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Date: | Tue, 24 Jul 2007 12:37:27 EDT |
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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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