PCB’s Comparative Tracking Index (CTI) and Its Importance in Design

In the world of Printed Circuit Boards (PCBs), selecting the right base material is critical for ensuring long-term reliability and safety—especially in high-voltage applications. One key property that determines a material’s ability to withstand electrical stress is the Comparative Tracking Index (CTI).

But what exactly is CTI, and why should PCB designers care about it? Let’s break it down.

What is the Comparative Tracking Index (CTI)?

CTI is a measure of a material’s resistance to electrical tracking, which occurs when an insulating surface becomes damaged due to repeated electrical discharges, leading to conductive paths. This phenomenon can cause short circuits, breakdowns, and even fires.

CTI is expressed in volts (V) and is determined through standardized testing (IEC 60112 or ASTM D3638), where the material is exposed to an electrolyte under controlled conditions. The higher the CTI value, the better the material resists electrical tracking.

CTI Classification and Material Selection

PCB materials are categorized into different CTI classes based on their performance:

Most standard PCB materials, such as FR-4, fall within CTI Class 3 (175-249V), while high-reliability applications may require materials with CTI Class 1 or higher.

Why is CTI Important for PCB Design?

1?? Safety and Reliability in High-Voltage Applications

In power electronics, automotive, industrial, and aerospace applications, high voltage stress can cause material breakdown. A higher CTI rating ensures better insulation and reduces the risk of failures due to electrical tracking.

2?? Influence on PCB Creepage Distance

Creepage distance is the shortest path between two conductive elements along the surface of an insulating material. Higher CTI values allow for reduced creepage distances, enabling more compact PCB designs without compromising safety.

3?? Compliance with Industry Standards

Regulatory standards like IEC 60950, UL 746, and IPC-2221 require PCB materials to meet specific CTI ratings, especially in high-voltage and industrial applications. Choosing the right CTI-rated material ensures compliance and avoids costly redesigns.

4?? Cost vs. Performance Trade-offs

While higher CTI-rated materials offer better electrical insulation, they can be more expensive. Designers need to balance cost, performance, and safety based on the application’s requirements.

How to Choose the Right CTI Material for Your PCB?

When selecting a PCB material based on CTI, consider:

? Application Type

– Is the PCB used in a high-voltage, high-power, or harsh environment?

? Creepage & Clearance Requirements

– Does your design need a compact layout with minimal creepage distance?

? Regulatory Compliance

– Does the application need to meet strict safety standards?

? Material Costs

– Does a higher CTI justify the additional cost for your specific use case?

Conclusion

The Comparative Tracking Index (CTI) is a crucial factor in PCB material selection, directly impacting safety, reliability, and design flexibility. As electronics continue to push the limits of power density and miniaturization, understanding and optimizing CTI can help engineers build more efficient and robust products.