Annular Ring in PCB Design: Calculation, Standards & Common Mistakes

After fifteen years of designing PCBs for everything from consumer electronics to aerospace systems, I’ve learned one thing: the smallest details often cause the biggest headaches. The annular ring is a perfect example. It’s just a thin copper band around a drilled hole, yet getting it wrong can tank your entire production run.

I’ve seen engineering teams spend weeks debugging connectivity issues that traced back to insufficient annular ring width. I’ve watched manufacturers reject entire batches because of breakout issues that could have been prevented with proper calculations upfront.

This guide covers everything you need to know about the annular ring—from the basic formula to IPC standards and the mistakes that trip up even experienced designers. Whether you’re working on a simple two-layer board or a complex HDI design, getting your annular rings right is fundamental to building reliable PCBs.

What Is an Annular Ring?

An annular ring is the copper area that surrounds a drilled hole on a PCB. When you look at a via or plated through-hole from above, you’ll see a ring-shaped copper pattern encircling the hole—that’s your annular ring.

The term comes from the Latin word “annulus,” meaning ring. In practical terms, the annular ring provides two essential functions: it creates the electrical connection between the copper trace and the plated via, and it provides the mechanical anchor that keeps everything together during assembly and operation.

Think of it this way: when you drill a hole through a copper pad to create a via, you need enough copper left around that hole to maintain a solid connection. Too little copper, and you risk losing connectivity. The width of this remaining copper band is what we measure and control as the annular ring.

Key Components of the Annular Ring Structure

• Copper Pad: The outer conductive area where the annular ring forms. This is the landing zone for traces and component leads.
• Drilled Hole: The via or through-hole created during PCB manufacturing. For plated holes, this gets copper-plated to conduct electricity between layers.
• Annular Ring Width: The radial distance from the hole edge to the pad edge. This is the critical dimension you need to calculate and verify.
• Plated Barrel: The copper coating inside the hole that connects layers together.

Why Annular Rings Matter

The annular ring directly affects three critical aspects of your PCB: electrical reliability, mechanical durability, and manufacturing yield. Let me break down each one.

Electrical Connectivity

An insufficient annular ring means less copper connecting your trace to the via barrel. This increases resistance, can cause signal integrity problems in high-speed designs, and may lead to intermittent connections. In RF applications, even small variations in annular ring size can affect impedance matching.

Mechanical Strength

The annular ring provides the anchor point for the plated barrel. Without adequate copper, thermal cycling and mechanical stress during assembly can cause pad lifting or barrel cracking. I’ve seen boards fail in the field because thermal expansion broke the connection between an undersized annular ring and its via.

Manufacturing Yield

Drill registration isn’t perfect. Every manufacturer has a tolerance for how accurately they can position drill hits. If your annular ring design doesn’t account for this tolerance, you’ll get tangency or breakout issues that cause board rejection. Designing with adequate annular rings improves first-pass yield and reduces costs.

How to Calculate Annular Ring Size

The basic calculation is straightforward. The annular ring width equals the difference between the pad diameter and the finished hole diameter, divided by two:

Annular Ring Width = (Pad Diameter – Finished Hole Diameter) / 2

Calculation Examples

Pad Diameter	Hole Diameter	Calculation	Annular Ring
22 mil (0.56 mm)	10 mil (0.25 mm)	(22 – 10) / 2	6 mil (0.15 mm)
28 mil (0.71 mm)	12 mil (0.30 mm)	(28 – 12) / 2	8 mil (0.20 mm)
0.60 mm	0.25 mm	(0.60 – 0.35*) / 2	0.125 mm
0.80 mm	0.40 mm	(0.80 – 0.50*) / 2	0.15 mm

*Note: Actual drill diameter is typically 0.10 mm larger than finished hole to account for plating.

The Real-World Calculation

Here’s what many tutorials miss: the finished hole size isn’t the same as the drill size. Manufacturers drill larger holes to account for copper plating. The actual drill diameter is typically 0.10 mm (4 mil) larger than the finished hole specification.

When calculating for IPC compliance, especially for Class 3 products, you need to use this formula:

Minimum Pad Size = Finished Hole + 2(Min Annular Ring) + Fabrication Allowance

The fabrication allowance accounts for drill wander and registration errors. Per IPC-2221, Class C (highest fabrication grade) allows 8 mil (0.20 mm) tolerance. This means for a 10 mil finished hole requiring a 2 mil annular ring, you’d need: 10 + 2(2) + 8 = 22 mil minimum pad diameter.

