When designing PCBs, even the smallest details matter—and annular rings are a perfect example. But what are they, and why should you care?

An annular ring is the area of copper that surrounds a drilled hole (via or pad) in a PCB. It ensures a secure electrical and mechanical connection between layers.

If you’ve ever looked closely at a PCB design or manufacturing spec, you’ve likely seen the term “annular ring.” This small copper ring around a via or component hole might seem minor, but it plays a huge role in ensuring structural integrity and signal reliability in multilayer boards.

In this article, I’ll explain what an annular ring is, why it’s important for PCB design, and what can go wrong if it’s not sized correctly.

What Is an Annular Ring in PCB Design?

An annular ring is the circular area of copper that surrounds a drilled hole on a PCB. This hole can be a via, which connects layers electrically, or a plated through-hole (PTH) used for mounting components.

Imagine the hole as the center of a target, and the ring as the copper area surrounding it. The size of the ring is calculated by subtracting the diameter of the hole from the diameter of the pad, then dividing by two.

For example, if a pad is 0.6 mm and the drilled hole is 0.4 mm, the annular ring would be 0.1 mm.

The annular ring helps ensure that electrical connections remain intact—even if the drill is slightly misaligned. It also provides enough copper surface for proper soldering, which is especially important in through-hole assembly.

Why Are Annular Rings Important?

Annular rings may be small, but their role is critical in both electrical and mechanical performance of the board.

A properly sized annular ring:

• Maintains solid electrical connection between layers (via plating)
• Provides a reliable solder joint area for components
• Compensates for manufacturing tolerances in drilling
• Enhances overall PCB robustness and long-term reliability

Without a sufficient ring, you risk a break in conductivity, especially if the via wall delaminates or the plated hole becomes damaged during thermal cycling.

In high-density PCBs, where space is limited, designers often try to minimize annular ring size. But reducing it too much—without understanding your fab’s drill tolerance—can lead to breakout (where the hole breaks through the edge of the ring) or open circuits.

To learn more, check out this excellent visual guide on JLCPCB’s annular ring tutorial.

Minimum Ring Requirements and Manufacturing Tolerances

Most PCB manufacturers define a minimum annular ring size, usually around 0.15 mm (6 mils), to ensure that even if the drill bit is slightly off-center, the copper ring still fully surrounds the hole.

Designers need to account for:

• Drill diameter tolerance: slight variation in actual vs. nominal hole size
• Pad alignment: errors in layering or exposure misregistration
• Copper etching: which may reduce pad diameter slightly

Because of these factors, it’s common to “overbuild” the ring a bit to avoid edge cases where copper disappears on one side.

If you’re working with HDI boards or microvias, manufacturers may allow reduced or staggered annular rings, but these designs require tight process control.

Common Annular Ring Defects

Design or manufacturing errors can cause several issues with annular rings:

• Breakout: The drill hole cuts into the ring, weakening connection
• No ring (missing pad): Usually due to mismatched layer data or over-etching
• Thin rings: Lead to soldering problems or poor thermal transfer
• Off-center holes: Caused by drill alignment errors or machine wear

Preventing these issues starts with good communication between the designer and PCB fabricator. Always review the fab’s design rules before finalizing your drill and pad sizes.

Conclusion

Annular rings may be tiny in size, but they play a huge role in PCB reliability. These copper borders around drilled holes ensure proper connectivity, solderability, and manufacturability. Skipping over their importance can lead to failures that are costly to diagnose and fix.

Whether you’re designing a simple two-layer board or a high-speed multilayer system, always check your annular ring dimensions and consult with your fabricator. In PCB design, precision truly matters—and the annular ring is where electrical and mechanical precision meet.

To dive deeper into the subject with illustrations, I recommend this guide from JLCPCB.