PCB Soldermask: Everything You Need to Know

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18 Apr, 2023

PCB Soldermask: Everything You Need to Know

The purpose of this article is to provide a comprehensive, in-depth guide to PCB soldermask, exploring the key concepts, principles, and practical applications that will help you excel in PCB design and manufacturing.

Introduction 

In the world of electronics, printed circuit boards (PCBs) serve as the foundation for virtually all electronic devices. As technology continues to advance, the importance of understanding and optimizing PCB soldermask grows exponentially. Throughout this article, we will discuss the various types of PCB soldermask materials, their applications, and their advantages and disadvantages. We will delve into the importance of soldermask colors, application techniques, design considerations, inspection methods, and common issues that may arise. 

1. What is PCB Soldermask?

PCB soldermask, also known as solder resist or solder stop, is a critical component of printed circuit boards (PCBs). It is a thin, protective layer of polymer material that is applied to the surface of the PCB, serving as a barrier against various elements, such as moisture, dust, and oxidation. The primary function of soldermask is to prevent solder bridges from forming during the soldering process, which could otherwise lead to short circuits and affect the overall performance of the electronic device. 

Soldermask serves as a primary defence against corrosion and oxidation of your PCBs. By reducing performance deterioration and the reduction of an electronic's operational lifespan, it has an impact on the PCB quality. Most crucially, a soldermask prevents the production of solder bridges by forming a barrier during assembly between soldered joints and other conductive components of the PCB. Soldermasks are a rarity on hand-made circuits, but they are always visible on PCBs that have been built in large quantities and soldered using equipment like solder baths. The soldermask is a distinct PCB layer in PCB manufacture, exactly like the copper and silkscreen layers.

2. Types of PCB Soldermask Materials

Soldermask materials play a vital role in the overall performance and reliability of a PCB. Several types of soldermask materials are available, each with its unique properties.

The soldermask layer is created by putting a polymer layer over the copper traces on the PCB. The PCB conductors are covered with liquid epoxy using a silkscreen to create the most popular soldermask solution. The ideal option for use will depend on the price and the purpose of your PCB among the various soldermasks available on the market. Following are the types of soldermask materials that might be suitable for your PCB project.

2.1. Epoxy Liquid

Epoxy liquid soldermask is one of the most common and widely used soldermask materials in the PCB industry. It consists of a two-component system, typically comprising an epoxy resin and a hardener. When mixed together, they create a liquid that can be applied to the surface of the PCB.

The primary benefits of epoxy liquid soldermask are its excellent adhesion, good chemical resistance, and cost-effectiveness. Additionally, epoxy liquid provides a smooth, uniform surface, which is ideal for fine-pitch components and intricate PCB designs. 

This involves the application of liquid epoxy to your board using a silkscreen. A liquid epoxy soldermask is not only a typical soldermask choice, but it is also a cheap technique. To support ink-blocking designs, a woven mesh is all that is necessary. As a thermosetting polymer, epoxy liquid solidifies during thermal curing. The liquid epoxy is combined with the soldermask dye, which cures to the desired hue.

2.2. Dry Film Soldermask

Dry film soldermask is a pre-laminated, photoimageable material that is available in sheets or rolls. It consists of a thin layer of photoactive polymer, sandwiched between a protective polyester film and a release liner. The dry film is applied to the PCB surface using a lamination process, which adheres the photoactive layer to the board.

The advantages of dry film soldermask include precise registration and alignment, excellent resolution for fine-pitch components, and reduced risk of contamination during application. However, dry film soldermask can be more expensive and less flexible compared to epoxy liquid, making it less suitable for certain applications and PCB designs.

In this process, a photolithography-like approach is used to expose the soldermasks. The dry film is put in sheets of soldermask film utilising a vacuum lamination procedure as opposed to being employed in its liquid form. The procedure reduces bubbles from the soldermask film and forces it to adhere to the PCB.

