Hi Guenter -
     
     I have always looked at the development of an empirical model that 
     predicts solder joint reliability in the electronics industry as one 
     of those after-the-fact dilemmas. Everyone has used solder as the 
     electrical and mechanical connection without thinking about the 
     material itself - it has always worked so why try to understand what 
     mechanisms are at work. Now that the electronics industry has pushed 
     the assembly pitchs down to .3mm (and lower!) the need to really 
     understand the deformation mechanisms occurring in solder are becoming 
     more critical. Lots of different opinions of the deformation 
     mechanisms are being suggested (e.g. Werner, Solomon, Lau, Frear, 
     Grossman) and with those resources dedicated to solving the problem 
     I'll bet a predictive formula that will have widespread industry 
     acceptance will emerge and be integrated into the design community. 
     I'm not a big Coffin-Manson equation fan because too many people try 
     to use the equation blindly without understanding how it applies. But 
     I do use the Coffin-Manson equation based on some stuff John Hagge has 
     published for first order estimates. When you complete your research 
     please publish the results so the rest of us can benefit.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Test temperature range
Author:  [log in to unmask] at ccmgw1
Date:    1/29/97 6:21 AM


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=B0C the strain rate activating primarily  GBS is approx. 10E-6, at 125= 
=B0C
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=B0C/min, and 500=B0C 
/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=B0C/min. This induces mainly GBS at temperatures
around - 0=B0C. In office equippment we measured approx. 0.5=B0C/min. I thin= 
k
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=B0C / min. from -20=B0C to -10=B0C, 2=B0C / min from -10=B0C to 0=B0C, 4= 
=B0C / min.
from 0=B0C to 20=B0C and above 20=B0C as fast as your equipment runs. 
Another point is the dwell time. At high temperatures ( 100=B0C ) 5 minutes 
dwell time are enough to relieve all the stress induced. The lower the 
temperatures the longer one must wait. At -20=B0C 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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