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54 DESIGN007 MAGAZINE I JULY 2020 This is usually accomplished by using selected ceramic fillers, which offer a combination of thermal conductivity and chemical stability. 2. Which resin chemistry types typically allow the widest operating temperature ranges, and why? Providing exceptional performance at high temperatures, silicone resins have the broad- est temperature range (-50 to +250°C), but these are generally soft resins and are not as chemically resistant as some of the other resins. Some polyurethanes can go down to temperatures lower than silicones (-60°C) but have a maximum operating temperature of +150°C, while epoxies are designed more for higher temperature applications (-40 to +200°C) but have excellent adhesion to a wide range of substrates and excellent chemi- cal resistance. 3. Why are some resins suited to different applications? It's all in the chemical bonds! Epoxies are tough yet can be brittle due to the high cross- link density that is possible. But this high crosslink density also means that the resins are very resistant to chemicals and have excel- lent adhesion to a wide range of substrates. Polyurethanes are generally made up of long flexible polyols linked together by reactive iso- cyanates, giving rise to the classic hard and soft segment polymer, which means the res- ins are normally tough yet flexible. The reac- tivity of the isocyanates also means the resins have good adhesion. Silicones are soft yet very flexible resins due to the presence of silicon in their chemical structure. This makes them very temperature stable and capable of withstand- ing a wide temperature range, particularly high temperatures. 4. Is it a good idea to use elevated temperatures to accelerate the curing process? Elevated cure temperatures are used to speed up the production process and reduce the cycle time. However, there are a few points that need to be considered. It is best to wait until the material has reached its gel time before sub- jecting the resin to a high temperature; if this is not possible or desirable, then the use of a temperature ramp is advised. In the case of epoxy resins, care must be taken due to the exothermic nature of the epoxy curing reaction, particularly with fast- curing unfilled resin systems. Also, the amount of resin being cast at one time in a unit is criti- cal. A large amount of resin has the potential to generate a lot of heat, which speeds up the curing reaction. For silicones, care must be taken when cur- ing as the catalysts used are very susceptible to being poisoned. It is recommended that sili- cone resins be cured in a separate oven from other resin types. If they are to be cured in the same oven as other resins, then the oven should be well ventilated before putting the sil- icone resin inside—no other resin types should be present. 5. Why would I potentially require a flexible encapsulation resin for my application, and what types of applications would typically suit this type of encapsulation resin? Flexible resins find a wide range of appli- cations as they can accept and absorb physi- cal and thermal stresses well. If a unit will be subjected to thermal cycling, either continu- ously or infrequently, then a flexible resin is designed to withstand the stresses induced under such conditions. Similarly, in the case of physical shock, where the electronics need to be protected against vibrations, then a flexible resin will absorb the stresses far more effec- tively than, perhaps, a more rigid resin. Conclusion Every customer and customer project is dif- ferent; while we can advise a customer as to which products are best suited to their needs based on our years of experience, it all boils down to the unit, dispensing method/equip- ment to be used, curing times, and tempera- ture limitations that may be imposed during

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