When you’re designing a new PCB, you’ll need to route on more than one layer on all but the simplest of designs. Vias are the standard structures for layer changes during design, and PCB design applications have made it exceedingly easy to route into multiple layers on a PCB. But just because it’s easy to route between layers doesn’t mean that you should.
Via inductance and parasitic with respect to other conductors will affect signal integrity in different ways, depending on the frequencies involved. Since vias are inevitable in modern PCB designs, how can a PCB designer ensure their new board will operate correctly in the required frequency range? If you know how via inductance affects signal behavior, it becomes easier to judge when and how many vias to use on high speed/high frequency interconnects.
If you start looking through the literature, you’ll find at least 2 different theoretical (semi-empirical) descriptions of via inductance in terms of the via length and radius. Via inductance can be determined empirically, or by approximating the structure as a simple cylindrical conductor. The two prominent equations for via inductance are shown below.
The second equation is more accurate as it includes an extra correction factor. These equations rely on a particular approximation, namely that the inner diameter is nearly equal to the outer diameter. They also always assume that the via is electrically short, which is only appropriate up to mmWave wavelengths (e.g., ~10 mm for a through-hole via in FR4, or about 15 GHz). Finally, by considering the via inductance and resistance, we have the total impedance of the via assuming the reference plane is an infinite distance away from the via:
Eq. (2): Via impedance due to via inductance, DC resistance, copper roughness (K), and skin effect resistance.
The problem with this equation is that, in reality, there is some capacitance back to some reference plane, which is the case for any conductor. Therefore, the via acts like a very short transmission line section. It will have some return loss and insertion loss, and its S-parameters can be measured. No matter how the via inductance is calculated, different sections of an interconnect will have different input impedance values along the length of the line.
Because a transmission line with vias will basically have an impedance discontinuity along the line, we have the following sources of signal loss on the interconnect:
It can be difficult to see the various electrical sources of impedance and transmission line behavior on a via without looking at the via’s structure. These parasitics, particularly parasitic capacitance, are what determine the transmission line behavior observed in mmWave PCBs. The parasitics along a via can be seen more clearly when we look at the structure of a via.
The image below shows were parasitics arise in a real PCB via. The via inductance and resistance are shown along the length of the via, which accounts for the impedance defined above in Eq. (2). Here, we have a complex capacitance across the via barrel, which accounts for the loss tangent and capacitive coupling between each side of the via barrel. If there is any nearby conductors outside the via barrel, there is also capacitive coupling to the nearby plane. In addition, the pad connected to the via will also have capacitive coupling to the nearby plane. We could draw similar images for blind/buried vias, and for filled/plated-over vias.
Structure of a typical through-hole via with parasitic elements marked in the via and surrounding reference plane/pad.
The point here is not necessarily to create a circuit model for a through-hole via. Rather, it is to show that the via’s characteristic impedance, and thus the input impedance for a (via + transmission line) section, are more complicated than just looking at the via inductance. At low frequencies, will be electrically short and thus the input impedance of a (via + transmission line) section will appear to be the characteristic impedance of the transmission line only.
At high frequencies, the via is no longer electrically short, and it will modify the input impedance of a (via + transmission line) section. In the case where a via stub is present, the via should be backdrilled to minimize the remaining stub length. This will set the input impedance of the stub section to infinity to ensure that no power is transferred into the stub.
At NWES, we provide PCB design and layout services to SMBs, large enterprises, and aerospace and defense electronics companies. We know how to deal with via inductance, high frequency parasitics, and major signal integrity problems in advanced RF systems, high speed backplanes, power systems, IoT devices, and much more. We’ve also partnered directly with EDA companies and multiple ITAR PCB manufacturing firms, and we help our clients get through the PCB manufacturing process with ease. Contact NWES for a consultation.