PCB annular ring: how to size it and how to avoid breakout
The annular ring is the copper left around a drilled hole. Too small and the hole can miss the pad. Here is how to calculate it and where our limits are.
What the annular ring is
The annular ring is the ring of copper between the edge of a drilled hole and the edge of the pad around it. On a plated hole it is the copper that connects the hole barrel to the trace on that layer. Its width on each side is
ring = (pad diameter − drill diameter) ÷ 2
At PCB-ELECTRA the smallest standard via has a 0.3 mm drill and a 0.5 mm pad, so the ring is (0.5 − 0.3) ÷ 2 = 0.1 mm on each side. Our drill limits are 0.3 mm to 6.4 mm (see vias and hole sizes).
Why it matters
A drill never lands exactly on its coordinates, and the layers of a board are never aligned perfectly. A generous ring absorbs this. A tight ring is the most common source of yield loss: if the hole shifts, it can cut through the edge of the pad. This is called breakout. A broken-out ring leaves a weak or incomplete connection, which is why the acceptance rules for breakout are stricter in IPC class 3 than in class 2.
How big to make it
- Vias: use the standard 0.3 mm drill with a 0.5 mm pad, or larger when space allows. Larger pads are more forgiving in production.
- Component holes: a common rule of thumb is a pad about 0.5 mm larger than the finished hole, which gives a ring of roughly 0.2 to 0.25 mm. This is a design guideline, not a limit of ours.
- Where there is room, give more. Use the smallest ring only where the layout forces you to.
- Multilayer boards: inner-layer pads are as exposed as outer ones, so apply the same margin on every layer.
Ways to protect the ring
- Teardrops widen the copper where a trace meets a pad or via, so a small drill shift leaves more copper.
- Avoid vias inside solder pads of fine-pitch parts unless you have agreed it with us first; solder can flow into the hole.
- Keep the hole away from the pad edge on pads shared with a trace: a ring that is wide on one side and thin on the other is the first to break out.
Set the rule in your CAD tool
Design rule checkers have a minimum annular width. In KiCad it is under Board Setup → Design Rules → Constraints; Altium has a minimum annular ring rule. Set it from the pad and drill limits above, then run the check before you export. The values for KiCad are in KiCad design rules for a PCB manufacturer, and the whole list of checks is in the DFM checklist.
When the layout is ready, configure the board in the price calculator and send the Gerber and drill files (drill file guide).
Frequently asked questions
What is the annular ring on a PCB?
The copper ring between the edge of a drilled hole and the edge of its pad. Its width on each side is the pad diameter minus the drill diameter, divided by two.
What is the minimum annular ring at PCB-ELECTRA?
The smallest standard via has a 0.3 mm drill and a 0.5 mm pad, which leaves 0.1 mm on each side. Give larger pads wherever there is room.
What is annular ring breakout?
The drilled hole reaches or crosses the edge of the pad, so the ring is incomplete on one side. It weakens the connection, and IPC class 3 is stricter about it than class 2.
Does the annular ring depend on the finished hole size?
It is measured from the drilled hole. Plating reduces the finished diameter by roughly 0.05 mm, so specify the finished size in the drill file.
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