Understanding CTI and Its Significance
Comparative Tracking Index (CTI) is a measure of a material’s ability to resist electrical tracking and erosion under high voltage stress and humid conditions. It is an essential parameter in PCB design, particularly for applications that require high electrical insulation and resistance to surface contamination.
CTI is determined by subjecting the material to a standardized test procedure described in the IEC 60112 standard. The test involves applying a voltage between two electrodes placed on the surface of the material and gradually increasing the voltage until tracking occurs. The CTI value is then assigned based on the highest voltage at which the material can withstand 50 drops of a specified electrolyte solution without exhibiting tracking.
The CTI values are categorized into four main groups:
| CTI Range | Material Group | Typical Applications |
|---|---|---|
| 600 ≤ CTI | I | High-voltage, outdoor, and harsh environments |
| 400 ≤ CTI < 600 | II | Industrial control systems and power electronics |
| 175 ≤ CTI < 400 | IIIa | Consumer electronics and general-purpose applications |
| 100 ≤ CTI < 175 | IIIb | Low-voltage and non-critical applications |
High CTI boards, typically those in Material Group I or II, are essential for applications that demand superior electrical insulation and resistance to surface contamination. These include:
- High-voltage power electronics
- Automotive and aerospace electronics
- Outdoor and harsh environment applications
- Medical devices and equipment
- Industrial control systems
Soldermask Properties Affecting CTI Performance
When selecting a soldermask for high CTI boards, several key properties must be considered to ensure optimal performance:
1. Dielectric Strength
Dielectric strength is the maximum electric field that a material can withstand without breaking down and allowing current to flow. A soldermask with high dielectric strength is essential for high CTI boards to prevent electrical breakdown and ensure reliable insulation.
2. Surface Resistance
Surface resistance is a measure of a material’s ability to resist the flow of electrical current along its surface. A soldermask with high surface resistance is crucial for preventing leakage currents and maintaining high insulation resistance.
3. Moisture Resistance
Moisture can significantly impact the electrical properties of a soldermask, leading to decreased insulation resistance and increased risk of tracking. A soldermask with excellent moisture resistance is vital for high CTI boards, especially those exposed to humid environments.
4. Thermal Stability
High CTI boards often operate in environments with elevated temperatures, such as power electronics and automotive applications. A soldermask with good thermal stability is essential to maintain its electrical and mechanical properties over a wide temperature range.
5. Adhesion Strength
Proper adhesion between the soldermask and the copper traces is crucial for ensuring the long-term reliability of high CTI boards. A soldermask with strong adhesion helps prevent delamination and other failures that can compromise the board’s performance.
Soldermask Material Options for High CTI Boards
Several soldermask material options are available for high CTI boards, each with its unique set of properties and advantages:
1. Epoxy-based Soldermasks
Epoxy-based soldermasks are the most common choice for high CTI boards due to their excellent electrical insulation, moisture resistance, and thermal stability. They offer a good balance of performance and cost-effectiveness, making them suitable for a wide range of applications.
2. Polyurethane-based Soldermasks
Polyurethane-based soldermasks provide superior flexibility and elongation compared to epoxy-based soldermasks, making them ideal for flexible and Rigid-Flex PCBs. They also offer excellent chemical resistance and thermal stability, making them suitable for harsh environment applications.
3. Acrylic-based Soldermasks
Acrylic-based soldermasks are known for their excellent UV resistance and color stability, making them a good choice for outdoor applications. They also provide good electrical insulation and moisture resistance, although not as high as epoxy-based soldermasks.
4. Silicone-based Soldermasks
Silicone-based soldermasks offer the highest temperature resistance among all soldermask materials, making them suitable for high-temperature applications such as automotive and aerospace electronics. They also provide excellent flexibility and moisture resistance, but their relatively high cost limits their use to specialized applications.
