Hi Roger,
Indeed, the combined effects of non-uniform plating current distribution 
[higher in the PTHs near the surface] and mass transport limited [not enough 
copper ions] plating in the holes are the causes for dog-bone plating and reduced 
strength and ductility of the Cu near the PCB center. The former is addressed 
with lower overall plating current densities, high acid/low copper plating 
solutions, current leveling additives, pulse plating, etc. and the latter by 
efforts like impingement plating.
The z-expansion strains/stresses are highest near the PTH center where the 
plating is thinn est and the properties worst; that is why barrel cracks occur 
near the PCB center [and they occur predominantely at the prepreg layers, 
because they have a higher CTE than the laminate layers]. 
The land rotation is highest near the PCB surface; that is the reason why 
shoulder cracks and inner-layer separations occur at layers 1, 2, 3, and n-2, n-1 
and n. The land rotation is augmented by large tensile stresses in the radial 
direction due to the high 'hydro-static' compression along the whole barrel 
wall.
Shoulkder cracks typically start in the Cu laminate layer—many people skimp 
on the surface layer Cu quality choosing E1 instead of E3 copper foil.
 

Werner



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