In a message dated 07/26/99 14:31:52, [log in to unmask] writes: >Werner: Can you please elaborate on your concern about the low-T extreme >below -20C? Thanks. >Dongkai Shangguan Hi Dongkai, I wrote: >> That is one of the faults of the infamous MIL-test of -55<->+125C; the upper >> temperature extreme is 25C too high, falsifying all results for FR-4 with Tg of >> <150C. >> The other problem with this T-cycle is on the low temperature end because the >> test straddles the homologues temperature of the solder; when cycling where the >> low T-extreme is below -20C and the high T-extreme is above +20C (not exactly >> the homologues temperature), two different damage mechanisms are confounded >> making a valid extrapolation and a valid determination of an acceleration >> factor impossible. The problem is, that in creep-fatigue, which takes place above the homologues temperature, micro-cracks are starting to form early (at ~30 to 40% of cycles-to-failure) in the fatigue life of a solder joint. These micro-cracks do not have a significant stress concentration effect in the solder structure above the homologues temperature, because at these temperatures the solder readily creeps and thus the thermal expansion mismatches do not built up stresses. However, at lower temperatures, where solder no longer creeps readily and thus blunts stress concentrations, these micro-cracks——and somewhat later macro-cracks——become significant stress risers because the non-creeping solder responds to the thermal expansion mismatches with an increase in stress rather strain. Therefore, the two damage mechanisms, creep-fatigue causing early micro-crack formation at the higher temperatures, and stress built-up causing these cracks to act as stress risers at the lower temperatures, reinforce and accelerate each other. Now, of course, for product seeing these type of temperature excursions, this is precisely what happens in the field, and proper "Design for Reliability" requires rather large safety margins, because proper testing would take too long. The problem arises when people think they have an accelerated test by -55<->+125C cycling, or some similar cycle, from which they can extrapolate some reliability information, when in fact it is not possible to derive a valid acceleration factor. Roger Wild has experimentally shown the effect of these type cycles. Werner Engelmaier Engelmaier Associates, L.C. Electronic Packaging, Interconnection and Reliability Consulting 7 Jasmine Run Ormond Beach, FL 32174 USA Phone: 904-437-8747, Fax: 904-437-8737 E-mail: [log in to unmask], Website: www.engelmaier.com ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################