ENGINEERING ANSWER
A controlled-impedance line needs a controlled return path. Plane splits, voids, connector cutouts and poorly placed layer transitions can cause discontinuities even when the straight trace width is correct.
What to decide before release
Follow the return current through each transition. Keep the reference continuous under RF routing and provide a close, appropriate ground connection when changing layers or entering a connector. Include any required antipads in simulation.
Checks to put in your design brief
- Inspect planes beneath every RF route
- Check layer-change return vias
- Model connector ground attachments
Important distinction
A good impedance coupon does not prove that a routed path crossing a plane gap is acceptable.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.