Drill and barrel
The drill sets the barrel diameter; plating thickness is controlled by the process. An aspect ratio beyond the process window produces thin or voided barrels regardless of the drill size.
Pad and ring
The pad must leave an annular ring after drill tolerance. Fine-pitch parts trade ring width for routing space, which is why via-in-pad and laser vias appear.
Practical rules
Keep the aspect ratio inside the fabricator's window, size vias for current and not only for routing, and keep pads on rework-prone parts generous.
