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April 2008

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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:
Mon, 28 Apr 2008 14:11:03 EDT
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text/plain
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Hi David,
you are asking the wrong questions.
Q 1: The higher yield strength [both in shear and tension] of SAC solders
causes higher stresses at the interfaces, everything else being equal, and thus
produce more interfacial brittle failures.

Q 2: Higher cooling rates cause increased stresses at the interfaces,
everything else being equal, and thus produce more interfacial brittle failures or
pad cracking.

Of course, everthing else is NOT equal. Since SAC has a Solidus of 217C—34C
higher than SnPb, the thermal expansion strains and stresses are that much
higher. Sac also has a modulus of elasticity about 40% higher than SnPb—the
stiffer solder will also result in higher stresses at the interfaces and thus
produce more interfacial brittle failures or pad cracking.


Werner



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