Hi Werner, Hi Dave

I think, if one performs accelerated tests on solder joints it is more
important not to activate a deformation mechanism which does not occur in
reality than stick to any temperature range. Tin lead solder deforms with
two different deformation mechanisms: Grainboundary sliding (GBs)and
dislocation climb (DC). Which deformation mechanism is activated depends on
the deformation rate ( temperature exchange rate ) and the temperature.
Grain boundary sliding is diffusion controlled and occurs at lower
deformation rates and at higher temperatures than dislocation climb. At
-40°C the strain rate activating primarily  GBS is approx. 10E-6, at 125°C
approx. 10E-2. For a ceramic SMT capacitor 1210 on FR4 with a soldergap of
30um this results in temperature exchange rates of 0.1°C/min, and 500°C
/min respectively.
In performing accelerated testing with a Coffin- Mansion plot to perform a
lifetime prediction one must run temperature cycle tests with 2 or 3
different temperature ranges. In these test you shouldn't change the ratio
of GBS and DC to keep the Coffin Mansion exponent constant. Here lays the
rabbit in the pepper ( just a joke for those who know German ). It is not
easy to determine this ratio. We are working on it to prepare some diagrams
that bring all that theoretical stuff in a form easy to use. However,
measurements in automotive under hood electronics showed temperature
exchange rates of approx. 2°C/min. This induces mainly GBS at temperatures
around - 0°C. In office equippment we measured approx. 0.5°C/min. I think
for accelerated testing of the majority of electronic applications mainly
GBS can be assumed. In this case it might be wise to extend the temperature
range to higher temperatures  (avoiding the glass transition temperature of
the board). If one extends the temperature range to lower temperatures use
1°C / min. from -20°C to -10°C, 2°C / min from -10°C to 0°C, 4°C / min.
from 0°C to 20°C and above 20°C as fast as your equipment runs.
Another point is the dwell time. At high temperatures ( 100°C ) 5 minutes
dwell time are enough to relieve all the stress induced. The lower the
temperatures the longer one must wait. At -20°C at least 30 min dwell time
are necessary, otherwise a lot of strain is stored elastically in the PCB
or in the leads of the IC's. It is questionable, whether one safes time if
the temperature range is extended to lower temperatures.

Best regards
Guenter Grossmann


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