Rich, your name sake at HP, Rich Nute, posted a message on this subject recently which I have taken the liberty of cutting and pasting because I thought it covered the basic principals well, although it was written to address creepage inner layers of multilayer pcbs.
Hi Chris: > To me, it's sort of funny in that it just says that the Creepage and > Clearance distances do not apply on inner layers of void free PCBs. > That's nice; but I can't find where a distance is specified. I mean, I > would think that there should be some minimum distance between an AC line > and a 5V SELV line on an inner layer of the board!!!! To answer this comment, we need to look at what a creepage is and its role in the scheme of the product. Almost all product constructions employ solid and air insulations, both in parallel and in series, between conductors. We call solid insulation "solid insulation." We call air insulation "clearance." We call the interface between solid insulation and air insulation "creepage." Note that solid insulation and air insulation are truly electrical insulations. Creepage is NOT an insulation. Creepage is not a material. It is simply a surface at which solid and air insulations meet. The surface of solid insulation is subject to deposition of airborne "pollution." Typical products provide little or no control of airborne materials to prevent deposition of the "polluting" material onto the surface of a solid insulating material. Polluting material is a solid, uncontrolled (i.e., not a known insulating) material in parallel with the solid (and air) insulations. The polluting material bridges the solid insulation, and therefore could jeopardize the safety function provided by the solid insulation. When sufficient polluting material accumulates on the surface of the solid insulation, the voltage across the insulator and the pollution causes micro-arcs in the pollution. These micro-arcs are high temperature, and cause thermal decomposition of the surface of the solid insulation. When organic materials decompose, they free up the carbon atoms, leaving a tiny carbon resistor on the surface of the solid insulation. Each tiny carbon resistor is in parallel with the adjacent solid insulation beneath the surface of the solid insulation. So the tiny carbon resistor is shorting out a small part of the surface of the solid insulation. This phenomenon is known as "treeing" due to the tracking pattern of the carbon path on the solid insulation surface. To account for the effect of pollutants on the surface of organic solid insulations, we require the creepage distance to be larger than the air distance (clearance). Because some materials are more resistant to tracking across the surface, the creepage distance is a function of the "relative tracking index" characteristic of the insulation. Where a solid insulation is not subject to pollution, there is no requirement for a creepage distance. Many standards specify that a hermetically sealed assembly is not subject to creepage distance requirements. Likewise, the inner layer of a multi-layer printed wiring board is not subject to pollution and therefore is not subject to creepage distance requirements. Best regards, Rich Rich Nute <[email protected]> Best Regards Ted Rook, Console Engineering, ext 4659 Please note our new location and phone numbers: Crest Audio Inc, 16-00 Pollitt Drive Fair Lawn, NJ 07410 USA 201 475 4600 telephone receptionist, 8.30 - 5 pm EST. 201 475 4659 direct line w/voice mail, 24 hrs. 201 475 4677 fax, 24 hrs. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc 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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
