Check Which Vias the CAD Backdrill Setup Actually Selects

A backdrill requirement written in a drawing does not prove that the CAD output selected every intended via. Selection depends on the design’s connectivity, stack definitions, and configured rules. Compare the expected location set with the actual generated drill data before release.

Altium’s controlled-depth drilling documentation describes start-and-stop layer definitions together with rules used to control backdrilling. These software definitions need to match the engineering plan.

Build an expected location list

Identify the nets and specific transition structures intended for backdrilling. Record the intended drilling side and protected connection layer for each group. Use stable feature identifiers or coordinates tied to the released board frame.

Include examples that should remain undrilled. A rule that selects every via in a broad class may remove copper needed by a different connection topology. Reviewing exclusions is as important as counting selected locations. Compare individual exported drill positions rather than only total counts: one missing intended location and one unintended extra location can cancel numerically while leaving two important engineering errors undiscovered.

Compare selection and output

Inspect representative vias in cross-sectional or layer-aware views. Confirm which layers are electrically connected and which portion is intended to be removed. A net name alone cannot establish the topology of every via on that net.

Challenge changes that affect selection

After a stackup edit, net-class change, or routing revision, regenerate the selection and compare it again. A previously correct rule can produce a different result when a connection moves to another layer or the layer naming changes.

Review the actual exported drill files and accompanying drawing information. Keep backdrill operations distinguishable from ordinary through drilling and other controlled-depth operations. The fabricator should not need to infer process identity solely from a hole diameter.

Coordinate the physical depth and residual-stub requirements separately with fabrication engineering. CAD selection establishes which locations and layer relationships are intended; it does not by itself establish the supplier’s achieved depth capability or inspection method.

Retain the expected-versus-generated comparison with the release. When an engineering change affects only a small subset, that record makes the affected operations visible and provides a practical basis for confirming that the new drill data implements the revised intent without broadening the selection accidentally.