When routing high-speed signals that transition between layers, ensuring a low-inductance return path is critical for signal integrity and EMI control. Whenever a signal trace jumps through a via, its reference plane changes. If the accompanying return-path stitching via is placed too far away, the high-frequency return current is forced to take a wide detour, increasing loop area and generating severe ground bounce and electromagnetic radiation.
Wavelength-Based Stitching Rules of Thumb
To prevent the space between reference planes from acting as a parallel-plate waveguide or antenna, stitching and shielding ground vias must adhere to strict pitching limits based on the signal's knee frequency (fknee) and subsequent dielectric wavelength (λ).
- Maximum Spacing (λ/10): Standard high-speed guidelines recommend a maximum pitch of λ/10 to contain high-frequency components.
- Extreme Spacing (λ/20): For high-integrity or ultra-fast edges, tightening the pitch to λ/20 prevents cavity resonance and provides robust field localization.
- Example for 200pS Edge Rate
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The Return Path Via is not needed when the routed signal is referenced to the same plane.
Siganl Path Graphics are from High-Speed Constraint Values 2nd Edition, Charles Pfeil
