As data rates climb past 56 Gbps and 112 Gbps per channel, high-speed SerDes (Serializer/Deserializer) interfaces are pushing the limits of traditional PCB design. At these frequencies, signals behave less like electrical currents and more like electromagnetic waves.
Core Impedance & High-Frequency Signal Integrity Challenges
High-frequency SerDes routing is extremely vulnerable to signal degradation, primarily insertion loss, crosstalk, and impedance mismatches. Traditional FR4 substrates introduce massive dielectric loss, causing severe signal attenuation at multi-GHz frequencies. Furthermore, trace copper roughness creates a skin effect that increases resistance and limits high-speed performance.
Advanced Low-Loss Substrates and Differential Routing Strategies
To overcome signal degradation, engineers must shift to high-performance, low-loss laminates like Rogers or Megtron 6, which maintain highly stable dielectric constants (Dk) and low dissipation factors (Df). Along with material upgrades, critical routing strategies must be implemented:
- Controlled Impedance Routing: Enforcing strict stack-up parameters to maintain uniform characteristic impedance across the channel.
- Differential Pair Phase Matching: Ensuring zero trace-length skew within differential pairs to prevent common-mode noise conversion.
- Via Stubs back-drilling: Eliminating resonant stubs via back-drilling to prevent reflections and signal dips at higher PVT corners.
- Guard Trace Shielding: Placing coplanar ground shielding traces to reduce inter-channel crosstalk and electromagnetic radiation.
Simulation Toolchains and Testing Frameworks
Advanced pre-layout and post-layout signal integrity (SI) simulations using Cadence Sigrity, Ansys HFSS, and Synopsys PrimeSim are indispensable. These tools model transmission line behaviors, via transitions, and package-to-board breakouts, allowing designers to resolve impedance discontinuities long before fabrication.
Conclusion
Mastering material selection and advanced layout design is no longer optional for high-speed channels. Adopting a unified material-routing strategy ensures first-time-right silicon and board execution.