Via Stubs: Why They Act Like Antennas and How to Back-Drill.

Unused parts of vias that extend past the signal layer (stubs), when used for high-frequency signaling, act as small antennas and result to reflections and loss of signals. Vias generate impedance discontinuities, and have resonant (not beneficial) frequencies. Use back-drilling to eliminate the stub by removing the leftover via barrel after production. It is very critical with multi-gigabit signaling since even small discontinuities are important. Minimizing the length of a via is important; ensure that poor performance isn’t caused by the board’s PCB emitting a signal.