Automated Via Stub Mitigation: Backdrilling vs. HDI

Via stubs are one of the most common sources of signal degradation in multi-layer PCBs above 5 Gbps. A via that passes through the full board thickness but exits the signal path at an inner layer leaves a stub — a quarter-wave resonator that creates a deep null in the insertion loss at its resonant frequency.

The Resonant Null

For a via stub of length L in a medium with effective dielectric constant Er, the resonant null occurs at: f_null = c / (4L√Er). A 1 mm stub in FR4 (Er≈4) resonates at approximately 37.5 GHz. At 25 Gbps NRZ, this is clear of the Nyquist frequency of 12.5 GHz. But at 56 Gbps PAM4 (14 GHz Nyquist) or 112 Gbps PAM4 (28 GHz Nyquist), the same stub creates significant problems.

Backdrilling

Backdrilling removes the via stub by drilling from the opposite board side to within a controlled depth of the signal exit point. The residual stub after backdrilling is typically 5–10 mils. A 5 mil residual stub resonates above 200 GHz — safely above any foreseeable digital signaling frequency.

The limitation of backdrilling is process tolerance: ±3 mil depth control means the actual residual stub varies from 2 to 8 mils, resulting in manufacturing variation in the resonant frequency. For 112 Gbps applications, this variation is acceptable; for 224 Gbps and beyond, more control is needed.

HDI Approach

High Density Interconnect (HDI) with blind and buried vias eliminates stubs entirely by using vias that start and stop exactly at the signal routing layers. The trade-off is cost: HDI processes add 15–40% to bare board cost depending on via aspect ratio and layer count.

FlowSim's via optimization engine automatically selects between backdrilling and HDI based on data rate, stub length, and a user-defined cost function, ensuring the minimum-cost solution that meets signal integrity requirements.