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
##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe: SUBSCRIBE TECHNET <your full name>
To unsubscribe: SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
847-509-9700 ext.5365
##############################################################
|