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50 DESIGN007 MAGAZINE I APRIL 2019 are temperature sensitive and would not sur- vive the exothermic reactions that occur when two-part epoxies and polyurethanes are mixed and cured; this is particularly so when a large volume of material is needed for pouring into a single unit. Silicones are also suitable for ap- plications where a high degree of flexibility is required, such as on flexible PCBs, and unlike epoxies, are easy to remove if circuit modifica- tions or repairs are required. Silicone resins have the broadest continu- ous operating temperature range of any of the available resin chemistries, so they are a nat- ural choice for both high- and low-tempera- ture applications. They maintain their flexibil- ity over this temperature range with very lit- tle signs of degradation over time. Due to their high flexibility, they place very low stresses on delicate components, particularly those with weak and fragile connecting legs. However, there is a downside to silicones, particularly the effectiveness of their adhesion to certain substrates. Moreover, their chemical resistance is not as good as that provided by an epoxy resin. Another category of resins— polyurethanes—would be the better choice for applications operating in the -30°C to +120°C region because they offer similar levels of flex- ibility and better adhesion to many substrates. 3. What key pain points are associated with resin selection? Resin selection is the art of compromise; it is deciding which characteristic or property of the resin is more important to your application compared with those previously outlined. Of- ten, the main areas of potential problems lie in deciding what are the realistic maximum and minimum requirements compared, for in- stance, to the design limits, which are likely to include large safety margins. Viscosity is another property that must be considered. Normally, where the lowest mixed viscosity resin is desired to promote excellent flow and coverage, there are thixotropic resins that behave somewhat differently in that their viscosity increases rapidly after mixing. This apparent change should not be confused with curing. The resin is still able to flow and has a useable life after it has stopped moving before reaching a final cure. 4. How do I overcome these pain points? In most cases, many of the potential sticking points come down to the design brief and dis- cussion between the designers and engineers as to what is feasible. If the high and low tem- peratures are only needed for a short period of time on an infrequent basis, and the normal operating temperature range is more modest, then this often opens up the choice to a much wider range of resins. Similarly, when considering chemical resis- tance, determine whether the resin is actually the primary point of exposure. For example, if an LED is potted with an optically clear resin, but then a plastic cover with a gasket is placed over the top of it, then the level of protection that the resin needs to provide is significant- ly reduced. Although it delivers the primary electrical insulation layer, it is only providing a secondary barrier against the environment. 5. To pot or not to pot? Why do we pot? Naturally, we would recommend that all electronic components—boards and units—be either coated or potted to extend the life of the finished unit and protect the components against the environment. The level of protec- tion required depends on the environment to which the finished unit may become exposed. It might be indoors in a domestic setting where a light layer of dust might be expected. Con - trast that with a unit submerged in a garden pond for four months of the year, or one that is There is a downside to silicones, particularly the effectiveness of their adhesion to certain substrates.

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