Buried via will be different from PTH because it is typically filled with
resin from the prepreg or some via fill material.

Our finite element results suggest that there might be some increase in the
strain in copper for buried via when filled with resin.  This is because
resin has a low elastic modulus (does not help to bear stress) and a high
CTE (help stretch the copper barrel even more).  We have observed failures
in buried via, which prompted us to do this FEA study.

Type II, HDI design has buried via from L2 to Ln-1.  The reliability of
these vias should be looked into further.

Regards,

Mason

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