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58 DESIGN007 MAGAZINE I NOVEMBER 2022 Have you ever been convinced that an epoxy would be the best solution, but a PU resin won the day? is isn't something we typically see when we work with a manufacturer from the outset. Usually, it is clear once we have collaborated with them to identify their key requirements and end-use parameters to assess which chem- istry would be best. In fact, we are oen able to narrow it down to one or two specific res- ins to take to the testing stage. However, it is something that we have seen in the past when a manufacturer has chosen a resin for them- selves without any of our specialist input and find that their resin choice is not working well in the field. By way of an example, an EMS company was potting remote vehicle unlocking units for an OEM and had self-selected an epoxy for their units. Once the units reached the thermal cycling stage of approval testing, it was found that many units were failing, and further inves- tigation revealed that this was because compo- nents were being pulled off the board. What happened was the rigid, inflexible epoxy was adhering well to the assembly but expanding and contracting to a different degree than the board and components due to a CTE differ- ence. is was causing so much stress that the solder joints were breaking. We recommended that they test one of our resins instead that is a tough but flexible polyurethane, and they found when they used this particular resin, they saw no failures during thermal cycling as this was able to flex better with the assembly during rapid temperature changes. Do you have a most popular resin and why? is is a tricky one, many of our resins are very popular, but if I had to highlight specific resins it would be our general purpose ER2188 and UR5604 because they are excellent "all- rounder" resins. ER2188 is a tough and rigid epoxy, while UR5604 is a more flexible poly- urethane, although still tough enough for many environments. Both are flame retar- dant, to UL94 V-0 level, and have a good level of thermal conductivity to allow for heat dis- sipation away from hot spots in the assembly. Unlike many other resins with good thermal conductivity, they have excellent flow, allow- ing them to be used even in tightly spaced units that need the resin to flow into small gaps. Both resins offer excellent electrical performance and dielectric strength, and excellent general environmental protection. All these proper- ties make them an excellent starting point for many applications. In the early days of potting and encapsula- tion, epoxy resins were certainly the materi- als of choice. But with epoxy resin technol- ogy maturing over recent years and most of the exciting developments in resin technology now taking place in polyurethane chemistry, use is gradually switching across. Polyurethane resins are now becoming more dominant and taking market share from epoxies to an increas- ing extent. DESIGN007 Beth Massey is head of encapsulation resins at Electrolube. To read past columns from Electrolube, click here. Additional content from Electrolube: • The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments by Phil Kinner • "Coatings Uncoated!" a free micro webinar series • You can also view other titles in our full I-007eBooks library