Ulrich,
I agree with your findings on ceramic boards and would also look into the
attachment reliability of Alloy42 TSOPs on FR-4 (although reliability is
relative and  depends on your requirements, use environment, product life,
etc...).

To add to Werner's comments (with which I agree), here are some test results:
I have seen thermal cycling tests of 32 I/O Alloy42 TSOPs on FR-4, where
solder joint characteristic lives (cycles to 63.2% failures) were about 1000
to < 2000 cycles (conditions: 0-100Celsius, ~ 5 minute dwells, ~ 48
cycles/day). The distribution of  failure times was tight (with high values
of Weibull shape parameters). Typically, one side of the attached TSOPs would
fail by "zipper" effect. In the same experiments, 32 I/O Copper TSOPs
survived about 5 times longer. 
Test results for both Copper and Alloy42 TSOPs were confirmed by several
solder joint reliability models (classical and FEA) that include local CTE
mismatch effects. 
For reference, see the following paper (for example): D. Noctor et al.,
"Accelerated testing and predictive modeling of the attachment reliability of
alloy 42 and copper leaded TSOPs", Proceedings, Nepcon East, Boston, MA, June
14-17, 1993, pp. 193-206.

I hope this helps. I do not know your test frequency but I would guess that,
if your test has been going since end of  November, you should have had
failures by the New Year. Can you share that information (off-line or thru
TechNet)?

Jean-Paul
________________________________________________________________
Jean-Paul Clech
EPSI Inc., P. O. Box 1522, Montclair, NJ 07042 - USA
tel: +1 (201)746-3796  fax: + 1 (201)655-0815
<A HREF="http://members.aol.com/Epsiinc1/index.html">http://members.aol.com/Ep
siinc1/index.html</A>
(click, or copy: http://members.aol.com/Epsiinc1/index.html )
__________________________________________________________________

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