A Complete Guide to PCB Solder Mask Material

Introduction

In order to fabricate PCBs appropriately, the solder mask material should be appropriate as per the related and application requirements. Solder resist or solder mask protects copper from oxidation damage. It is also commonly known as Solder Mask. Not only does it prevent dangerous electrical shorts (solder bridges) during assembly, it also ensures the long-term reliability of your product. With this guide from PCBMay, you will learn about the various types of solder mask, like Liquid Photoimageable Solder Mask (LPI Solder Mask), the exact solder mask application process, and how different solder mask color options affect performance and inspection. You’ll be able to specify the right protective coating for your next design by knowing this.

What is a Solder Mask?

Solder mask is a significant process that covers the copper on the PCB to prevent oxidation of copper. The value of this coating is that it prevents oxidation, as well as protects copper from environmental pollutants. Opting for a sturdy material like resin or fiberglass is a smart choice as it provides excellent insulation. Furthermore, it will fight off the humidity and handle high temperatures well. Plus, it won’t get dirty over time.

Most folks think of the PCB solder mask as just green. But, there are many other solder mask color options like blue, black, and white. Your color choice commonly depends on the purpose. For instance, designers often use red solder mask material on prototypes to differentiate them from production units on NPI boards. On the other hand, if you would like your PCB to blend into the color of the final product, a matte black solder mask is a good choice.

What is the Purpose of a Solder Mask?

The main purpose of a solder mask, also known as a solder resist material, is to protect your PCB from corrosion and oxidation. By applying this protective layer to a top-quality PCB, the performance is prevented from degrading for a longer duration. A solder mask provides a very important barrier during assembly to prevent solder bridging (shorts) between adjacent conductors.

You might not see solder masks on a hand-built circuit, but there is a need for them on mass assembled PCBs. Machines deposit solder mask material on an identified layer that is similar to a copper layer. When looking closely at a finished board, the solder mask is transparent over the non-copper areas. This indicates that it is indeed part of a separate layer.

Solder Mask Material Used

The type of PCB coating material you use will affect the protection of your PCB and the cost. Epoxy liquid is the most common and affordable material to protect electronic and electrical PCBs with a lot of connections and components. You may choose a Dry Film Resist material for boards with a completely flat surface.

The top solder mask specifications in the design process are primarily its thickness. The solder mask thickness typically falls between 0.2 mil and 1.6 mil. Be sure to note that the size of the solder dam should be 4 mils for the green solder mask material and 5 mils for all other custom solder mask colors. The diameter of a solder mask hole plug should be between 10 and 25 mil.

Types of Solder Mask

You have lots of choices when it comes to solder mask materials, and each of them offers varying types of applications and cost characteristics. Epoxy liquid is the top choice because it is affordable and offers reliable protection, even over busy circuit boards. Another option is the Dry Film Resist, which works best if your PCB has a perfectly flat surface.

1. Top and Bottom Side Masks

Examine the manuals on PCB solder mask. You’ll often see top-side and bottom-side references. The soldermask is the material on the top and bottom of your PCB. The term refers to the board surface or surfaces that it describes and not to a specific material or production process.

2. Epoxy Liquid Solder Masks

The most widely used is epoxy liquid solder mask, which is one of the least expensive types of solder resist materials. To create a solder mask using an epoxy liquid polymer, an ink blocking pattern is applied using a mesh. The liquid hardens through thermal curing. The solder mask color will depend on the dye of the soldering material.

3. Dry Film Photoimageable Solder Masks

The LPSM process involves applying dry film solder mask material in thin sheets. A liquid is not used in this process. This technique is a photographic process where you laminate the dry film to the PCB using a vacuum machine to eradicate air bubbles.

You then subject the film to light exposure. The areas that are not exposed to light are removed with the help of a solvent, and the remaining film is cured with the help of heat.

How is Dry Film Solder Mask Applied?

In order to ensure better performance, you need to store the boards you plan to use with dry film PCB solder mask material in a controlled temperature and humidity atmosphere. During the first step of the application process, form a printed circuit board by cleaning it thoroughly so that the film adheres completely. If you do not do this, the solder mask may delaminate with moisture or contaminants.

