2. Via Design:
f
Purpose: Vias used for layer transitions can introduce impedance discontinuities, affecting signal quality in high-speed
designs.
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Implementation:
ɢ Use controlled impedance vias for high-speed signals, defining via geometry (drill size, pad size) in Constraint
Manager.
ɢ Keep high-speed vias as short as possible in the Z axis (whether using back-drilling or blind and buried vias) and avoid
unnecessary vias on high-speed signal paths.
ɢ For differential pairs, use closely spaced vias to preserve coupling and impedance consistency.
Practical Tips for High-Speed Layout
Layout with proper component placement, routing and via placement
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Use Ground Planes for Shielding: Ground planes reduce EMI and provide a stable return path for high-speed signals.
f
Limit Layer Transitions for High-Speed Signals: Each transition introduces potential impedance mismatches; minimize
transitions to maintain signal integrity.
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Simulate Routing Choices Early: Run pre-layout simulations to test high-speed routing strategies, making adjustments to
placement and routing before finalizing the layout.
20 www.cadence.com
OrCAD X High-Speed Digital Design Guide
