Identify the unused stub
A plated through hole may connect a signal to an internal layer while leaving an unused barrel segment beyond that connection. That segment is a via stub. In a sufficiently demanding interconnect, the stub can affect the electrical response and become a candidate for removal.
PCB back drilling is a controlled-depth operation that removes the unwanted plated section using a larger drill. It is distinct from the original through-hole drilling operation. Determine the need from the interface and interconnect analysis rather than from board thickness alone.
Define the drilling side and stopping relationship
Identify which side is drilled, which vias are affected and which functional layer must remain connected. State the allowed residual stub and the required clearance from the protected connection. The fabricator must account for stackup variation and depth-control capability when proposing the process.
A depth number measured from the surface is meaningful only with the corresponding stackup and tolerances. Do not copy it unchanged after a dielectric or layer-order revision. Some designs may require operations from different sides or to different depths, each explicitly identified.
Allow room for the larger tool
The back-drill diameter is larger than the original plated opening, so the keepout must be checked on every affected layer. Review nearby copper, planes and other holes against the actual removal envelope and positional tolerance. The original via clearance is not automatically sufficient.
Coordinate the resulting hole with mask, component placement and assembly requirements. Review reference paths around the transition and any differential-pair symmetry requirements. Removing a stub addresses one interconnect feature; it does not automatically resolve every impedance or return-path issue.
Make acceptance measurable
Include an unambiguous back-drill table or layer data, stackup, diameters, protected layers and residual-stub requirement in the fabrication package. Ask the supplier to confirm the proposed interpretation and any needed design changes before release.
Agree dimensional or structural verification and any signal-integrity test structures required by the project. Retain the results with the approved revision. A design change to layer assignment, board thickness or via use must trigger review of the back-drill data so a formerly unused segment is not removed after it becomes functional.