Comprehensive Analysis of Solder Mask in PCB Manufacturing

What is solder mask made of?

Solder masks are usually made of resin (such as epoxy resin, polyurethane, or acrylic resin), photopolymer, or liquid UV curing material (LPI). These materials are heat, insulation, and corrosion resistant and can firmly adhere to the surface of the PCB to form a solid protective film.

The core function of solder mask

In PCB board fabrication, the most critical function of solder mask is to prevent solder bridging, ensuring welding quality and circuit safety. During the welding process, the molten solder is fluid. Without the restriction of solder mask, the solder may form a bridge between adjacent pads, causing a short circuit or damaging the entire electronic system.
The solder mask effectively guides the solder to adhere only to the pad by forming an insulating protective film in the non-welding area, thereby improving welding accuracy.

The solder mask can also isolate air and moisture, prevent copper foil oxidation, and enhance the durability and long-term reliability of the circuit board. It also has good electrical insulation, which can avoid signal crosstalk and electrical interference in high-density wiring. For multi-layer boards and high-frequency circuit boards, the solder mask also plays an important role in improving overall performance and anti-interference capabilities.

Coating, exposure and development on both sides of circuit board

Visual appearance of solder mask

The visual appearance of solder masks, an indispensable protective coating in PCB manufacturing, has received attention, but it is not the primary criterion for judging quality. Although green is the mainstream color in the industry, other colors such as red, blue, black, white, and yellow are also widely used, especially in consumer electronics, medical equipment, and high-end customized products. Color selection is often based on identification, branding, or aesthetic considerations.

Many factors affect the appearance of solder mask, including coating method, dry film thickness, curing temperature, board reflectivity, and circuit distribution. Even if the same color is applied to copper wire, pad, and substrate area, due to different reflectivity and coating thickness, slight differences in brightness or hue may appear under naked eye observation. This slight difference is a normal process phenomenon and will not affect electrical performance or welding quality.

It should be noted that the solder mask is not designed for decorative purposes, but rather prioritizes technical performance. Slight bubbles, scratches, or rework marks on the surface are acceptable without affecting the function. In actual production, its appearance compliance needs to be evaluated according to standards such as IPC-A-600 to ensure that key areas are intact and meet application requirements.

Relationship between component holes and solder mask

In PCB board design, the coordination between through-holes (PTH) and solder mask directly affects the welding quality and manufacturability. For metallized through-holes used for THT (through-hole mounting) components, the industry usually requires that the hole walls and hole openings remain free of solder mask coverage to ensure sufficient solder wetting and form reliable solder joints. However, on tiny through-holes (such as vias or microvias), due to the limitations of the fluidity of solder mask ink and the control accuracy of the PCB board manufacturing process, it is often difficult to completely avoid solder mask residues in extremely fine apertures.

Especially when the aperture is less than 0.45mm, the solder mask may partially accumulate in the hole;
The solder mask may even completely block the hole when the aperture is further reduced to less than 0.3mm.

This type of situation is not a manufacturing defect but a natural phenomenon acceptable to modern PCB processes.

Edge coverage and solder mask retreat distance

The solder mask should accurately cover the copper foil, but a 0.1-0.15mm retreat distance must be reserved to avoid covering the pad. This is especially critical in High-frequency PCB or BGA packages.

appearance and circuit distribution of solder mask

Requirements for solder mask coating under different PCB structures

The solder mask layer is usually applied to both sides of the circuit board in the printed circuit board assembly to form a standard double-sided solder mask structure, which is the most common design form and is suitable for most double-sided circuit boards. In this configuration, both the front and back sides need to be coated, exposed and developed to protect the copper wires, pads and isolate external contaminants.

However, for multilayer PCB and high-density interconnect boards (HDI PCB) with more complex structures, the requirements for solder mask coating are also more refined. This type of PCB usually contains a large number of micro blind holes, laser drilling and dense wiring areas. Local thickening or multiple coating processes may be used in the manufacturing process to enhance the protection of key circuit areas and improve welding reliability and durability.

In addition, in some high-frequency or high-voltage applications, in order to improve the insulation performance of the solder mask layer, customized protection may also be achieved by adjusting the thickness or using special materials (such as ceramic solder mask). Therefore, the number of solder mask layers depends not only on the board structure, but also on the comprehensive influence of electrical performance, packaging density and terminal application requirements.

