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March 2007

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Fri, 16 Mar 2007 14:53:40 EDT
Content-Type:
text/plain
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Hi Gregg,
Last I looked MIT was in Cambridge.
Solder joint strength is never the issue with a properly 'wetted' solder 
joint. Under sufficient force you will rip the soldering pad out of the resin 
matrix, but not separate the solder joint.
The strength, or more accurately the yield strength, comes into the picture 
when thermal cycling, because the yield strength is highly 
temperature-dependent [as are almost all other properties of solder].
When T-cycling leadless solder joints the cyclic stress range will be bounded 
by the yield strength of the solder at the prevailing T; the reason why 
compliant leads work is that they do not allow the stress range to come close to 
the yield strength=f(T).
The high variability of lead geometries, and some of the above, make SJ 
strength standards impractical to say the least, and even if you could do it, what 
would you do with the information?
What is much more important is the failure mode—either brittle interfacial 
showing   there is something wrong with the SJ arrangement, or pad rip-out 
meaning the SJ is good.

Werner
P.S.: Bev, which part of the above do you disagree with?



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