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July 1999

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 27 Jul 1999 09:21:08 EDT
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In a message dated 07/26/99 14:31:52, [log in to unmask] writes:
>Werner: Can you please elaborate on your concern about the low-T extreme
>below -20C? Thanks.
>Dongkai Shangguan

Hi Dongkai,
I wrote:
>> That is one of the faults of the infamous MIL-test of -55<->+125C; the 
upper
>> temperature extreme is 25C too high, falsifying all results for FR-4 with 
Tg of 
>> <150C. 
>> The other problem with this T-cycle is on the low temperature end because 
the 
>> test straddles the homologues temperature of the solder; when cycling 
where the 
>> low T-extreme is below -20C and the high T-extreme is above +20C (not 
exactly
>> the homologues temperature), two different damage mechanisms are confounded
>> making a valid extrapolation and a valid determination of an acceleration
>> factor impossible.

The problem is, that in creep-fatigue, which takes place above the homologues 
temperature, micro-cracks are starting to form early (at ~30 to 40% of 
cycles-to-failure) in the fatigue life of a solder joint. These micro-cracks 
do not have a significant stress concentration effect in the solder structure 
above the homologues temperature, because at these temperatures the solder 
readily creeps and thus the thermal expansion mismatches do not built up 
stresses. However, at lower temperatures, where solder no longer creeps 
readily and thus blunts stress concentrations, these micro-cracks——and 
somewhat later macro-cracks——become significant stress risers because the 
non-creeping solder responds to the thermal expansion mismatches with an 
increase in stress rather strain. 
Therefore, the two damage mechanisms, creep-fatigue causing early micro-crack 
formation at the higher temperatures, and stress built-up causing these 
cracks to act as stress risers at the lower temperatures, reinforce and 
accelerate each other. Now, of course, for product seeing these type of 
temperature excursions, this is precisely what happens in the field, and 
proper "Design for Reliability" requires rather large safety margins, because 
proper testing would take too long. 
The problem arises when people think they have an accelerated test by 
-55<->+125C cycling, or some similar cycle, from which they can extrapolate 
some reliability information, when in fact it is not possible to derive a 
valid acceleration factor. Roger Wild has experimentally shown the effect of 
these type cycles.

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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