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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