Annular rings are one of the biggest concerns of PCB designers. You know that you may place your via right in the middle of the pad in the design files, but it just does not mean that you will 100 percent avoid tangencies, or even worse, breakouts. Let’s talk about annular rings, shall we?
What is an Annular Ring?
An annular ring is the area of copper pad around a drilled and finished hole. The finished hole we are talking about here is nothing but a copper plated via. All around this via there should be enough copper to form a solid connection between the copper traces and the via in a multi-layer PCB. Therefore, the main purpose of an annular ring is to establish a good connection between a via and the copper trace.
Construction of an Annular Ring
When you need to connect traces to another layer in a multi-layered board, you typically have to place a copper pad on your circuit board and drill a via on it to make the connection. The outer ring surrounding the via after drilling process constitutes the annular ring.
The Cosy Annular Ring Width
Ideally, designers want their annular rings with holes located dead-center in order to get the best connection possible between the vias and the layers.
The perfect annular ring width is the difference between the diameter of the copper pad and the diameter of the finished hole divided by two.
- Annular ring width = (Diameter of the pad – Diameter of the finished hole) / 2
For example, if your pad diameter equals 22 mils and the hole diameter equals 10 mils, then the annular ring width is calculated in this manner: (22 – 10) / 2 = 6 mils.
- Undesired annular ring
All the above may arise due to insufficient annular ring width in the design.
Even though the designers calculate and place the perfect annular ring in their CAD design, manufacturing issues often cause vias to be drilled off-center. Indeed, the drill bit may slightly wander and miss the middle of the pad. Another explanation could be that some layers may slightly shift during the lamination process. Or, the registration may not be 100% dead-center during the imaging process, and so on. This could lead to an inaccurate circuit and you will be left with a piece of junk.
Undesired Annular Ring, Tangency, and Breakout
The manufacturing issues mentioned earlier can result in three different problems.
If the PCB designer provides a wide annular ring area, chances are that the via will be drilled approximately in the middle of the pad. Even though it will not be dead-center, this will still retain good electrical connectivity.
If the PCB designer doesn’t provide a wide enough annular ring area, then the hole could almost end up touching the boundaries of the pad. This leads to an annular ring width that equals 0. Here, the drilled hole forms a tangent with the outer rim of the annular ring. This is called tangency. This will lead to connection problems between the via and the copper traces.
A designer’s worst nightmare is when the hole shifts over the copper pad. The drill bit might deviate outside the pad during the drilling process. This is what we call an annular breakout. Annular breakouts can lead to connection problems between the via and the layers. It will also cause problems with component placement, solderability, and so on.
Special Case (When God really hates you!):
When two vias are placed too close to each other, the current through them can be too high and can short the two vias. This results in a breakout or tangency. This phenomenon is referred to as Conductive Anodic Filament (CAF) formation.
One of the best ways to reduce the annular ring issues on your PCB is tear dropping. This trick provides an extra space on the pad for the drilling process. Here the copper pads are elongated towards the copper trace side making it look like a comet. This technique reduces breakout by providing more pad area to the drill bit. Teardrops are mandatory for designing medical and military boards (Class 3 boards).
How to Keep your Annular Rings Safe
Getting the perfect annular rings predominantly depends on your PCB manufacturer. The minimum annular ring varies from manufacturer to manufacturer. So, it is always good to find out their capabilities before placing an order. Some of the manufacturers do offer smaller annular rings at an extra cost. But smaller the annular ring more problems may arise within the hole. That’s something you need to look out for.
For a high-density PCB design, smaller annular ring assists in more component placement and saves more space on the PCB. This leads to efficient real estate utilization. As a result, there will be more room for traces and spaces.
The onus lies on the designer to make sure that there is enough annular ring width that can be manufactured by a manufacturer. Not to forget, sometimes the CAD tool that the designer uses doesn’t add an annular ring by default. Hence, these things must be taken care of before the design is sent to the manufacturer.
Quick Inspection Guide
Here are a few things that can help you achieve a healthy annular ring.
- As a quick inspection check if the copper pads are present for plated drills on all copper layer
- Check if the annular ring called out in fab can be maintained
For example, in order to have minimum 2Mil annular ring the following must be considered:
2 layer job 4 Mil in working data
4 layer job 5 Mil in working data
6 layer job 5-6 Mil in working data
>6 layer 7 Mil in working data
The Annular ring plays a critical role in PCB design and manufacturing. Hence, it’s a good practice to make sure all your annular rings are carefully crafted.
How Sierra Can Help
Designing for manufacturing is the key to success. The free Better DFM is an online tool that checks your Design For Manufacturability. When using this tool, you have the possibility to go to Advanced Options and choose what you need to check for in your annular rings. You now have several options:
- Tangency is the default option. If you are willing to accept tangency on pads in your manufactured board, the Better DFM will look for a minimum of 5-mil (0.005″) annular ring width in your design.
- When you need to have a minimum 1-mil annular ring on pads in your manufactured board, then the Better DFM will look for a minimum of 6-mil (0.006″) annular ring width in your design.
- If you are targeting an annular ring of minimum 2 mils on pads in your manufactured board, the tool will look for a minimum of 7-mil (0.007″) annular ring width in your design.
- Lastly, if you do not mind having breakouts on pads in the manufactured board, the Better DFM will practically not look for any minimum annular ring width in your design. However, this option is not recommended.
The Better DFM does not only check for annular rings. The tool does a comprehensive Design For Manufacturability analysis on your files.
The 40-point checklist includes the following DFM checks:
|Solder Mask Checks
Silk Screen Checks