Annular Rings are the width of the copper area around via. The purpose of an annular ring is to create a sound electrical connection between a specific copper plated via and a specific copper trace. An annular ring also allows the via holes to have some tolerance, that is the via does not need to be exactly in the middle of the annular ring to ensure the electrical connection is made.
For example at Bittele we can control the hole size tolerances as below:
Plated Holes: +/- 3 mil
Pressfit Holes: +/-2 mil
Non-Plated Holes: +/-2 mil
Thus to ensure a plated via is drilled somewhere within an annular ring the annular ring should be at least 1 mil wider in diameter than the tolerances of the plated holes, that is 4 mils. The calculations for an Annular Ring Width when looking at only the diameter of the pad and the diameter for the hole are:
Annular Ring Width = (diameter of the pad – diameter of the hole)/2
At Bittele we can fabricate annular rings as follows:
Copper weight (oz.)
Minimum annular ring needed
0.5 oz. or 1.0 oz.
4.0 mil (using 3.5 mil in a few places is Acceptable)
6.0 mil (using 5.0 mil in a few places is Acceptable)
8.0 mil (using 6.0 mil in a few places is Acceptable)
Sometimes due to the tight spacing of nearby elements, a designer may opt to make very small annular rings. However this increases the chances of the drilled via being very close to the edge of the annular ring and in the worst case possibly breaking out of the annular ring. This can cause a broken trace connection or make the connection too thin to operate effectively.
To prevent the above issues Bittele recommends tear dropping the annular rings. This adds extra surface area between the annular ring and the copper trace and thus reduces the chances of a break out scenario.
The teardrop design feature also enhances the structural integrity of the annular and trace against possible thermal and mechanical stresses. At Bittele we recommend adding teardrops if an annular ring is smaller than 7 mil.