The Tech Blog

Last week we discussed antipads, and I mentioned we’d touch back on back drilling. So, let’s dive into that today!

First of all, what are via stubs? When a signal travels through a PCB via, it may only need to pass through certain layers, but the unused sections of the via remain in the board. These unused sections are known as via stubs. For example, routing from Layer 1 to Layer 3 on an 8-layer board causes a via stub between Layer 3 and Layer 8. While they might seem harmless, they can become a major source of signal degradation at high frequencies.

Now for some basic transmission theory, When a signal travels along a transmission line, and the line length is a quarter of the wavelength, the signal will reflect back to the start after traveling another quarter-wavelength, totaling half a wavelength. This creates a 180-degree phase shift, inverting the signal. The same concept applies to via stubs. When a via stub’s length is a quarter wavelength of the signal, it reflects back with a 180-degree phase shift, interfering with the original signal. This results in signal degradation, dips in signal quality, and a distorted eye diagram, indicating poor signal integrity. Check the chart in images to see what at max data rates via stubs starts becoming a problem in the high-speed domain.
Back drilling is a technique used to remove these unwanted via stubs. By drilling from the opposite side of the PCB, we remove the unused section of the via, leaving just the necessary portion for signal transmission. As explained in the antipads post, you can have drill offset due to tolerances so size your antipads properly. If you want to know and calculate the stub lengths possible for your signal, I would suggest searching online for Bert Simonovich’s blog post on via stubs. He explains it in greater detail.

Via stubs are why full-length through-hole connectors are fading for high-speed use. The pin acts as a stub to the last layer, causing signal issues. Now, shorter pin/SMD connectors are getting preferred.