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84 The PCB Design Magazine • July 2017 RESINS: CUTTING THROUGH THE TECHNICAL JARGON Q. What are the main parameters to consider when making a resin selection and what pitfalls should I look out for? A. The size and geometry of a PCB assembly are important parameters to consider before you choose an encapsulation/potting resin. Size will dictate the required useable life and gel time of a two-part resin, and it will also have quite a big impact on cure time. Usable life and gel time data are typically calculated on a 100g mix size at room temperature (20-23°C). Short gel times are good for rapid turnaround of units, but may cause problems by allowing air to be released; long gel times, on the other hand, can extend production cycle times and introduce bottlenecks into the manufacturing process. It is important to note here that very small volumes of resin will take longer to cure and special care should be taken to ensure that the correct mix ratio is maintained. In the case of larger volumes of resin, while the cure times will be reduced, the user must consider the corresponding rise in temperature of the resin/ hardener mix, the "exotherm," which may be high enough to damage vulnerable components. Epoxy resins are much more exothermic than polyurethanes. With complicated geometries, the flow of the resin around the components and wiring must be taken into account. Vacuum potting should be considered where there are lots of components and/or wiring and a void-free encapsulation is required. Essentially, try to match the thermal and physical properties of the resin to the components and the substrate. Ideally, you want all three to have very similar thermal properties to minimise the stresses and strains on the PCB assembly during potting/encapsulation and subsequent curing. Q. What are the most sought-after resin require- ments for LED lighting? A. When choosing a resin for LED encapsulation, optical clarity is the prime consideration, closely followed by toughness. We have a variety of options that have achieved considerable success in these applications, and can even provide a diffused lighting effect, for example. Air entrapment is the bane of any LED lighting encapsulation as it will degrade the performance of a lighting unit. I offer a key tip here: An often-overlooked property is the mix viscosity of the resin, which will affect the way it flows around the components. The greater the number of components in a given area, the greater the chances of turbulent flow and subsequent air entrapment. Q. With RF applications, it is important that the resin does not attenuate the RF signal. How are resins formulated to achieve this? A. A polymer resin itself is transparent to radio frequencies, it is the fillers that are added—usually to render the cured resin flame retardant— that can cause RF signals to be scattered and attenuated. Clearly then, the best performance is obtained with resins with little or no filler in them (epoxies such as ER1137, and polyurethanes such as UR5048 are suitable for RF applications). We also have an epoxy, ER2141, which is filled with nickel and acts as an RF shield. Q. What are the steps for effective product selection? Are there any hard and fast rules or do these steps vary? A. Every customer and customer project is different; while we can advise a customer as to which products are best suited to their needs based on our years of experience, it all boils down to the unit, the dispensing method/equipment to be used, the curing times, and the temperature limitations that may be imposed during the production process. And the more information that the customer can provide regarding the resin's ultimate operating conditions— temperature range, likely chemical exposures and so on—then all the better. PCBDESIGN Alistair Little is technical director for Electrolube's Resins Division.