Where PCB tooling holes are used for alignment on a test fixture, consider these points during board layout to improve testability.

Ensure there are at least three tooling holes within the board outline. These can be used for alignment during assembly (e.g. solder paste printing) and are also useful for alignment on fixtures. Remember that tooling holes on panels, outside the board, may no longer be present during test.

Ensure tooling holes are un-plated. Plating is not needed and just introduces variability into the finished diameter. Un-plated holes will be the most accurately sized, ensuring good fit on tooling pins without binding.

Place tooling holes as far apart as possible in both X and Y for best registration accuracy. The same considerations apply as for fiducial marks, and it can often be convenient to place fiducials and tooling holes near each other.

If tooling holes will be use to support the board, as well as aligning it, during test, for stability ensure that the center of mass of the board is within the polygon described by the tooling hole locations.

Read more tips like this in our free PCB Design Guide.