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50 The PCB Design Magazine • September 2016 achieve the desired cure—provide that all-im- portant moisture resistance, as well as excel- lent electrical insulation, flexibility and good adhesion to most substrates, both metal and plastic. Some polyurethane resin systems pro- vide exceptional resistance to sea water, as well as temperature extremes, making them an ideal choice for marine applications such as sensor encapsulation. A good outdoor example of a polyurethane resin encapsulated component that requires maximum protection against wa- ter ingress is an LED lighting unit; these resins are also optically clear and UV stable, making them ideal for the task. There are one-part moisture curing poly- urethane resins available, but these are a no-no if you want to achieve moisture resistance, as moisture penetration is necessary in order to obtain full cure on potting or encapsulation. Some epoxy resins, too, offer good moisture resistance, as well as a fast cure, hardness and high stability, particularly in variable tempera- ture environments. These also adhere well to most substrates, even difficult substrates with a very low surface tension, such as PTFE, ensuring a watertight encapsulation. You might also need to protect electrical or electronic components that are likely to come into contact with chemicals, including acids, alkalis, solvents and a whole raft of other sub- stances that pose a threat to delicate circuits and components. Chemical resistance is very much the province of epoxy resins, though some of the tougher polyurethane products—as well as some silicone based formulations—will provide a degree of protection. Epoxy resin products are available to protect electrical/electronic units that undergo frequent or permanent immersion in solvents such as diesel fuel, leaded and un- leaded petrol and cellulose thinners. Protecting components from mechanical shock and vibration may also be on your must- do list. Here, a more flexible cured product is likely to provide the best solution as it adds a level of compliance which will help insulate the potted components against adverse mechanical movements. In addition to their moisture resis- tant properties, polyurethane resins also pro- vide this desired level of flexibility and so they are often the first choice for shock and vibration protection. Silicone resins, too, offer excellent flexibility and, in addition, will tolerate high operating temperatures. One more "shock" that I should mention is thermal shock, which can have a devastat- ing effect on electronic components, shorten- ing their operating lives at best, and destroying them completely at worst. The adverse effects of thermal stress can be ameliorated by using two-part epoxy resins, which are best for those applications that undergo severe temperature cycling or which are likely to suffer thermal shock. Some also provide the added bonus of flame-retardant technology. Finally, it's worth mentioning that, quite apart from providing all the protections listed above, opaque potting and encapsulation resins also conceal what lies beneath. This could pro- vide an effective foil against counterfeiters or those wishing to copy a circuit layout, helping you to protect your intellectual property. When it comes to the choice and applica- tions of resins, there's a great deal more to dis- cuss, and over the following months I hope to provide some useful tips and design advice that will help you in your quest for reliable circuit protection. PCBDESIGN Alistair Little is global business/ technical director for the Resins Division of Electrolube. THE LITTLE GUIDE TO RESINS " Some polyurethane resin sys- tems provide exceptional resistance to sea water, as well as temperature extremes, making them an ideal choice for marine applications such as sensor encapsulation. "