What is an annular ring?
An annular ring is the ring of copper that surrounds a drilled and plated hole on a PCB pad. It is measured from the edge of the hole to the outer edge of the pad. A sufficient annular ring ensures a reliable connection between the hole and the trace, even when the drill is slightly off-centre.
Annular rings explained
Every via and through-hole pad on a PCB consists of a drilled hole and a copper pad around it. The annular ring is the width of copper between the hole wall and the edge of the pad. In design, it is set by choosing a pad diameter larger than the drill diameter; the nominal annular ring is half the difference between the two.
In manufacturing, the actual annular ring is usually smaller than the nominal one. The drill can wander slightly, the layers can shift during lamination, and imaging and etching have their own tolerances. Fabricators therefore compensate by adjusting pad sizes and use a minimum design annular ring that leaves room for these tolerances.
When the hole is so far off-centre that it cuts through the edge of the pad, this is called breakout. Tangency is the limit case where the hole just touches the pad edge. Breakout reduces the copper connecting the hole to its trace and, at the junction with a trace, can reduce the conductor width or cause a weak point.
Annular ring is specified separately for external layers and internal layers. Internal layers are affected by layer registration during lamination, while external layers are affected mainly by drilling accuracy. Requirements are defined in IPC-6012 for rigid boards, with stricter criteria in Class 3 than in Class 2.
Design techniques to protect the connection include larger pads, teardrops at the junction between pad and trace, and keeping the drill size realistic for the pad. Microvias and HDI designs use their own rules for capture and target pads.
When annular ring size matters
Annular ring matters in every design, but especially on dense boards where designers shrink pads to fit routing, on boards with many layers where layer registration becomes harder, and on high-reliability products built to IPC Class 3.
The main failure mode is breakout. A small amount of breakout on a via is often acceptable under Class 2 requirements, but it reduces the margin for reliability, and Class 3 does not accept it. Where a trace enters the pad at the point of breakout, the conductor can be narrowed, which can lead to cracks or open circuits under thermal cycling and vibration.
Annular ring issues are typically found during DFM review. If the design uses the fabricator's minimum values with no margin, the manufacturer may need to enlarge pads or reduce drill sizes, which can conflict with spacing rules. Setting realistic pad-to-drill ratios early avoids late changes.
Annular ring in IPC Class 2 vs Class 3
IPC-6012 defines annular ring requirements per class for rigid boards. Class 2 accepts some breakout as long as the connection meets the conditions in the standard. Class 3 requires a positive minimum annular ring on both external and internal layers, which in practice means larger pads or tighter manufacturing control. Check the current revision of IPC-6012 for the exact values.
Annular rings in your quote request
In your quote request, state the IPC class the bare boards must meet, as this sets the annular ring requirement. Make sure your drill files and pad sizes reflect the finished hole sizes you need, and note if you have used teardrops or deliberately small pads in dense areas. Our free DFM check flags pads where the annular ring is likely to fall below the requirement after drilling tolerances.
Need boards built? Upload your Gerber files for a free quote and DFM check from the right manufacturer in our network.
PCB manufacturing →FAQ about annular rings
What is an annular ring in a PCB?
An annular ring is the ring of copper surrounding a drilled and plated hole on a PCB pad. It is measured from the edge of the hole to the edge of the pad. It provides the connection between the plated hole and the traces, and must be large enough to remain intact after drilling and registration tolerances.
What is annular ring breakout?
Annular ring breakout occurs when a drilled hole is so far off-centre that it breaks through the edge of its copper pad. It is caused by drill wander, layer misregistration or pads that are too small for the drill. Breakout weakens the connection between hole and trace and may be accepted in Class 2 within limits, but not in Class 3.
How do I avoid annular ring problems?
Use pad sizes that leave enough margin around the drill for manufacturing tolerances, rather than designing at the minimum the fabricator allows. Add teardrops where traces join pads, use realistic drill sizes and state the IPC class required. Running a DFM check before ordering identifies pads at risk of breakout so they can be enlarged early.
Related terms
- Via
A via is a small plated hole in a PCB that electrically connects copper on two or more layers. Vias carry signals, power and ground between layers and can also conduct heat. The main types are through-hole vias, blind vias, buried vias and microvias, which differ in how many layers they span and how they are made.
- DFM
DFM (Design for Manufacturability) is a structured review of your PCB design against a fabricator's process limits — trace widths, drill sizes, annular rings, solder mask clearances, and stack-up choices — to catch issues before tooling starts. A good DFM review prevents rework, scrap, and missed delivery dates.
- IPC class 2 vs class 3
IPC class 2 and class 3 are two acceptance levels defined in IPC standards (A-600 for bare boards, A-610 for assemblies). Class 2 covers dedicated-service electronics; class 3 covers high-reliability electronics where failure cannot be tolerated. Class 3 demands tighter tolerances, stricter inspection, and typically costs more.
- IPC-A-600
IPC-A-600 is the IPC standard that defines visual acceptance criteria for bare printed circuit boards. It specifies what surface, dimensional, and structural conditions are acceptable across three class levels, and is the reference document fabricators use to inspect boards before shipment.
- Microvia
A microvia is a small laser-drilled blind via, typically 150 µm or less in diameter (per IPC-T-50M definition), used to connect adjacent layers in HDI boards. Microvias enable routing for high pin-count BGAs and densely packed components that conventional mechanical vias cannot support.
- Plating
Plating refers to the electroplating or electroless deposition of metal onto a PCB, primarily copper plating inside drilled holes to make them conductive, and final surface finish plating (ENIG, immersion silver, etc.) on exposed pads. Plating thickness affects current capacity, reliability, and IPC class compliance.