Hi Guenter,
Well, I don't think what we are doing is arguing—its more a matter of trying 
to understand each others observations and and their interpretations.
The 'slip bands' you are speaking of (except you confuse me be stating you 
observed a "breakdown of the coarse solder structure after soldering into 
smaller grains—a refinement")  where first reported by Prof. Bill Morris, 
Berkeley, who called them 'grain coarsening bands'. This bands occur in the 
regions of highest loading showing a coarse grain structure surrounded by the 
original fine grain structure. These bands can be very pronounced in 
accelerated testing; in SJ field failures of product the bands are barely 
distinguishable, because the surrounding solder has also significantly 
coarsened due to time and temperature (activation energy ~0.52 eV). I have 
lots of SEM pictures showing that.
These coarsened bands are the precursors of micro-voiding at grain boundary 
intersections (first reported by  Prof. John Tien, Columbia—he called them 
'cavitations'); the micro-voids grow into coalescing and growing micro-cracks 
leading ultimately to SJ failure (again, lots of photos).
Just because these bands can be very narrow for localized high loading 
regions, does not mean that higher solder heights (columns) do not reduce the 
strains resulting from some given global thermal expansion mismatch. I have 
lots of data showing the increase in fatigue life with increasing solder 
joint height from about 2 mils (51 um) up to the 70 mils (1780 um) IBM has 
used. When you have SJs less than 2 mils (50um) thick, how much solder 
material do you have actually left after you account for the IMC layers on 
both sides and the solder volume from which the Sn has been depleted to form 
the IMCs?
Depending on the SJ geometry (including height), the coarsed 'slip bands' 
will be at different locations. I have even seen multiple bands as a slowly 
advancing crack tip relocated the bands of highest cylic loading.

Werner Engelmaier

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