Specifying Backdrill Depth and the Residual Via Stub

A note saying “backdrill high-speed vias” does not define a manufacturable operation. The fabricator needs to know which holes are affected, the approach side, the electrical layers that must remain connected, and the allowed residual barrel. The design team also needs to know whether the proposed process protects adjacent copper.

Start with the electrical span

Backdrilling removes an unused portion of a plated through-hole barrel using a larger drill under depth control. Altium’s documentation distinguishes the connected via span from the remaining stub and provides rules for checking stub length. Its published example values are design guidance, not a capability guarantee for every supplier. Altium: controlled-depth drilling.

For each affected via family, record the first and last electrically connected layers. Then identify the unused barrel above, below, or on both sides of that span. Review the actual net connections; a pad that appears unused in a top-view image may connect to a plane or an internal trace. Do not derive drill depth from overall board thickness alone.

Make the mechanical limits explicit

• Identify the backdrill tool and its clearance envelope, including agreed positional tolerance.
• Specify the approach side and protected connection layer for each operation.
• State the permitted residual stub using the agreed measurement definition.
• Provide the approved stackup and its thickness tolerances.
• Check nearby traces, planes, pads, and holes against the enlarged drilled region.

A concrete review case is a through via connecting the top layer to an internal signal layer. Drilling from the bottom must remove enough unused barrel to meet the electrical objective while retaining the intended internal connection. The process window depends on both depth control and the position of that internal layer in the finished stackup. A nominal depth without tolerance hides that relationship.

Align CAD outputs with the agreed process

Check the generated drill files and fabrication drawing together. Every backdrill tool should have an identifiable layer span and quantity. Confirm that the fabrication viewer displays the intended operations and that ordinary through drills remain distinguishable. If the fabricator proposes a different tool or depth, run the clearance and signal-integrity review again before accepting it.

Decide how compliance will be demonstrated

Agree on representative inspection evidence, such as a suitable section or other validated depth verification, and retain the association with the production lot. The appropriate sampling and acceptance criteria belong in the purchase agreement. A passing electrical continuity test alone cannot establish the remaining stub length. Finally, keep the requirement tied to the channel analysis: backdrilling adds process complexity, so apply it where the electrical design needs it and where the verified fabrication process can deliver the intended geometry.