Aspect ratio is the relation between the hole diameter and the thickness of
the via/board.
A 10 mil hole in a 62 mil board is an aspect ratio of 6:1
Buried vias maybe 1:1 like 4 mil hole that is 4 mil deep, or even less like a
six mil hole that is only 4 mil deep.
Hope this helps
George Milad
Shipley company