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Date: | Mon, 30 Oct 2000 16:32:42 +0100 |
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The time necessary for the relaxation of elastically stored deformation depends on the temperature as well as on the temperature gradient. Since the creep rate depends on temperature and stress the relaxation is a asymptotic process that leaves always a remaining amount of strain to be induced into the solder joint after a certain dwell time. However this remaining strain becomes negligible if the dwell time is sufficiently small. We did some calculations for ceramic chip capacitors 2220 on 1.6mm FR4 with a plastically deformed layer of 30um based on our deformation model for Sn62Pb36Ag2. I have the feeling that these calculations are quite a good guideline to estimate the dwell times necessary to leave a certain remaining strain to total relaxation in various applications.
In the case of 125°C I am sure that a dwell time of 5 Minutes after the specimen has reached the 125°C is sufficient to have less than 0.04 % remaining strain.
At -55°C The creep behaviour of Sn62Pb36Ag2 is quite slow. In my opinion the following dwell times are necessary:
Remaining strain Temperature gradient Dwell time
0.1% 2°C/min. 5min
0.06% 2°C/min 50min
0.04% 2°C/min 170min
0.1% 10°C/min 15 min
0.06% 10°C/min 150min
0.04% 10°C/min >500min
0.1% 40°C/min 350min
I didn't calculate any further since the dwell times for small amounts of remaining strain at 40°C/min and faster become very long. You can find these data as a graph for 2, 10, 40 and 80°C/min in the proceedings of the 1997 IEMT symposium in Austin TX.
Have a great day
Guenter
Guenter Grossmann
Swiss Federal Institute for Materials Testing and Research EMPA
Centre for Reliability
8600 Duebendorf
Switzerland
Phone: xx41 1 823 4279
Fax : xx41 1823 4054
mail: [log in to unmask]
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