Apparently, my message got garbeled somewhere in the transmission. Therefore, here it is again: Hi Andy, The concept of an MBTF makes sense only if you have a product which has a constant failure rate for some reason. That is essentially the assumption underlying the MIL-HBK-217, which has been shown to be inadequate for electronic assemblies, because electronic products contain wearout failure mechanisms which cause a continuously increasing failure rate with time, and thus render the MTBF concept meaningless. For constant failure rates you can express the reliability in FITs (1 FIT = 1 failure/10**9 device-hours), a concept developed way-back by Bell Labs. But both FITs and MBTF are purely statistcal concepts and do not take into account the physics-of-failure of the machanisms and loading conditions the actually cause failures. At Bell Labs, we tried to get around this by using iFITs (time-dependent instantaneous FITs) to account for changing failure rates—while it works within the FIT frame work, it is cumbersome and really does not answer the basic question of the expected system reliability. In IPC-D-279, 'Design Guidelines for Reliable Surface Mount Technology Printed Board Assemblies', equations are given allowing you to get a cumulative failure probability for all the SM solder joints in a system. 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] ################################################################ 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 web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################