Hello Werner,
See answers below.
JP
In a message dated 1/10/2005 5:46:24 PM Eastern Standard Time, Engelmaier
writes:
I do not know if I am misunderstanding you, please clarify for me.
1) I fully agree, that SAC will have higher stresses during T-cycling. These
higher stresses will result in somewhat higher creep rates than at the same
stresses as SnPb, but I do not see how the creep rates would be "higher than for
SnPb joints." The stresses will be higher in the range of about 30% or so,
but the SAC basic creep rates are lower by almost a factor of 100.
Looking at raw creep rate data (creep rate vs. stress) in the high stress
region (10-20 MPa) , creep rates for typical SAC alloys are as high or slightly
higher than for SnPb, regardless of temperatures. See Figures 1 and 2 in my
SMTAI'04 paper, for example. Similar observations have been made by others, e.g.
at CALCE and IZM-Fraunhofer institute.
According to SAC hysteresis loop simulations (my own and others), the above
stress levels are reached on both the hot and cold sides of typical thermal
cycling profiles. In-situ measurements (Shi et al., SMTAI'04) indicate peak
stresses of about 15MPa in real solder joints during ramp-ups.
The higher stresses for SAC during accelerated thermal cycling, and the
subsequent higher creep rates during ramps and at the beginning of dwell periods
also explain why profiles with slower ramps are more damaging (as observed by
Dusek et al., SMTAI'04).
2) You say "with longer dwells (in test but also under use conditions for
many types of applications)the hysteresis loops for SAC joints grow at a faster
rate than for SnPb joints (unpublished work)." I just do not understand this.
With long enough dwells, full stress relaxation will be reached with SAC but in
much longer times than for SnPb.
When you go to longer and longer dwell times, the width of SAC joint
hysteresis loops increases at a faster rate than for SnPb joints. This is because
during ramps, at the beginning of dwell periods, AND during a significant
fraction of dwell periods (more so on the cold side), SAC displays higher stresses
than in SnPb (as discussed above).
This is in agreement with the thermal cycling data of Bartelo et al. (APEX
2001) who showed that cyclic lives for SAC and SnPb joints would be shifted
around when testing under longer dwell conditions, with longer dwells leading to
SAC cyclic life being less than SnPb life. I have done hysteresis loop
simulations for the Bartelo et al. test vehicles and it appears that predictions of
relative lives (SAC compared to SnPb) based on strain energy criteria for dwell
times of 7 to 112 minutes are in very good agreement with Bartelo et al.'s
test results.
I'll probably present some of that material in the lead-free rel. workshop at
the upcoming APEX.
Regardless of the above details, I am now more concerned about applications
with long dwell times.
JP
__________________________
Jean-Paul Clech
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http://jpclech.com/
EPSI Inc.
P.O. Box 1522, Montclair, NJ 07042, USA
tel: +1 (973)746-3796
fax: +1 (973)655-0815
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