Annular Ring: Why Your Vias Fail a Design Check

Annular ring is the ring of copper left around a drilled hole. It is one of the least considered numbers in a design and one of the most likely to limit what a fabricator can build, because it depends on drilling accuracy rather than on how finely copper can be etched.

The arithmetic

Annular ring = (pad diameter − drill diameter) ÷ 2.

A 0.8 mm pad with a 0.4 mm hole leaves (0.8 − 0.4) ÷ 2 = 0.2 mm of copper all the way round. A 0.6 mm pad on a 0.3 mm hole leaves 0.15 mm. A through-hole resistor pad at 1.6 mm with a 0.8 mm hole leaves a comfortable 0.4 mm.

Note what that means: vias are almost always the tightest annular ring on a board, and they are the part of a layout most people never touch after setting a default.

Why it is about drilling, not etching

Copper features can be imaged very precisely. Drills are mechanical: the bit deflects slightly, the stack shifts a fraction, the panel expands with heat. Registration — how accurately the hole lands in the middle of the pad — is where the tolerance goes.

If a hole drifts far enough off centre, it breaks out of the pad on one side. A partial breakout leaves a weak connection; a full one leaves no connection at all. That is why annular ring is specified as a minimum rather than treated as decorative.

Why default vias sit close to the limit

Most EDA tools ship with via defaults that are perfectly sensible and also close to what a typical prototype line can hold. If you have never changed the via size in your board setup, your board is very likely using one of those defaults — and it is probably the tightest annular ring in the design.

This catches experienced designers as often as beginners, because the value is set once in a template and never revisited. It is worth checking, not assuming.

How to check yours

• Find your via settings in the board setup or design rules — you are looking for a via diameter and a via drill.
• Subtract the drill from the diameter and halve it. That is your annular ring.
• Do the same for the smallest through-hole component pad on the board.
• The smaller of the two is what a fabricator will measure.

Compare that against the published limits on our capabilities page rather than a number in an article — capability changes, and the table is the version that takes or refuses an order.

How to fix a board that fails

In order of preference:

• Increase the via pad diameter and leave the drill alone. This adds copper without changing the hole and is usually the least disruptive change.
• Reduce the via drill, if the via does not need to carry much current and your fabricator supports the smaller hole. This also improves the ring, and often improves routing density as a side effect.
• Change the via type or use fewer, larger vias in the tight area. Sometimes the real problem is a dense cluster rather than the via size itself.
• Reroute so the tight vias are not needed. Slowest, but occasionally the honest answer.

Changing the global via default fixes the whole board at once, which is usually what you want. Editing individual vias is for the handful that need to stay small.

A note on what gets measured

Annular ring is measured from your actual files, not from your design rule settings. A board whose rules say one thing and whose geometry says another will be judged on the geometry — so a rule you set and then overrode locally will not save you.

Upload a board and the measured value comes back with the rest of the design check, alongside the trace width, spacing and hole sizes it found. If something is out of spec you are told which measurement and by how much.