In the field of PCB (printed circuit board), CTI (Comparative Tracking Index) is a key indicator for measuring the insulation performance of PCB substrate, used to evaluate the ability of materials to resist leakage tracking in humid, polluted or high-voltage environments. The following is a detailed analysis of CTI:
Table of Contents
1、 Definition and Testing Principles of CTI
1. Definition
CTI represents the highest voltage value at which a material surface can withstand 50 drops of electrolyte (usually 0.1% ammonium chloride solution) without forming a conductive channel under the combined action of an electric field and pollutants (unit: V）。 Leakage tracing refers to the phenomenon where the surface of a material gradually carbonizes and forms conductive pathways due to pollutants and electric fields, which may lead to short circuits, breakdowns, or even fires.
2. Testing methods
According to the international standard IEC 60112, a conductive solution is dropped onto the surface of the material during testing, and the lowest voltage at which conductive traces are formed is recorded. The testing conditions include:
-Electrode spacing: 4.0 mm ± 0.1 mm;
-Droplet height: 30-40 mm;
-Voltage range: usually gradually adjusted from 300V until the critical value is determined.
2、Level classification
CTI values are classified into different levels according to UL and IEC standards, used to guide material selection and application scenarios:
| CTI value range (V) | UL Rating | IEC level | Examples of applicable materials |
| ≥600 | 0 | – | Ceramic substrate (such as Al ₂ O ∝) |
| 400 ≤ CTI < 600 | 1 | – | High performance resin substrate |
| 250 ≤ CTI < 400 | 2 | IIIa | High Tg FR-4 |
| 175 ≤ CTI < 250 | 3 | IIIb | Ordinary FR-4 |
| 100 ≤ CTI < 175 | 4 | – | Low grade materials (use with caution) |
| <100 | 5 | – | Not recommended for insulation scenarios |
Note:
-UL standards are divided into 6 levels, IEC standards are divided into 4 levels, and CTI ≥ 600 is the highest level.
-The CTI value of ordinary FR-4 is usually 175-250V, suitable for consumer electronics; Industrial or automotive grade PCBs require higher CTI values.
3、 The significance of CTI in PCB design
1. Security guarantee
High CTI materials can reduce the risk of short circuits in high voltage or humid environments, making them suitable for harsh scenarios such as electric vehicles and medical equipment.
2. Long term reliability
Leakage tracing will gradually deteriorate the insulation layer, and high CTI materials can extend the service life of PCBs under harsh working conditions.
3. Regulatory compliance
Industry standards such as UL and IEC have clear requirements for CTI, for example, automotive electronics must comply with AEC-Q100 certification, requiring CTI to be ≥ 300V.
4. Creepage distance design
According to IEC 60335, the higher the CTI value, the smaller the allowed minimum creepage distance, thereby optimizing high-density PCB layout.
4、 Comparison of CTI values of common PCB substrates
| Substrate type | Range (V) | Typical applications |
| Ordinary FR-4 | 175-250 | Consumer electronics (mobile phones, computers) |
| High Tg FR-4 | 250-300 | Industrial control equipment, power module |
| Polyimide (PI) | ≥300 | Aerospace, high-frequency and high-voltage scenarios |
| Substrate | ≥600 | High frequency, high pressure, and high temperature resistant scenarios |
5、 How to improve CTI value?
1. Optimize resin composition: Choose resins with low carbon content and high thermal stability to reduce easily carbonized groups.
2. Adding inorganic fillers: such as sheet-like fillers can interrupt the carbonization path and improve insulation.
3. Surface treatment: Use hydrophobic coatings or moisture-proof processes to delay the accumulation of pollutants.
Summarize
CTI is one of the core parameters for PCB material selection, which directly affects the safety, reliability, and compliance of products. When designing, it is necessary to select substrates with appropriate CTI values based on the application environment (such as humidity, voltage, and pollution level), and verify them with reference to UL or IEC standards.
