Back To Basics: Via Stubs and Back Drilling

Posted under Back to Basics, Educational, Technology on November 10th, 2024 by

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.

If you liked the post, Share it with your friends!

Back To Basics: AntiPads

Posted under Back to Basics, Educational, Product Design, Technology on November 3rd, 2024 by

In PCB design, antipads might sound like a minor detail, but they’re crucial for ensuring signal integrity, especially in high-speed designs. Let’s break down today what antipads are and why they’re essential.

An antipad is a void (or clearance area or no-Copper region) around a via within a PCB inner layer. Its purpose is to prevent electrical connection between the via and the plane, except where it’s intentionally needed. Simple enough. For newbies who might not know, most PCB CAD software handles antipad sizing automatically. That’s why you don’t get a short on the inner layers when a via passes from the top to the bottom. It has a pull-back region defined already. They are usually round and are defined by a diameter larger than the via pad diameter. But in some cases, folks do use oval-shaped antipads.

One key factor to keep in mind is not to make anti pad diameter very small. There are 2 issues I see with it, one is drill wandering, meaning if for some reason your via drill is slightly offset from the actual drill center, you can touch the plane before when plating happens and a short can happen in production. Another case is when you do backdrilling(Will do a detailed post on this in the future) to remove a via stub. For back drilling, manufacturers usually use a larger drill than standard drill holes so the chances of colliding with the planes while drilling are higher. So talk to the PCB manufacturer for your tolerances.

Another consideration is that an antipad should be designed such that it maintains the impedance of a transmission line when moving across the layers. This becomes critical in high-speed designs. There are field solvers and some direct analytical equations to get the exact anti-pad diameters. Search online to find them. Another issue I have seen is when a bunch of signals(think parallel buses) change layers, relatively close to each other. you will have multiple antipads in the return plane close by without clearance, effectively becoming a slot in the return plane(Return planes are ideally unbroken). Take care to prevent that while routing.

If you liked the post, Share it with your friends!
1 … 15 16 17 18 19 … 89
Copyright © 2025 Amaldev. All Rights Reserved.