Soldermask requirements for high CTI boards

Understanding Soldermask and CTI

Soldermask, also known as solder resist or solder mask, is a thin layer of polymer applied to the copper traces of a printed circuit board (PCB). It serves several important functions:

Prevents solder bridges from forming between closely spaced solder pads
Protects the copper traces from oxidation and corrosion
Insulates the copper traces from accidental contact with other conductors
Provides a surface for labeling and marking components

The Comparative Tracking Index (CTI) is a measure of the electrical breakdown (tracking) properties of an insulating material. It indicates the material’s ability to resist the formation of conductive paths on its surface when subjected to high voltage gradients and contamination. CTI is expressed as a numerical value, with higher values indicating better performance.

Typical CTI values for PCB soldermasks range from 175 to over 600. For high voltage applications, soldermasks with a CTI of 400 or higher are generally recommended to ensure adequate insulation and prevent electrical discharge.

CTI Test Method

The CTI test is performed according to the IEC 60112 standard. A test specimen is prepared by applying the soldermask to a flat, non-conductive substrate. Two electrodes, consisting of platinum wires, are placed on the specimen surface at a specified distance apart.

The test is conducted by applying AC voltage across the electrodes and gradually increasing it until tracking occurs. The CTI value is determined based on the maximum voltage withstood before failure, using a table provided in the standard.

CTI	Voltage (V)
600	>600
550	550-600
500	500-550
450	450-500
400	400-450
350	350-400
300	300-350
250	250-300
200	200-250
175	175-200

Table 1. CTI values and corresponding voltage ranges per IEC 60112.

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Importance of High CTI Soldermask

Preventing Electrical Discharge

In high voltage PCB Applications, there is a risk of electrical discharge (arcing) between adjacent conductors if the insulation is inadequate. This can occur due to:

Contamination or moisture on the PCB surface reducing the insulation resistance
Manufacturing defects in the soldermask, such as pinholes or thin spots
Degradation of the soldermask material over time due to environmental factors

Using a high CTI soldermask helps mitigate these risks by providing a more robust insulation layer. The increased tracking resistance makes it much less likely for conductive paths to form, even in the presence of contamination.

Ensuring Regulatory Compliance

Many industries have specific requirements for the electrical safety and reliability of PCBs used in their products. For example:

Medical devices (IEC 60601-1)
IT equipment (IEC 60950-1)
Measurement/control equipment (IEC 61010-1)
Household appliances (IEC 60335-1)

These standards often specify minimum CTI values for insulating materials based on the working voltage and pollution degree of the end application. Using a soldermask with a sufficiently high CTI helps ensure compliance with these requirements.

Improving Product Reliability

Even in applications where high voltage is not a concern, using a high CTI soldermask can improve the overall reliability of the PCB. The enhanced insulation properties provide an extra margin of safety against unforeseen circumstances, such as:

Voltage spikes induced by switching loads or electrostatic discharge
Condensation or other contamination in the end-use environment
Degradation of the soldermask due to aging or exposure to harsh chemicals

By reducing the likelihood of insulation breakdown, high CTI soldermasks contribute to a longer service life and fewer field failures for the end product.

Choosing a High CTI Soldermask

Material Composition

Soldermask materials are typically based on epoxy, acrylic, or polyimide polymers. The choice of base resin and additives determines the final properties of the soldermask, including the CTI value.

Epoxy soldermasks are the most common type and offer a good balance of properties. They have excellent adhesion to copper, good chemical resistance, and can achieve CTI values of 550 or higher with appropriate formulation.

Acrylic soldermasks are known for their high resolution and fast curing times. However, their CTI values are generally lower than epoxies, typically in the range of 300-400.

Polyimide soldermasks offer the highest CTI values, often exceeding 600. They also have exceptional heat resistance and mechanical toughness. However, they are more expensive and require higher curing temperatures compared to other types.

