Characterization of a Printed Circuit Board Via

"Design Guidelines

[1] Use the minimum size drill bit for creating the via cylinder. This has less to do with lowering the capacitance of the via and more to do with raising its inductance. Since a via looks like a region of low impedance compared to a traditional printed circuit board transmission line, raising the inductance will increase its characteristic impedance to better match the connecting lines. 

[2] Use the minimum size pad that the PCB manufacturer allows. The pad is the source of the most capacitance. The ideal case would be to connect the transmission lines directly to the via cylinder. 

[3] Do not use the minimum size ground clearance radius. This is counterintuitive since in most cases, smaller is better. By having a small portion of the connecting traces near the via NOT run over a ground plane, two regions of higher impedance immediately before and after the via are introduced. These regions of higher impedance will counter the lower impedance characteristic of the via and better match the line. This effect can also be accomplished by placing very small surface mount inductors in series with the via immediately before and after. 

[4] Use the thinnest printed circuit board possible. This will reduce the overall height of all vias on the board. Reducing the height of the via will decrease the length of the discontinuity that the signal has to pass through. 

[5] Place vias that connect the ground planes together near the signal vias that pass through multiple ground planes. This provides a low impedance path for the return current to flow when the signal changes layers. This will reduce the discontinuities caused by the via."

Source: www.coe.montana.edu/ee/lameres/vitae/publications/a_thesis/thesis_002_msee.pdf

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