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.
