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?
When you need to connect traces to another layer, you typically have to place a pad on your circuit board and drill a via on it to make the connection. The area on the pad that surrounds the via is called an annular ring.
Ideally, you want your annular ring to be dead-center in order to get the best connections possible between the vias and the layers. The perfect annular ring width is the (diameter of the pad – diameter of the hole) / 2.
Example: If your pad diameter equals 22 mils and the hole diameter equals 10 mils, the annular ring width is calculated in this manner: (22 – 10) / 2 = 6 mils.
However, even though you calculate and place the perfect annular ring on your 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.
Annular ring, tangency and breakout
These manufacturing issues can result in three different ways. If the PCB designer provided 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, and will still retain a good electrical connectivity. However, when the annular ring area is not wide enough, the hole could almost end up touching the boundaries of the pad and have an annular ring that equals 0. This is called tangency. The worst case scenario is when the hole shifts over the pad. This is what we call an annular breakout. Annular breakouts can lead to connection problems between the via and the layers.
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.
- If 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