A via is created by drilling a hole through a copper pad etched on each layer of a High frequency PCB. An Annular Ring is the area between the edge of the drilled via and the copper pad associated with that hole. The greater the width of an annular ring, the greater the copper connection around the drilled via will be.

In a multilayer PCB, traces are routed from one layer to another layer with the help of vias. These vias are the holes drilled through copper pads on the surface of the PCB. The amount of copper left around the via on both top and bottom sides of the PCB is called the annular ring.

 Mathematically, an annular ring is the difference between the diameter of the hole and diameter of the pad divided by 2. For instance, if the diameter of the pad is 24 mils and the diameter of the hole is 12 mils then the width of the annular ring is [(24-12)/2] = 6 mils

Calculation of the width of an annular ring plays an important role at the time of High frequency PCB manufacturing. If the width of the annular ring is not enough then the hole could touch the boundaries of the pad, this condition is called as ‘Tangency’. In the extreme situation, the hole could be outside the boundary of the pad which is termed as ‘Breakout’. Both these situations should be avoided during the process of High frequency PCB fabrication.