An annular ring is the area of copper pad that sits around a drilled and finished hole. The drilled and finished hole is nothing but a via.
An annular ring provides a connection surface and establishes a ceaseless connection between a via and copper traces. All around a via, there should be enough copper to form a solid connection between the copper traces and the via in a multi-layer PCB. In a multi-layer PCB, the inner layers are connected to the via by means 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 the drilling and copper plating process, constitutes the annular ring.
The width of the annular ring is an important aspect in design and manufacturing. Ideally, designers require their annular rings with holes located at the 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.
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