particularly in the case of LED lighting assemblies; opacity, which is often desired when protecting circuit designs from potential IP theft; and RF signal compatibility. It is always recommended to undertake some testing to confirm the suitability of the selected resin as every application is unique in terms of the operating parameters, conditions, and the geometry of the unit. Q: What resin chemistries are available and how do they differ? A: There are three major classes of resin chemistry: epoxy, polyurethane, and silicone. Epoxy is the strongest and most chemically resistant of the three, but it is brittle, difficult (if not impossible) to remove for rework and repair, and is typically limited to operating temperatures between -40°C and +150°C. However, epoxy resins offer excellent adhesion to a wide range of substrates. Polyurethanes are both tough and flexible and are suitable for applications operating at lower temperatures. However, due to the limitations of polyurethane chemistry, this type of resin is only suitable for applications reaching a maximum temperature of 110°C (though some can go to 130°C). The chemical resistance of a polyurethane resin is generally lower than that of epoxy, but polyurethanes—depending on the chemical backbone—outperform epoxies in water splash or immersion and high- humidity environments. It is best practice to draw up a list of the standard operating conditions that the unit will be exposed to in order of importance, and then a list of what the extremes of those operating conditions are likely to be.

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