IPC Standards for Annular Rings

The IPC (Institute for Printed Circuits) defines three product classes with different annular ring requirements. Understanding these standards is essential for specifying your design correctly.

Specification	Class 1	Class 2	Class 3
Application	Consumer Electronics	Industrial, Telecom	Military, Medical, Aerospace
Min. External AR	Breakout allowed	90° breakout OK	2 mil (0.05 mm)
Min. Internal AR	Breakout allowed	90° breakout OK	1 mil (0.025 mm)
Breakout Permitted	180° allowed	90° with teardrop	Not permitted
Tangency Allowed	Yes	Yes, limited	No (with exceptions)

Key IPC Standards to Reference

• IPC-6012: Qualification and Performance Specification for Rigid Printed Boards—the primary standard for annular ring acceptance.
• IPC-2221: Generic Standard on Printed Board Design—provides land pattern and fabrication allowance guidelines.
• IPC-A-600: Acceptability of Printed Boards—visual inspection criteria for annular ring defects.
• IPC-6012ES/EM/EA: Addenda for Space, Medical, and Automotive applications with stricter requirements.

Common Annular Ring Problems and How to Avoid Them

After reviewing thousands of designs, I’ve seen the same mistakes repeatedly. Here are the problems that cause the most grief and how to prevent them.

1. Tangency

Tangency occurs when the drilled hole touches the edge of the copper pad. This happens when drill registration is off-center and your annular ring doesn’t have enough margin to absorb the error.

Solution: Design for at least 5 mil annular ring width if you can accept tangency, or 6+ mil if you need a guaranteed 1 mil ring after manufacturing tolerances.

2. Breakout

Breakout is worse than tangency—the drill actually exits the pad boundary, leaving little or no copper connection. This typically causes board rejection for Class 2 and 3 products.

Solution: Account for manufacturer’s drill registration tolerance (typically ±2-3 mil) in your pad sizing. Use the formula: Pad = Hole + 2(Annular Ring) + Fabrication Allowance.

3. Ignoring Inner vs. Outer Layer Differences

A common mistake is applying the same annular ring rules to all layers. Inner layer annular rings are measured from the drill diameter, while outer layers are measured from the finished (plated) hole diameter.

Solution: For Class 3, specify 2 mil minimum for external layers and 1 mil minimum for internal layers. The plating thickness (typically 1 mil) accounts for the difference.

4. Undersized Pads for HDI Designs

High-density interconnect designs push everything smaller, but some designers reduce pad sizes without understanding the impact on annular ring reliability.

Solution: For microvias, laser drilling is more accurate (±0.5 mil vs. ±2-3 mil for mechanical), so you can design tighter. But still maintain minimum 3-4 mil annular rings for reliability.

5. Forgetting Layer Registration in Multilayer Boards

Each lamination cycle introduces registration error. A 20-layer board with multiple sub-laminations will have worse registration than a 4-layer board, requiring larger annular rings.

Solution: Per IPC-2221, add 1 mil to your annular ring requirement for boards with more than 8 layers. For multiple sub-lamination structures, add 2 mil.

Using Teardrop Pads for Reinforcement

Teardrop pads are an underutilized technique that can save your design when annular rings are marginal. A teardrop adds copper at the junction between the trace and the pad, creating a gradual transition that improves both electrical and mechanical performance.

When to Use Teardrops

• Traces narrower than 20 mil connecting to vias
• Flex PCBs where mechanical stress is a concern
• Designs where tangency is acceptable but you want extra reliability
• High-vibration environments (automotive, aerospace)
• When your annular ring is at or near the minimum specification

Most PCB design software can add teardrops automatically. If your tool doesn’t support it, your manufacturer may be able to add them during CAM processing—just ask.

Typical Manufacturer Requirements

Different manufacturers have different capabilities. Here are typical specifications you’ll encounter when ordering PCBs:

Parameter	Typical Specification
Minimum Annular Ring (Standard)	4-6 mil (0.10-0.15 mm)
Minimum Annular Ring (Advanced)	3-4 mil (0.075-0.10 mm)
Drill Registration Tolerance	±2-3 mil (±0.05-0.075 mm)
Minimum Drill Size (Mechanical)	8 mil (0.20 mm)
Minimum Drill Size (Laser)	4 mil (0.10 mm)
Plating Thickness (PTH)	0.8-1.2 mil (20-30 μm)

Always verify specifications with your manufacturer before finalizing designs. Capabilities vary significantly between fab houses.

