Jason, My comments relate to PTV (not SMT) reliability which [I think] would be the dominant failure mode for the board you described. I've taken Werner's course on high aspect ratio PTV reliability and, IMO it's among the best tutorials IPC has to offer - backed by solid theory and years of empirical data. One of IPC-TR-579's conclusions, as I remember, was that the PWB fabrication and assembly process alone consumes about 16% of a PWB's "life" (assuming 5 thermal excursions). A couple of years ago, while working in your neighborhood (i.e., Toronto), I wrote an MS Excel program based on the barrel-fatigue formulas that Engelmaier, et.al. used in IPC-TR-579. It usually predicts mean fatigue life anywhere from 8 - 24 cycles to failure for high aspect ratio plated-through-vias under reflow-type thermal excursions (i.e., from 20 deg.C to 210 deg.C). Failure, here, is carefully defined; for a thick board with several thousand high-aspect ratio PTVs (e.g., 8:1 or higher), it is a virtual statistical certainty that, after assembly, the barrel of at least one via has cracked though it may retain electrical continuity and complete functionality. [This is a conclusion on my part and I have no evidence to support it - Werner, please jump in and correct me if it's too much of a leap...] High-Tg laminates greatly enhance PTV reliability by reducing the strain on the copper barrels and microvias may eventually eliminate the problem altogether. Anyway, I'd be happy to run your numbers through my little spreadsheet and fax you the results; just e-mail me your fax number along with the following parameters: - Laminate Tg: typically 140C or 170C - PWB z-axis CTE below the Tg: default is 50 ppm/deg.C - PWB z-axis CTE above the Tg: usually 275-350 ppm/deg.C for a high-layer resin-rich build - minimum temp in thermal cycle: typ. ambient temp 20-25C - max. temp in thermal cycle: peak reflow temp. 205-230 C - PWB thickness: .100" in your case - drilled PTV diameter: typ. .010", .0135", .018" - number of vias per PWB: anywhere from 200 to 20000 - barrel plating thickness: default is .0012" - plating quality index: refer to IPC-TR-579; my default is 8 (I don't have TR-579, but as I remember, 8 is good but not superior) - PTH barrel copper ductility: typ. 15% - 22%; this is one of the most important parameters - brittle copper breaks early. - Weibull distribution shape factor: default = 3 which is a probability curve skewed just left of normal Since your PWB is OSP coated it has seen at least one less thermal excursion than if it were HASL. Also, since most of the PTV fatigue damage is done at temperatures above the Tg - and service temperatures never [??] exceed the laminate Tg - chances are if it survives assembly and rework, it's good for a long time. Joe Felts Independent Engineer mobile office on world tour - currently in... Phoenix. -----Original Message----- From: Jason P. Bragg [SMTP:[log in to unmask]] Sent: Friday, February 12, 1999 8:14 AM To: [log in to unmask] Subject: [TN] PCB Rework Reliability Hi Jason, I have done and published extensive work in the area and give workshops and consult in the subject matter. However, you can find all you need in IPC- D-279, Design Guidelines for Reliable Surface Mount Technology Printed Board Assemblies, Appendix B. All it takes is a little work on your part. I give workshops on this topic both at NEPCONWest and IPC Expo. 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] ################################################################ Has anyone done a study on the correlation of board reliability vs. the number of heat cycles a board sees? Particularly, if a board has been through 2 smt processes and wavesolder, how much smt rework can one do to the board and still feel confident that the product won't fail in the field? I'm especially interested in large boards, .100" thick, 18 layer, OSP finish. ____________________________________________________________ Jason Bragg, Component Failure Analyst Materials Analytical Lab, TPA, Celestica Inc. 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For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################ ################################################################ TechNet E-Mail Forum 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's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################