The table below summarizes the key properties of different soldermask material options:
| Property | Epoxy | Polyurethane | Acrylic | Silicone |
|---|---|---|---|---|
| Dielectric Strength | High | High | Moderate | High |
| Surface Resistance | High | High | Moderate | High |
| Moisture Resistance | High | High | Moderate | High |
| Thermal Stability | High | High | Moderate | Very High |
| Flexibility | Moderate | High | Moderate | High |
| UV Resistance | Moderate | Moderate | High | High |
| Cost | Moderate | Moderate | Low | High |
Best Practices for Soldermask Application on High CTI Boards
To ensure the best performance of high CTI boards, it is essential to follow best practices during the soldermask application process:
- Surface Preparation: Prior to soldermask application, the PCB surface must be thoroughly cleaned and free of contaminants to ensure proper adhesion. This can be achieved through a combination of chemical cleaning, micro-etching, and plasma treatment.
- Soldermask Thickness: The soldermask thickness should be optimized based on the specific requirements of the application. A thicker soldermask provides better insulation and resistance to mechanical damage but may impact the fine-pitch component assembly. A typical soldermask thickness range for high CTI boards is 0.8 to 1.2 mils (20 to 30 μm).
- Curing Process: Proper curing of the soldermask is critical for achieving optimal performance. The curing process should be carried out according to the manufacturer’s recommended temperature and duration to ensure complete cross-linking of the polymer and maximum adhesion to the PCB surface.
- Inspection and Testing: After soldermask application, the PCBs should undergo thorough inspection and testing to verify the soldermask quality and performance. This includes visual inspection for defects, adhesion testing, insulation resistance testing, and CTI testing as per the IEC 60112 standard.
FAQ
1. What is the minimum CTI value required for high CTI boards?
A: The minimum CTI value for high CTI boards is typically 400, which corresponds to Material Group II. However, for some critical applications, such as high-voltage power electronics and outdoor equipment, a CTI value of 600 or higher (Material Group I) may be required.
2. Can a soldermask with a lower CTI value be used on a high CTI board?
A: While it is technically possible to use a soldermask with a lower CTI value on a high CTI board, it is not recommended as it can compromise the overall insulation and tracking resistance of the board. It is always best to select a soldermask with a CTI value equal to or higher than the required CTI value for the application.
3. How does the soldermask color affect the CTI performance?
A: The soldermask color itself does not directly affect the CTI performance, as the CTI value is primarily determined by the soldermask material composition and properties. However, some pigments used for colored soldermasks may influence other properties, such as UV resistance and thermal stability, which can indirectly impact the overall performance of the board.
4. Can a high CTI soldermask be used on a low CTI board?
A: Yes, a high CTI soldermask can be used on a low CTI board, and it may even provide additional benefits such as improved insulation and resistance to surface contamination. However, it is essential to consider the cost implications, as high CTI soldermasks are generally more expensive than their lower CTI counterparts.
5. How often should high CTI boards be tested for CTI performance?
A: The frequency of CTI testing for high CTI boards depends on various factors, such as the criticality of the application, the environmental conditions, and the expected service life of the product. In general, it is recommended to perform CTI testing during the initial qualification of the board and periodically during production to ensure consistency. For critical applications, additional testing may be required after exposure to environmental stresses, such as high temperature, humidity, or chemical contamination.
Conclusion
Selecting the appropriate soldermask is crucial for ensuring the performance and reliability of high CTI boards. By understanding the key properties affecting CTI performance, evaluating the available soldermask material options, and following best practices for soldermask application, designers and manufacturers can create PCBs that meet the demanding requirements of high-voltage and harsh environment applications.
As the electronics industry continues to evolve, the development of new soldermask materials and technologies will play a vital role in pushing the boundaries of PCB performance and reliability. By staying up-to-date with the latest advancements and collaborating closely with soldermask suppliers and PCB manufacturers, engineers can design and produce high CTI boards that excel in even the most challenging applications.