Annular Rings and Multilayer PCB Design: Stay Within Your Tolerances

Altium Designer
|  Created: February 13, 2018  |  Updated: November 3, 2020

Close-up of circuit board with vias

One of my high school teachers had a pretty successful business for when he wasn’t teaching: he harvested maple syrup. I learned that a bit more goes into the process than simply taking the maple syrup from the tree after tapping a hole into it. You’ve got to let it rest and let some of it evaporate so you can distill the maple syrup into the tasty, flavorful product that will then go onto all of your pancakes. 

While there’s a whole process attached to making maple syrup, it still starts with that first hole being tapped in the maple tree. Similarly, there’s a whole process attached to making multilayered PCB designs work; however, they would be nowhere without vias. Vias are essentially a vertically drilled shaft that bridges the gap in between any number of layers. 

But, even vias have considerations that need to be followed or your design may encounter failures. Annular rings come into play when considering the physical location of the drill holes through each via trace. There are a few factors that come into play when determining the proper annular ring size for your application.

Annular Ring Connection Strength: Vias and Multilayer PCBs

Multilayered PCBs are beneficial to some due to the greater capacity of complexity they can handle. Computers, phones, and medical equipment are some examples of applications in which are benefited via multilayered designs. Working with multilayered PCBs, however, poses a critical issue in connecting these layers to one another. Without connecting each layer to its corresponding points, you’ll just end up with multiple single layered PCBs glue to one another. 

Enter vias; our ingenious solution to vertically connecting each layer. Vias require an understanding of annular rings to work properly, though. These rings are defined as the minimum distance between the drilled hole and the edge of the via trace. The greater the annular ring, the greater the copper connection around the drill hole will be.

The use of your annular ring will often determine what size you should be shooting for. Are you soldering a component to one or both sides of the board? You’ll likely need a larger area for soldering’s sake. Are you simply using this via as a testing point and won’t solder? A smaller annular ring size will get you by.

Whatever the application you are using with your annular ring, you can easily determine the size by referencing our trusty old friend, the IPC-7251. This document recommends an annular ring width of 250 µm for maximum material condition (MMC). MMC simply means you’ll have the most robust solder joint. On the other hand, 150 µm is the recommended width of an annular ring to achieve the least material condition (LMC). LMC simply meaning that you’ll walk away with the least robust solder joint connection.

Obviously, these are just recommendations and can (and should!) be altered depending on your particular application.

Annular ring diagram
How an annular ring is measured.

Manufacturing Tolerances for Your Multilayer PCB Design

When the marriage of any multiple manufacturing processes happen within a production environment, there will often be slight errors that overlap due to tolerancing imperfections here and there. Specifically, when the process of etching the copper traces on your PCB and the process of drilling the vias through said traces come together, your drill holes will often not align completely on the center of these traces and will leave you slightly off-kilter. However, don’t fear; tolerances are here!

Since you already should be designing for manufacturing tolerance errors, designing for annular ring tolerance errors are no different. First, by identifying the manufacturer specific tolerances they’ll be able to accommodate for errors, and by determining the least amount of width your annular ring can safely be you can the de-risk the whole manufacturing process ensuring your minimum values are always reached.

In short, knowing that there will exist some manufacturing errors along the way but designing for said errors will keep you above your minimum values specifically speaking to annular rings of drilled via holes.

Arrangement of different sized drills
Drilling tolerances can play a huge part in determining your annular ring size.

Calculating Annular Ring Width

An easy way to verify that your width will be acceptable for your design is by calculating the maximum width you’ll be seeing post-production run. The following equation can be used:

((Diameter of the trace pad) - (Diameter of the drilled hole)) / 2 = (Max annular ring width)

The greater the manufacturing errors in tolerance are, the less the annular ring width will be. The greater the diameter of the drilled hole, the less the annular ring width will be. 

Knowing that the width of your annular ring should enable a strong enough connection for both electrical and mechanical connections, and being cognizant of manufacturing tolerancing will keep your annular ring width to acceptable distances and out of situations where the drilled hole doesn’t even touch the trace pad (heaven help us).

As vias are being designed into microvia territory, you’ll need PCB design software which can handle adequately designating annular ring widths and manufacturing tolerances for vias. Thankfully, Altium Designer can approach the sensitive layout complications with ease with their complete list of design rule checking as well as with intuitive board layout software. 

So go forth and design a strong and error-free annular ring that will keep your components on for the longevity of your design! If you are curious about what Altium software can do for your annular ring issues, talk to an Altium expert today.

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PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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