Hi,
IPC-SM-785 is not just for SMT; solder behavior is independent of
application.
The "15 minutes (min.)" dwell times comes from the limitations of commercial
thermo-cyclic chambers and has nothing to do with the solder behavior. With
modified chambers one can run down to about 3.5 minutes; then problems with
differential heating rates due to different thermal masses become too large
and one no longer has a sufficiently well-defined dwell time. As Günter
pointed out the time required to (lets say 98% stress reduction
(creep)=conversion of elastically stored strains to plastic strains) will
vary significantly with the temperature. I would however, recommend that you
do not use 40C/min or larger, because at those large rates of temperature
change thermal shock effects become significant and acclerated test results
become dominated by totally different loading conditions than your product
sees.
In accelerated testing, one takes advantage of empirical modeling which
accounts for incomplete creep due to short dwell times (see equations in
IPC-SM-785).
You do not say what "space-borne" application you are talking about; there is
a huge difference between a space probe, e.g. Magellan, and a LEO satelite.
Also, when thermal cycling between extremes of -55C and +125C, the solder is
damaged by more than one damage mechanism, and your accelerated testing
results will be confunded by these different mechanisms. If your CTEs between
parts & PWB will be matched to <5ppm/C difference, you may noy have much to
worry about unless the components are large.
I would recommend you read IPC-SM-785 and IPC-D-279, Appendix A, to get an
understanding of the behavior of solder. A workshop on solder joint
reliability (see my website) would also help a lot.
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
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