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58 DESIGN007 MAGAZINE I MARCH 2020 1. What are some examples of a critical application, and how do these differ from non-critical? Normally, the phrase "critical application" is used when referring to safety-critical sys- tems. These are applications that will not only seriously impact on the user and their organization but also potentially result in se- rious environmental damage, injury, or loss of life if they malfunction. Examples of criti- cal applications include aircraft; automotive and railway operating and control systems; emergency lighting; burglar and fire alarm systems; medical equipment required by doc- tors, nurses, and surgeons; as well as pow- er grid systems and first responder communi- cation systems. Non-critical applications are normally where if a failure occurs, lives are not immediately at risk. If we look at poten- tial malfunctions within automotive applica- tions, the brakes would be considered criti- cal, and climate control would be considered non-critical. 2. I have heard mention of some resins containing glass beads (or similar). Why would these be required and offer some potential applications? Glass beads are added to resins for a num- ber of different reasons. They can be used as a filler to produce a low density, chemical re- sistant resin, which is good for weight criti- cal applications. They also control the mini- mum resin thickness that can be applied. Aes- thetically, glass and/or plastic beads can be used to provide a matt surface to resins or, in the case of optically clear resins, a diffused effect when light is passed through. An ex- ample is the UR5635 two-part, flexible poly- urethane potting compound, which has been formulated to provide a light-diffusing effect for LEDs. It offers a unique light diffusion ef- fect thanks to its cloudy appearance and has been used to great success as an LED potting compound for both decorative and protective applications. 3. What are some key pointers to remember when manually potting? Think about the amount of resin that you are going to need and across how many units. If there are a large number of units, and a large amount of resin required, it is often better to use a number of small-sized resin packs, as you will have better control over the potting pro- cess. If a large resin pack is used, then due to the volume of material, once mixed, the resin will start to cure over a shorter time-scale com- pared to a small volume pack. Consider where to place the resin so that it can flow around and under the components. With thixotropic resins, it is more critical, as the resin is purposely de- signed to have a low flow when applied. 4. Does thicker coverage equal better performance? The simple answer to this is yes. In general, they do perform better; however, there will be a point at which increasing the resin thickness will not offer any significant gain in perfor- mance. This heavily depends on the specifics of the application and the resin used. Gener- ally, a thicker resin layer is better if high lev- els of chemical resistance are required, par- ticularly in the case of long-term immersion or for high voltage systems to increase long- term performance. Considering the variation in height of components on a PCB, the thin- nest coating thickness needs to be taken as being representative across the entire unit rather than the average. In the case of physi- cal shock, a thicker layer of resin can help to protect the components and dissipate forces better than a thin layer. The major downside with increasing the resin thickness is the in- crease in weight and volume occupied by the encapsulated board. 5. What are the potential issues to be faced by selecting an inappropriate resin for a particular application? Okay, there's no dancing around the issue here; the simple answer is a premature fail-

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