After exposure, you ought to use a solvent to remove the areas of the soldermask that weren't exposed, then thermally cure the remaining layer.

2.3. Liquid Photoimageable Soldermask (LPSM)

High-end soldermasks use a photolithography process, similar to the one used in photoresist exposure in semiconductor manufacture, with either a dry film or a liquid soldermask. The most economical application method for LPSM is to spray it over the board or to apply it like epoxy. Yet, there is a method that uses lithography to define the soldermask openings for pads, vias, and holes that is more precise.

With the LPSM approach, you must convert your Gerber files into a photographic mask in accordance with your selected soldermask. To remove dust from the soldermask that has become hardened, you should also thoroughly clean the panelization board. Also, use the liquid LPSM to completely coat your panels.

The PCBs should be dried in an oven after being covered with panels before being put on a UV developer. Once more, before shining UV light on the dry PCB, align the photography mask over it. UV light is used to cure the exposed LPSM material while a solvent is used to remove the covered areas to form a solid soldermask layer.

3. PCB Soldermask Colors:

Green, white, blue, black, red, yellow, matte colour, purple, chrysanthemum, bright green, matte black, matte green, and other colours are among the palette options for PCB soldermasks. The white ones are typically white PCBs that are required to create LED lighting goods.

 Different colours are primarily a result of the product classification system, which varies depending on the manufacturer. Some people use red to represent experimental boards, some people use blue to represent keyboards, and some people use black to represent computer boards.

3.1. Green

Green is the most widely used soldermask color, and for good reasons. It offers an excellent balance between visibility and contrast, making it easy to inspect traces, solder joints, and components on the PCB. Green soldermask also provides a high level of electrical insulation and performs well under various environmental conditions.

3.2. Blue

Blue soldermask is another popular choice for PCBs, offering a visually appealing appearance and good contrast between copper traces and the soldermask layer. Blue soldermask can provide slightly better heat dissipation compared to green, which can be advantageous for high-power or high-temperature applications.

However, blue soldermask can make it more challenging to inspect solder joints and components due to its darker color. It is essential to weigh the benefits of improved heat dissipation against the potential drawbacks of reduced visibility when considering blue soldermask for your PCB project.

3.3. Red

Red soldermask is often chosen for its striking visual appearance and good contrast with copper traces. This color can make it easier to identify components and traces on the PCB, which can be beneficial during assembly, inspection, and debugging processes.

However, red soldermask has a lower electrical insulation performance compared to green and blue options, making it less suitable for high-voltage applications. Additionally, red soldermask may not dissipate heat as effectively as other colors, which could impact the performance and longevity of the electronic components mounted on the PCB.

3.4. Black and White

Black and white soldermasks are less common options but can be chosen for their unique aesthetic appeal or specific project requirements. Black soldermask provides excellent contrast with copper traces and silkscreen legends, making it easy to read and identify components on the PCB. However, the dark color can make it more challenging to inspect solder joints and may retain more heat than other color options.

White soldermask offers a clean, minimalist appearance and can reflect light, making it a suitable choice for LED applications. However, white soldermask can be more prone to showing dirt and contamination, and it may not provide the same level of electrical insulation as other color options.

4. PCB Soldermask Application Techniques

There are several techniques used in the PCB industry to apply soldermask, each with its advantages and disadvantages. In this section, we will discuss the most common soldermask application techniques, including screen printing, curtain coating, spray coating, and electrostatic spray coating.

4.1. Screen Printing

In the screen printing process, ink and solder paste are transferred using a tool, such as a mesh or a stencil, onto a PCB surface or into PCB vias and microvias. A reverse stroke causes the screen to briefly touch the substrate along a line of contact after a blade or squeeze is used to move across the screen and fill the tool apertures with ink. As the screen springs back after the blade has passed, the ink wets the substrate and is drawn out of the tool apertures as a result.

The main advantages of screen printing include its cost-effectiveness, simplicity, and suitability for small- to medium-scale production runs. However, this method may not provide the same level of accuracy and resolution as other techniques, particularly for fine-pitch components and high-density PCB designs.

