Solder Mask Layer in PCBs: Guide for Beginners

What is a Solder Mask?

A solder mask, also known as solder resist or solder stop, is a thin protective layer applied to a printed circuit board (PCB) over the copper traces and pads. It is primarily used to protect the copper from oxidation and to prevent solder bridges during the assembly process. The solder mask is typically made of a polymer resin that is applied as a liquid and then cured to form a solid protective layer.

The main purpose of the solder mask is to prevent the solder from flowing onto areas where it is not intended to go, such as between closely spaced copper traces or onto the surface of the PCB. This helps to ensure that the electrical connections on the PCB are made only where they are supposed to be, improving the reliability and performance of the circuit.

In addition to its protective function, the solder mask also helps to improve the overall appearance of the PCB by providing a uniform and consistent surface finish. It can also help to improve the solderability of the PCB by providing a clean and smooth surface for the solder to adhere to.

Purpose of Solder Mask

The solder mask serves several important purposes in PCB manufacturing:

1. Corrosion Protection: The primary function of the solder mask is to protect the exposed copper traces on the PCB from environmental factors such as moisture, dust, and oxidation. This protection helps prevent corrosion and ensures the long-term reliability of the PCB.
2. Insulation: The solder mask provides electrical insulation between the copper traces and other conductive elements on the PCB, such as components and the PCB substrate. This insulation prevents unintended electrical connections and short circuits.
3. Solder Bridge Prevention: During the soldering process, solder is applied to create connections between components and the PCB. Without a solder mask, there is a risk of solder bridging between adjacent copper traces or pads, causing short circuits. The solder mask prevents this by covering and isolating the copper traces and pads.
4. Identification and Aesthetics: Solder masks are available in different colors, with green being the most common. The color of the solder mask can be used for identification purposes, such as distinguishing between different PCBs or indicating specific components or functions. Additionally, the solder mask enhances the aesthetics of the PCB by providing a uniform and professional appearance.
5. Solderability: The solder mask helps control the flow of solder during the soldering process, ensuring that it adheres to the intended areas and forms reliable connections. This is especially important for surface-mount components, where precise soldering is crucial for proper functionality.

Overall, the solder mask is a critical component of PCB manufacturing, providing essential protection, insulation, and solderability for the PCB, ultimately contributing to the overall reliability and performance of electronic devices.

What Solder Mask Should I Use?

When selecting a solder mask for your PCB, several key factors should guide your decision. Firstly, consider the physical dimensions and surface layout of your board. For boards with complex features, a liquid photoimageable solder mask (LPSM) may be more suitable, as it can provide better contact with copper traces and laminate. On the other hand, for boards with a flat surface, a dry film solder mask could be a better choice, offering a more uniform thickness across the board.

Secondly, take into account the industry standards applicable to your PCB’s intended application. Industries such as aerospace, telecom, or medical have specific reliability requirements that may dictate the type of solder mask you should use. Adhering to these standards ensures that your PCB meets the necessary quality and safety regulations.

Lastly, consider the application method and finish of the solder mask. The application method, whether liquid or dry film, can impact the mask’s thickness uniformity and adhesion to the board surface. Additionally, the finish of the solder mask layer, such as matte or glossy, can affect the performance of your PCB during processes like solder reflow.

Overall, by carefully evaluating these factors and consulting with your PCB manufacturer, you can choose a solder mask that meets your PCB’s requirements and ensures optimal performance and reliability.

Types of Solder Masks

Solder mask plays a crucial role in protecting PCBs from environmental factors and ensuring their long-term reliability. There are several types of solder masks available, each offering unique advantages and suitable for different applications. Understanding these types can help you choose the right solder mask for your PCB manufacturing needs.

• Epoxy Liquid Solder Masks: This is the most common type of solder mask and is applied as a liquid epoxy over the PCB using a silkscreen. The liquid epoxy cures during thermal processing, forming a protective layer over the copper traces. Epoxy liquid solder masks are cost-effective and widely used in the industry.
• Dry Film Photoimageable Solder Masks (DFSM): DFSM is applied as a dry film on the PCB using a vacuum lamination process. The film is exposed to UV light through a photomask, which defines the areas where the solder mask should be removed. After exposure, the unexposed areas are washed away, leaving behind the desired solder mask pattern. DFSM offers precise control over mask openings and is suitable for high-density PCB designs.
• Liquid Photoimageable Solder Masks (LPSM): LPSM is applied as a liquid and cured using UV light. It offers high resolution and is ideal for applications requiring intricate designs and precise mask openings. LPSM is commonly used in industries where high-quality PCBs are required, such as aerospace and medical devices.
• Top and Bottom Side Masks: These refer to the solder masks applied to the upper and lower sides of the PCB, respectively. They do not refer to any specific fabrication method or material but are essential for protecting both sides of the PCB.
• Thermal Cure Liquid Solder Masks: These solder masks are cured using heat, which helps in achieving a durable and reliable protective layer over the PCB. Thermal cure liquid solder masks are known for their excellent performance and reliability.
• UV Curable Liquid Solder Masks: UV curable solder masks are cured using ultraviolet light, offering a fast and efficient way to protect PCBs. They are suitable for applications where quick curing times are required.

Choosing the right solder mask depends on various factors such as cost, application requirements, and desired performance characteristics. It is essential to consider these factors carefully to ensure the best results for your PCBs.

