The microvia you have sectioned will be an enormous problem to  electroplate
- as you have found out.

It appears that very significant etchback of the resin/glass  under the
copper foil has occured. The "shelf" left by the copper surface foil  is requiring
the copper to plate back under itself.  This type of "desmear"  was common
when plasma was used to remove resin only dielectric some years  ago.  The way
the overhang problem was solved was to agressively  microetch the surface foil
to remove all the overhang.  I suggest the same  thing here.

How was the original copper foil removed for this  microvia?  You say CO2,
but it appears to me that too much copper is  present for the CO2 to cut through
it.  Either, this was a combined UV/CO2  microvia drill where the UV cut the
initial foil and the CO2 removed the epoxy  and glass fibers, or the original
hole in the foil was chemically etched.   Notice how the microvia drill
stopped nicely at the level two copper foil - the  way CO2 drills do.

I think your microvia will be easier to plate with more of a  "V" shape, and
certainly no copper foil overhang.

Denny Fritz
MacDermid, Inc.

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