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50 DESIGN007 MAGAZINE I MARCH 2018 underlying circuit from being copied, then a resin will not only provide excellent protection due to its toughness, chemical resistance and adhesion to the substrate and components, but with pigmented resins, its opacity will ensure that the circuit detail is visually obscured. It is well to remember that some filled resins are also X-ray opaque! Why are silicone resins appropriate choices for high temperature applications that also demand protection against harsh environmen- tal conditions? Silicone resins have the broadest continu- ous operating temperat ure range of any of the resin chemistries that we currently offer, so they are a natural choice for both high and low temperature applications. They maintain their flexibility over this temperature range with very little signs of degradation over time. Due to their high flexibility, they place very low stresses on delicate components, particu - larly those with weak and fragile connecting legs . There is, however, a downside to silicones, particularly the effectiveness of their adhesion to certain substrates. Moreover, their chemical resistance is not as good as that provided by an epoxy resin. Another category of resins— polyurethanes—would be the better choice for applications operating in the -30 to +120°C region as these offer similar levels of flexibility and better adhesion to many substrates, and all for a lower price. How do resins work to reduce damage to PCBs from vibration? Due to the crosslinking that takes place dur- ing the curing of the resin, a three-dimen- sional structure is created which can absorb stresses and strains and help to distribute the forces involved over a larger volume. By doing so, the magnitude of the forces acting upon a component is considerably reduced compared to that of a non-potted component. The same reasoning applies to both physical and thermal shocks. In these cases, the thicker the resin layer applied, the greater the level of protec- tion that is provided. Optically clear resins are ideal for LED applications (for example protection of the LED itself), so why is UV stability so important for these resins? For LED lighting units, particularly those installed outdoors and in all weather, UV radiation is a fact of life, and the single greatest source is the sun. Most of resins have an aro- matic backbone (they contain benzene rings), which will turn yellow when exposed to UV light. They will then start to break down over time as the processes that cause yellowing also result in the formation of free radicals, which leads to the breaking of chemical bonds within the structure. Resins with an aliphatic backbone do not contain aromatic (benzene) rings, and are less likely to turn yellow and subsequently dete- riorate. Free radicals are still formed within aliphatic resins, but at a much lower concen- tration and they do not have as many sites to attack compared to aromatic resins. Of course, while being the main source of UV light on earth, the sun is not alone in emit- ting this potentially damaging radiation. Artifi- cial light sources (tungsten, fluorescent, metal halide, for example) all emit certain levels of UV light, which can attack the resin, not to mention the LEDs themselves, which also gen- erate some UV radiation. I hope this has been a useful overview that has provided further insight into the effective- ness of resin chemistries. Feel free to get in touch with any of your own queries. Until next time, folks. DESIGN007 Alistair Little is global business technical director of Electrolube's Resins Division. Q Q Q A A A Download Electrolube's free e-book, The Printed Circuit Assembler's Guide to... Conformal Coatings for Harsh Environments, here.