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Last month, I started this series of columns on resins by going back to basics, questioning the core rationale for potting and encapsulation with resins, their fundamental chemistries and how each resin type differs one from the other—indeed, how their individual properties can be exploited to maximise performance under a wide range of environmental conditions. I hope readers found this useful. Of course, when it comes to the choice and applications of resins, there’s a great deal more to discuss, 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.
For this month's column, I'm placing a bit more emphasis on the critical points that arise when you embark on resin selection. We need to get down to the real nitty gritty here because it’s important that circuit designers have an understanding of some of the basic properties of resins before deciding which is the most suitable for the task in hand.
But let's not get too technical at this stage; instead, I'd like to simplify the selection criteria and try to make life a bit easier for those who have to make the decisions. So, picking up on my colleague Phil Kinner's format of providing a five-point guide in his previous articles on conformal coatings, let's take a look at five key resin properties that, if not taken carefully into consideration, could have serious implications for the performance of your application.
Perhaps counterintuitively, bearing in mind that a resin is there to protect, the cured hardness of the resin is actually rather important as the higher the hardness, typically the stiffer the resin will be. For example, if cured too hard, the resin may place too much stress upon components, particularly those with fragile connection legs. On the other hand, if too soft, then the resin will be easy to remove and it will provide reduced protection against chemicals and leave components vulnerable to mechanical shock and vibration. Cured resin hardness is measured on the Shore A (softer polyurethanes) and D (harder epoxies) scales.
Of course, there are applications in which the encapsulation resin must have an extremely low hardness—in order to maintain flexibility at low temperatures, for example. This is particularly critical for devices that not only have sensitive components but which are also frequently exposed to rapid changes in temperature. Moreover, as some devices may need to be reworked, it will be necessary to remove the resin. In general, encapsulation resins can be difficult to remove; however, specialist formulations are available that allow the bulk of the resin to be cut out of the unit for this purpose.
What many might consider to be the least important property—colour—could actually be critical for certain applications. Optically clear resins may be desirable for a variety of reasons and this property is of particular importance when it comes to the potting of LED lighting fixtures, as the cured resin will obviously need to retain its clarity for the life of the unit. It must also be UV resistant, particularly for outdoor LED fixtures. Colourless resins are also useful for prototyping applications as the encapsulated components are easily viewed during and after environmental and mechanical testing.
To read this entire article, which appeared in the October 2016 issue of The PCB Design Magazine, click here.