PCB Solder Mask: The Most Comprehensive Guide
After PCB manufacturing, the copper traces on the substrate are left exposed to environmental conditions, like water and dust that can lead to oxidation. The best method of prolonging the PCB’s lifespan is applying a protective coating known as a solder mask on the PCB surface.
This article offers a comprehensive analysis of solder mask and how you can use it to inhibit problems, like solder mask relief. Additionally, you will also learn how to make a mask, where you can get custom colored masks, finished boards, and PCB assembly services for your project.
What is Solder Mask?
A solder mask also referred to as solder resist or solder stop, is a thin layer of polymer you apply into the PCB copper traces to prevent oxidation and protect them from environmental contaminants. Resin is the standard material for making solder masks since it has good properties regarding humidity resistance, insulation, solder resistance, aesthetics, and high-temperature resistance.
Some people use the color green to define PCBs, but that is actually the color of solder mask green oil. Nevertheless, solder masks are available in various colors, such as green, blue, black, white, yellow, red, etc. The type of color you use depends on different factors.
For example, most designers use red solder masks for prototypes in the New Product Introduction (NPI) stage to distinguish them from mass-produced PCBs. Black solder mask is used to conform to the color of the end product enclosure when PCBs should be partially or fully exposed.
What is the Purpose of a Solder Mask?
A solder mask acts as your PCB’s primary protector against corrosion and oxidation. It affects the PCB quality by preventing performance degradation and the shortening of an electronic’s operational lifespan. More importantly, a solder mask creates a barrier between soldered joints and other conductive aspects of the PCB during assembly, thereby preventing the formation of solder bridges.
You will rarely see solder masks on hand-made circuits, but they are always present in mass-assembled PCBs, soldered using machines, like solder-baths. In PCB manufacturing, the solder mask is a separate PCB layer, just like the copper and silkscreen layers.
PCB Solder Mask Types
Each solder mask layer is made up of a polymer layer applied over the PCB copper traces. There are several solder masks in the market, and the best choice for use depends on the cost and the function of your PCB. The most common solder mask option uses silkscreen to print liquid epoxy over the PCB conductors.
Below are the common types of solder masks:
1. Liquid Epoxy Solder Mask
As mentioned earlier, the most common solder mask option is the use of silkscreen to print liquid epoxy over your board. Apart from being a common solder mask option, a liquid epoxy solder mask is also a low-cost method. You simply use a woven mesh to support ink-blocking patterns. The epoxy liquid acts as a thermosetting polymer that hardens during thermal curing. The solder mask dye is mixed with the liquid epoxy and cures to the chosen color.
2. Liquid Photoimageable Solder Mask (LPSM)
High-end solder masks apply a photolithography technique with either a dry film or a liquid solder mask, similar to the one used in photoresist exposure in semiconductor fabrication. You can apply LPSM like epoxy or spray it over the board – this is the most cost-effective application method. However, there is a more accurate technique of using lithography to define solder mask openings for pads, vias, and holes.
In the LPSM method, you need to create a photography mask from your Gerber files according to your preferred solder mask. You should also carefully clean your panelization board to clear dust from the hardened solder mask. Besides, you should completely cover your panels with the liquid LPSM.
After covering the panels, you should dry the PCBs in an oven before placing them on a UV developer. Again, you should align the photography mask over the dry PCB before illuminating it with UV light. The exposed parts of LPSM material are cured by UV light, while the covered parts are washed off with a solvent to create a solid solder mask layer.
3. Dry Film Solder Mask (DFSM)
You can use the LPSM method to apply a dry film solder mask. In both cases, the solder masks are exposed to a photolithography-type technique. Instead of being used in its liquid form, the dry film is applied in sheets of solder mask film using a vacuum lamination process. The process compels the unexposed solder mask film to stick to the PCB and eliminates bubbles from the film.
After exposure, you should remove the unexposed solder mask parts with a solvent and thermally cure the remaining film.
4. Top and Bottom-side Masks
If you carefully analyze other PCB solder mask guides, you will discover two more solder masks – the top-side and bottom-side masks. Generally, they refer to the solder masks applied on the upper and lower PCB sides, respectively. They do not refer to any fabrication method or solder mask material.
Solder Mask Vs. Paste Mask
Often, solder masks and paste masks are mentioned in the same scenario, making it harder for beginners to distinguish them. If you have come across a printed circuit board, you are definitely familiar with their predominantly green color. The thin green coating on the upper and lower sides of a PCB is known as a solder mask.
As mentioned earlier, there are instances where you only need to coat one side of your board. A solder mask fundamentally covers the whole upper or lower side, leaving out the sections where parts would make PCB shopping.
