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50 The PCB Design Magazine • May 2017 TO ENCAPSULATE OR COAT: THAT IS THE QUESTION cuit board into a bath containing the coating material. Where large numbers of circuits have to be treated in a fast-moving production line, coatings are more likely to be applied by closely controlled robotic selective coating systems for maximum consistency. Potting compounds and encapsulation res- ins can be applied in thicknesses from 0.5 mm, but are generally applied much thicker than this, which can lead to a significant weight gain for the assembly. Weight gain aside, this increased thickness does mean that the PCB is far better protected against chemical attack, particularly in cases of prolonged immersion. A resin can also provide superior protection against physi- cal shock (depending upon the formulation), since its bulk will help to dissipate the forces across the PCB, rather than allowing them to be concentrated. And for those of you who are keen to protect your intellectual property and design advantages, a layer of dark coloured res- in can obscure the circuit layout and compo- nents from prying eyes. One thing to remember though is that attempting to remove the resin will damage the PCB, severely limiting opportu- nities for component replacements. Potting compounds and encapsulation res- ins are generally two-part systems, in which a resin is mixed with a hardener in a precise ratio to form a cross-linked polymer when cured. It's also possible to add mineral substances (fillers) to resins to improve their performance under certain operating conditions. Like conformal coatings, most resins will cure at room tempera- ture, and while this can be a relatively slow pro- cess in the case of potting resins, cure time can be reduced by applying heat. An important thing to be aware of when mixing a resin with its hardener is the inevitable rise in temperature of the mix as the majority of curing reactions are exothermic in nature. The rise in temperature can be controlled by care- ful judgement of the amount of material that needs to be cast in a single process, as too much resin applied at once may result in components becoming overheated and possibly irreversibly damaged. Before we get to the criteria for making a choice between potting and coating, it might interest readers to learn that, in certain applica- tions, where a two-part resin formulation may have been the first choice for circuit protection, a two-part conformal coating may turn out to be the better approach, thanks to its superior mechanical properties, compared with one-part coatings. For example, Electrolube has recently launched the 2K range of solvent-free coating materials, based on two-part chemistry that is similar to resins, but designed to be applied by selective coating equipment in the 200—400 mi- cron range, combining many of the advantages of both technologies and minimising many of the drawbacks of each. Moreover, switching from a resin to a conformal coating will elimi- nate the weight penalties of the former, which may be critical to some applications. So, how do we choose which approach is best for a particular application? Clearly, there will be applications where the choice of tech- nology is obvious, for example, where harsh environment duty demands the highest level of protection. In this case, users can be con- fident that potting and encapsulation will provide the necessary long term protection, so long as the correct resin has been selected, tested, and approved for the prevailing envi- ronmental conditions. As I mentioned, if a PCB housing provides adequate primary protection against adverse en- vironmental conditions, then a conformal coat- ing will protect that PCB should the primary protection fail in any way, as well as protecting against high humidity and condensation with- in the housing itself. And where ease and speed " One thing to remember though is that attempting to remove the resin will damage the PCB, severely limiting opportunities for component replacements. "

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