What is Solder Mask? A Comprehensive Guide.

PCB manufacturing is a complex process, even multi-layered if one can say so. You have to decide on a substrate material, metal selection is required that will form the conductive traces in your PCB. These two layers when laminated together, have to further go through numerous other processes, for example, drilling and etching. It can certainly be a handful job.

So where does a Solder Mask come in? What is the use of a Solder Mask? And how does it make PCBs more efficient? This article will give you answers to each of these questions! We’ll discuss an important step in PCB manufacturing that often goes neglected and yet is one of the most important procedures to ensure the smooth functioning and future reliability of a PCB.

Green Solder-Mask

What is Solder Mask?

A Solder Mask, also known as a Solder Resist, is a protective layer of polymer material that is applied on the surface of Printed Circuit Boards. A Solder Mask helps protect the PCB from any possible oxidation and to prevent accidental solder bridges between adjacent traces during the Soldering process. It should be noted that The Solder Mask is usually applied after the copper traces and vias have been etched and before the solder pads are plated. 

The mask material is typically a thermoset epoxy or a liquid photo imageable (LPI) Solder Mask. The mask is applied to the surface of the PCB in a pattern that exposes the copper pads and vias that are intended to be soldered while covering the areas that are not to be soldered. The exposed areas are sometimes referred to as "lands". 

Solder Masks are available in different colors like red, blue, Yellow, and white. However, Green is the standardized color as it is the most used, helps developers to work on PCBs easily, and is useful when Silk Screen is applied to a PCB. Some PCB manufacturers also offer custom colors or patterns for Solder Masks, which can be used for branding or aesthetic purposes.

Solder Mask: Cross-Sectional View

What is the Purpose of Solder Mask?

Solder resist is a crucial component of a printed circuit board (PCB). Here are several functions it serves in detail:

• Protection: One of the primary purposes of a solder resist is to protect the PCB from the environment. The copper traces on the PCBs are vulnerable to oxidation and corrosion which can lead to reducing in their electrical conductivity and reliability. Hence, the masks act as a physical barrier between the copper traces and the environment.
• Insulation: The Solder Mask also provides insulation between the copper traces on the PCB. This insulation helps to prevent short circuits between adjacent traces that could be caused by stray solder or other types of contamination.
• Solder Bridging Prevention: The Solder Mask helps to prevent the solder from adhering to areas of the PCB where it is not intended to be applied, which reduces the risk of solder bridging.
• Aesthetic: The Solder Mask can be used to create a visual contrast between the copper traces and the substrate material of the PCB, making it easier to visually inspect the board for defects or mistakes. Masks of different colors can be used to differentiate different types and brands of PCBs.

Multiple PCBs with Solder Masks of Different Colors

Different Types of Solder Masks

Here are some different types of Solder Masks, and their pros and cons, to help you consider when selecting a Solder Mask:

Liquid Photo-Imageable (LPI) Solder Masks

LPI Solder Masks are the most commonly used type of Solder Mask. They are applied as a liquid and then exposed to UV light to cure and harden. LPI Solder Masks can create very fine features on a PCB and are often used for high-density PCBs.

• Pros: High resolution and fine features, available in a wide range of colors.
• Cons: Can be more expensive than other types of Solder Masks, and requires exposure to UV light to cure.

Dry Film Solder Masks

Dry film Solder Masks are applied as a dry film to the surface of the PCB and then laminated to create a permanent bond. They are often used in high-temperature applications, such as automotive or aerospace electronics.

• Pros: High chemical and mechanical resistance, good for high-temperature applications.
• Cons: Limited color options, not suitable for fine features.

Thermoset Solder Masks

Thermoset Solder Masks are applied as a liquid and then cured using heat. They are often used in applications where durability is important, such as industrial control systems or medical equipment.

• Pros: Excellent adhesion and mechanical strength, suitable for durable applications.
• Cons: Limited color options, can be more difficult to apply than other types of Solder Masks.

Factors to Consider When Selecting Solder Mask

Cross-Sectional PCB View

Choosing the right Solder Mask is important for all PCB manufacturers. These are a few points to keep in mind while selecting a Solder Resist:

• PCB Design: The design of the PCB can impact the type of Solder Mask that is required. For example, high-density PCBs may require a Solder Mask with fine resolution and high accuracy.
• Application: The application of the PCBs is important to decide the type of PCB required. PCB that is going to be exposed to mechanical stress, or high temperatures requires a mark having high resistance.
• Cost: Some types of Solder Masks may be more expensive than others, and the budget for the project may impact the choice.
• Color: Different colors may be needed to differentiate between different parts of the PCB or to create a specific aesthetic.

Applying Solder Mask to a PCB

The process of applying a Solder Mask to a printed circuit board (PCB) typically involves the following steps:

• Cleaning and Preparation: The PCB is cleaned thoroughly to remove any dirt, grease, or other contaminants that may interfere with the adhesion of the Solder Mask. The board may be treated with a surface finish or coating to improve adhesion and prevent oxidation of the copper traces.
• Applying Solder Mask: The Solder Mask is applied to the PCB using a variety of techniques, including screen printing, spray coating, or inkjet printing. The most common method is screen printing, where a stencil is used to apply the Solder Mask to specific areas of the board. The excess material is then removed, leaving the desired pattern.
• Curing Solder Mask: Once the Solder Mask is applied, it is cured by exposing the PCB to heat or UV light. Curing hardens the mask and ensures that it adheres properly to the PCB. The specific curing process will depend on the type of Solder Mask used.
• PCB Inspection: After the Solder Mask is cured, the PCB is inspected for defects, such as missing or incomplete coverage of the Solder Mask. Any defects are repaired before moving on to the next stage of PCB fabrication.

Overall, the application process of a Solder Mask is a critical step in PCB fabrication. Proper application and curing of the Solder Mask are essential to ensure a reliable and functional PCB.

Conclusion

In conclusion, Solder Mask plays an important role in the Printed Circuit Boards manufacturing process. It provides a variety of benefits to a PCB, from proper insulation to prevention from the environment. Therefore, it is vital to make a good decision while selecting the material of your PCB resist, the type of resist required, and having a clear vision of what will be the application of the PCB in the future.