On the other hand, a paste mask is a material you apply to the sections that have been left uncovered by a solder mask to facilitate the soldering process. Designers often use this paste to link surface-mount parts to pads on a board during soldering. You can also use it in through-hole pin-in-paste parts with a stencil, a syringe, or a jet printing to apply the mask in or over the holes.
The solder paste’s robust adhesive features allow it to hold a component firmly. This enables designers to heat their PCBs, melt the mask, and form electrical and mechanical bonds between the parts and the board.
Therefore, you need a solder mask in PCB manufacturing to inhibit the oxidation of copper traces and the creation of solder bridges. On the other hand, you apply a paste mask into the sections where component pins make contact with the board to enable a strong and reliable bond between them during assembly.
Even stencils designed for solder mask application differ from those intended for paste mask. The other difference between a solder mask and a paste mask is that the latter is gray while the first is available in different colors.
What is the Common Color for Solder Mask?
While the standard color for solder masks is green, more shades have emerged to satisfy designs with specific color requirements. These requirements include distinguishing prototype runs and modifications and making boards appear trendy in a see-through enclosure. With OEMs increasingly applying modern equipment and processes, we have adopted colored solder masks to give our products an exclusive identity.
However, the most common solder mask colors are red, black, white, yellow, and blue. Clear, not technically a color type, is also a common choice nowadays. Apart from these five colors, MKTPCB also provides other choices, like purple, pink, orange, and any additional colors according to the customer’s requirements.
Though the solder mask color spectrum has significantly expanded in the last five years, most boards are still soldered green. The primary reason why most PCB manufacturers and designers prefer color green to other colors is that it helps in the inspection. Research shows that the human eyes are more sensitive to green color than other colors. This facilitates the quality inspection process of the copper pads, traces, and blank spaces.
Besides, the green mask sticks more uniformly, cures correctly, and displays the highest resolution. This implies it can create minor mask dams in a widely spaced board.
Designers, particularly those from modern technological fields, like Artificial Intelligence (AI) and the Internet of Things (IoT), prefer red solder masks to other colors for aesthetic reasons. Sometimes, color red creates a more striking color contrast between the PCB components and traces.
Blue and green solder masks play almost similar roles, but some designers prefer color blue to color green because of their uniqueness. PCBs soldered blue are pretty rare on the market. However, if you want them for a custom project, you can get them from MKTPCB.
Though black solder masks are dully in color and not the most eye-catching, they help applications that need light reflection. Since black absorbs heat more than other colors, black soldered PCBs require less heat during a reflow.
Color white is rarely applied in solder masks as it offers poor visibility. Furthermore, most low-quality masks turn yellowish after some time, especially when exposed to high temperatures for a long time. Nonetheless, white solder masks are often ideal for LED systems since they are brighter than any other color.
6. Custom Colors
If none of the above colors meets your design requirements, we provide rarer but more uniquely appealing options, like orange, purple, and yellow solder masks. These options are common among businesses that deal with niche electronics and those that want to distinguish their products from those of their competitors.
Choosing the Right Solder Mask Color
While experimenting with various shades can make your PCB project more interesting than the typical green, it is important to realize that the color you use will, to some extent, affect your solder mask performance. For beginners, black, white, and yellow have low resolutions since UV light fails to pass through the entire layer. A board masked with these colors will display fewer tracks than a similar PCB with a green, red or blue mask.
On the other hand, though clear solder masks provide the best resolution, they are vulnerable to color shifts, especially during high temperatures and chemical treatment.
The performance of a color mask is also influenced by solder mask quality. Top PCB manufacturers like MKTPCB have custom shades that guarantee high resolution, temperature, and chemical resistance.
Businesses often apply different-colored solder masks for aesthetic purposes. Still, different shades can help you quickly identify your PCBs throughout the production process if you do not have custom color specifications. You can apply red, blue, and black solder masks to mark various design phases and then choose green or white for your end product.
Solder Mask Design Tips
A solder mask might not be an important aspect of PCB manufacturing, but failure to use it causes severe issues that will significantly minimize your PCB’s lifespan. Therefore, if you are a diligent designer, it is essential to consider the solder mask that your PCB service provider will use.
Tip 1: Consider the Solder Mask Type and Thickness
Selecting the suitable mask depends on various factors, most notably the dimension of your PCB, the surface layout, parts, conductors, and the intended application of your PCB. If you are creating a PCB for use in regulated industries such as healthcare, telecommunication, or aerospace, the industry standards will also influence the solder mask you will use.
