http://www.ieee-pses.org/symposium http://www.emc2004.org/ -------------------------------------------------- Ralph, In the case of IEC 60950-1 it is because that standard follows the basic standard for protection against electric shock IEC 60664-<don't recall>. Perhaps you think this is a cop-out when you were looking for an engineering answer, but it is also how standards are written in practice: product standards generally have to follow Basic Safety Publications. To provide a partial engineering answer however, it is true that the surface of the pcb under the encapsulating conformal coating cannot suffer surface contamination caused by dust. I have been told by a UK test house that certain conformal coatings actually degrade the insulation characteristics under high humidity conditions - presumably this occurs when the coating is more hydroscopic than the base laminate. These points hopefully provide some food for thought as regards creepage distances. Clearances exist in a vacuum as well as in air, so if you did get de-lamination then you would end up with a clearance even if there were no trapped air bubbles present. Of course, standards such as IEC 60950-1 have a number of tests in place to ensure that delamination is unlikely, which is why the standard permits relaxations when this technique is used (and all the type-tests have been past and there is a routine test 'just in case'). Regards, Richard Hughes In a message dated 08/05/2004 22:40:51 GMT Standard Time, [email protected] writes:
Dear Group, 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? 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? Ralph McDiarmid, AScT Compliance Engineering Group Xantrex Technology Inc. ------------------------------------------- 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
