Defeating Via Stubs and Resonance at Frequencies Beyond 110 GHz
At frequencies exceeding 110 GHz, even microscopic via stubs act as accidental notch filters. Here is how advanced backdrilling tolerances impact high-speed signal pathways.
As next-generation AI computing clusters and 6G backhaul networks push operating frequencies past 110 GHz, micro-via stubs have become a premier source of signal degradation. A via stub—the unused portion of a plated through-hole via—acts as an open-ended transmission line. At ultra-high frequencies, the physical length of this stub matches a fraction of the signal's wavelength, creating a severe quarter-wave resonance loop that absorbs and reflects energy.
This resonance acts as an accidental notch filter, completely dropping out specific frequency bands from the transmission spectrum. The standard mitigation technique, backdrilling, removes the unused copper, but manufacturing tolerances pose a severe challenge.
A backdrill target tolerance variation of just a few mils can leave behind a microscopic stub large enough to disrupt signal integrity above 100 GHz. Consequently, layout engineers are forced to pivot toward blind or buried micro-vias, sequential lamination stacks, or pad-on-pad connector placements to completely eliminate the need for traditional through-hole transitions.