Understanding Annular Rings in PCB Design

The annular ring is a critical PCB design element that directly affects manufacturability and reliability. Understanding annular ring requirements helps you create designs that manufacture successfully and provide reliable connections.

What is an Annular Ring?

An annular ring is the copper area remaining around a drilled hole after the drilling process. It's the difference between the pad diameter and the drill hole diameter, divided by two.

Formula: Annular Ring = (Pad Diameter - Hole Diameter) / 2

Example: A 1.0mm pad with a 0.5mm hole has an annular ring of 0.25mm.

Why Annular Rings Matter

The annular ring serves several critical functions:

Electrical Connection: Provides the conductive path between the copper trace and the component lead or via barrel.

Mechanical Strength: A larger annular ring creates a stronger solder joint and more robust pad attachment.

Manufacturing Tolerance: Accounts for drill registration errors during fabrication.

Minimum Annular Ring Requirements

Different manufacturers have different capabilities:

Manufacturer Type	Minimum Annular Ring
Standard (budget)	0.15-0.20mm (6-8 mil)
Advanced	0.10-0.15mm (4-6 mil)
High-end	0.075-0.10mm (3-4 mil)
HDI	0.05-0.075mm (2-3 mil)

Recommendation: Design for 0.15mm (6 mil) minimum for standard manufacturing. This provides margin for drill wander and registration errors.

Breakout and Tangency

Breakout: When the drill hole extends beyond the pad edge, breaking the annular ring. This can cause open circuits or weak connections.

Tangency: When the drill hole just touches the pad edge, leaving zero annular ring at that point. Also problematic.

Both conditions reduce reliability and may cause manufacturing rejection.

Calculating Proper Pad Size

To determine pad diameter for a given hole:

Pad Diameter = Hole Diameter + (2 × Minimum Annular Ring)

Example: For a 0.4mm via hole with 0.15mm annular ring requirement:
Pad = 0.4mm + (2 × 0.15mm) = 0.7mm

Design Considerations

For Through-Hole Components

• Use component manufacturer's recommended pad sizes
• Typical annular ring: 0.25-0.5mm
• Larger pads provide easier hand soldering

For Vias

• Signal vias can use smaller annular rings
• Power vias often use larger rings for current capacity
• Micro-vias in HDI have very tight annular rings

For Multi-Layer Boards

• Inner layer annular rings may differ from outer layers
• Consider layer-to-layer registration accuracy
• Some manufacturers require larger inner layer annular rings

IPC Standards

IPC-2221 provides annular ring guidelines:

Class 1 (General): Minimum 0.05mm
Class 2 (Dedicated Service): Minimum 0.05mm with specific conditions
Class 3 (High Reliability): Minimum 0.05mm with stricter requirements

Note: These are minimums; actual design rules should include manufacturing margins.

Common Problems and Solutions

Problem: Breakout on inner layers
Solution: Increase pad size or improve drill accuracy specification

Problem: Via breakout on outer layers
Solution: Use larger via pads or specify tighter drill tolerance

Problem: Inconsistent annular ring
Solution: Check drill compensation and registration in manufacturing

Design Tips

1. Always consult your manufacturer's design rules
2. Use design rule checks (DRC) in your CAD tool
3. Add margin beyond minimum requirements
4. Review Gerber files before ordering
5. Consider manufacturing class requirements for your application

Conclusion

Proper annular ring sizing ensures manufacturable, reliable PCBs. Design with adequate margins above minimum requirements, and always verify with your specific manufacturer's capabilities. A little extra copper around holes costs nothing but prevents costly manufacturing issues.