Hi 'mtaylor,'
> 1) I posted a peripheral question about determining if a peak temperature
> could
> be detected by looking at the microstructure of solder joints.
> A: No, you cannot tell from the solder microstructure what the peak
temperature reach was. The solder micro structure depends solely on the rate of
cooling--fine grain structure=fast cooling; coarse grain structure=slower cooling. No
practical advantage is to be gained from cooling products faster to get finer
grain structures--however, you may get fractured SJs because of thermal shock
loading. Fine grain structures result in more cycles during accelerated
testing, however, SJ grain structure is inherently unstable, and thus the apparent
gain in reliability is an illusion.
> 2) Can you look at a board that has failed in a lead-free process and see
> if the board saw the same thermal experience as other boards in the lot? By
> thermal analysis? By metallurgical examination?
> A: No, you cannot. However, you can tell whether or not it sustained the same
level of damage. You get similar levels of damage by multiple exposures to
lower T-levels or one exposure to a high T-level. Moisture content will also
play an important role as does resin content and the degree of cure of the resin.
> 3) Within a board, could you look at a BGA for example, near a failure,
> and
> see if it saw more thermal experience (rework) than the surrounding board?
> A: Yes, but that determination is destructive, because you have to section
the PCB.
> Do you know if an acceptable procedure has been published that we could
> reference in support of an analysis? The answers help direct where to go to
> find a solution to prevent a future failure.
> A: No, there is no established procedure to my knowledge. I can tell you
where to go to prevent future occurrences--engage a consultant, if you do not have
the in-house expertise.
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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