Printed circuit boards (PCBs) require insulation as part of the manufacturing process. The insulation helps to protect non-conductive materials while also preventing the conductive components from coming into contact with each other. It separates the conductive paths to prevent shortages and failures.
However, insulation can experience a range of adverse conditions that impact its quality and protective properties. Determining the comparative tracking index (CTI) allows manufacturers to use the right types of insulation on PCBs to prevent tracking and creeping of the conductive paths.
What is CTI in Relation to Printed Circuit Boards?
The comparative tracking index (CTI) is a measurement that defines the insulation resistance of the PCB substrate to unwanted current flows that may occur with traces and pads. This metric allows manufacturers to know how the PCB substrate reacts to different environments and if tracking can occur along the deteriorated part of the insulation. The CTI represents the maximum voltage that the substrate base can withstand when liquid is applied during testing.
CTI (comparative tracking index) isn’t something you can see or feel rather something you can trust that is there and confirm with testing. The properties of the insulating material are tested under IPC-TM-650 for electrical breakdown on the surface.
Printed circuit boards are used in a variety of environments including marine, aerospace, chemical, and defense applications that may experience excessive moisture, contamination, or corrosion. When PCBs experience contamination, electrical stress, or humidity, unwanted conductive paths can form which could cause electrical shorts. In addition, arcing can occur between two conductive components placed close to each other.
How is CTI Processed and Measured?
All CTI value testing must be done in accordance with IEC-22 "Test Method for Comparative Tracking Index of Substrates, Printed Boards and Printed Board Assemblies" standards. Every base material is tested and given a CTI rating based on the amount of resistance it shows when exposed to contaminated liquids. The contaminated liquid is an electrolyte that contains 0.1% of ammonium chloride. The manufacturer applies 50 drops of this liquid to the substrate with each drop spaced out at every 30 seconds. At the same time, a test voltage of 600 volts is applied to the sample material.
If the test sample fails at 600V, then the test voltage is lowered, typically by 25V. Then another sample of the same base material is tested at this lowered voltage setting. A failure is indicated when tracking happens while the 50 drops are applied to the surface. If the substrate material burns, the test becomes inconclusive as the material’s thickness is increased to start the test again.
A base sample has passed the test when all 50 drops are applied without tracking at a specific voltage. The test is repeated 5 times at different locations along the sample. The CTI rating is the maximum voltage that the base material can withstand without failing and represents the material's performance in that environment. Once the material is given a CTI rating, it becomes placed into a material category developed by IEC and Underwriters Laboratories (UL).
The material categories are as follows:
- Material Category I: 600V </=
- Material Category II: 400V </= CTI < 600V
- Material Category IIIa: 175V </= CTI < 400V
- Material Category IIIb: 100V </= CTI < 175V
So, if the material experiences 200V but fails at 350V, it would fall into Material Category IIIa. It has a CTI value that is equal to or greater than 175V but is less than 400V. Knowing the CTI value provides important information to the manufacturer so the right PCB substrate is selected that can provide optimal performance even when experiencing a certain level of humidity, dust, pollen, moisture, chemicals, or other stresses. It also allows them to deal with leakage and creepage.
What is the Difference Between Leakage and Creepage?
There is a major difference between what is considered leakage and what is considered creepage for PCB manufacturing. Different solutions can be employed depending on the problem that is present.
Leakage involves the insulating materials forming a conducive path along the deteriorated portion of the material. The deteriorated section becomes carbonized and conductive. For example, if the insulated material becomes moldy due to wet and humid conditions, that moldy location can create an electrical current which could pose a serious safety hazard to the user of the device. If there is leakage, manufacturers can select a different material at a thickness that meets CTI value standards for the application while the substrate's properties are resistant to mold growth.
Creepage involves when there are two conductive parts. The conducive path spreads outward from each part and toward each other as this can create arcing between the conductive parts. The CTI value of the material can allow a manufacturer to know how much creepage can occur when placing in traces and pads, called creepage distance.
Knowing the creepage distance factors into how close traces and pads can be placed in relation to each other without arcing. A manufacturer can select a material with a higher CTI so that they can place traces and pads at the minimum creepage distance allowable for the application. This aspect is important for high-performance applications that require smaller PCBs.
CTI for FR4, Ceramic, and Teflon Materials
PCM substrate materials come with varying CTI values that determine their resistance over long lengths of time. The lower CTI value indicates lower resistance to moisture, humidity, and other environmental elements to voltages that are higher than the CTI value.
Here are the CTI values for some materials.
- FR-1, XPC, and other standard paper-based copper-clad laminates: </= 150V
- CEM-1 and CEM-3 standard composite-based copper-clad laminates: 175V to 225V
- Ordinary FR4 and glass fiber cloth-based copper-clad laminates: 175V to 225V
- Teflon (PTFE): 600V
There are no CTI values for glass, ceramics, or other inorganic materials that experience no breakdown along the surface during testing. There may be variants of copper-clad laminates and FR4 that offer higher CTI values, such as high voltage Teflon (HVPF) or PCB materials that are backed with aluminum to prevent corrosion and damage from high humidity and pollution. High CTI copper-clad materials can be used in both high-pollution and high-voltage applications. Keep in mind that substrates that have a high CTI still may not be suitable for certain environmental conditions depending on their properties.
Why is the CTI value of a laminate material crucial for PCB designers?
- Electrical Safety: CTI value indicates the electrical insulation properties of the material. A higher CTI value implies better resistance to electrical tracking, which is important for preventing short circuits and ensuring the safety of the circuit.
- High Voltage Applications: In high-voltage applications, such as power electronics, a higher CTI value is necessary to withstand the high potential differences between conductive elements without breakdown.
- Reliability: A material with a high CTI value is less likely to degrade over time due to tracking, moisture absorption, or environmental contaminants. This enhances the reliability and longevity of the PCB.
- Compliance: Many industries have standards and regulations regarding the electrical safety and reliability of electronic products. Designing PCBs with materials that meet specific CTI requirements ensures compliance with these standards such as UL and CE.
Industries that may require high CTI PCBs include oil and gas exploration, automotive, and aerospace. Some applications that use high CTI PCBs include:
- Power generation
- Heavy machinery operation
- Power distribution
- Motor drives
What is PTI and How Does It Compare to CTI?
CTI should not be confused with the proof tracking index (PTI). While CTI indicates the maximum voltage value of the materials as they can withstand the applied electrolyte, PTI is a metric that indicates the amount of voltage that can be withstood under the same testing parameters without forming leakage traces at a single point. So, a material can have a PTI of 175V, meaning it can withstand this voltage.
A PCB substrate can have a low PTI value and a higher CTI value. For example, a material can withstand 175V for PTI, yet the material can undergo voltage changes from 175V up to 300V for CTI before it begins to experience failure.
Summary
By understanding the CTI of PCB materials, customers can have a diverse range of substrates to select from for their specific applications. Knowing the working environment, the strength of voltages experienced, and other critical factors can significantly impact how the PCB is manufactured, ensuring both safety and high performance.
This knowledge enables customers to make informed decisions, optimizing the reliability and longevity of their electronic devices while adhering to industry standards and regulations. Ultimately, a thorough grasp of CTI can lead to more efficient designs, reduced maintenance costs, and enhanced overall system performance.
