Issue link: https://iconnect007.uberflip.com/i/1315894
DECEMBER 2020 I DESIGN007 MAGAZINE 31 industry standards (IPC-CC-830), or national safety standards (UL746), while for resins, it is very much dependent on the application, and there are very few standards. Due to the coating thickness, coatings occupy less volume and have a lower increase in overall weight compared to resins. There are both coatings and resins that are based on epoxy, polyurethane, and silicone chemis- tries, but there are also acrylate, acrylic, and parylene coatings that do not have a direct resin equivalent. Ninety-nine percent of resins are 100% solid systems, so they have low or no VOCs released during curing, while many coatings are high in solvent content, although there are two-component (2K) and UV-cur- able acrylate systems that are also available as 100% solids. 5. Why would I choose a resin instead of a coating? Fundamentally, the choice will often depend on the design of the housing. If the housing is designed to be the primary protection, then a coating will often be used to provide secondary protection or additional insulation. If the hous- ing is not the primary barrier to the environ- ment, then an encapsulation resin will often be used to augment or replace the housing. The choice between a resin or a coating nor- mally comes down to application specifics. If the unit involved is to be subject to long-term immersion in various chemicals or 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 sometimes better to use a resin to encapsulate these than to coat them. Another scenario where a resin would be pref- erable is when the unit will be used in a situa- tion where it is not easily accessible, or a long continuous service life is required. In this case, a resin would be recommended to provide the extra protection and durability needed. However, the 2K conformal coatings have demonstrated excellent performance in con- densing environments and immersion tests. In recent trials simulating highly condensing and immersion conditions, a urethane coating, 2K301, gave the highest overall values in terms of circuit protection and showed the least change during condensing/immersion events. The very large difference in thickness between the 2K conformal coating and a urethane resin potted assembly didn't show a large increase in performance. Indeed, the 2K coating achieved much the same results as the resin applied at one-tenth of the thickness. 2K conformal coatings can be applied more thickly than previous coat- ing technologies without the risk of cracking. They can also be readily applied by selective coating techniques to provide a coating with better thickness and sharp edge coverage that has performance somewhere between where a conventional conformal coating fails and pot- ting is required. Conclusion Every customer and their project is different. Whilst 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 environment it will be subjected to, the dispensing method/equipment to be used, the curing times, and the temperature limitations that may be imposed during the production process. The more information that a customer can provide regarding the ultimate operating conditions—temperature range, likely chemical exposures, and so on—then all the better. Look out for my next column, where I will look at more considerations to improve the lifetime and reliability of electronic assem- blies. DESIGN007 Phil Kinner is the global business and technical director of conformal coatings at Electrolube. To read past columns or contact Kinner, click here. Download your free copy of Electrolube's book, The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments, and watch the micro webinar series "Coatings Uncoated!"