Application Method

The soldermask application method can also influence the final CTI value. The most common methods are:

Screen printing: A thick film of soldermask is deposited through a fine mesh screen and then cured. This method is simple and economical but may result in a thicker, less uniform coating.
Liquid photoimageable (LPI): The soldermask is applied as a liquid photopolymer, exposed through a photomask, and developed to create the desired pattern. LPI allows for thinner, more precise coatings and is widely used for high-density PCBs.
Dry film: A pre-made film of photoimageable soldermask is laminated onto the PCB, exposed, and developed. Dry film offers good thickness control and is suitable for large-volume production.

In general, LPI and dry film methods produce more consistent soldermask layers with fewer defects, which can contribute to higher CTI values. However, the base material properties still have a larger impact on the final performance.

Color Considerations

Soldermasks are available in a variety of colors, with green being the most common. However, for high voltage applications, it is often desirable to use a different color to visually distinguish these boards from standard ones.

Some popular color choices for high CTI soldermasks include:

White: Provides good contrast for component markings and allows for easy inspection of the soldermask surface.
Black: Offers high opacity and can help hide underlying copper features for a cleaner appearance.
Red, blue, yellow: Bright colors that stand out and make it clear the board has special requirements.

It’s important to note that the pigments used to achieve these colors can slightly affect the final CTI value. In general, lighter colors like white tend to have higher CTI than darker ones. However, this difference is usually minor compared to the base material properties.

Designing PCBs for High CTI

Clearance and Creepage

When designing PCBs for high voltage applications, it’s crucial to consider the clearance and creepage distances between conductors. Clearance refers to the shortest distance through air, while creepage is the shortest path along the insulating surface.

The required clearance and creepage distances depend on several factors:

Working voltage: Higher voltages require greater spacing to prevent arcing.
Pollution degree: The amount of conductive contamination present in the end-use environment. Higher pollution degrees necessitate larger distances.
Material group: Insulating materials are classified into four groups based on their CTI value (I, II, IIIa, IIIb). Higher group numbers allow for smaller creepage distances.

Soldermasks with higher CTI values (group I or II) can enable more compact PCB layouts by reducing the necessary creepage distances. However, it’s still important to follow the spacing rules prescribed by the relevant safety standards.

Here are some example creepage distances for different material groups and working voltages, assuming pollution degree 2:

Working Voltage (V)	Group I (CTI ≥600)	Group II (400≤CTI<600)	Group IIIa (175≤CTI<400)	Group IIIb (100≤CTI<175)
≤50	0.6 mm	1.2 mm	1.8 mm	3.0 mm
100	0.7 mm	1.4 mm	2.1 mm	3.5 mm
150	0.8 mm	1.6 mm	2.4 mm	4.0 mm
300	1.5 mm	3.0 mm	4.5 mm	7.5 mm
600	3.2 mm	6.4 mm	9.6 mm	16.0 mm
1000	5.0 mm	10.0 mm	15.0 mm	25.0 mm

Table 2. Example creepage distances for different material groups and working voltages per IEC 60664-1.

Soldermask Thickness

The thickness of the soldermask layer can also affect its insulation properties. A thicker coating generally provides better protection against pinhole defects and Dielectric Breakdown.

Typical soldermask thicknesses range from 0.5 to 2.0 mils (12.7 to 50.8 μm). For high voltage applications, a minimum thickness of 1.0 mil (25.4 μm) is often recommended. However, the exact requirements may vary depending on the specific soldermask material and the PCB design.

It’s important to work closely with the PCB manufacturer to ensure the soldermask is applied at the appropriate thickness and consistency across the board. Some factors to consider include:

Copper weight: Heavier copper weights may require a thicker soldermask to achieve adequate coverage.
Surface finish: The type of finish (HASL, ENIG, OSP, etc.) can affect how well the soldermask adheres and conforms to the surface.
Via tenting: If vias are tented with soldermask, a thicker layer may be needed to prevent dimpling or cracking.

Ultimately, the soldermask thickness should be sufficient to provide reliable insulation without compromising the PCB’s manufacturability or functionality.

Testing and Qualification

Production Testing

Before a high CTI soldermask can be used in production, it must undergo thorough testing and qualification to ensure it meets the required performance standards. This typically involves a combination of:

Material property tests: Measuring the CTI value, Dielectric strength, insulation resistance, etc. of the soldermask material itself.
PCB-level tests: Subjecting finished boards to various electrical, environmental, and mechanical stresses to verify the soldermask’s performance in the final application.

