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98 DESIGN007 MAGAZINE I SEPTEMBER 2020 ceramic fillers, which offer a combination of thermal conductivity and chemical stability. 2. What are some key differences between resins and coatings? The most noticeable differences are the methods of application (aerosol, conformal coating spray equipment, manual spray gun, and brushing for coatings, compared to mix- ing and dispensing equipment and resin packs for resins), the thickness of application (<100 microns for conformal coatings, <500 microns for thick coatings and >500 microns for res- ins), and approval ratings (coatings are gener- ally approved to UL746, while for resins, it very much depends on the application). Because of the coating thickness, coatings occupy less space and have a low increase in overall weight compared to resins. There are both coat- ings and resins that are based on epoxy, poly- urethane, and silicone chemistries, but there are also acrylate, acrylic, and parylene coat- ings that do not have a direct resin equivalent. 99% of resins are 100% solid systems, so they have low, or no VOCs released during cur- ing, while many coatings are solvent-based, although there are two-component (2K) and UV curable acrylate systems that are also avail- able as 100% solids. 3. Why would I choose a resin instead of a coating? The choice between a resin or a coating is normally down to application specifics. If the unit involved is to be subject to long-term immersion in various chemicals, and subject to long-term thermal, and/or physical shock cycling, then a resin is generally preferred. Also, if there are a large number of large com- ponents on a PCB, it is generally better to use a resin to encapsulate these than to coat them. Another scenario where a resin would be preferable is when the unit will be used in a situation where it is not easily accessible, or long continuous service life is required. In this incidence, a resin would be recommended to provide the extra protection and durability needed. 4. What are the consequences of having air bubbles trapped in encapsulation resins, and does this impair performance? Air bubbles can have a number of impacts upon the performance of the cured resin. Depending on the number and distribution of the bubbles, the actual thickness of the poly- mer layer applied will be decreased; hence, the level of protection will also be reduced, particularly against chemical attack. If there are air bubbles next to components, wiring, or tracks, then particularly when high volt- ages are applied, corona discharge can build up inside these voids and ultimately destroy the components or wiring. Voids also act as a weak point for thermal and physical shock, which can lead to the resin cracking in service. 5. Talk me through the best method for mixing a resin pack. Explain what can go wrong and why. First, you need to remove the resin pack from the outer packaging. In the case of poly- urethane and silicone resins, don't remove them from the foil pack until ready to use. Lay the pack out onto a flat surface and remove the centre clip. Use the clip to push the resin from one half of the pack to the other, then pick the pack up in both hands and mix in a circular motion for a couple of minutes. Place the pack back on the flat surface and use the clip to push the resin from the corners into the centre. Pick the pack up in both hands and continue mixing for a further minute. Repeat pushing the resin from the corners into the centre of the pack and then continue to mix for another minute to ensure that the mate- rial is fully mixed and uniform colour. Use the clip to push the mixed resin to the side of the pack with the angled seam. Roll the pack film up so that the pack fits into the hand. Cut off the corner of the pack and then tilt the pack to dispense the resin, applying slight pressure as required to maintain the flow. If the material from the corners of the pack is not pushed into the centre of the pack, then unmixed material

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