Hi Charlie,
Get yourself a copy of IPC-SM-785, Guidelines for Accelerated Reliability
Testing of Surface Mount Solder Attachments, IPC-D-279, Design Guidelines for
Reliable Surface Mount Technology Printed Board Assemblies, would also be of
help.
There are formulas for relating the results of accelerated testing to actual
use environments, however if you T-cycle from -65 to +125C you do NOT have an
accelerated test—just a test leading to failure quickly. The difference is
that an accelerated test causes failures to result from the same damage
mechanism as in the field. T-cycling from -65 to +125C compounds (and
confounds) the effects of a number of damage mechanisms, some of which you
are not likely to see for product in the field. If you do T-shock (>~30
C/min) it the confounding is even worse. Thus, "programs or formulae readily
available which will predict joint failure after so many cycles at certain
temps, freqs, and g's" can not be valid. When you combine T-cycling with
vibration, the sequence of the tests become very important. Yopu will get
radically different answers if you T-cycle first and vibrate afterwards vs.
vibration followed by T-cycling.
I know the pressures management can put on their technical people to do
things fast (not as fast as realistically and validly possible) and with
minimum resources. When you do accelerated testing wrong, you are better off
not doing any testing at all, because then you at least do not have a set of
misleading data. You do not see any 'Comet' jet aircraft build by DeHaviland
flying anymore, do you. They got killed (and passengers along with them) as
the result of just such inappropriate "accelerated testing" "validating" an
inadequate design.
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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