PCB Annular Ring Guide: Design Rules and Manufacturing

What the Annular Ring Is

The annular ring is the band of copper that surrounds a drilled hole on the pad, measured from the edge of the finished hole to the outer edge of the pad. It is what keeps the barrel of a plated through hole connected to the trace that enters it, and it is the feature that absorbs the difference between where the drill was supposed to go and where it actually went. Everything about the ring is a tolerance question: the drill wanders, the artwork shifts, the laminate moves during lamination, and the ring has to be wide enough that the copper still surrounds the hole after all those errors are added together.

Minimum Ring Width by Class

The IPC requirements set the minimum in absolute terms rather than as a percentage of the pad. For a Class 2 board the common minimum is 0.05 mm of remaining ring with an allowance for breakout, and for Class 3 it is tighter, typically 0.05 mm with no breakout permitted on the outer layers and stricter internal requirements. Internal layers usually have a slightly more generous minimum because they are not exposed to rework. Those numbers assume the hole is drilled in the nominal position; in practice designers add margin on top, and a working minimum of 0.15 mm for a standard board and 0.2 mm or more for a high reliability board is a sensible default on outer layers.

What Causes a Ring to Break Out

Breakout occurs when the accumulated error exceeds the ring width. The main contributors are drill position tolerance, typically plus or minus 0.075 mm on a good machine and wider on an older one; pad artwork registration, affected by the exposure and development of the inner layers; and material movement during lamination, which is worse on thick or asymmetric stacks. Drill size tolerance and bit wander on deep holes add to the picture. The errors add rather than cancel, so the ring must be sized for the sum. A design that specifies a nominal ring of exactly the minimum will produce breakout on a significant fraction of boards, which is why the nominal value should be the minimum plus the expected error stack.

The Relationship Between Hole Size and Pad Size

The pad diameter is the finished hole plus twice the ring, and the drill diameter is larger than the finished hole because of plating. A common rule is to add 0.15 mm to the required finished hole to get the drill size for standard plating, so a 0.3 mm finished hole is drilled at about 0.45 mm. Designers who specify the finished hole directly and forget the plating allowance end up with pads that are too small. The other constraint is the space between pads: on a 0.5 mm pitch device there is not much room, and the ring has to shrink to keep the pad within the ball or lead footprint. That tension is exactly why fine pitch routing moves to microvias and to via in pad, where the ring can be smaller because the via is filled and covered.

Special Cases That Change the Rule

Via in pad. The via is filled and plated over, so the ring is not exposed and can be reduced, but the fill and planarisation must be complete or the pad surface will not accept a component. Non functional pads. Removing unused inner layer pads reduces barrel stress and drill wear, but it means those layers have no ring at all, which is a deliberate choice rather than a defect. Board edges. A hole close to the outline has less support and needs a wider keep out from the edge, typically at least the panel edge clearance. Castellated holes. The ring is cut in half, so the remaining copper has to be sized as if the hole were a full pad. Heavy copper. Etch compensation changes the finished pad size, so the drawn pad must be compensated for the copper weight.

Design and Check Workflow

Start from the finished hole sizes the component needs, add the plating allowance, then add the ring for the class and the expected drill tolerance. Check the resulting pad against the component land pattern and against the minimum pad to pad spacing for the assembly process. Run a design rule check with the fabrication rules loaded so that pad to hole ratios, minimum ring and copper to edge clearances are all verified together. Finally, send the stackup and the drill table to the fabricator and ask for the achievable minimum ring on that specific construction, because the answer depends on the layer count, the laminate and the drill equipment rather than on a general table.

Common Mistakes

Specifying the finished hole as the drill size is the most common error and produces undersized pads. Using a single ring value for all layers ignores the fact that inner layers move more during lamination. Copying a land pattern from a device that uses via in pad onto a design that uses standard vias gives a pad that is too small. Ignoring the coupon and the fabricator’s process capability leads to a design that is technically legal but has poor yield. Each of these is cheap to fix at the layout stage and expensive to fix after the panels are drilled.

The ring is a tolerance budget, so it belongs with the rest of the manufacturing review. Read how plating and drilling behave in PCB manufacturing, apply the pad and hole rules from PCB design and layout, and check the design and manufacturing considerations before release. A prototype PCB assembly build confirms the pads take the paste and the placement as intended.

FAQ

What is a typical minimum annular ring? Around 0.05 mm is the IPC minimum for many classes, but a working design value of 0.15 mm on outer layers is common to absorb drill and registration error.

Does the drill size equal the finished hole size? No. The drill is larger because plating reduces the hole; a 0.15 mm allowance is a typical starting point.

Can the ring be smaller for a via in pad? Yes, because the via is filled and covered, but the fill has to be complete and planar.

Why is breakout more common on inner layers? Because inner layers move during lamination and are registered from a different artwork, so the accumulated error is larger.

Conclusion

The annular ring is the tolerance budget of every plated hole. Size it from the finished hole plus the plating allowance plus the ring for the class and the drill capability, then verify it with a design rule check that uses the fabricator’s rules. Sized that way in 2026, the pads will survive drilling, plating and rework without breakout.