After this, vacuum lamination happens, in which the pre-cleaned PCB with dry film is further heated to remove any air so that it can make full contact. This film is cured by exposing the circuit to light. After each exposure, develop the board in an aqueous solution and then rinse and dry to remove ionic contaminants. At last, you apply UV light – and maybe heat as well – to cross-link the polymer molecules so that the film bonds with the PCB.

4. Liquid Photoimageable Solder Masks Material

Liquid Photoimageable Solder Masks (LPSM) material is a high-quality photoresist-like material that can be applied in liquid or film form. This process defines the holes for pads and vias on your PCB correctly. Using epoxy liquid or spraying for the application of the material is an economical choice for making holes of a smaller diameter than those punched in the board.

To start the LPI Solder Mask, you need a phototool (mask) from your Gerber files. Make sure to carefully clean your panelized boards of hardened dust before processing as well. Make sure to cover all the panels as the liquid LPSM cures. This will help ensure that the surface stays protected over time. You’ll put the boards into an oven set to 265 degrees Fahrenheit to dry them out prior to the UV exposure, where a photography mask is placed over the dry PCB. When the solder mask ink is exposed to UV light, the layer of solder mask ink exposed to the UV light gets hard, while the non-exposed areas of solder mask ink get washed away with a solvent. The result is a final blank layer.

How is the LPI Solder Mask Applied?

The application process for Liquid Photoimageable Solder Mask (LPI Solder Mask) is crucial yet simple. It is important to keep all activities clean and controlled to ensure the solder resist material is of correct quality.

1. Preparation (Cleaning): Ensure PCB panels are cleaned thoroughly to remove all dust, oily deposits, and contaminants. The pre-treatment step is critical to prevent delamination between the liquid solder mask ink and the copper substrate.
2. Application: The liquid LPI solder mask material is uniformly applied over the entire area of the PCB. Most of the time, a printed method that includes the use of an automated screen-printing method, spraying method, or curtain-coating method is adopted to control the thickness of the solder mask with a high degree of precision.
3. Tack-Drying: The PCB that’s wet goes into the oven for a tack-drying cycle. The mask is baked at a low temperature to remove excess solvent and set the mask so that it can be handled without smearing.
4. Patterning (Exposure): You accurately align a patterned phototool (mask), made from your design files, over the board. The setup uses UV light for curing and hardening the solder mask, wherever the solder mask is meant to remain on the PCB.
5. Development: You are washing the board with a developer solution. This step removes the liquid mask’s soft, unexposed areas that were there before the exposure for your pads and vias, with the wastewater efficiently processed by the water treatment system.
6. Final Curing: The last solder mask, which features a pattern, undergoes a final curing cycle that is usually a heat treatment or application of UV light. This ensures complete cross-linking of the polymer, ensuring maximum hardness, adhesion, and high temperature resistance of the UV curable solder mask.

Colors & Finishes of PCB Solder Masks

Other colors of solder mask, other than green or black, are also available in a wide range. The choice of a distinct color may be used to make prototypes look more different than the finished product or provide a matching color with their theme of casing or branding on the product. While green, black, red, yellow, and blue are the standard colors, clear finishes are also gaining popularity as an alternative to white or black.

Many OEMs are currently updating processes and equipment. As a result, the sedimentation of solder masks has expanded. It would be nice if you used a custom color solder mask as a way to differentiate your product more aesthetically. PCBMay provides these five regular colors as well as specialized custom options, which can get you the perfect look and feel that you need for your end product.

1. Green

Green solder mask material has now become the preferred choice of PCB circuitry due to its increased color range. This popularity is that they are much easier to inspect. Studies show that green color out of the color palette has the best ability for contrast. Therefore, a green PCB color will help the copper pads, copper traces, and blank space be easier for the human eye to review. This conventional green masking achieves the maximum resolution, proofs correctly, and sticks very well, meaning that it is ideal for fine-pitch features across big boards.