Applying soldermask to a PCB requires precision and accuracy to ensure proper coverage, alignment, and thickness. Applying soldermask to a PCB requires precision and accuracy to ensure proper coverage, alignment, and thickness. 

4.2. Curtain Coating

As PCBs have grown more complicated with higher track density and smaller track widths, curtain coating has become increasingly popular in the circuits sector. The printed circuit board is coated with liquid photoimageable (LPI) using the curtain-coat process as it passes through a sheet of low viscosity ink that is falling through a small slit. With little material loss, the curtain coating may apply liquid material at very high speeds.

This procedure seems to cause "shadowing," a coating variation phenomena. Reduced soldermask on the leading edge of traces parallel to the curtain as compared to the trailing edge of those traces causes shadowing.

Curtain coating offers several advantages over screen printing, such as higher accuracy, improved thickness control, and better coverage for complex PCB designs. This technique is well-suited for high-volume production runs and PCBs with fine-pitch components. However, curtain coating may be more expensive and require specialized equipment compared to other methods.

When using the curtain coating technique, it is essential to control the flow rate and viscosity of the soldermask material to ensure a consistent and even coating across the entire PCB surface.

4.3. Spray Coating

Spray coating is another method for applying liquid soldermask materials, involving the use of a spray gun or nozzle to distribute the material evenly across the PCB surface. This technique can provide more precise control over the soldermask application, particularly for PCBs with irregular shapes or complex designs.

The benefits of spray coating include its flexibility, adaptability to different PCB designs, and the ability to achieve uniform coverage and thickness. However, this method may require more frequent cleaning and maintenance of equipment to prevent clogging or contamination of the soldermask material.

When using the spray coating technique, it is important to maintain proper nozzle distance, spray pattern, and material viscosity to ensure an even and consistent application of soldermask.

4.4. Electrostatic Spray Coating

Electrostatic spray coating is a specialized method for applying soldermask that involves charging the soldermask material with an electric field. As the charged material is sprayed onto the PCB surface, it is attracted to the grounded areas of the board, creating a uniform and precise coating.

This technique offers several advantages, such as improved accuracy, excellent coverage for complex PCB designs, and reduced material waste. Electrostatic spray coating is particularly suitable for high-density PCBs and those with fine-pitch components. However, this method can be more expensive and require specialized equipment compared to other application techniques.

When using electrostatic spray coating, it is crucial to maintain proper equipment settings and material properties to ensure optimal results and minimize the risk of defects or inconsistencies in the soldermask layer.

Each soldermask application technique has its unique advantages and limitations, making it essential to select the most appropriate method for your specific PCB project requirements. By understanding the capabilities and trade-offs of each technique, you can make informed decisions that ensure the highest quality and performance of your PCB soldermask.

5. PCB Soldermask Design Considerations

Designing a PCB involves many critical factors, and soldermask plays a significant role in ensuring the proper functioning and reliability of the final product. When incorporating soldermask into your PCB design, it is essential to consider several aspects, such as material selection, thickness, tolerances, and clearance requirements. In this section, we will discuss these design considerations and provide guidelines to help you optimize your PCB design for soldermask application.

5.1. Material Selection

As discussed earlier, there are various types of soldermask materials available, each with its unique properties and performance characteristics. When selecting a soldermask material for your PCB design, consider factors such as thermal stability, chemical resistance, flexibility, and electrical insulation.

  • Evaluate your PCB's operating environment and requirements to choose the most suitable material.

  • Consult with your PCB manufacturer to understand the available soldermask material options and their compatibility with your specific design.

5.2. Soldermask Thickness

The thickness of the soldermask layer can impact the PCB's overall performance, including thermal management, electrical insulation, and mechanical stability. Determining the appropriate thickness for your design requires balancing these factors and ensuring compliance with industry standards.

