A method for easily accessing inner layers

Nope, just held the bare bit and spun it with my fingers. It was quick, and worked easily.

The 0.036" hole was a snug fit for the 0.025" post. The soldering operation pushed the 0.025" post into the solder pool below, displacing the melt up around the post & forming a solid anchor plug for the post's base.

That worked out better than planned. It's surprisingly satisfactory--rigid, good contact, and strong enough to hold a 'scope probe.

Cheers, James

Make a box on the drill table that fits the BGA part, use that to aim the drill them replace the part with the full board using the box to align the board.

Or if you have DRO you could just aim using the coordinates from your PCB design.

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That would work. I was thinking more low-techish, I 'spose.

Two extra tips from my experiment: I practiced on a scrap board, first. Though I was expecting all sorts of difficulties (such as punching through the copper), it went tremendously well. That was very encouraging, so the next two were on the real board.

Secondly, while clearing out the bottom of the hole I used the continuity rig on the steel needle, too, so I could tell when an area was cleared. That was invaluable.

The main consideration in my case was not to puncture the groundplane I was targeting. A BGA would add 'avoid trace runs' to that, so, if it's busy under there, your DRO--or at least a caliper measurement to some landmarks, then center-punching--might be the way to go.

Hand-holding the drill bit works quite well, easy to control, and it's

a few centuries out of place, to handle a high-tech assignment.

Cheers, James Arthur