2. Red

These days, a red PCB solder mask is quite the favorite among many designers, especially those in high-growth sectors like AI and IoT. One of the reasons why it is a popular color is that red offers a strong contrasting color to sugar red.

Some designers find that the bright red aesthetic helps since their creativity is moving, or think it is visually more appealing for their design or brand case.

3. Blue

A blue solder mask is the same as a green solder mask, but only the colors differ. The designers are now opting for blue because it is less popular in the market, which is why it is being chosen more. Giving a unique look. You will rarely see a blue solder mask on a PCB, but we can make you custom PCBs with this color so your product can stand out.

4. Black

A matte black solder mask may not instantly grab your attention. But, it’s a worthy choice for circuitry needing great durability with a long lifespan. Examples are automobile headlights. Black paint absorbs heat better than other colors. The solder on a black PCB has a lower chance of reflowing in subsequent high-temperature soldering processes due to this key property.

5.White

A white solder mask PCB is not often used as it is usually not easy to inspect. Also, a low-quality white mask will start to show a yellowish hue after wearing it for too long in hot weather. Due to these problems, a white solder mask is often the perfect choice for an LED system because it will help enhance the brightness and reflect the output better than any other color.

6. Custom Colors

If you’re not happy with the standard solder mask color palette, you can choose from our custom solder mask colors. You may choose a special solder resist color option, like purple, yellow, or orange. When you select one of these personalized colors, your item will look more memorable and distinctive. This is often the case with niche high-end electronics industries.

How Solder Masks Are Made and Applied

The solder mask application process helps you understand the quality of your final printed circuit board (PCB). While the particulars of the method will differ according to the types of solder mask you choose, the basic steps involved in making up and applying this layer are similar. The mask should have the insulating and protective functions, which can only be guaranteed by selecting a good solder resist material manufacturer.

Step 1: Getting The Circuit Board Ready

Prior to applying the solder mask for the PCB board, you need to ensure that the surface is clean and doesn’t have any dirt or contaminants. The solder mask material will not adhere properly due to the presence of oil and dust contaminants on the copper traces. The board must be regularly washed with chemical solutions or using abrasive procedures in order to achieve a very smooth and tolerant surface.

Step 2: The Second Step Is the Solder Mask Application

The manner in which you apply the solder mask material all depends on the type of solder mask material that you are dealing with. There is a relatively inexpensive method to apply an epoxy liquid solder mask known as screen-printing. If you go with Liquid Photoimageable (LPI) Solder Mask, you’ll be applying the liquid across the whole PCB surface by spraying or rolling it on. This will give you greater thickness control. Another way to go is to use a Dry Film Solder Mask. This mask is laminated onto the PCB as a solid sheet. Heat and pressure allow for uniform thickness.

Step 3: Defining the Pattern (Photoimageable Masks)

You need to define the precise pattern of the openings for both LPI and dry film solder mask materials. You lay a photo mask: a translucent film containing your design, on top of the board, and expose it to UV light. A light shines through the solder mask ink; it hardens the areas that will stay on the board. The unexposed, soft areas are removed later using a developer solution. Your pads and vias will have the exact openings that this process creates.

Step 4: Getting The Solder Mask to Cure

After you have applied and patterned the solder resist material, it must be cured for permanent hardening and bonding to the PCB. Whether you apply heat (thermal curing) or additional exposure to a UV curable solder mask depends on the material. The reason why proper curing is important is that the mask must be in a position to be able to withstand any soldering and environmental pressures that follow.

Step 5: Quality Checks and Inspections

After the mask cured, you must inspect the PCB carefully to check that the solder mask application was correct. Look for common issues like poor coverage, misalignment, or flaws in the small openings. To check that their quality meets all relevant IPC standards for solder mask, manufacturers frequently rely on Automated Optical Inspection (AOI) for high-volume jobs. Your PCB manufacturer handles this whole process, but sharing the proper design files is critical for avoiding errors.

How to Work with Solder Masks: Tips that Beginners Should Read

If you are new to PCB design, there are some suggestions that you can apply in real life for solder mask material selection that will help you achieve successful production. By adhering to these guidelines, you will be able to avoid common mistakes and speed up production.

