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Design007-Sept2019

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74 DESIGN007 MAGAZINE I SEPTEMBER 2019 should be given to the ease of removal of the coating. During the rework or repair process, it is likely that components will be replaced. Thus, it is highly advantageous that the coat- i ng can be easily and cleanly removed from the required areas to facilitate these repairs and that the coating has good adhesion to it- self to ensure that an effective and permanent seal can be made to the repaired area. Com- plex issues can be avoided entirely if the re- workability factor is accounted for during the coating selection process. Manufacturers like us offer products for the effective removal of conformal coatings, including those that are solvent-resistant, but with the right conformal coating product for your application, it can be easy to completely remove the entire confor - mal coating prior to repair. Keep reworkabil- ity in mind when selecting your coating prod- uct to save yourself a lot of aggravation in the long term. 2. Does a Thicker Level of Coating Coverage Equal Greater Protection? This is a good question that we get asked a lot, and the simple answer is, "It depends." Thicker coating coverage can achieve great- er protection of your assembly from an end- use environment. However, it is not unusual for end users to push the limits on coating thickness, favouring thicker films of coating for a number of reasons, such as protecting the assembly from a harsh environment or simply for additional peace of mind. That be - ing said, conformal coatings should not be applied in thicknesses greater than necessary nor exceed the thickness they were designed to be applied at. In fact, very thick application of conformal coatings can actually result in harmful levels of stress on components, vastly extend the manufacturing/curing times, and depending on the cure method, cause inadequate adhe - sion to the substrate. Designers should not specify coating fillets around bottom-terminat- ed components (BTCs)—such as ball grid ar- rays (BGAs) or quad flat no-leads (QFNs)—as capillary forces will pull conventional coatings under the part, which may result in reduced solder joint lifetime, especially during thermal transitions. Conformal coatings are normally applied to circuit boards by dipping or spray - ing, typically at a thickness of 20–50 microns, although conventional silicone and other spe- cialist coatings can be applied at up to 200 mi- crons. However, we have created an interesting option for the industry by developing the two- part (2K) coatings range, which can be ap- plied at much greater thicknesses, from 100– 300 μm greater than the average coating. But there is a reason for this. 2K conformal coat- ings are solvent-free and have been formu- lated to perform at these greater thicknesses with the same ease of application as a con- ventional conformal coating and are partic- ularly suitable for protection against harsh, condensing environments. So, there certain- ly won't be any need for angst over coating thickness with 2K coatings! However, to obtain the maximum levels of protection available from the coating, it is im- perative that the coating is applied in the rang- es of thickness specified on the datasheet. If you are in any doubt about appropriate coat- ing thickness, always consult the datasheet or get the advice of a reputable supplier; they have laboured hard to establish optimum coat- ing thicknesses for their products in all kinds of operating environments. You really don't need to apply coatings any thicker than speci- fied for additional protection or it could result in diminishing returns later on. Instead, work on achieving the very best degree of coverage for a given thickness range. Coverage is often more important than thickness, except in the harshest environments. Keep reworkability in mind when selecting your coating product to save yourself a lot of aggravation in the long term.

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