Hi Al,
No, you are not wrong. Many failures, excluding brittle interfacial failures 
due to some adhesion weakness, due to creep-fatigue of the solder are 
near-interfacial. That has 2 underlying reasons 1) many solder joints have their 
smallest cross-section near the interfaces, particularly solder balls, resulting in 
much higher loading near the interface, and 2) near the interfaces the solder 
alloy is somewhat depleted of Sn used up in the formation of IMCs, and there 
is evidence that solder alloy with an abundance of Pb-rich phase [sorry, no 
data yet on LF-solders] has less fatigue resistance.
So, your FEA model does not need to include the IMC layer(s), but in order to 
do what you are aiming at, you need the properties if the near-interfacial 
solder alloy—good luck with that!



Werner



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