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Hi Ralph: > Why is conformal coating not considering solid insulation, but rather only > providing a reduction in pollution degree? If the coating were of > sufficient thickness and free of trapped air, then how can creepage or > clearance exist under it? Conformal coating MUST be considered solid insulation because it is a solid material between two conductors, and it must be an electrical insulator. A conformal coating necessarily prevents pollution >from accumulating on the conductors beneath the coating. As such, a conformal coating lengthens the path between two conductors on which pollution can accumulate. The pollution accumulates on the conformal coating instead of directly on the PCB. Clearances and creepages relate to air insulation. When two solid insulations butt up to each other, clearances and creepages continue to exist at the joint. A coating applied to a PWB does not automatically mean the joint is free of air and therefore would have no clearance or creepage. To be considered a continuous solid insulation, the construction of the joint between the two solid insulations must exclude air throughout the distance between the conductors. So, there are two factors, one being the construction of the joint, and two being the consistency of the construction of the joint throughout the joint. The construction of the joint must include a cement or similar mechanical adherence such as an amalgam. This would be evaluated by means of a peel strength test. Then, the cement or amalgam must be uniform throughout the area of the joint. This is much more difficult to evaluate. So, a simple coating does not necessarily conform to the construtional requirements of a contiguous solid insulation. Indeed, unless proven as contiguous, conformal coatings are treated as having air between the two solid insulations. > Some standards even consider encapsulation or potting for providing a > reduction in pollution degree, even where it provides void-free moulding > over the part. Isn't encapsulation the same as the moulding on a plastic > integrated circuit? Yes and no. Encapsulation or potting is evaluated the same way... the joints must exlude air and be contiguous. Not all encapsulations or potting processes exclude air in the joints between two solid insulations. The joint must be cemented, or the two materials must form an amalgam or equivalent at the joint. Best regards, Rich This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. IEEE PSES Main Website: http://www.ieee-pses.org/ To post a message send your e-mail to [email protected] Instructions for use of the list server: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
