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Design007-Feb2020

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72 DESIGN007 MAGAZINE I FEBRUARY 2020 If there are air bubbles next to components, wiring, or tracks, then particularly when high voltages are applied, corona can build up in- side 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. Bubbles in optically clear resins not only look unattractive but will also distort the light passing through the resin either by the chang- es in the refractive index or by diffusion of the light. Bubbles on the surface of the resin may break over time, with dirt and detritus collect- ing in the resulting crater. 2. What are some key differences between resins and coatings? The most noticeable differences are the methods of application, such as aerosol, con- formal coating spray equipment, manual spray gun, and brushing for coatings. This is com- pared to mixing and dispensing equipment and resin packs for resins, thickness of application (<100 microns for conformal coatings, <500 microns for thick coatings, and >500 microns for resins), and approval ratings (coatings are generally approved to UL746, whereas for res- ins, it heavily 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 coatings and resins 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. Ninety-nine percent of resins are 100% solid systems, meaning they have low, or no, VOCs released during curing, while many coatings are solvented. However, it is important to mention that there are two-component (2K) and UV-curable acrylate systems with 100% solids available. These 2K coatings, in particular, can be ap- plied up to 500 microns thick without crack- ing during thermal shock testing and enable a greater degree of component lead coverage to be achieved, resulting in improved perfor- mance during thermal shock, powered salt- spray testing, MFG testing, and condensation testing (i.e., traditionally gruelling test regimes that are commonly used during automotive qualification campaigns). The 2K series is also VOC-free, solvent-free, and fast curing. 3. Why would I choose a resin instead of a coating? The choice between a resin or a coating is usually down to application specifics. If the unit involved is to be subject to long-term immersion in various chemicals, continuous thermal, and/or physical shock cycling, then a resin is generally preferred. Additionally, if there are a large number of big components on a PCB, it is generally better to use a resin to encapsulate these than to coat them. Further consideration should be given if the unit is to be used in a situation where it is not easily ac- cessible or if a long continuous service life is required, in which case a resin would be rec- ommended to provide the extra protection and durability needed. 4. Are encapsulation resins and potting compounds basically identical, or are there differences? There are no real differences between the two in terms of the resins used, but there is a difference in what the resin is required to do. An encapsulation resin will totally cover the PCB and the components and can act as the protective support structure, while a pot- ting compound is used to fill a housing or en- closure containing the PCB and components. An encapsulation resin will adhere to the PCB and the components, and its outer faces will act as the primary barrier to protect the unit. However, a potting compound has to ad- here not just to the PCB and components, but also the housing. In this case, the differences in CTE (coefficient of thermal expansion) be- tween all the materials used can become a crit- ical factor, as the resin will be subjected to dif- ferent rates of expansion and contraction due to the materials concerned, as well as its own CTE. This can put a resin under extreme stress and, over time, lead to failure.

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