ENGINEERING ANSWER
Unused via barrels can create resonant structures. Their effect depends on length, surrounding planes, dielectric environment and the frequency content of the signal, so a single frequency threshold for back-drilling is unreliable.
What to decide before release
Review the complete via path in a three-dimensional model or validated transition model. Consider layer placement, blind structures or controlled-depth drilling where appropriate. Include fabrication tolerances in any residual-stub requirement.
Checks to put in your design brief
- Identify the active and unused barrel lengths
- Check antipad geometry
- Agree any back-drill tolerance
Important distinction
Do not request back-drilling solely because a signal is “RF”; first determine whether the actual stub is a problem.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.