You did not say where your 'post separations', now renamed 'inner layer
separations  (ILS)' occurred, nor what the design (MLB thickness, PTH/PTV
diameter, plating thickness, foil type & thickness) and processing (HASL,
wave, reflow, manual soldering) details of your MLB are. ILS can occur in one
of three failure modes: (i) within the inner layer copper foil, (ii) at inner
layer copper foil/electroless copper flash interface, (iii) at electroless
copper flash/electrolytically plated copper interface.  Depending on the
failure mode, the solution is different, but etchback may not be adequate.
 You may have manual soldering operations (run too hot) that simply put too
much loading on your PTH/PTV structures. For (i) change to HTE copper foil,
for (ii) you may have inadequate smear removal or some problem in your
processing, and for (iii) you have some processing problem.  In any case, I
would recommend using IST testing to monitor the effects of your changes on
the reliability of your PTH/PTV interconnect structures. 

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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