What you are saying concerning ball deformation is interesting. However, I
don't believe this applies here as no solder wetting was effected with the
balls in question, if I read the original post correctly.

If, during the cool down cycle, after reflow and wetting, the CTE mismatch
became a factor, the deformation would occur with some possible difficulty
with eutectics going from solids to liquids, and back, without going through
a plastic state. Therefore, little chance exists that x-y ball deformation
could be effected.

If the ball deformed, to its "normal" pumpkin shape, without "side"
deformation, during reflow, but failed to wet to the aformentioned solder
"mound" (supposedly "melted" and wetted to the solder termination pads)
there would be no x-y deformation. The squatted, pumpking shape would be
farily uniform as normally found during soldering operations.

The thermal profile, affecting whatever physical/chemical activity (not
enough heat to effect total melting or too much to diminish flux activation
properties thus rendering wetting ineffective) seems the culprit. This is
true provided no other factors (contamination, oxidation, or wrong solder
ball composition) are apparent.

Earl Moon

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