The innerlayer failure you describe typically is the result of the z-axis
thermal expansion mismatch between the copper PTH barrel and the PWB resin
during soldering operations being too large for the interconnection to
withstand.  The actual failure mode (innerlayer foil, foil/electroless
interface, electroless/electrolytic interface) will tell what is the
'weakest' link. The weakest link needs to be 'strengthened' or loading
conditions need to be reduced; sometimes both. 
This subject is complex; I give a one- day workshop on the issue of design,
manufacturing, and reliability of plated-through holes and vias.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-Mail: [log in to unmask] 

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