Boards stored in ODB++ already contain the required information in various attributes (Thickness, Copper Weight and Thickness of Material) and individual drill tables.
Drill Checks report the aspect ratio for the smallest through hole and vias (standard, blind and buried) on a drill layer. Each category reports the aspect ratio calculated as <drill_span>/(<hole_size>+const).
<drill_span> — As determined by v_aspect_ratio_calc_drill_span, is the value of board attribute Thickness (recommended where all drills are through hole and standard vias) or the calculation of the drill span obtained from the layer attributes Copper Weight and Thickness of Material (recommended where blind and buried vias are present and these attributes have been accurately defined).
For regular via drills, the span is considered as piercing all copper layers—a drill span of 3 layers pierces layer 1, 2 and 3. For vias of type laser or photo, the span is to the last layer, but does not pierce it.
<hole_size> — The size of the smallest plated through hole or via in the layer (diameter for circular pads or diagonal of the bounding box for non-circular pads). Plated slots are ignored. Whether to use finished or unfinished drill diameter is determined by ERF variable v_aspect_ratio_by_finished_size.
const — The correction constant determined by ERF variable v_pcb_aspect_ratio_correction.
If v_aspect_ratio_calc_drill_span = 0 and attribute Thickness does not exist, or its value is zero or a negative number, a very large value (10000) is reported to the relevant categories to indicate a critical problem. One of these messages is printed to the log:
Job attribute <Thickness> does not exist. (Unreliable results in Aspect Ratio categories.)
Illegal value for job attribute <Thickness> (0.000000). (Unreliable results in Aspect Ratio categories.)