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58 DESIGN007 MAGAZINE I JULY 2021 Another important factor to consider is the duration of time that the unit will be exposed to in the most extreme of operating conditions. For example, there is a vast difference between specifying a chemically resistant resin that can withstand fully permanent immersion in anti- freeze, and one that only requires resistance to occasional splashes of antifreeze that are wiped clear aer short periods of exposure. Similarly, if an application reaches a maxi- mum 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 maxi- mum is 90°C, then it is sufficient to specify a resin with a maximum operating temperature of, say, 100°C. Most polymeric materials will tolerate wider temperature excursions than originally quoted for continuous operation over short periods, similar to those seen in typical reflow profiles. Other application requirements for consid- eration might include flame retardancy. Is UL certification essential as part of material quali- fication or is it customary to accept a manufac- turer's internal test report? For LED lighting assemblies, optical clarity and UV resistance is important. Information on changes to corre- lated colour temperature (CCT), colour space (L*a*b*) and total colour difference (delta E*) can be useful when qualifying a resin to pot directly over a PCB with LEDs and lenses. Or perhaps a design engineer wants to protect their circuit design from potential IP the, in which case it is better to select an opaque resin. RF communication is used in many industries; these communication devices can benefit from encapsulation resins with low loss tangent and low dielectric constant properties to provide electrical insulation. Electrification in the transport industry requires high power density electrical compo- nents to sustain long ranges and high horse- power. To manage the heat generated by small, high power devices, thermally conduc- tive encapsulation resins can effectively dissi- pate heat away from hot components to a heat sink to sustain long-term performance. ere are several different thermal characterisation techniques including steady state and transient methods; each method gives some variety in results so it's worth bearing that in mind when comparing literature from different manufac- turers. What Types of Resin Chemistries are Available? Resin chemistries fall within three major classes: epoxy, polyurethane, and silicone. Epoxy is the strongest and most chemically resistant of the three, but it is brittle, challeng- ing if not impossible to remove for rework, and is typically limited to operating tempera- tures between -40°C and +150°C. Epoxy res- ins offer excellent adhesion to a wide range of substrates. e tough and flexible polyurethanes are suitable for applications operating at lower temperatures. Typically, polyurethane resins are only suitable for applications reaching max- imum temperatures of 110°C for long periods (though some can go to 130°C). e chemical resistance of a polyurethane resin is generally lower than that of an epoxy, but polyurethanes outperform epoxies in water and high humid- ity environments. Polyurethane resins are typically used in marine applications, where water penetration resistance is critical, and for applications subject to a high level of physical stress, such as the potting of accelerometers, Another important factor to consider is the duration of time that the unit will be exposed to in the most extreme of operating conditions.