Handling 10 Gbps Signals: Why FR-4 Might Be Your Enemy

Once you reach 10Gbps data rates, standard FR-4 becomes limited due to its high dielectric loss (loss tangent) and consequently attenuated high frequency signals, causing eye diagram closure and a loss of signal integrity. Eye diagram closure and loss of the signal's integrity will worsen with longer traces and is significantly worse at higher harmonics since the insertion loss will be much more significant with longer traces. In addition the different manufacturers of FR-4 have different property variances from one manufacturer to another. In order for the designer to maintain data integrity at high-speed, they generally will use a low-loss laminate (Rogers, Megtron). In some cases where FR-4 must be used, keep the traces short and maintain a tight impedance control, don't use vias. When operating at these speeds, your PCB material is no longer just a substrate, it is actually part of the signal path.