Hi Tarun,
As Thomas Ahrens said, researchers have found that most LF-solders creep
slower than Sn/Pb-solders. The consequence is, that hold times at T-extremes need
to be longer during ATC than for Sn/Pb-solders [IPC-SM-785 and IPC-9701 make
the case for minimum 5 minute dwells at the T-extremes; for specially built
chambers 3.5 minutes work as an absolute minimum]. In order to have definitive
answers, acceleration models are necessary--they do not as yet exist.
So, what to do in the mean-time--after all engineering is the art to do the
right thing with insufficient information? Until appropriate acceleration
models are developed, I would suggest using the testing suggested in IPC-9701 at
the lowest delta-T [we know that LF-solders do have different more complex
responses to accelerated fatigue testing], but with T-extreme dwells at least
five-times longer [even that may be non-conservative].

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com

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