  • Follow IPC guidelines for soldermask thickness recommendations.

  • Consult with your PCB manufacturer to determine the achievable soldermask thickness for your design and manufacturing process.

Recommended Reading: PCBs Thickness: Understanding Thickness Variations

5.3. Tolerances and Registration

Soldermask tolerances and registration play a crucial role in the accurate alignment of the soldermask layer with the underlying copper traces and pads. Ensuring proper alignment is essential to avoid solder bridging or shorts during assembly and prevent exposure of sensitive components to environmental factors.

  • Use appropriate soldermask design rules and tolerances based on your PCB manufacturer's capabilities.

  • Include fiducial marks or alignment features in your PCB design to facilitate accurate soldermask registration during manufacturing.

5.4. Soldermask Clearances and Openings

Soldermask clearances and openings refer to the areas on the PCB where the soldermask is intentionally omitted to allow for soldering, testing, or other purposes. Properly defining these areas is crucial to ensure successful PCB assembly and functionality.

  • Design soldermask clearances based on component pad sizes, soldering requirements, and manufacturing tolerances.

  • Include additional openings for test points, fiducial marks, or other functional requirements as needed.

By carefully considering these soldermask design aspects, you can optimize your PCB design for performance, reliability, and manufacturability. 

Working closely with your PCB manufacturer and understanding their capabilities and limitations will help you make informed decisions throughout the design process, ultimately resulting in a high-quality final product.

6. PCB Soldermask Inspection and Quality Control

Ensuring the quality and consistency of the soldermask layer on a PCB is crucial for the board's functionality and reliability. To achieve this, it is essential to implement proper inspection and quality control measures during the manufacturing process. In this section, we will discuss the key aspects of soldermask inspection and quality control, including visual inspection, automated optical inspection (AOI), and electrical testing.

6.1. Visual Inspection

Visual inspection is a fundamental step in assessing the quality of the soldermask layer. It involves examining the PCB for any visible defects, such as misalignment, insufficient coverage, uneven thickness, or other anomalies.

  • Conduct a thorough visual inspection of the soldermask layer to identify any obvious defects or issues.

  • Use magnification tools, such as microscopes or magnifying lenses, to inspect fine-pitch components and high-density areas.

6.2. Automated Optical Inspection (AOI)

Automated Optical Inspection (AOI) is an advanced inspection technique that utilizes high-resolution cameras and image processing software to automatically detect defects in the soldermask layer. AOI can quickly identify issues such as misregistration, insufficient coverage, or bridging between pads and traces.

  • Implement AOI during the manufacturing process to catch soldermask defects early, reducing the risk of costly rework or scrap.

  • Fine-tune AOI settings and parameters to ensure accurate detection of defects while minimizing false positives or negatives.

6.3. Electrical Testing

Electrical testing is a crucial aspect of quality control that verifies the functionality and performance of the PCB, including the effectiveness of the soldermask layer. Various electrical tests can be performed on the PCB, such as continuity tests, insulation resistance tests, and high-voltage tests.

  • Conduct electrical testing on the PCB to ensure proper functioning and compliance with design specifications.

  • Use electrical tests to validate the performance of the soldermask layer, such as its insulation properties and resistance to electrical breakdown.

7. Common PCB Soldermask Issues and Troubleshooting

Identifying and troubleshooting these problems is crucial to ensure the functionality and reliability of your PCBs. In this section, we will discuss some common PCB soldermask issues and provide guidance on how to address and prevent them.

7.1. Soldermask Misalignment

Soldermask misalignment occurs when the soldermask layer is not properly aligned with the underlying copper features, potentially leading to solder bridging or exposed traces. This issue can be caused by inaccurate registration during soldermask application or insufficient tolerances in the PCB design.

  • Review your PCB design for appropriate soldermask tolerances and alignment features, such as fiducial marks.

  • Work with your PCB manufacturer to identify and correct any registration issues during the soldermask application process.

