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.
Annular Ring Calculation
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.
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 Better DFM 40-point checklist includes the following DFM checks:
|Solder Mask Checks
Silk Screen Checks
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