Soldermask requirements for high CTI boards

Understanding Comparative Tracking Index (CTI)

Comparative Tracking Index (CTI) is a standardized measure of a material’s ability to resist electrical tracking along its surface when exposed to high voltage gradients and contamination. CTI testing involves applying a voltage between two electrodes placed on the material’s surface and gradually increasing the voltage until tracking occurs. The higher the voltage at which tracking initiates, the higher the material’s CTI rating.

CTI ratings are classified into several categories based on the voltage at which tracking occurs:

CTI Category	Voltage Range
0	Less than 400V
1	400V to 599V
2	600V to 799V
3	800V to 999V
4	1000V and above

For high-reliability applications, such as those in the automotive, aerospace, and medical industries, PCBs are often required to have soldermask with a high CTI rating (category 3 or above) to ensure robust electrical insulation and minimize the risk of surface tracking.

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Factors Influencing Soldermask CTI

Several factors can impact a soldermask’s CTI rating, including:

Material Composition

The chemical composition of the soldermask plays a significant role in determining its CTI performance. Soldermasks are typically made from polymeric materials, such as epoxy, acrylic, or polyurethane, which can be formulated with various additives and fillers to enhance their electrical and mechanical properties.

To achieve high CTI ratings, soldermask formulations often incorporate:

Flame retardants: These additives help prevent the propagation of electrical tracking by inhibiting the formation of conductive carbonaceous paths on the soldermask surface.
Inorganic fillers: Materials like silica, alumina, or ceramic particles can improve the soldermask’s thermal stability and resistance to surface degradation.
Crosslinking agents: Higher crosslink density in the soldermask polymer network can lead to improved mechanical strength and resistance to moisture absorption.

Surface Roughness

The surface roughness of the soldermask can also affect its CTI performance. A smoother surface can help prevent the accumulation of contaminants and moisture, which can contribute to electrical tracking. Conversely, a rougher surface may provide more sites for contaminants to accumulate and create conductive paths.

To optimize surface roughness, soldermask application processes, such as screen printing or spray coating, must be carefully controlled to ensure a uniform and smooth finish.

Soldermask Thickness

The thickness of the soldermask layer can also influence its CTI rating. A thicker soldermask can provide better insulation and resistance to surface tracking, as it creates a greater distance between the conductive copper traces and the surface.

However, increasing soldermask thickness can also present challenges, such as:

Longer curing times
Reduced feature resolution
Increased risk of soldermask cracking or delamination

Balancing soldermask thickness with other design and manufacturing requirements is essential to achieve optimal CTI performance without compromising the overall PCB Quality.

Soldermask CTI Requirements for High-Reliability Applications

For PCBs used in high-reliability applications, soldermask CTI requirements are typically specified by industry standards or customer-specific requirements. Some common standards that address soldermask CTI include:

IPC-SM-840: This IPC standard defines the qualification and performance requirements for permanent polymer soldermask coatings used on PCBs. It specifies a minimum CTI rating of 300V (category 1) for standard soldermasks, with higher ratings required for more demanding applications.
UL 94: This Underwriters Laboratories (UL) standard evaluates the flammability of plastic materials, including soldermasks. While not directly related to CTI, the flammability rating of a soldermask can be an indicator of its ability to resist electrical tracking.
IEC 60112: This International Electrotechnical Commission (IEC) standard outlines the method for determining the proof tracking index (PTI) and comparative tracking index (CTI) of insulating materials. It provides a standardized testing procedure for evaluating a material’s resistance to electrical tracking.

In addition to these standards, specific industries may have their own requirements for soldermask CTI:

Industry	Typical CTI Requirement
Automotive	Category 3 (800V to 999V) or higher
Aerospace	Category 3 (800V to 999V) or higher
Medical	Category 2 (600V to 799V) or higher
Industrial	Category 1 (400V to 599V) or higher
Consumer	Category 0 (Less than 400V) or higher

Meeting these CTI requirements involves careful selection of soldermask materials, optimization of application processes, and thorough testing to validate performance.

Testing and Qualification of High CTI Soldermasks

To ensure that a soldermask meets the required CTI rating for a given application, thorough testing and qualification processes must be carried out. The testing procedure typically involves:

Sample preparation: Soldermask is applied to a standardized test coupon, which is then cured according to the manufacturer’s recommendations.
Conditioning: Test samples are conditioned in a controlled environment (temperature and humidity) to simulate real-world operating conditions.
CTI testing: The test coupon is subjected to the CTI testing procedure as described in IEC 60112, with the voltage gradually increased until tracking occurs.
Evaluation: The voltage at which tracking occurred is recorded, and the corresponding CTI category is determined.

