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54 DESIGN007 MAGAZINE I MAY 2018 eventually be exposed to is often quite limited, and almost certainly not the broad range that is frequently listed at the design stage, just to be on the safe side. Environmental factors that normally affect a PCB are temperature, chemical attack, physical shock (vibration) and thermal shock; the trick is to decide which of these is likely to have the greatest impact upon your PCB and then concentrate on making an appropri- ate resin choice. Each of the three main resin types (epoxy, polyurethane and silicone) have strengths and weaknesses. Silicone resins have the broadest continuous operating temperature range of any of the resin chemistries, so they are a natural choice for both high- and low-temperature applications, as well as those subject to thermal shock. They also maintain their flexibility over this temper- ature range with very little sign of degradation over time. On the downside, silicones have poor adhesion on certain substrates and their chemical resistance is not as good as that pro- vided by an epoxy resin. As well as having excellent chemical resis- tance, epoxies provide good temperature per- formance. However, due to their rigid nature, epoxies are not so 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. They do, however, provide similar levels of flexibility and better adhesion to many substrates compared with silicones, and at a lower cost. Where the PCB is mounted in an enclo- sure into which resin is poured to fully encapsulate it, the material that the enclosure is made from needs to be taken into consider- ation. In the case of plastic enclosures, these are normally injection moulded, so there might be traces of release agent on the surfaces that will result in poor resin adhesion unless the agent is removed beforehand. Some plastics are very moisture sensitive and are likely to undergo dimensional and other physical changes in humid conditions, which will impose physical stresses on the enclosure, the encapsulation resin, the PCB and the components. Where the enclosure is made from steel, aluminium or other metals, then the differences in the coeffi- cient of thermal expansion between the enclo- sure and the resin will have to be considered, as well as the surface treatment used, which might affect the adhesion of the resin. Typically, the thicker the resin layer, the better the level of protection; however, unless all the components on the PCB are of a uniform height, then the thickness of the resin layer will vary across the board, and poten- tially slightly different levels of protection will be provided for individual components. Good board design and component selection will go a long way towards mitigating this type of problem, but the thinnest resin layer must be assumed to be the level of protection offered across the board. Naturally, with the desire/ need to reduce weight and/or volume, design- ers are inclined to reduce the amount of resin applied. Nonetheless, the expected service life needs to be factored in, with thicker layers gen- erally providing better long-term protection. Just 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, par- ticularly in cases of chemical resistance (as it provides an easier route for chemicals to pene- trate). In addition, contaminants will adversely affect the resin's ability to absorb physical and thermal shocks due to the weak layer forma- 3 4 5 2 Each of the three main resin types (epoxy, polyurethane and silicone) have strengths and weaknesses.