Liquid photo-imageable solder masks are the commonest solder masks in the electronic world right now. They are cost-effective, reliable, and they facilitate easy detection of PCB faults during quality inspection.
The copper trace width primarily determines the solder mask thickness. You require about 0.5 mils of mask over the traces. A liquid mask brings unpredictable consistency to a PCB. It can be as thick as 1.2 mils over blank laminate regions and as thin as 0.3 mils over complex features, like the circuit knee.
Tip 2: Always Include a Solder Mask in Your Design
PCB design rules require the solder mask to be its own 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. Don’t worry about that since the mask file will indicate the regions where you do not want the mask to be applied.
Generally, you are required to cover the whole PCB with solder masks, leaving out the regions for mounting components and maybe the points you plan to use as test points. The mask apertures rely on your product, but generally, you should adjust the solder mask to 4 mils wider than the copper pad. If you print your mask to the same size as your pad, it would be centered wrongly, and the mask may cover some of the copper pads.
Sometimes, a project calls for a defined pad, which forms when the solder mask relief – a PCB region that is not masked to enable easy detections of PCB defects – is of the same or smaller size than the exposed pad.
Narrow mask openings are useful when mask dams are needed between pads, but there is inadequate space to accommodate them and still abide by the 2-mil-border standard. Your PCB service provider will require you to include a fabrication note in your Gerber document whether you need a mask dam, but you do not desire some holes to be altered.
Tip 3: Inhibit Solder Mask Relief Problems
Solder mask relief regions typically engulf surface-mount pads, through-hole pads, test points, and vias as error margins during PCB fabrication. Contrary, the mask 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, such as black, white, and yellow, will not stick to the surface in small area sizes. This means the PCB will have fewer pads than if it had a higher-resolution mask shade.
For any PCB size, the bigger the solder mask relief, the smaller the solder mask dam. However, more extensive reliefs can condense the dams beyond the recommended scope, exposing large areas that it is tolerable and creating solder bridges during fabrication. Contrary, little relief can lead to the contamination of pads by the mask, creating uneven surfaces that are challenging to solder.
You can prevent solder mask relief issues by keeping the reliefs to the recommended 2-mil border. It is also essential to consider your solder mask color right from the beginning of your design. High-resolution shades, such as green, red, blue, and clear permit smaller dam reliefs even in dense patterns.
Solder Mask Manufacturing Process
Some people believe that you can only make solder masks using cutting-edge technology. Contrary to this, you can DIY a solder mask for use in your PCB design. However, a DIY solder mask is only suitable for simple PCB projects as it is challenging to guarantee product reliability.
In industrial PCB manufacturing, the process of making solder mask is not a walk in the park. First, you must follow strict regulations, like ISO9001, UL, and RoHS standards. Secondly, solder mask manufacturing comprises multiple stages that require high accuracy levels springing from advanced technologies, high manufacturing experience, and state-of-the-art facilities.
This is the basic solder mask manufacturing process:
Step 1: Board Cleaning
Here, you are required to thoroughly clean your board to clear dust and other environmental contaminants from the surface. After cleaning and rinsing your board, dry it well before proceeding to the second step.
Step 2: Solder Mask Ink Coating
In this step, you load your clean and dry board into a vertical coating machine to coat it with a solder mask ink. The thickness of the coat will depend on various factors, like the reliability demand of PCBs, application, and board thickness. It is good to note that the PCB surface may not be as smooth as you think.
The solder mask ink thickness varies when applied to different PCB parts, such as on traces, substrates, and copper foil. At MKTPCB, we have laid down specific coating thicknesses because of our equipment capacity and manufacturing experience considerations.
Step 3: Pre-hardening
This step strives to make the coatings relatively firm. This allows easy removal of undesired layers during the development stage.
Step 4: Imaging and Hardening
Here, you mount a transparent film containing the circuit images to the PCB and then expose it to UV light. The aim is to harden the solder mask covered by the transparent film and pre-harden the section of the film covered with images. For good results, you must ensure the proper alignment during the hardening process to prevent the exposure of non-designated copper foil to create shortcuts.
Step 5: Developing
In step five of the PCB solder mask manufacturing process, you place your board into a developer to clean all the undesired solder masks. This will expose the designated copper foil well.
Step 6: Final Hardening and Cleaning
Lastly, it would be best to carry out final hardening to mount the solder mask ink to the board properly. Afterward, clean all the PCB sections covered with solder masks before assembling your products.
As a PCB service provider with more than ten years of experience in PCB manufacturing, fabrication, and assembly, MKTPCB has a rich background in providing solder mask color and design recommendations. Kindly request an online PCB quote to get PCB design suggestions now.