An annular ring is the area of a copper pad surrounding a drilled hole on a PCB. Its primary function is to ensure a solid electrical connection between the via and traces.
The width of the annular ring depends on the type of via and the expected drill wander. For laser-vaporized microvias, a minimum of 2 mil is acceptable, while mechanically drilled vias require at least a 4 mil annular ring considering a 2 mil drill wander.
If these tolerances are not met, it can lead to annular ring breakout, where the drilled hole extends beyond the copper pad, potentially causing connection failures.
The width of the annular ring can be calculated using the formula:
Annular ring width = (diameter of the pad – diameter of the finished hole) / 2.
