Influence of Via Stub Length and Antipad Size

Alexander Ippich
Multek Inc.
Boblingen, Germany

The growing transmission speed and volume of digital content increases the pressure on reduction of overall insertion loss of printed circuit boards permanently.

In today's circuit boards, it is not only the transmission line itself, but also the via structure that impacts the insertion loss profile. To optimize the via, the stub length needs to be reduced by methods like backdrilling the copper out of the unused portion of the PTH.

In this paper, the influence of remaining stub lengths - varied between a couple of mils and 100mils - on the insertion loss profile is evaluated. As a second variable the size of the antipad is chosen and a two factor, multiple level DOE is performed.

Both, single ended and differential insertion loss is investigated and an 'analysis of variance' approach is used to determine the level of influence of the variables stub length and antipad size at various frequencies up to 40GHz. The frequency of the quarter-wave-length-resonance is correlated to the stub length as well as the increase of the insertion loss well below the resonance point is discussed.

The paper describes the test vehicle, the performed measurements and discusses the electrical performance characteristics of the various test cells. A recommendation for an acceptable stub length is given.

In this investigation, data were generated to predict the additional insertion loss generated by via stubs of the launch vias. The effect on the frequency of the resonant notch in the loss profile was also demonstrated. Both parameters were evaluated over various stub lengths and antipad sizes.

The data confirmed that a larger via stub reduces the resonant frequency and increases the overall insertion loss. It was demonstrated in addition, that a smaller antipad size has the same effect, but to a much smaller degree.