What Is an Annular Ring in PCB Design?

This Gold Phoenix PCB Knowledge Center article explains what an annular ring is and how to calculate annular ring width in PCB design.

When traces on a circuit board need to connect to another layer, they typically end up on a pad and then through a via on that pad go to connect to another layer. An annular ring is the area on the pad that surrounds the via.

The width of the annular ring is an important design and manufacturing consideration.

During manufacturing there are many conditions that can cause the drilled hole to not be dead-center (even though that is how it is on the CAD design).

For example, during the drilling process, the drill bit may wander slightly, or during the lamination process some of the layers may have shifted slightly, or during imaging the registration is not 100% dead center and so on.

In fact, it is almost never the case that the manufactured hole is exactly dead center on the pad as designed in the CAD.

Thus, if the designer provides a wide annular ring area in the design, it is likely to ensure that in the manufactured printed circuit board good electrical connectivity between pad and the hole is still retained.

If the annular ring width is not enough, then during manufacturing, the hole could be almost touching the boundaries of the pad (called “Tangency”) or in the worst case, even go outside the boundary of the pad (“breakout”).

As long as the manufactured hole is inside the pad and even touching the boundaries of the pad, that is generally an acceptable situation. On the other hand, for certain applications or situations a designer may need absolutely that there should be a minimum left over annular ring (typically 1 or 2 mils) after manufacturing.

How Do You Calculate Annular Ring Width?

An Annular Ring Width is the (diameter of the pad − diameter of the hole) / 2.

Example 1: If diameter of the pad is 22 mils and diameter of hole is 10 mils then annular ring width = (22 − 10) / 2 = 6 mils.

Example 2: If we know that on a certain pad, the annular ring width is 8 mils, that means that pad diameter is (8 × 2 =) 16 mils more than the hole diameter.

Annular Ring Options in DFM Checking

When running DFM checking, you can go to Advanced Options and choose what you want the DFM to check for in your annular rings.

• Tangency (default): If you are okay with accepting tangency on pads in the manufactured board, the DFM will look for a minimum of 5 mils (0.005") annular ring width in your design.

• Minimum 1 mil annular ring: If you wish to have minimum 1 mil annular ring on pads in the manufactured board, the DFM will look for a minimum of 6 mils (0.006") annular ring width in your design.

• Minimum 2 mil annular ring: If you wish to have minimum 2 mils annular ring on pads in the manufactured board, the DFM will look for a minimum of 7 mils (0.007") annular ring width in your design.

• Accepting breakouts: If you are okay with accepting breakouts on pads in the manufactured board, the DFM will practically not look for any minimum annular ring width in your design. (NOTE: accepting breakouts is typically NOT recommended.)