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Dangling via stubs can distort signals passing through your interconnect, and decrease the usable bandwidth of the signal. A via stub acts as a transmission line antenna, and has a resonant frequency determined by the quarter wavelength of the structure. At this frequency, the transmitted signal is greatly attenuated, by up to 3dB. For low-frequency signals, this is not much of an issue because these signals are significantly lower than the resonant frequency of the via stub.
However, for higher-frequency signals (>1GHz), which are becoming more common as performance specifications are increased, this issue becomes a problem because the signals are transmitted at frequencies near or at the resonant frequency of the via stub. Harmonic components that are odd multiples of the fundamental frequency can also be highly attenuated.
The conventional solution to this problem is to back-drill (or control-depth drill) the vias to bore out the via stub barrels, so that the via stubs are reduced in length, if not completely removed.
If the via is short, compared to the signal rise time, then it acts mostly as excess shunt capacitance. The entire length of the via contributes to the capacitance, while only the section where the signal current actually flows makes up the inductance. However, a long via stub can develop resonance that exacerbates the effects of its capacitance. I should point out that it is fine to have a plated through-hole (PTH) via, providing the signal goes in at one end and out at the other, using the entire length of the barrel.
When a via’s stub length is equal to a quarter wavelength of the signal frequency, the signal travels from the trace to the end of the stub and then bounces off the open circuit end-point and back to the trace for a total distance of a half wavelength. This half wavelength travel has the effect of shifting the phase of the signal by 180 degrees, creating resonance in the via stub. The phase-shifted, reflected signal has a maximum value at a time when the signal has a minimum value, and vice-versa.
The Nyquist frequency of a discrete signal is defined as a half of the sampling rate of the signal and will have a strong frequency component at this frequency. In addition, the signal can have strong power spectrum harmonic components at frequencies greater than the Nyquist frequency typically up to the 5th harmonic. The resonant frequency of the via stub is inversely proportional to the dielectric constant of the material, surrounding the via, with a wavelength of four times the length of the unused portion of the via.
To read this entire article, which appeared in the August 2016 issue of The PCB Design Magazine, click here.