On 5 April 2014 12:54, David VanHorn wrote: *> I saw a PCB which was entirely UL-94V0 go up in flames while connected to a * *> brick supply in "hiccup" mode, delivering only 1w average to the board.* Was it FR4 material or something like paper-phenolic (rated to UL-94 V0). > I've mentioned it here before, but a web search on "Low voltage, the > incompetent ignition source" will turn up a very interesting article. > > The full title tells you what it's about (although Dave's citing it is a ra ther strong clue). LOW VOLTAGE THE *IN*COMPETENT IGNITION SOURCE DISPELLING THE MYTH from: http://www.kodiakconsulting.com/page19/assets/Low%20Voltage_The%20Incompete= nt%20Ignition%20Source_Final.pdf ABSTRACT The present, widely held belief is that products powered by "low voltage" (typically less than 24V) electrical energy are incapable of starting fires since there exists an insufficient level of energy to produce sufficient heat and ignition. By analyzing commonly implemented electronic circuits found in a variety if household electronic devices, manufactured using standard wave solder or surface mount technology (SMT) processes, single point failures will be discussed and induced that demonstrate ignition at low voltages, or more precisely, low power levels. Through the application of a thoughtfully designed series of experiments, this paper will demonstrate that voltages as low as 3VDC and power levels in the 3W range or less, are entirely capable of igniting proximate fuel packages sufficient to produce sustained fire in a variety of electronic assemblies. __________ UL 94 V-x requirements http://web.rtpcompany.com/info/ul/ul94v012.htm UL 94 - Wikipedia: http://en.wikipedia.org/wiki/UL_94 Digikey - 3 pages.Notes that UL 94 V0 starts on page 2. Page 1 is the much less demanding UL94-HB. Not actually stated there !!!! but contains a section that reads ~~=3D "allows flaming incendiary arrows to rain down on hapless crowd below". _____________________ I dislike PCB materials which are cited as UL94-V0 as an effective MATERIAL spec, rather than as a safety spec. eg a device may be said to use FR4 and UL94V0 PCBs. The FR4 will be for the main PCB(s) latter is usually for small assemblies such as switches or sensors. This may well be a material made of paper laminated with class B clarified Yak's butter, or perhaps Phenolic material. This is usually used instead of FR$ either due to lower cost or (more likely) due to ability to be punched. FR$ tends to blunt punching dies in under 1000 operations. Such materials may fracture due to relatively mild stresses or just because they want to. If the fracture line is crossed by a PCB track or tracks it is liable to be broken. (Ask me how I know). Some manufacturers offer only FR4 or equivalent and use more expensive routing instead of punching. Russell --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .