The solder mask is among the many critical components of a printed circuit board. This layer is vital in the manufacturing process as it helps to protect the exposed copper circuitry from damage, oxidation, dust, and other contaminants. The solder mask also provides a consistent surface for soldering components onto the board.
Like other aspects of PCB production, the solder mask requires careful attention to detail when it comes to materials, manufacturing processes, and quality control. Failure to account for any of these factors can result in faulty boards that are difficult or impossible to use or repair.
By understanding the many facets of the PCB solder mask layer covered in this article, startups and other small businesses can ensure that their printed circuit boards are of the highest quality possible.
The PCB Solder Mask
Even unknowingly, most people are familiar with what a solder mask is: it’s that green, black, or sometimes red coating found on the finished product of most PCBs.
The solder mask coating is quite ingenious, specifically designed to limit the soldering material (solder) that can come into contact with the exposed copper circuitry. Too much solder can result in “bridging” between two or more conductive paths, which can cause all kinds of problems, including shorts, open circuits, and intermittent connections.
The polymer’s oxidation resistance is due to its non-conductive material, meaning that it won’t corrode when exposed to the air like copper will. This characteristic is essential for maintaining the long-term reliability of the board.
The PCB Solder Masking Process
There are many steps to getting a proper solder mask on a printed circuit board. Each has a unique function that helps to ensure a perfect outcome. Skipping steps or failing to execute them properly will likely result in a less-than-satisfactory board.
Board Cleaning
Cleanliness is important when it comes to applying the solder mask. Any dust, grease, or other contaminants on the board’s surface will prevent the solder mask from properly adhering. This contamination can lead to delamination, cracking, and other problems down the road.
Solder Mask Ink Coating
First, you apply a thickness of approximately 0.8mil (20 microns) to the board. The material is typically dispensed from a syringe, although some manufacturers may use other methods. Ultimately, the design and function of the PCB will determine the desired thickness of the solder mask.
Pre-Hardening
To facilitate a strong bond between the coating and the PCB, allow a pre-hardening phase to happen. This stage has the added benefit of making the unwanted substrate easier to remove later.
Imaging & Hardening
This stage involves the application of a photo film that reacts to UV light. The light allows the ink to cross-link and harden. This action results in a strong and durable solder mask that will protect the board for years to come.
Developing
Unwanted solder mask is cleaned away in the developing process. This process is crucial as any remaining material will prevent proper soldering from happening later on. Additionally, this stage ensures that all needed copper foil is adequately exposed.
Final Hardening & Cleaning
The final hardening process uses heat to cure the solder mask further, ensure long-term reliability, and permanently mount the solder mask to the board.
Cleaning is the final step and involves the removal of any remaining developer solution as well as any other contaminants that may have found their way onto the board during the process.
When you complete these stages, the PCB is ready for its final assembly and surface finish.
Types of Solder Masks
Knowing the PCB solder masking process only provides half the information needed to create a top-quality board. The other half involves understanding the different types of solder masks and which one is best suited for the job.
Top & Bottom Side Masks
While not a kind of mask per se, how you apply the mask to the board can make a big difference. The most common method is to apply the solder mask to both the top and bottom sides of the board.
Top and bottom side masks are typically used to create the standard green PCB seen in most electronics on the planet. These masks act as better markers for holes and openings in the PCB, making it easier to solder component pins in the right places.
Epoxy Liquid Solder Masks
Chosen for their cost-effectiveness when making PCBs in large quantities, epoxy liquid solder masks are common in the industry. This type of solder mask allows for the silkscreening of the epoxy onto the substrate, making it easier to identify open areas for ink transfer. Once applied, you cure the epoxy with heat.
The biggest advantage to using an epoxy solder mask is that you can quickly and easily apply it to the board. Additionally, this solder mask is highly durable and can withstand a fair amount of abuse.
Liquid Photoimageable Solder Masks
This solder mask is made using two mixtures that you must blend together perfectly to create the final product. Once applied, the mixture is photo-cured with UV light, resulting in a tough solder mask.
The main advantage of using a liquid photoimageable solder mask is that you can apply it very thinly to the board. Taking this measure results in better electrical properties and a lower risk of shorts.
Dry Film Photoimageable Masks
Dry film photoimageable solder masks require vacuum lamination to apply them to the board. Once in place, the film is exposed to UV light which hardens it in place. Afterward, you identify openings in the PCB and smolder components onto the copper pads.
This process is ideal for high-density wire boards as it limits the risk of flooding through holes and provides extra insulation to space high voltage traces closer together on the board.
PCB Solder Mask Colors
While the color of a PCB doesn’t enhance or detract from its function (outside of added protection), it does go a long way in identifying issues and defects with a PCB.
- Green – The most common color for solder masks, green is used in the majority of PCBs due to its low cost and easy availability. It provides the highest level of trace visibility, making it easy to identify issues during the design process.
- Yellow – Yellow is often used in boards that will be used outdoors. This is because it provides a high level of contrast against green and brown backgrounds. It doesn’t screen print well with white, so it’s not often used on boards with a lot of text or writing.
- Red – Most commonly used when the interior hardware needs to be showcased in the design. It doesn’t have the contrast of other colors on this list, so engineers performing repairs on them usually need additional magnification tools.
- Blue – Best suited for installations behind LCD screens, blue masks offer good silkscreen contrast for prototyping and open-source development.
- Black – Though it comes with a higher price point and added manufacturing time, black solder masks offer the benefit of reduced light reflection, making them the perfect choice for circuits sensitive to bright lights.
- White – The least invasive of the solder mask colors, white is often used in LED applications for seamless blending of color when turned on and limited visual disturbance when turned off.
Always Solder Mask Your PCB
A solder mask offers greater protection, durability, and longevity to a printed circuit board. Not only does it prevent oxidation and shorts, but it also makes the board more resistant to physical damage from dust, scratches, and UV light. By knowing how the process works and what colors offer the best benefits for your project, you can be sure to get the most out of your solder mask – and your PCB.
Be sure to discuss the details of your PCB solder mask with a certified and reputable PCB manufacturer. These experts can help you choose the right color, thickness, and type of solder mask for your project to ensure optimal results.