As always Rich provides excellent technical information . While a different standard was originally referenced in this thread, I'd like to consider for a moment Chris Maxwell's question "...there should be some minimum distance... on an inner layer of the board"L The 950-based standards have guidelines in this regard.
In section 2.9.6 'Enclosed or Sealed Parts' "For components or sub-assemblies which are enclosed or hermetically sealed against ingress of dirt or moisture, and which satisfy the following compliance requirements, the minimum internal CREEPAGE DISTANCES and CLEARANCES can be the values for Pollution Degree 1." The paragraph that follows states, "Compliance is checked by inspection, measurement and by subjecting the component or sub-assembly to the thermal cycling test of 2.9.5" That section was developed for printed circuit boards. The samples spend a month going through thermal cycling 0-100C. If the printed circuit fab was of low quality, the test would uncover the development of voids due to layer separation where there could be subsequent arcing then tracking. John A. Juhasz GE Interlogix Fiber Options Div. Bohemia, NY -----Original Message----- From: Rich Nute [mailto:ri...@sdd.hp.com] Sent: Wednesday, August 14, 2002 12:12 PM To: chris.maxw...@nettest.com Cc: richard.pa...@exgate.tek.com; emc-p...@majordomo.ieee.org Subject: Re: Creepage on PCB Internal Layers 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 ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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