Solder Mask Design Tips

While the solder mask may not seem like a critical aspect of PCB manufacturing, its absence can lead to severe issues that significantly reduce the lifespan of your PCB. Therefore, as a diligent designer, it is essential to consider the solder mask your PCB service provider will use.

Consider the Solder Mask Type and Thickness

The selection of the suitable solder mask depends on various factors, including the dimensions of your PCB, surface layout, components, conductors, and intended application. If your PCB is for use in regulated industries like healthcare, telecommunications, or aerospace, industry standards will also influence your choice.

Liquid photo-imageable solder masks are the most common in the electronics industry due to their cost-effectiveness, reliability, and ease of detecting PCB faults during quality inspection.

The thickness of the solder mask, typically around 0.5 mils over copper traces, is primarily determined by the copper trace width. A liquid mask can vary in thickness, ranging from 0.3 mils over complex features to 1.2 mils over blank laminate regions.

Always Include a Solder Mask in Your Design

PCB design rules require the solder mask to be its layer in the Gerber files. Since the mask layers in the Gerber files are negative images, the colored areas will not be covered by the mask. You should cover the entire PCB with solder mask, leaving out areas for mounting components and possibly the points you plan to use as test points.

The mask apertures should be adjusted to be 4 mils wider than the copper pad. Printing the mask to the same size as the pad can cause it to be centered wrongly, potentially covering some of the copper pads.

Sometimes, a project may require a defined pad, which occurs when the solder mask relief (a PCB region not masked to enable easy detections of PCB defects) is the same or smaller size than the exposed pad.

Narrow mask openings can be useful when mask dams are needed between pads, but there is inadequate space to accommodate them while still adhering to the 2-mil-border standard. Your PCB service provider will require you to include a fabrication note in your Gerber document if you need a mask dam but do not want some holes to be altered.

Inhibit Solder Mask Relief Problems

Solder mask relief regions typically surround surface-mount pads, through-hole pads, test points, and vias as error margins during PCB fabrication. The space between two immediate reliefs is known as a mask dam.

The correct solder mask dam measurements depend on the mask’s resolution, which also relies on the color type. Low-resolution shades like black, white, and yellow may not adhere to the surface in small area sizes, resulting in fewer pads visible on the PCB compared to higher-resolution mask shades.

For any PCB size, the larger the solder mask relief, the smaller the solder mask dam. However, extensive reliefs can compress the dams beyond the recommended scope, exposing large areas that are tolerable and creating solder bridges during fabrication. Conversely, minimal relief can lead to pad contamination by the mask, creating uneven surfaces that are challenging to solder.

You can prevent solder mask relief issues by keeping the reliefs within the recommended 2-mil border. Additionally, it is essential to consider your solder mask color right from the beginning of your design. High-resolution shades like green, red, blue, and clear permit smaller dam reliefs even in dense patterns.

Solder Mask Manufacturing Process

While some may think that creating solder masks requires cutting-edge technology, it’s possible to DIY a solder mask for simple PCB projects. However, ensuring product reliability can be challenging. In industrial PCB manufacturing, the process of making solder masks is far from simple. Manufacturers must adhere to strict regulations such as ISO9001, UL, and RoHS standards.

The solder mask manufacturing process involves multiple stages that require high accuracy levels, advanced technologies, extensive manufacturing experience, and state-of-the-art facilities. Here’s an overview of the basic solder mask manufacturing process:

Step 1: Board Cleaning

Thoroughly clean the board to remove dust and other contaminants from the surface. After cleaning and rinsing, dry the board thoroughly before proceeding to the next step.

Step 2: Solder Mask Ink Coating

Load the clean, dry board into a vertical coating machine to apply a solder mask ink coating. The thickness of the coating depends on factors such as PCB reliability requirements, application, and board thickness. The PCB surface may not be as smooth as expected, so coating thickness varies when applied to different PCB parts such as traces, substrates, and copper foil. Manufacturers often have specific coating thickness guidelines based on equipment capacity and manufacturing experience.

Step 3: Pre-hardening

Pre-harden the coatings to make them relatively firm. This facilitates the removal of undesired layers during the development stage.

Step 4: Imaging and Hardening

Mount a transparent film containing circuit images onto the PCB and expose it to UV light. This process hardens the solder mask covered by the film and pre-hardens the film’s section covered with images. Proper alignment during hardening is crucial to prevent exposure of non-designated copper foil, which could create shortcuts.

Step 5: Developing

Place the board into a developer to remove all undesired solder masks, exposing the designated copper foil.

Step 6: Final Hardening and Cleaning

Perform final hardening to ensure proper mounting of the solder mask ink to the board. Clean all PCB sections covered with solder masks before assembling the products.

Conclusion

The solder mask is a critical component of PCB manufacturing, providing essential protection, insulation, and solderability for the PCB. It plays a crucial role in ensuring the reliability and performance of electronic devices by preventing solder bridges, protecting against corrosion, providing insulation, and enhancing the overall appearance of the PCB.

When choosing a solder mask, consider factors such as the physical dimensions of your board, industry standards, and application requirements. Liquid photoimageable solder masks are commonly used due to their cost-effectiveness and reliability, but the choice between liquid and dry film masks depends on the surface topography of your board.

Overall, selecting the right solder mask and adhering to best practices in solder mask design and manufacturing are essential for creating high-quality, reliable PCBs for a wide range of applications. By understanding the importance of the solder mask and following these guidelines, you can ensure that your PCBs meet the necessary standards and perform optimally in their intended applications.