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72 DESIGN007 MAGAZINE I DECEMBER 2018 for both high- and low-temperature applica- tions as well as those subject to thermal shock. They also maintain their flexibility over this temperature range with very little sign of deg- radation over time. On the downside, silicones have poor adhesion on certain substrates, and their chemical resistance is not as good as that provided by an epoxy resin. As well as offering excellent chemical resis- tance, epoxies provide good temperature per- formance. However, due to their rigid nature, epoxies are not as good at protecting against physical shock. Polyurethanes, on the other hand, have excellent moisture and physical shock resistance but deliver poor high-temper- ature performance. As a result, polyurethanes are best restricted to applications operating in the -40 to +120°C region. However, they do provide similar levels of flexibility and better adhesion to many substrates compared with silicones at a lower cost. 3 Where the PCB is mounted in an enclosure (into which resin is poured to encapsu- late it fully), the enclosure material must be considered. In the case of plastic enclosures, these usually are injection moulded, so there might be traces of the release agent on the surfaces, which will result in poor resin adhe- sion unless the agent is removed beforehand. Some plastics are very moisture sensitive and likely to undergo dimensional and other physi- cal changes in humid conditions, which will impose physical stresses on the enclosure, encapsulation resin, PCB, and components. Where the enclosure is made from steel, alu- minium, or other metals, then the differences in the coefficient of thermal expansion (CTE) between the enclosure and resin will have to be taken into account as well as the surface treatment used, which might affect the adhe- sion of the resin. 4 Typically, the thicker the resin layer, the better the level of protection. However, unless all of the components on the PCB are of a uniform height, then the thickness of the resin layer will vary across the board and potentially slightly different levels of protection will be provided for individual components. Good board design and com - ponent selection will go a long way towards mitigating this type of problem, but the thin- nest resin layer must be assumed to be the level of protection offered across the board. Naturally, with the desire and need to reduce weight and/or volume, designers are inclined to reduce the amount of resin applied. None - theless, the expected service life needs to be factored in with thicker layers generally pro- viding better long-term protection. 5 Remember, before you even consider resin encapsulation or potting, the PCB needs to be thoroughly cleaned. Surface con- tamination can have a negative impact on the protection levels offered by encapsulation, particularly in cases of chemical resistance as it provides an easier route for chemicals to penetrate. In addition, contaminants will adversely affect the resin's ability to absorb physical and thermal shocks due to the weak layer formation between the resin and the PCB, which ultimately leads to delamina - tion. Of course, after cleaning, any solvent or cleaning solution must be removed, and the PCB needs to be thoroughly dried before resin application. Conclusion By paying attention to these basic design pointers, you are likely to achieve the levels of reliability and long service life that will ensure happy relationships with your customers. Over the coming months, I will take a look at a wide range of issues concerning the correct choice and application of resins, so be sure to check out this column in future issues of Design007 Magazine. DESIGN007 Alistair Little is global business/ technical director of resins at Elec- trolube. To read past columns from Electrolube, click here. To download your copy of Electrolube's micro eBook, The Printed Circuit Assem- bler's Guide to… Conformal Coatings for Harsh Environ- ments, click here.

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