Design Best Practices

Based on years of production experience, here are the practices that consistently produce reliable boards:

1. Start with manufacturer specs. Before beginning layout, get your fab house’s design rules. Their minimum annular ring, drill tolerances, and registration capabilities should drive your pad sizing.
2. Build in margin. If the minimum is 4 mil, design for 6 mil. The extra copper costs nothing but dramatically improves yield and reliability.
3. Run DFM checks early. Most EDA tools have design rule checkers that flag annular ring violations. Use them throughout design, not just at the end.
4. Consider the IPC class upfront. Designing a Class 2 board? Your annular ring rules are different than Class 3. Define this in your design rules from the start.
5. Document your requirements. Include annular ring specifications in your fabrication notes. Don’t assume the manufacturer knows your requirements.
6. Use teardrops on critical connections. For thin traces or high-reliability applications, teardrops provide extra insurance.

Useful Resources and Tools

Here are resources I regularly use and recommend to other engineers:

IPC Standards (Available from IPC)

• IPC-2221B: Generic Standard on Printed Board Design
• IPC-6012E: Qualification and Performance Specification for Rigid Printed Boards
• IPC-A-600K: Acceptability of Printed Boards

Free Online Tools

• Sierra Circuits Better DFM: Free online tool that checks your Gerber files for annular ring violations and other manufacturability issues.
• PCBWay Online Gerber Viewer: Upload and visualize your design with layer-by-layer analysis.
• Altium 365 Viewer: Free PCB viewer that can help identify potential issues.

Design Software with Good DRC

• Altium Designer: Comprehensive design rules including annular ring checks.
• Cadence OrCAD: DFM Constraint Manager for manufacturing compliance.
• KiCad: Free and open-source with configurable design rules.

Frequently Asked Questions

What is the minimum annular ring for PCB?

The minimum depends on your IPC class and manufacturer capabilities. For most standard PCBs (IPC Class 2), manufacturers typically require 4-6 mil (0.10-0.15 mm) minimum annular ring. For high-reliability Class 3 boards, IPC-6012 specifies 2 mil (0.05 mm) minimum on external layers and 1 mil (0.025 mm) on internal layers. However, you should design with more margin to account for manufacturing tolerances—I recommend at least 6 mil for standard designs.

How do you measure annular ring in PCB?

For external layers, measure from the inside edge of the finished (plated) hole to the nearest edge of the copper pad. For internal layers, measure from the drill diameter to the pad edge. CAM software typically performs this measurement automatically during DFM checks. Physically, you can measure using a calibrated microscope or optical inspection system. The key is understanding that internal and external measurements use different references—drill size vs. finished hole size.

What causes annular ring breakout?

Breakout occurs when the drilled hole extends beyond the copper pad boundary. Common causes include: undersized pads that don’t account for drill registration tolerance, CNC drill wander during manufacturing, layer misregistration in multilayer boards, and material movement during lamination. The primary prevention is designing larger pads that incorporate fabrication allowances—typically 6-8 mil beyond the minimum hole size plus desired annular ring.

What is the difference between annular ring and via?

A via is the complete plated hole structure that connects layers in a PCB—including the drilled hole, copper plating on the barrel walls, and the pads on each layer. The annular ring is specifically the copper area surrounding the hole on each layer’s surface. Think of the via as the entire structure, while the annular ring is just the copper “doughnut” visible on each layer. Every via has annular rings on the layers it connects, but the annular ring itself isn’t the via—it’s part of the via structure.

Can teardrop pads fix annular ring problems?

Teardrops can help mitigate marginal annular ring situations but shouldn’t be relied upon to fix fundamentally undersized designs. They work by adding copper at the trace-to-pad junction, which provides an extended connection path if there’s tangency at the opposite side of the pad. Teardrops are particularly valuable for narrow traces (under 20 mil), flex circuits, and high-vibration applications. However, if your annular ring is significantly undersized, the proper fix is increasing pad diameter—teardrops are a supplement, not a substitute for proper design.

Conclusion

The annular ring may be a small feature, but it has an outsized impact on PCB reliability. Getting it right requires understanding the relationship between pad size, hole size, and manufacturing tolerances—and designing with enough margin to accommodate real-world variations.

Key takeaways to remember:

• Use the formula: Annular Ring = (Pad Diameter – Hole Diameter) / 2
• Account for fabrication allowance (6-8 mil for Class C manufacturing)
• Know your IPC class requirements—Class 3 has no tolerance for breakout
• Internal and external layers have different measurement references
• Design with margin—minimum specs should be your floor, not your target
• Use teardrops to reinforce marginal connections

When in doubt, communicate with your manufacturer early. They can advise on their specific capabilities and help you optimize your design for their process. A quick conversation about annular ring requirements can prevent expensive board spins and production delays down the road.