Guenter (or anyone),  How would the ramp time affect solder joint
reliability.  For example, how would thermal cycling with a ramp of 5
degrees C per minute compare to a thermal shock condition (going directly
from hot to cold), assuming the same temperature extremes and dwell times?

Thanks,
Jim M.


        -----Original Message-----
        From:   Guenter Grossmann [SMTP:[log in to unmask]]
        Sent:   Monday, October 30, 2000 10:33 AM
        To:     [log in to unmask]
        Subject:        [TN] Antw: [TN] BGA Thermal Cycling

        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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