Influence of via stub and how to minimize its impacts

We worry about via stubs in high-speed designs because via stubs present more loss to the signal path and impedance discontinuity, they cause unwanted resonant frequency nulls which appear in the insertion loss plot.

Enclosed figure shows the insertion loss comparison by varying via stub lengths. The resonance occurs at different frequencies when the via stub length changes.

There are several techniques commonly employed to mitigate the effects of via stubs, such as routing layer optimization of PCB traces, using blind hole and/or buried hole, removing the via stub by back-drilling.

We will be happy to discuss with customers about issues relating to via stubs on boards, PCB design guidelines and PCB manufacturing of blind/buried hole and back-drilling.