On high density boards, they are generally built on a "normal" core, then have microvia layers added simultaneously to top and bottom, one layer at a time (also called "sequential layup").

To increase density, microvias can be filled, then outer layer vias can be stacked directly on top.

Is it OK to do the same with the core layer vias?  Fill, overplate, then stack microvias on top of them?

Thanks,

Wayne Thayer

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