Hi Karl,
In general you are correct, however, solder, accept for low temperatures
and/or high strain rates, neither of which occurs in typical applications to
an extent to be significant, does not experience significant elastic strains
(be careful however, because the PCB, the component, and the leads can store
elastic deformations that covert into plastic deformation in the solder given
enough time for completion of the creep process). Thus, considering plastic
deformations only is appropriate for both use conditions and proper
accelerated testing (IPC-SM-785). However, the equation given by Pratap should
be considered only a very coarse acceleration transform; the exponent is
temperature and dwell time dependent because of the temperature- and time-
dependence of the solder creep dominant in the plastic strain formation.

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'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
################################################################