Some common PCB-level tests for high voltage applications include:

Hipot (dielectric withstanding voltage): Applying a high voltage between conductors and checking for leakage current or breakdown.
Insulation resistance: Measuring the resistance between conductors at a specified voltage.
Thermal cycling: Exposing the PCB to alternating high and low temperatures to check for cracking or delamination of the soldermask.
Humidity resistance: Subjecting the PCB to high humidity conditions and checking for degradation of the soldermask’s insulating properties.

The specific test requirements and acceptance criteria will depend on the end-use application and the relevant industry standards.

Ongoing Quality Control

Once a high CTI soldermask is qualified and put into production, it’s important to implement ongoing quality control measures to ensure consistent performance. This may include:

Incoming inspection of soldermask materials to verify CTI values and other key properties.
Process monitoring during PCB fabrication to check soldermask thickness, registration, and curing conditions.
Periodic testing of finished boards to confirm they meet the specified electrical and environmental requirements.

By maintaining tight control over the soldermask material and application process, manufacturers can minimize the risk of field failures and ensure the long-term reliability of their high voltage PCBs.

Frequently Asked Questions

What is the minimum CTI value required for high voltage applications?

The minimum required CTI value depends on the specific application and the relevant industry standards. In general, a CTI of 400 or higher is considered suitable for most high voltage PCBs. However, some applications may require even higher values (550 or 600) for added safety and reliability.

Can a high CTI soldermask be used on flexible PCBs?

Yes, there are soldermask materials specifically formulated for use on flexible PCBs. These typically have a lower modulus and higher elongation to allow for bending and flexing without cracking. However, the achievable CTI values may be somewhat lower than for rigid PCBs due to the different material properties.

How does the soldermask color affect its CTI value?

The pigments used to color the soldermask can have a slight effect on its CTI value. In general, lighter colors like white tend to have higher CTI than darker colors. However, this difference is usually minor compared to the base material properties. It’s important to work with the soldermask supplier to ensure the desired color can be achieved without compromising the required CTI value.

What is the shelf life of a high CTI soldermask?

The shelf life of a soldermask depends on its specific formulation and storage conditions. Most liquid soldermasks have a shelf life of 6-12 months when stored at room temperature in sealed containers. Dry film soldermasks may have a longer shelf life of 12-18 months under appropriate conditions. It’s important to use the soldermask within its stated shelf life to ensure optimal performance and avoid any degradation of its properties.

How does the PCB Surface Finish affect the soldermask adhesion and performance?

The PCB surface finish can have a significant impact on how well the soldermask adheres and conforms to the surface. Some finishes, like HASL (hot air solder leveling), have a rough topology that can make it challenging to achieve a uniform soldermask coating. Others, like ENIG (electroless nickel immersion gold), provide a smoother surface that allows for better soldermask coverage and adhesion.

In general, smoother surface finishes are preferred for high voltage PCBs to minimize the risk of soldermask defects and ensure reliable insulation. However, the specific requirements will depend on the soldermask material and the PCB design. It’s important to work closely with the PCB manufacturer to select a compatible surface finish and optimize the soldermask application process.

Conclusion

Choosing the right soldermask is critical for ensuring the electrical safety and reliability of high voltage PCBs. By understanding the key properties and requirements of high CTI soldermasks, designers and manufacturers can make informed decisions that will help their products meet the necessary performance standards.

Some key considerations include:

Selecting a soldermask material with a sufficiently high CTI value for the intended application and working voltage.
Designing the PCB layout with appropriate clearance and creepage distances based on the soldermask’s material group.
Specifying the appropriate soldermask thickness and application method to ensure adequate coverage and minimize defects.
Qualifying the soldermask through rigorous testing and maintaining ongoing quality control measures in production.

By following these guidelines and working closely with experienced suppliers and manufacturers, companies can produce high voltage PCBs that offer reliable performance and long-term durability in demanding applications.