In the world of PCB design and manufacturing, we often focus on traces, vias, and signal integrity. However, the very foundation of any printed circuit board—its base material or substrate—holds secrets critical to long-term reliability. One of the most important, yet sometimes overlooked, properties of this substrate is its CTI.
If you’re designing boards for high-voltage or harsh environments, understanding CTI is not just an option; it’s a necessity. Let’s demystify what CTI is and why it’s so vital for your PCB’s performance and safety.
What Does CTI (Comparative Tracking Index) Mean?
CTI is short for Comparative Tracking Index, means the maximum voltage measured in volts to verify the electrical breakdown properties of an insulating material that used in the electronic devices. At this voltage the specific material can withstands 50 drops of contaminated water without any tracking. Tracking means the conductive paths (or called PCB current leakage path) formed due to heat, electrical tress, humidity and contamination. Therefore, it is also called as PCB leakage current resistance.
Here’s a more detailed breakdown:
- The “Tracking” Phenomenon: When a PCB is exposed to contamination, moisture, or dust, a conductive path can slowly form on its surface. If a high enough voltage is applied across this contaminated path, it can cause small electrical discharges (arcing). Over time, these discharges carbonize the insulating material, creating a permanent, and increasingly conductive, “track.”
- The “Index” or Measurement: The CTI value is the highest voltage (in Volts) at which the material can withstand 50 drops of a standardized conductive solution without failing (i.e., without forming a conductive track). A higher CTI number indicates a better resistance to electrical tracking.
How is CTI Measured and Classified?
CTI is tested according to the IEC 60112 standard. Based on the test results, materials are assigned a Performance Level Category (PLC):
- PLC 0 (CTI ≥ 600V): Highest resistance. Used in very demanding applications.
- PLC 1 (400V ≤ CTI < 600V): Good resistance. Common for high-quality, high-reliability boards.
- PLC 2 (250V ≤ CTI < 400V): Moderate resistance. This is the category for standard FR4 materials.
- PLC 3 (175V ≤ CTI < 250V): Lower resistance. Suitable for low-voltage, benign environments.
- PLC 4 (100V ≤ CTI < 175V) & PLC 5 (CTI < 100V): Poor resistance. Generally unsuitable for most commercial PCB applications.
| Grade | Comparative Tracking Index (CTI) in Volts | Laminate Reference |
|---|---|---|
| I | CTI≥600V | |
| 400 <= CTI < 600 | N/A | |
| IIIA | 175 <= CTI < 400 | |
| IIIB | 100 <= CTI < 175 | Most FR1 base material can meet this value |
Why is CTI So Important in PCB Manufacturing?
The CTI of your base material directly impacts the board’s reliability, safety, and lifespan, especially in specific use cases.
- Prevents Catastrophic Failure: Electrical tracking is a progressive failure. Once a carbon track forms, it can lead to short circuits, leakage currents, and in the worst case, fire hazard. A high CTI material acts as a robust barrier against this failure mode.
- Enables Miniaturization: For high-voltage designs, a higher CTI rating allows for reduced Creepage Distances. Creepage is the shortest distance between two conductive parts on the surface of an insulator. With a better-tracking-resistant material (high CTI), you can safely place high-voltage traces closer together, enabling more compact and powerful board designs.
- Ensures Compliance and Safety Certification: Many international safety standards, such as UL and IEC, have mandatory CTI/PLC requirements for end-products. Using a material with an insufficient CTI rating can prevent your product from receiving necessary safety certifications, blocking it from the market.
- Critical for Harsh Environments: Any application where the PCB might be exposed to humidity, condensation, dust, or chemical contaminants is a prime candidate for high CTI materials. This includes:
-
- Automotive electronics (under-hood controls)
- Industrial motor drives and power supplies
- Outdoor telecommunications equipment
- Medical devices
- Appliances and consumer electronics in humid environments (e.g., washing machines)
CTI of Common PCB Materials
- Standard FR4: Typically has a CTI around 175-250V, placing it in PLC 2 or 3. This is sufficient for many consumer electronics.
- Medium CTI FR4: Specially formulated FR4 with CTI ratings of 275V to 400V (PLC 2).
- High CTI FR4 / Specialty Materials: These materials, including polyimide and certain ceramic-filled laminates, can achieve CTI ratings of 600V and above (PLC 0), making them ideal for the most demanding applications.
How to Choose the Right CTI Material for Your PCB
Selecting the appropriate CTI rating is a risk-based decision. Follow this practical guide:
1. Assess the Operating Environment:
- Benign (Air-conditioned Office): Standard FR-4 (CTI ~200-300) may be sufficient.
- Moderate (Consumer Automotive, Indoor Industrial): Specify a material with a CTI ≥ 300.
- Harsh (Outdoor, High Humidity, High Altitude, Automotive Under-hood): Do not compromise. Use a high-CTI material (≥400, PL 1) to ensure lifetime reliability.
2. Consider the Operating Voltage:
The higher the voltage difference between adjacent conductors, the greater the risk of tracking. For power electronics, motor controllers, or AC/DC converters, always opt for a higher CTI material.
3. Check Your Manufacturer’s Data Sheets:
Do not assume all “FR-4” is the same. Laminate manufacturers like Isola, Shengyi (S1600), and ITEQ offer a range of FR-4 grades with varying CTI values. Always review the technical data sheet for the specific CTI rating.
4. Consult with Your PCB Fabricator:
An experienced PCB manufacturer like Weller PCB can provide invaluable guidance. We can help you navigate material options, balancing CTI requirements with other critical properties like thermal performance and cost.
Conclusion: Don't Let CTI Be an Afterthought.
While it may be an invisible property on a finished board, the Comparative Tracking Index is a fundamental pillar of PCB reliability. By understanding CTI and proactively selecting a base material with a rating that matches your electrical and environmental demands, you invest in the safety and longevity of your product
Ready to ensure your next PCB project is built for reliability? Contact WELLER today to discuss your material requirements. Our engineering team is here to help you navigate the complexities of base material selection and build boards you can count on.
