Via Capacitance
CHAPTER 4 4.13 Fringe Effect and Capacitance of Combination of structureVias are often part of the signal routing. In a high speed signal path, via acts as capacitor and degrades the quality of the signal. The extent of the degradation will depend upon the rise time of the signal.
The most common type of via is a through hole via. For signal layer, the through hole via typically starts from the top layer. There is a via pad which has a diameter, typically 10 mils larger than the via hole size. The via is drilled all way down to the bottom layer. If the signal has to come out of the bottom layer there is pad at the bottom layer. All inner layers also have the pads. The capacitance of the via increases with the increase in the pad size. A large pad increases the area of overlap between the signal pad and the return power planes, resulting in the increase in the capacitance. Similarly if the drill size in increased the capacitance increases. The large drill diameter results in the increase is the surface area of the via. The cylinder of the via couples with the returning power planes. It is not very tightly coupled to the power planes and hence the increase in the capacitance is not as dramatic as the increase in via pad size. The third factor is the presence or absence of the pad in the inner layers. If there are pads in the inner layers, it increases the capacitance of the via. Stripping off the inner layer pads decreases the capacitance but also reduces manufacturability. A fourth factor, upon which the via capacitance depends is the number of the inner power or ground layers. The capacitance increases if there are more inner layer power or ground planes.
The capacitance of via can be calculated using a 3D field solver. A typical PCB via has a capacitance of 0.30 pf.
In general the more the area of overlap or the interaction between the via and the return signal, the more is the capacitance. Often, vias are routed with pads in place in the inner layers. Presence of these pads increases the capacitance of these vias. Stripping of these inner pads reduces the capacitance, but also makes it unsuitable for manufacturing.
If you are routing power signals, use bigger vias with bigger pads. In addition to increasing the current carrying capacity, they provide additional capacitance. For signal layers, use smaller vias with smaller pad size. Note that a very small pad size increases chances of manufacturing error. This is because the mechanical drills have a typical tolerance of 2 to 3 mils. If the pad size is not substantially bigger than the drill size, the drill may strip off the pad. Keep the pad size minimum possible, at the same time, keep them manufacturable by adhering to the manufacturing tolerance.
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