Printed circuit boards used in power electronics, industrial equipment, automotive systems, energy products, and electrical control systems may operate at voltages where insulation performance becomes a critical design consideration. One important characteristic used to evaluate PCB insulating materials is the Comparative Tracking Index (CTI).
CTI helps engineers understand how a PCB material behaves when exposed to electrical stress and contamination under specified test conditions. It is particularly relevant when designing circuits that require appropriate creepage distances between conductive features.
Understanding CTI, material ratings, creepage, and related safety considerations can help PCB designers select suitable materials and develop more reliable high-voltage assemblies.
What Is PCB CTI?
The Comparative Tracking Index (CTI) is a material characteristic associated with the electrical tracking resistance of an insulating material. Engineers can learn more about pcb cti when evaluating insulation performance and electrical safety in PCB materials.
Electrical tracking occurs when contamination and moisture on an insulating surface create a conductive path between energized conductors. Repeated electrical stress can progressively damage the surface and potentially create a permanent conductive path.
CTI testing provides a standardized way of comparing the tracking behavior of insulating materials under specified test conditions.
A higher CTI value generally indicates greater resistance to the formation of conductive tracking paths under the defined test conditions.
CTI is therefore one of the material properties engineers may consider when selecting PCB laminates for applications involving electrical insulation and closely spaced conductors.
Why CTI Matters in PCB Design
CTI can become particularly important when conductive features need to be placed relatively close together.
For high-voltage PCB designs, engineers need to control the distance between conductors while considering factors such as:
- Operating voltage
- Pollution level
- Insulating material
- Environmental conditions
- Creepage distance
- Clearance distance
- Insulation requirements
- PCB surface contamination
- Applicable safety standards
Using a material with an appropriate CTI rating can provide greater flexibility in meeting insulation requirements, although CTI alone does not determine the complete safety design.
CTI Material Ratings
PCB insulating materials can be grouped into CTI ranges according to applicable material classification systems and standards.
A commonly referenced classification uses the following ranges:
| Material Group | CTI Range |
| Group I | CTI ≥ 600 V |
| Group II | 400 V ≤ CTI < 600 V |
| Group IIIa | 175 V ≤ CTI < 400 V |
| Group IIIb | 100 V ≤ CTI < 175 V |
The applicable classification should always be confirmed against the relevant safety standard, material documentation, and PCB manufacturer’s data.
A higher CTI material can be advantageous where PCB geometry requires reduced creepage distances, but engineers should not select a material based on CTI alone.
CTI and Creepage Distance
CTI and creepage are closely related but describe different things. Designers can review high-voltage PCB creepage and clearance considerations when developing boards for elevated-voltage applications.
CTI is a material characteristic. It describes the resistance of an insulating material to electrical tracking under a defined test method.
Creepage is a physical distance. It is the shortest path along the surface of an insulating material between two conductive parts.
For example, if two high-voltage copper areas are separated by a PCB surface, the creepage distance follows the surface of the board between them rather than traveling directly through the air.
The required creepage distance depends on the applicable safety standard and factors such as working voltage, material group, pollution degree, and other application conditions.
CTI vs Clearance
Clearance and creepage should not be confused.
Clearance is the shortest distance through air between two conductive parts.
Creepage is the shortest distance along the surface of an insulating material between those conductive parts.
A PCB design may satisfy a clearance requirement while failing to provide sufficient creepage, or vice versa. Both distances therefore need to be evaluated independently.
This distinction becomes particularly important in high-voltage power supplies, industrial control systems, motor drives, battery systems, and other equipment where electrical insulation is critical.
Steps for Using CTI in PCB Design
Step 1: Identify the Operating Voltage
Begin by determining the relevant voltage levels in the PCB assembly.
Consider the normal operating voltage as well as applicable transient or overvoltage conditions defined by the relevant safety requirements.
Different portions of the same PCB may operate at different voltage levels, so insulation requirements should be considered for each relevant circuit boundary.
Step 2: Determine the Applicable Safety Standard
Identify the safety standard applicable to the end product and application.
Different equipment categories can have different requirements for insulation, creepage, clearance, material properties, and testing.
The PCB designer should not rely on a generic creepage value without first identifying the appropriate standard and product category.
Step 3: Determine the Required Pollution Conditions
The environment in which the PCB operates can influence insulation requirements.
Dust, moisture, condensation, chemicals, and other contaminants can increase the possibility of surface leakage and electrical tracking.
The applicable standard may define pollution degrees or environmental categories that must be considered when determining creepage requirements.
Step 4: Check the PCB Material CTI
Obtain the material datasheet or certification information from the PCB laminate supplier.
Confirm the CTI value or material group applicable to the exact laminate construction being used.
Do not assume that two materials with similar electrical properties have identical CTI characteristics.