• When you send in your design to get it made, always make sure that your design files say what the solder mask layers’ Gerber files are. By including such details, a PCB designer is ensuring that the manufacturer understands exactly where to put the mask and where to leave openings for soldering.
• When you are designing your board, add sufficient clearance around all pads on the board. Your manufacturer’s capability will influence the gap. It is often 0.1 mm or more, so solder resist does not overlap or misalign.
• To save money, stick with standard colors. Unless your project requires a particular color for branding or functionality, we recommend that you pick the green solder mask material. This option maintains low complexity in your manufacturing and normal settings and production processes.
• Make sure you run a Design Rule Check (DRC) in your design tool to ensure the solder mask is aligned with the copper layers. When you check the DRC, you meet the manufacturing tolerances and prevent errors such as exposed traces or mask-covered pads.
• If you are unsure about anything, like the solder mask specifications or the various materials available, make sure to contact your manufacturing partner. They can give you guidance on how to select the best solder mask type and color for your project.

How is Solder Mask Applied on PCB

Applying a solder mask to a printed circuit board can prevent copper traces from oxidation. The series of steps includes Copper Preparation, Board Coating, and Mask Curing. These steps give it excellent durability and ensure long-lasting performance.

Solder mask needs to be consistent at every stage to meet IPC specifications and get stable results. You start by cleaning the copper surface. After that, you use liquid or dry film methods to apply the PCB solder mask. Afterward, you cure, check, and complete the board to complete a solder mask application.

Step 1: Surface Preparation

To begin, the copper has to be degreased automatically or manually to remove oil, grease, and other contaminants. The solder mask material ensures adhesion on the clean surface of the copper foil material. When the surface is clean, it is treated with a mild chemical etchant to get a microscopic roughening of the copper. This will provide for enhanced adhesion of both LPI solder mask and dry film. You then rinse the board to remove any residue. In some cases, you can take an ionized water rinse. The board must be completely dry before commencing the coating process.

Step 2: Application of Solder Mask (Lamination and Screen Printing)

The PCB solder mask can be applied using dry film lamination or screen printing. Screen printing, a patterned mesh screen controls where liquid photoimageable solder mask lands as it is pushed through a squeegee at a controlled pressure. The required thickness of the solder mask is built up over passes. A heated laminator applies a process of lamination to a PCB solder mask that utilizes a pre-cut dry film. The solder mask film is tightly glued to the copper by heating and the application of pressure. Both methods prepare the mask for UV exposure and curing.

Step 3: Drying/Curing

Depending on the type of solder you use, you cure the solder mask either with liquid solder mask ink or dry film solder mask. Liquid coatings are heated in a drying tunnel (or oven), where bubbling is prevented and solvents are gradually removed. UV exposure or controlled heating fully cures dry film coatings through polymer cross-linking. This step creates a hardened, durable PCB coating material. If properly cured, the properties of the solder mask will be strong enough for insulation and temperature resistant.

Step 4: Inspection

You check the cured solder mask on the AOI equipment. Defects that High-resolution cameras look for are pinholes, bridging, contamination, and uneven coverage. These challenges can influence the dielectric strength of the solder mask, as well as the reliability over time. If you notice any blemishes, you can work over or reapply the mask. The camera inspection confirms that the coating meets both customer and IPC solder mask criteria.

Add a drill mask for extra protection of plated through-holes during the soldering process. This additional coating acts as a barrier against solder from making its way into the holes. You screen print this, meaning only over the holes you need, like the original mask. Using drill masking is normal when working with BGA designs or boards with dense components.

Step 6: Marking/Finishing

Add marks like logos, labels, or tracking codes after the application of the solder mask. Usually, specialized inks are utilized in the application that adheres well to the LPI solder mask or dry film.

You can apply a surface finish at this stage if you would like to improve solderability before assembly. When marking and finishing are finished, the boards get packaged, shipped, or moved to the next step of production.

The General Problems of Solder Masks and Their Prevention

If you know how they form, you can prevent other solder mask issues and make your PCB with sufficient clearances. Issues can come from misalignment, contaminants, and/or poor adhesion.

