Definition

The annular ring is the copper area between the edge of a drilled hole and the edge of the copper pad. Adequate annular ring prevents pad lift-off during assembly, ensures reliable solder joints, and survives thermal cycling without cracking. IPC standards specify minimum annular ring based on class: 50 µm for Class 2 (general electronics), 75 µm for Class 3 (high reliability). Insufficient annular ring causes intermittent connections and field failures. HDI designs with micro-vias can use smaller annular rings due to better laser drilling accuracy.