The 448 Gbps PAM4 Transition Chaos: Is Standard PCB Routing Dead?
DesignCon 2026 exposed a major crisis: traditional PCB copper routing is hitting a brick wall at 448 Gbps PAM4. We analyze why over-board twinax cables are displacing board-level traces.
Pushing data rates to 448 Gbps per channel using Pulse Amplitude Modulation 4-Level (PAM4) signaling completely breaks traditional assumptions about printed circuit board (PCB) layouts. At these extreme frequencies, standard copper traces behave less like pathways and more like high-attenuation antennas. The loss budget for standard ultra-low-loss dielectric substrates is so tight that even a few inches of trace can completely destroy the eye diagram.
The primary bottleneck is the physical transition from the PCB trace to the high-speed connector footprint. The via structures, breakout channels, and surface-mount pad geometries introduce parasitic capacitance and inductance that cause devastating return loss.
To bypass this physical barrier, hardware architects are bypassing the PCB substrate entirely. High-speed signals are routed straight from the processor package into vertical over-board twinax cable assemblies. By utilizing ultra-thin coaxial shielding directly over the board, engineers can bypass the lossy dielectric layers of the motherboard, maintaining signal integrity over much longer distances.