PCB Design: Getting Creepage and Clearance Right

 

Understanding the Basics

Defining Creepage and Clearance

Term
Definition
Measurement Method
Primary Concern
Creepage
Shortest path between conductors along surface
Following surface contours
Surface contamination
Clearance
Shortest distance through air
Direct line measurement
Air breakdown voltage

Why They Matter

Critical Factors Affecting Safety

Factor
Impact on Design
Risk if Ignored
Voltage
Higher voltage requires larger distances
Electrical arcing
Pollution
Affects surface conductivity
Current leakage
Altitude
Affects air insulation properties
Premature breakdown
Material
Influences tracking resistance
Surface degradation

Standards and Regulations

Key PCB Design Standards

Standard
Scope
Application
IPC-2221
Generic PCB Design
General electronics
IEC 60950-1
Safety of IT Equipment
Computer hardware
UL 60950
Safety Requirements
North American market
EN 60335-1
Household Appliances
Consumer products

Pollution Degrees

Degree
Description
Environment Type
Example Applications
1
No pollution
Hermetically sealed
Medical implants
2
Non-conductive pollution
Office/home
Consumer electronics
3
Conductive pollution
Industrial
Factory equipment
4
Persistent conductivity
Outdoor
Weather stations

Calculating Required Distances

Basic Clearance Requirements

Working Voltage (V)
Clearance (mm)
0-50
0.13
51-100
0.25
101-300
0.7
301-600
1.7
601-1000
2.8

Creepage Distance Requirements

Working Voltage (V)
Pollution Degree 1 (mm)
Pollution Degree 2 (mm)
Pollution Degree 3 (mm)
0-50
0.2
0.5
1.2
51-100
0.3
0.7
1.4
101-300
0.8
1.5
2.5
301-600
2.0
3.0
4.0
601-1000
3.2
4.5
6.0

Altitude Correction Factors

Altitude (m)
Multiplication Factor
0-2000
1.00
2001-3000
1.14
3001-4000
1.29
4001-5000
1.48
5001-6000
1.70

Material Considerations

Comparative Tracking Index (CTI)

Material Group
CTI Range
Common Materials
Typical Applications
I
≥600
High-grade FR4
Medical devices
II
400-599
Standard FR4
Consumer electronics
IIIa
175-399
Basic laminates
Industrial equipment
IIIb
100-174
Basic materials
Low-cost applications

Material Selection Guide

Application
Recommended Material
Minimum CTI
Rationale
High voltage
Group I
≥600
Maximum safety margin
Industrial
Group II
≥400
Good balance
Consumer
Group IIIa
≥175
Cost-effective
Low voltage
Group IIIb
≥100
Minimal requirements

Design Implementation

PCB Layout Techniques

Technique
Purpose
Implementation
Slot placement
Increase creepage
Cut slots between conductors
Guard rings
Prevent leakage
Surround high-voltage areas
Component spacing
Maintain clearance
Space components based on voltage
Layer separation
Improve isolation
Use separate layers for voltage levels

Common Design Patterns

Pattern
Application
Advantages
Disadvantages
Gridded ground
High-frequency
Good isolation
Complex routing
Star ground
Mixed signal
Reduced interference
More space needed
Isolation barrier
Safety critical
High reliability
Increased cost
Interleaved traces
EMI reduction
Better noise immunity
Harder to modify

Testing and Validation

Test Methods

Test Type
Purpose
Equipment Needed
Standard
Hi-pot test
Voltage breakdown
Hi-pot tester
IEC 60950-1
Insulation resistance
Leakage current
Megohmmeter
IEC 60601-1
Environmental
Pollution effects
Climate chamber
IEC 60068-2
Thermal cycling
Stress testing
Thermal chamber
IEC 61189-5

Quality Assurance Checklist

Check Point
Verification Method
Frequency
Design rules
DRC software
Every design
Clearance measurements
Physical inspection
Prototype stage
Material certification
Documentation review
New material
Environmental testing
Lab testing
New product

Special Considerations

High Voltage Applications

Voltage Range
Special Requirements
Additional Measures
>1000V
Double clearance
Conformal coating
>2000V
Triple clearance
Potting compound
>5000V
Custom design rules
Special materials
>10000V
Expert consultation
Full encapsulation

Extreme Environments

Environment
Challenges
Solutions
High humidity
Moisture conductivity
Conformal coating
Salt spray
Corrosion
Special coatings
Chemical exposure
Material degradation
Protective barriers
High altitude
Air insulation
Increased spacing

Frequently Asked Questions

Q1: How do I determine the minimum creepage distance for my PCB?

A: To determine minimum creepage distance, you need to consider:

1. Working voltage
2. Pollution degree of the environment
3. Material group (CTI value)
4. Altitude of operation Use the tables provided in this guide and multiply the base value by any applicable correction factors.

Q2: What's the difference between functional and basic insulation?

A: Functional insulation is the minimum insulation needed for proper operation of the equipment, while basic insulation provides fundamental protection against electric shock. Basic insulation typically requires larger creepage and clearance distances than functional insulation, often 1.5 to 2 times greater.

Q3: Do I need to consider creepage and clearance between traces on different layers?

A: Yes, creepage and clearance requirements apply to conductors on different layers. Through-board clearance must be considered, and the PCB material's properties become crucial. The distance should be measured through the shortest possible path, including through-holes or vias.

Q4: How does conformal coating affect creepage requirements?

A: Conformal coating can reduce creepage requirements as it protects against pollution and moisture. However, it doesn't affect clearance requirements. Type 1 coating can reduce creepage requirements by one pollution degree, while Type 2 coating can reduce them by two degrees.

Q5: What are the common mistakes in creepage and clearance design?

A: Common mistakes include:

• Not considering altitude correction factors
• Ignoring pollution degree requirements
• Forgetting about through-hole and via spacing
• Not accounting for component height in clearance calculations
• Overlooking temperature effects on material properties