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 Via 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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In practice, for a signal-to-return via transition pair, you want to place them as close as physical fabrication rules allow. Which is typically Via Pad to Via Pad ~6mil (0.15mm) or Hole Edge to Hole Edge ~15mil (0.4mm).
The Return Path Via is not needed when the signal is routed on adjacent signal layers that are referenced to the same plane.
Signal Path Graphics are from High-Speed Constraint Values 2nd Edition, Charles Pfeil