• Solder Mask Misalignment: Make sure your design files are accurate and follow the manufacturer’s minimum clearance specifications. This will prevent the mask from covering pads or exposing traces.
• Incomplete Coverage: To prevent contamination or uneven application of solder resist material, choose a manufacturer with robust process controls to avoid missing areas.
• Peeling or Delamination: To avoid peeling, especially in severe situations, it is necessary to avoid poor adhesion by ensuring correct surface preparation and curing.
• Color Inconsistencies: For color consistency, use standard manufacturer colors for solder mask, or coordinate closely with your PCB manufacturer for a custom color.

Other Terminology You Should Know About Solder Mask

To better work with PCB solder mask specifications, we need to understand the basics first. It describes the definition of opening, clearance, and via coating in your design. They also affect solder flow and assembly reliability. Knowing them helps you avoid common manufacturing issues.

• Tented Vias: When the solder mask covers the via, there will be a dome-shaped structure covering the via in order to plug the solder flow and avoid solder bridges between the vendors.
• Solder Mask Clearance: A solder mask clearance defines the space left between a pad and the solder mask material surrounding it. It is often in the realm of 100 micrometers for a copper area in a PCB, allowing for clearance during reflow.
• Solder Mask Opening: In this region, the mask is taken off to reveal the copper pads, which will be soldered. Thus, proper alignment is necessary for strong and reliable soldering of electronic components to the PCB.
• Solder Mask Expansion: The mask extends slightly beyond the edge of the pad to provide a safe buffer zone during assembly. This latter protects against the risk of solder bridging.

PCBMay: Trustworthy Manufacturer of PCB Solder Mask

As a professional PCB manufacturer, we specialize in tailored solder mask solutions to match your project’s unique requirements. We offer a full range of solder mask types, including epoxy liquid, Dry Film, and Liquid Photoimageable (LPI) solder masks, along with versatile color options: green, blue, red, black, white, and custom color to suit branding or functional needs.

Our skilled operators ensure precise application, and every production process strictly adheres to IPC Class 2/3 standards. Each PCB undergoes rigorous quality checks, including visual inspection and Automated Optical Inspection, to guarantee consistent solder mask coverage, alignment, and reliability.

Have a PCB project needing high-quality solder mask? Send your Gerber files, solder mask specifications, or project details to sales@pcbmay.com today. Our team will provide a customized solution to meet your quality and delivery expectations

FAQs

Could You Make a PCB Board without a Solder mask?

It is definitely not advisable to manufacture a PCB that does not possess a solder mask layer. If the traces on the PCB are not coated, then they will produce oxidation. Soldering can also remove another trace and pads, leading to potentially dangerous shorts. The improved visibility of the traces and pads allows for easier inspection of your design.

Does the Solder Mask Color Matter?

The solder mask color doesn’t influence the board’s functionality, but it does impact visibility and inspection quite a bit. Green is mainly selected because it shows good contrast with fluid and pad traces. Dark color, for example, a matte black solder mask, makes it harder to visually inspect. Any imperfections or spots, on the other hand, can be seen through white or yellow.

Does a Solder Mask Repel Solder?

Solder resist can be defined as a coating that prevents solder from making contact with certain areas. The material will either inhibit or reduce surface soldering and allow one to control the part to be soldered. It is essential to avoid the accidental solder bridging of pads that are close together.

How Thick Should a Solder Mask Be?

All in all, a standard solder mask thickness of between 20 and 50 microns (20μm−50μm) is preferable. Thickness below 20μm is not going to protect the copper sufficiently. On the other hand, higher thicknesses, for example, over 50μm, do not confer any additional significant benefits and can hinder the visibility of underlying traces through the mask. We recommend 25μm to 35μm as the most suitable range.

Conclusion

Choosing the right solder mask for your PCB is a crucial ingredient for a dependable, high-quality PCB. The material type (epoxy liquid solder mask or Dry Film Solder Mask), the ultimate solder mask surface finish, and the required solder mask thickness all matter.