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Design007-Aug2020

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AUGUST 2020 I DESIGN007 MAGAZINE 79 sing on what is most important to the success- ful coating operation. 5. How is it possible that some areas of a coated board may never cure fully? There are several reasons why conformal coating in some areas of a coated board may never fully cure, each depending upon the cure chemistry. With water-based materi- als, for example, a film develops as the water evaporates, but the drying process can take an extremely long time, which can increase exponentially with applied thickness. If the material gets underneath a component and the component is tented with coating material, the material will dry much faster at the component leads and may impede the subsequent drying of the material beneath the component. The evaporating water cannot readily pass through the tent, so there is no kinetic driver for the remainder of the water to evaporate, and the coating will remain conductive. With UV curable materials, the primary reac- tion mechanism is initiated by UV radiation of suitable wavelength and intensity. Light only travels in straight lines, and due to the 3D topography of a typical PCB and tendency for material to wick beneath components, there will always be areas that don't see the UV radiation. To overcome this issue, formulators include a secondary cure mechanism, which is usually moisture initiated. The issue is similar to water-based in that very often the coating material will have cured around the component which was exposed to the UV light. If the coat- ing is a good barrier to humidity—which you would hope from a conformal coating—then it can take a very long time for moisture to dif- fuse through and initiate the secondary curing mechanism. Even if this happens, 50–70% of a typical formulation will not be involved in the moisture reaction and the product "cured" by the secondary mechanism only will have very different properties to the bulk material. The platinum catalyst used in certain heat- cured silicone materials is very sensitive to the presence of contaminants on the board, which can poison the catalyst and prevent cure. These contaminants include many amines and halides, flux acids, and other chemicals widely used in electronics assembly. Cleaning, cleanliness, and process control become very important with these types of material. Conclusion It's no easy task choosing the correct confor- mal coating for your product—let alone hav- ing the confidence that in applying it, you will have achieved the ultimate goal of protecting your electronics. Conformal coatings are avail- able in many generic types, and each type has its strengths and weaknesses. Choose the right coating for your intended use and operational environment, rather than one that is used by your subcontractor or qualified on another product line for a different end-use environ- ment. And, most importantly, be sure to test your design to ensure that it has sufficient robustness for the intended application. Just because a coating has an industry-standard approval doesn't mean it will solve your coat- ing needs. Testing is key to knowing what your material can and can't do. In my next column, I will look at more coat- ing-related tips. 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!" Conformal coatings are available in many generic types, and each type has its strengths and weaknesses.

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