The relationship between the diameter of the hole and its length. When a manufacturer states that their production has an “aspect ratio” of 8:1 it means, for example, that the hole's diameter is 0.20 mm in a 1.60 mm thick PCB. For HDI structures, the aspect ratio for microvia is limited to 1:1, but 0.7-0.8:1 is preferable to easy the plating.