What is Comparative Tracking Index (CTI) of the PCB base material?

Introduction to Comparative Tracking Index (CTI)

Comparative Tracking Index (CTI) is a critical parameter used to assess the electrical insulation properties of printed circuit board (PCB) base materials. It measures the ability of a material to resist electrical tracking, which is the formation of conductive paths on the surface of an insulating material due to electrical stress. CTI is an essential consideration in the selection of PCB base materials, especially for applications that require high reliability and safety.

What is electrical tracking?

Electrical tracking is a phenomenon that occurs when an insulating material is subjected to high voltage stress. Over time, the electrical stress can cause the formation of conductive paths on the surface of the material, leading to a reduction in its insulation properties. This can result in leakage currents, short circuits, and even electrical fires.

Importance of CTI in PCB design

In PCB design, the choice of base material is crucial to ensure the reliability and safety of the final product. The CTI value of the base material determines its ability to withstand electrical stress and maintain its insulation properties over the lifetime of the PCB. A higher CTI value indicates better resistance to electrical tracking and, consequently, a more reliable and safer PCB.

Factors Affecting CTI

Several factors can influence the CTI value of a PCB base material. Understanding these factors is essential for selecting the appropriate material for a given application.

Material composition

The chemical composition of the base material plays a significant role in determining its CTI value. Different materials, such as FR-4, polyimide, and ceramic, have varying CTI values due to their unique chemical structures and properties.

Environmental conditions

The operating environment of the PCB can also affect the CTI value of the base material. Factors such as temperature, humidity, and the presence of contaminants can accelerate the formation of conductive paths and reduce the material’s resistance to electrical tracking.

Manufacturing process

The manufacturing process used to produce the PCB base material can also impact its CTI value. Variations in the curing temperature, pressure, and duration can lead to differences in the material’s final properties, including its resistance to electrical tracking.

Measuring CTI

CTI is measured using a standardized test method described in IEC 60112 and ASTM D3638. The test involves applying a high voltage between two electrodes placed on the surface of the material and measuring the time it takes for a conductive path to form between the electrodes.

Test setup

The CTI test setup consists of two electrodes, typically made of stainless steel, placed on the surface of the material with a specified distance between them. A voltage is applied between the electrodes, and the time it takes for a conductive path to form is recorded.

Test procedure

The test procedure involves gradually increasing the voltage applied between the electrodes until a conductive path forms. The voltage at which this occurs is known as the tracking voltage. The CTI value is then determined based on the tracking voltage and the distance between the electrodes.

CTI rating system

The CTI rating system classifies materials into different categories based on their tracking voltage. The categories range from 25 to 600, with higher values indicating better resistance to electrical tracking. The table below shows the CTI categories and their corresponding tracking voltage ranges:

CTI Category Tracking Voltage Range (V)
25 25-49
50 50-99
100 100-174
175 175-249
250 250-399
400 400-599
600 600 and above

Common PCB Base Materials and Their CTI Values

There are several common PCB base materials used in the electronics industry, each with its own set of properties and CTI values. Some of the most widely used materials include:

FR-4

FR-4 is a glass-reinforced epoxy laminate and is the most commonly used PCB base material. It offers a good balance of mechanical, thermal, and electrical properties. The CTI value of FR-4 typically ranges from 175 to 250, making it suitable for most general-purpose applications.

Polyimide

Polyimide is a high-performance polymer known for its excellent thermal stability and mechanical strength. It is often used in applications that require high reliability and the ability to withstand harsh environments. The CTI value of polyimide is typically in the range of 400 to 600, making it an ideal choice for high-voltage applications.

Ceramic

Ceramic PCB base materials, such as alumina and beryllia, are known for their excellent thermal conductivity and electrical insulation properties. They are often used in high-power and high-frequency applications. The CTI value of ceramic materials is generally above 600, indicating excellent resistance to electrical tracking.

Selecting the Right PCB Base Material Based on CTI

When selecting a PCB base material for a specific application, it is essential to consider the required CTI value along with other factors such as the operating environment, thermal requirements, and mechanical constraints.

Application requirements

The first step in selecting the right PCB base material is to understand the specific requirements of the application. This includes factors such as the operating voltage, temperature range, and environmental conditions. Based on these requirements, a suitable CTI value can be determined.

Safety considerations

For applications that involve high voltages or require a high level of safety, such as medical devices or aerospace systems, it is crucial to select a PCB base material with a high CTI value. This ensures that the material can withstand electrical stress and maintain its insulation properties over the lifetime of the product.

Cost-performance trade-off

In some cases, there may be a trade-off between the cost and performance of the PCB base material. Higher CTI materials, such as polyimide and ceramic, tend to be more expensive than general-purpose materials like FR-4. It is important to strike a balance between the required performance and the cost constraints of the project.

Frequently Asked Questions (FAQ)

  1. What is the difference between CTI and dielectric strength?
  2. CTI measures a material’s resistance to surface electrical tracking, while dielectric strength measures the material’s ability to withstand electrical breakdown through its bulk.

  3. Can the CTI value of a PCB base material change over time?

  4. Yes, the CTI value of a material can degrade over time due to exposure to environmental factors such as temperature, humidity, and contaminants.

  5. Is it always necessary to use a high CTI material for PCB design?

  6. No, the choice of PCB base material depends on the specific requirements of the application. In some cases, a general-purpose material with a lower CTI value may be sufficient.

  7. How does the CTI of a PCB base material affect the overall reliability of the PCB?

  8. A higher CTI value indicates better resistance to electrical tracking, which can lead to improved reliability and safety of the PCB over its lifetime.

  9. Can the CTI value of a PCB base material be improved through additives or surface treatments?

  10. Yes, some additives and surface treatments, such as flame retardants and conformal coatings, can help improve the CTI value of a PCB base material.

Conclusion

Comparative Tracking Index (CTI) is a crucial parameter in the selection of PCB base materials, as it determines the material’s ability to resist electrical tracking and maintain its insulation properties. Understanding the factors that affect CTI, such as material composition, environmental conditions, and manufacturing processes, is essential for making informed decisions when designing PCBs.

By selecting the appropriate PCB base material based on the required CTI value and other application-specific factors, designers can ensure the reliability, safety, and performance of their products. As the electronics industry continues to evolve, with increasingly demanding applications and harsher operating environments, the importance of considering CTI in PCB design will only continue to grow.