Step 5: Calculate the Required Creepage
Use the applicable safety standard to determine the required creepage distance based on the relevant voltage, material group, pollution conditions, and other specified parameters.
The required distance should be applied between the appropriate conductive parts of the PCB.
Step 6: Calculate the Required Clearance
Determine the required clearance separately from creepage.
The shortest through-air distance between conductive features should meet the applicable requirement, taking into account working voltage and relevant transient conditions.
Step 7: Check PCB Layout Geometry
Review the physical layout for high-voltage areas.
Look for:
- Narrow conductor spacing
- Component lead spacing
- Via-to-pad distances
- Connector pin spacing
- Copper pours
- Mounting holes
- Slots and cutouts
- PCB edges
- Test points
All relevant conductive structures should be included in the insulation review.
Step 8: Consider Slots and Cutouts
In some PCB designs, slots or cutouts can increase the effective creepage path between conductive features.
However, slots should only be used when they are appropriate for the design and manufacturing process. Their dimensions, locations, mechanical effects, and cleanliness requirements should be evaluated.
Step 9: Verify Manufacturing Tolerances
The nominal layout distance is not necessarily the minimum distance that will exist on every manufactured board.
Manufacturing tolerances, etching variation, registration, drilling tolerances, and other production factors can affect actual spacing.
Designers should therefore provide sufficient margin rather than designing exactly at the minimum calculated value.
Step 10: Review the Finished PCB
Before releasing the design, verify that the final PCB layout, material selection, fabrication documentation, and applicable safety requirements are consistent.
For high-voltage products, appropriate inspection and testing should also be incorporated into the manufacturing and qualification process.
Factors That Can Affect Creepage
Several factors can influence the required creepage distance.
Working Voltage
Higher voltage generally requires greater insulation distances under applicable standards.
Material Group
The CTI classification of the insulating material can affect creepage requirements in standards that use material groups.
Pollution Degree
A clean controlled environment and a heavily contaminated environment can present different insulation risks.
Environmental Conditions
Humidity, condensation, dust, chemicals, and temperature can influence insulation performance.
PCB Surface Design
Copper geometry, solder mask, slots, cutouts, and component placement can all affect the actual creepage path.
Solder Mask and Creepage
Solder mask is an insulating coating, but designers should not automatically assume that solder mask can be used to satisfy every creepage requirement.
Whether solder mask can be considered in an insulation calculation depends on the applicable standard and its recognition for the particular application.
For safety-critical designs, engineers should follow the requirements of the applicable standard and verify the PCB material and coating qualifications.
Applications Where CTI Is Important
CTI considerations can be relevant to many types of electronic equipment, including:
- AC power supplies
- Industrial control equipment
- Motor drives
- Battery management systems
- Electric vehicle electronics
- Solar and energy equipment
- Power converters
- Industrial automation systems
- Telecommunications equipment
- Lighting systems
- High-voltage measurement equipment
- Aerospace and defense electronics
The exact insulation requirements depend on the product architecture, operating environment, voltage levels, and applicable standards.
Benefits of Considering CTI Early
Better Material Selection
Evaluating CTI during material selection helps engineers identify laminates appropriate for the intended insulation requirements.
More Efficient PCB Layout
Knowing the insulation requirements early can prevent major spacing changes after the PCB layout has been completed.
Improved Safety Planning
CTI evaluation can form part of a broader insulation coordination process covering creepage, clearance, materials, and environmental conditions. Proper PCB base material selection helps designers match electrical, thermal, and mechanical properties to the application.
Reduced Redesign Risk
Early consideration of CTI and creepage can reduce the likelihood of discovering insufficient spacing during certification or qualification testing.
Better Manufacturing Documentation
Clearly documented material and spacing requirements provide PCB manufacturers with more precise production expectations.
Common PCB CTI Design Mistakes
One common mistake is treating CTI as if it were a voltage rating for the entire PCB. CTI is a material property obtained under a defined test method; it does not by itself establish the safe operating voltage of a finished circuit board.
Another mistake is confusing creepage with clearance. These are different measurements and may have different requirements.
Designers may also overlook contamination, component leads, connector geometry, mounting hardware, and PCB edges when calculating insulation distances.
Finally, relying on generic online creepage tables without checking the applicable product safety standard can result in incorrect design assumptions.
Final Considerations
CTI is an important material characteristic for PCB designs where electrical insulation and surface tracking resistance matter. It provides a standardized method for comparing the tracking behavior of insulating materials and can influence material selection and creepage requirements.
A reliable high-voltage PCB design requires more than selecting a high-CTI laminate. Engineers should evaluate operating voltage, clearance, creepage, pollution conditions, environmental factors, PCB geometry, manufacturing tolerances, and the requirements of the applicable safety standard.
By addressing these factors during the early design and stackup stages, engineering teams can develop PCB layouts that are better aligned with electrical safety, manufacturing, and product qualification requirements.