7.2. Insufficient Coverage

Insufficient coverage refers to areas on the PCB where the soldermask layer is not adequately covering the intended areas, leaving traces or pads exposed to potential contamination or corrosion. This issue can result from uneven soldermask application, incorrect clearances, or material defects.

  • Inspect the soldermask layer for uniform thickness and coverage, addressing any inconsistencies in the application process.

  • Review your PCB design for proper soldermask clearances and openings, ensuring they meet your manufacturer's specifications.

7.3. Soldermask Delamination

Soldermask delamination is the separation of the soldermask layer from the PCB surface, which can compromise the board's electrical insulation and mechanical stability. Delamination can be caused by factors such as poor adhesion, excessive heat during soldering, or contamination during manufacturing.

  • Ensure proper surface preparation and cleaning before soldermask application to promote optimal adhesion.

  • Monitor temperature profiles during soldering processes to prevent excessive heat exposure that could lead to delamination.

7.4. Soldermask Bubbles or Voids

Soldermask bubbles or voids are air pockets or gaps within the soldermask layer, which can affect the electrical insulation properties and overall appearance of the PCB. These defects can result from trapped air during application, improper curing, or material issues.

  • Implement proper soldermask application techniques to minimize air entrapment, such as adjusting viscosity or applying multiple thin layers.

  • Monitor curing parameters, such as temperature and time, to ensure complete and consistent curing of the soldermask material.

8. Conclusion

PCB soldermask plays a crucial role in the overall performance, reliability, and longevity of printed circuit boards. By understanding the various aspects of soldermask, such as material types, colors, application techniques, and design considerations, you can make informed decisions throughout the PCB design and manufacturing processes. Implementing proper inspection and quality control measures, as well as addressing and troubleshooting common soldermask issues, will help ensure the highest quality and reliability of your final product.

10. Frequently Asked Questions (FAQs)

1. Can I use multiple soldermask colors on the same PCB?

Yes, it is possible to use multiple soldermask colors on the same PCB. However, it may increase the complexity and cost of the manufacturing process. Consult with your PCB manufacturer to determine the feasibility of using multiple colors on your specific design.

2. How can I ensure the best soldermask adhesion?

To ensure the best soldermask adhesion, it is essential to properly clean and prepare the PCB surface before application. This may involve processes such as chemical cleaning, brushing, or plasma treatment. Additionally, selecting the appropriate soldermask material and following the manufacturer's recommended application and curing processes will help promote optimal adhesion.

3. Can I apply soldermask to both sides of a PCB?

Yes, you can apply soldermask to both sides of a PCB. Applying soldermask to both sides can provide additional protection and insulation to the circuitry, especially in high-density or complex designs. However, this may increase manufacturing time and cost, so it is essential to consider the trade-offs when deciding whether to apply soldermask to one or both sides of your PCB.

4. What is the typical lifespan of a PCB soldermask?

The typical lifespan of a PCB soldermask depends on factors such as material type, operating environment, and exposure to mechanical or thermal stress. High-quality soldermask materials can generally provide effective protection and insulation for the lifetime of the PCB, which can range from several years to over a decade. Regular inspection and maintenance of the PCB can help extend the lifespan of the soldermask.

5. What is the minimum soldermask clearance for fine-pitch components?

The minimum soldermask clearance for fine-pitch components depends on the specific component dimensions, soldering requirements, and manufacturing tolerances. It is essential to consult with your PCB manufacturer to determine the appropriate soldermask clearances for your design, taking into consideration their capabilities and limitations. Following industry guidelines, such as IPC standards, can also help ensure proper soldermask clearances for fine-pitch components.

References:

https://www.mdpi.com/2072-666X/13/3/460

http://pen.ius.edu.ba/index.php/pen/article/view/2337

https://www.mktpcb.com/pcb-capability/solder-mask/

https://www.pcbaaa.com/pcb-solder-mask/#Why-are-the-PCB-solder-mask-colors-different-for-different-circuit-boards