Available coating processes

Solder mask can be applied by using the following methods:

Curtain Coating: Applicable to large-volume boards, uniform thickness
Inkjet spraying: Suitable for high-precision, low-volume products
Screen printing: Suitable for simple graphics, low-cost applications
LPI (liquid photoresist): The most commonly used technology, exposure and development after photosensitization

Exposure process of solder mask

The solder mask is developed through a graphic exposure process to ensure that the solder mask pattern is accurately aligned with the pad. The UV exposure equipment uses a mask to align the PCB, and cleans the unexposed area after ultraviolet exposure to form a solder mask opening. Common methods include:

Manual Film Exposure: Easy to operate, suitable for small-batch PCB boards production
Semi-Automated Film Exposure: Improve accuracy and efficiency through mechanical alignment
Fully Automated Film Exposure: With precise alignment and stability, it is suitable for high-precision production of large-scale printed circuit manufacturing
Manual Direct Imaging (Laser/LED): Direct exposure without film, manual material processing. Suitable for HDI, finer structures and smaller batches
Automated Direct Imaging (Laser/LED): Direct imaging does not require film, and directly uses laser or LED light sources to accurately draw graphics. It is particularly suitable for HDI PCB and fast proofing projects, greatly improving resolution and graphic consistency

adjust circuit board solder mask equipment

Classification of solder mask materials

Common solder mask materials are divided into:

● Thermal Curing Type

This type of material needs to be cured at high temperature, has excellent heat resistance and adhesion, and is suitable for demanding high-reliability applications such as multi-layer rigid boards, ceramic PCBs or military and automotive control modules. Its excellent insulation and chemical corrosion resistance make it the first choice for high-frequency and high-power applications.

● UV Curing Type

Rapidly cured with ultraviolet light, suitable for single layer circuit board, large-volume consumer electronic boards such as TVs, power boards, etc. This type of material has high production efficiency and low cost, and is widely used in the field of mid- and low-end electronic products.

● Hot Melt Type

The lowest cost and simple process, but poor heat resistance and mechanical properties. Generally used in simple Aluminum PCBs or low-end lighting products, not suitable for complex or high-temperature working environments.

Acceptance criteria and quality criteria for solder mask

For rigid PCBs with solder mask, the most important standard is IPC-6012 and its combination with IPC-A-600.

The standard for the solder mask itself is IPC-SM-840. For example:

No cracks, no peeling
The edges of the solder mask pattern are clear and there is no oil diffusion
No pinholes or uneven thickness
The coating thickness meets the design specifications (10–30μm)

Matt or glossy green solder mask varnish

Depending on the process, the surface of green solder mask can be matte, semi-matte or glossy. Matte is suitable for industrial control PCBs to reduce light reflection, while glossy has better visual effect and is often used in consumer electronic appearance boards. If you or your customers prefer to use conformal coating, or your end customers have requirements for appearance, please contact us.

Through-hole residue and solder mask contamination

If through-holes are not cleaned thoroughly after soldering, flux or oxide residues may contaminate the solder mask, causing blistering, peeling, or subsequent reliability reduction, so cleaning and baking treatments need to be strengthened.

Solder mask residues in through-holes are mainly caused by 3 factors:

● Through-hole aperture is too small

When the final aperture of the through-hole is less than Ø0.45 mm, especially less than Ø0.3 mm, the solder mask ink is difficult to completely remove during deposition or exposure and development, and it is easy to form residues on the hole wall or inside the hole, or even completely block the hole. This is because the ratio between the surface tension of the ink and the aperture is too large, resulting in incomplete cleaning.

● Too thick solder mask coating

If the thickness of the solder mask is not properly controlled during printing or spraying, especially on high-density boards, the amount of one-time coating is large, which is easy to cause ink to accumulate at the hole mouth. When the subsequent development and rinsing steps fail to completely remove the excess material in the hole, it will cause residues.

● Insufficient development and rinsing process

If the exposure energy of the solder mask is insufficient, the development time is too short, the water pressure is insufficient, or the nozzle angle is not good, the ink in the through hole may not be fully developed and discharged, and the residue will be difficult to remove after solidification. This problem is more likely to occur when the parameters of the automated processing equipment are not matched.

A few things you need to know about solder mask

Solder mask is not an insulating material and cannot completely replace the insulating layer design

The thickness of solder mask in high-frequency PCB will affect signal transmission and impedance

Black solder mask is not easy to detect, so it is recommended to use it with caution in industrial products

Some high-temperature applications require white ceramic solder mask materials

As an indispensable part of PCB manufacturing, the design and processing quality of solder mask directly affect the weldability, electrical performance and appearance quality of the finished product. Whether in consumer electronics or aerospace, automobile, or medical equipment, a reasonable solder mask process is the guarantee of high-reliability PCB. Choosing SCSPCBA printed circuit board manufacturers, the right materials, standards, and processes are the important cornerstones to help you move towards high quality.