Guenter,
        It seems you have done a bit more work in this area than I have and thanks for the good info.  After all is said and done however, I think we agree on the final result.  No real benefit and a possibility of damaged components.

John Guy
ACI / EMPF
714 N. Senate Ave.
Indianapolis, IN 46202-3112
voice:  (317) 655-EMPF ext.130
fax:    (317) 655-3699
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-----Original Message-----
From:   Guenter Grossmann [SMTP:[log in to unmask]]
Sent:   Wednesday, August 13, 1997 9:48 AM
To:     [log in to unmask]
Subject:        Re: [TECHNET] Rework cooling rate

John

Tin Lead solder is weird stuff. Many things you observe in microscope are
not what one thinks to see. In the last years, when I worked with this
material I got the following Know How:

Tin Lead that is subject to fast cooling does not form a fine tin
structure. The lead phases are small but the tin forms large domains
containing these lead phases. It may well happen, that an entire SMT solder
joint consists of 1 or 2 domains. However most deformation takes place in
the tin matrix. Therefore the size of the lead phases has only a secondary
influence onto the reliability of solder joints.

Due to the high homologous temperature of tin lead solder ( even at ambient
temperatures ) the material deforms readily even if a minor load is
applied. There is no such thing like a Youngs modulus. Tin lead solder
creeps like a chewing gum with a weight hanging at it. Deformation is not a
matter of load but of time. This means, it is impossible to store any
stress in a solder joint at 20 degrees C. Any stress induced into the
solder joint will be relieved within minutes.

If thermal cycles are applied to a solder joint the deformation will cause
the tin domains to break up into smaller units thus forming fine tin
grains. Due to diffusion the lead phases will grow giving the impression of
a coarse structure. That's why you find the coarse lead phases primarily
along the interphase between the solder and the pads / components. That's
where the largest deformations take place.

As an outcome of the whole blabla I think I can say, that the cooling rate
of a solder joint after any solder operation desn't matter very much. The
only thing is that the ceramic capacitor may crack if the temperature shock
is too severe.

I hope this was not too trivial. If you wish I can explain the thing more
complicated.
These statements are my personal knowledge and understanding of the
subject. They do not necessarily correspond with the opinion of other
people working in the field of soft solder joints.
( I think I am getting silly )

Best regards
Guenter Grossmann