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!

Back To Basics: DC Blocking Capacitors in High-Speed Communication

Posted under Back to Basics, Educational, Technology on October 27th, 2024 by

I was doing some high-speed PCB layout for a project and ran across DC Blocking capacitors or AC Coupling capacitors. These 2 names are used interchangeably and, in my opinion, remain the same for high-speed design. These caps play an essential role in maintaining signal integrity while enabling proper interfacing between different circuit sections.

A DC blocking capacitor is simply a capacitor connected in series that passes AC signals while blocking DC components in that line. It is effectively a high-pass filter. It removes the unwanted DC bias in the line. These can happen when there is some sort of encoding of signals in the line and it comes up as a non 50% duty cycle of 1s and 0s. Also when on an actual PCB, there may be 2 different chips communicating via a diff pair and each can be of a different DC operating point. So directly connecting them without a blocking capacitor can cause unwanted current flow or blow up the Tx or Rx drivers.

Since these caps are used in series on differential lines on PCBs with a particular line impedance, it causes a line impedance discontinuity. This can cause reflections for high-speed signals. So ideally we want to minimize the impedance changes. For that, we usually prefer using the smallest-sized resistors like 0201 rather than let’s say 0603 because to add a cap physically, you need to widen the controlled impedance traces to accommodate it. Now another option is to remove the return layers right below the capacitor as a slot. This will increase the impedance at the spot as the return layers are much farther. There are a few papers and a nice Intel app note which sums up the size of the slot. It shows how with a slot you effectively make the capacitor invisible(Check images) in terms of impedance changes. The usual chosen caps are 0.1uF or 0.01uF with low ESR.

In summary, DC-blocking capacitors are essential for interfacing high-speed communication lines like USB 3.0+, PCIe, SATA etc. Do take care on the placement position and PCB considerations when using them in your design.

If you liked the post, Share it with your friends!
1 … 14 15 16 17 18 … 86
Copyright © 2025 Amaldev. All Rights Reserved.