Soldermask manufacturers often work closely with PCB fabricators and end customers to develop and qualify soldermask formulations that meet specific CTI requirements. This collaboration involves iterative testing, process optimization, and validation to ensure consistent and reliable performance.

Design Considerations for High CTI PCBs

When designing PCBs that require high CTI soldermasks, several factors should be considered to ensure optimal performance:

Copper Trace Spacing

Increasing the spacing between copper traces can help reduce the electrical stress on the soldermask and minimize the risk of surface tracking. Designers should adhere to the minimum spacing guidelines specified by the soldermask manufacturer and the relevant industry standards.

Soldermask Coverage

Ensuring adequate soldermask coverage over the copper traces is crucial for achieving high CTI performance. Designers should allow for sufficient soldermask expansion during the curing process and avoid designing features that may lead to thin or inconsistent soldermask coverage.

Surface Finish

The choice of surface finish can also impact the soldermask’s CTI performance. Some surface finishes, such as immersion silver or gold, may be more susceptible to contamination and moisture absorption, which can degrade the soldermask’s insulation properties over time. Designers should consider the compatibility of the surface finish with the soldermask material and the environmental conditions the PCB will be exposed to.

Assembly Processes

During PCB Assembly, care must be taken to avoid damaging the soldermask layer. Processes such as soldering, cleaning, and conformal coating application can potentially degrade the soldermask’s CTI performance if not properly controlled. Designers should work closely with the assembly team to ensure that the high CTI soldermask is not compromised during the manufacturing process.

FAQ

What is the difference between CTI and PTI?
CTI (Comparative Tracking Index) and PTI (Proof Tracking Index) are both measures of a material’s resistance to electrical tracking. However, CTI is determined by gradually increasing the voltage until tracking occurs, while PTI is determined by applying a fixed voltage for a specified duration and observing whether tracking occurs.
Can a high CTI soldermask compensate for poor PCB design?
While a high CTI soldermask can provide enhanced insulation and resistance to surface tracking, it cannot completely compensate for poor PCB design. Proper design practices, such as maintaining adequate trace spacing and ensuring soldermask coverage, are still essential for achieving optimal performance.
How does the color of the soldermask affect its CTI rating?
The color of the soldermask itself does not directly impact its CTI rating. However, some pigments or dyes used to achieve certain colors may influence the soldermask’s material properties and, consequently, its CTI performance. It’s essential to work with the soldermask manufacturer to ensure that the desired color does not compromise the required CTI rating.
Are there any environmental factors that can degrade a soldermask’s CTI performance over time?
Yes, environmental factors such as temperature, humidity, and exposure to chemicals can degrade a soldermask’s CTI performance over time. High temperatures can lead to thermal degradation of the soldermask material, while humidity and chemical exposure can cause moisture absorption and contamination, which can reduce the insulation properties. Designers should consider the expected environmental conditions and choose a soldermask material that can withstand these factors.
Can a high CTI soldermask be reworked or repaired?
Reworking or repairing a high CTI soldermask can be challenging, as any damage or alteration to the soldermask layer can potentially degrade its insulation properties. If rework or repair is necessary, it’s crucial to follow the soldermask manufacturer’s guidelines and use compatible materials to minimize the impact on the CTI performance. In some cases, it may be necessary to strip and reapply the soldermask to ensure consistent performance across the PCB.

Conclusion

Soldermask plays a critical role in ensuring the electrical insulation and reliability of PCBs, particularly in high-voltage and high-reliability applications. The Comparative Tracking Index (CTI) rating of a soldermask is a key indicator of its ability to resist electrical tracking and maintain its insulation properties under challenging conditions.

To achieve high CTI performance, PCB designers and manufacturers must carefully consider factors such as soldermask material composition, surface roughness, thickness, and compatibility with the PCB design and assembly processes. Adherence to industry standards and close collaboration between soldermask manufacturers, PCB fabricators, and end customers is essential for developing and qualifying soldermask formulations that meet the required CTI ratings.

By understanding the importance of soldermask CTI and implementing best practices in design and manufacturing, PCB professionals can ensure the production of high-quality, reliable boards that can withstand the demands of high-voltage and high-reliability applications.