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The CTI grade (Comparative Tracking Index) in PCB is an indicator to measure the material surface's ability to resist tracking under the combined effects of electric field and moisture, which directly affects the insulation safety and long-term reliability of the circuit board.
Definition:CTI indicates the ability of a material surface to resist leakage current to form a conductive path under specific conditions (such as moisture, pollution, high voltage). The higher the value, the stronger the material's ability to resist leakage tracking.
Test standard: According to the international standard IEC 60112, by applying voltage (usually 100~600V) and dripping electrolyte, the lowest voltage value at which a permanent conductive path is formed on the material surface is recorded.
According to the IEC 60112 standard, CTI is divided into the following grades:
Class 1：CTI ≥ 600
Class 2：400 ≤ CTI < 600
Class 3：250 ≤ CTI < 400
Class 3a：175 ≤ CTI < 250
UL standard classification examples:
Class 0 (CTI ≥ 600)
Class 1 (400-599)
Class 2 (250-399)
Class 3 (100-249)
Insulation safety:
High CTI materials can prevent short circuits, arcs or fires caused by leakage tracking in high voltage or high humidity environments.
Creep distance design:
CTI directly affects the creepage distance (the minimum insulation path length between conductors). The higher the CTI level, the shorter the allowed creepage distance, which can optimize the PCB layout.
Compliance requirements:
Medical devices, automotive electronics (such as ISO 26262), industrial controls (such as UL 60950) and other scenarios require CTI levels (usually ≥ 2 levels).
FR-4: CTI 175~250 (level 2~3), suitable for most consumer electronics.
Polyimide (PI): CTI ≥300 (level 1), used for aerospace, flexible circuits.
Aluminum substrate: CTI ≥200 (level 2~3), used for high heat dissipation scenarios.
Ceramic substrate: CTI ≥600 (level 0), used in extreme high-voltage environments.
Choose CTI level 3 (FR-4):Low voltage/normal environment (such as mobile phones, home appliances):
Require CTI level 2 or higher (such as polyimide or aluminum substrate):High voltage/high humidity environment (such as electric vehicles, outdoor equipment):
Preferably choose CTI level 1~2, and comply with UL, IEC and other certifications:Critical safety equipment (such as medical equipment, rail transit):
Arc Resistance:
Measures the ability of materials to resist arc erosion, which is complementary to CTI.
Insulation Resistance (IR):
The overall insulation performance of the material, while CTI focuses on local failure on the surface.
Glass Transition Temperature (Tg):
CTI has no direct correlation with temperature resistance and requires comprehensive evaluation.
1.Environmental matching: Select the CTI grade according to the operating voltage and pollution level. For example, a humid and dusty environment requires CTI≥250.
2.Comprehensive performance balance: CTI is not the only indicator, and heat resistance (Tg value), mechanical strength (such as bending strength) and cost must be taken into account.
3.Standard differences: Different industries (such as automobiles vs. home appliances) may adopt different CTI grading standards, and specific requirements must be clarified before design.
For Example: The battery management system (BMS) of electric vehicles usually requires CTI ≥ 600 (Class 1) to ensure long-term stability under high voltage and vibration environments; while LED lighting driver boards may only require CTI ≥ 175 (Class 3a) in dry environments.
Summary: The CTI grade is the core parameter of the insulation reliability of PCB materials. The correct selection can significantly improve product safety and life. When designing, it is necessary to combine application scenarios, industry standards and cost factors for a comprehensive evaluation.