When studying circuit boards, especially as a beginner in the electronics industry, I always wondered why the top layer of PCBs is green. Answers vary, but everyone agrees on one thing: solder mask aids inspection, protects conductors, and reduces eye fatigue during manual assembly. Different types of PCB solder mask vary in application method, composition, and cost.
When studying circuit boards, especially as a beginner in the electronics industry, I always wondered why the top layer of PCBs is green. Answers vary, but everyone agrees on one thing: solder mask aids inspection, protects conductors, and reduces eye fatigue during manual assembly. Different types of PCB solder mask vary in application method, composition, and cost.
To determine the right type and thickness of solder mask for your board, you need to consider your manufacturer’s capabilities as well as inspection and assembly processes. Below are four common types of PCB solder mask:
- Liquid Epoxy Solder Mask
- Liquid Photo-Imaging Solder Mask (LPSM)
- Dry Film Solder Mask (DFSM)
- Top and Bottom Solder Mask Layers
What is a PCB Solder Mask?
Solder mask is a PCB process used to protect metal features on the board from oxidation and prevent conductive bridging between pads. It is a critical step in PCB manufacturing, especially with reflow or wave soldering—processes that offer little control over molten solder placement, while solder mask provides precise control.
Solder mask is sometimes called “solder resist,” which I find more accurate, as I once thought solder mask was a full layer of solder applied to the board.
Types of PCB Solder Mask
All solder masks consist of a polymer layer applied over the metal traces on a printed circuit board. Many types exist, and the best choice depends on cost and your application. The most basic option is screen-printed liquid epoxy, like stenciling paint onto conductors. Solder mask can be produced in nearly any color.
1. Liquid Epoxy Solder Mask
The most basic and lowest-cost popular option. A woven mesh is used to define the solder mask pattern. Liquid epoxy is a thermoset polymer that hardens during heat curing. Dye is mixed into the liquid epoxy to produce the desired color after curing.
2. Liquid Photo-Imaging Solder Mask (LPSM)
A more advanced solder mask using photolithography similar to photoresist processes in semiconductor manufacturing. LPSM can be screen-printed like epoxy or sprayed (often cheaper). The more precise method uses photolithography to define openings for pads, vias, and mounting holes.
[Illustration: LPSM photolithography process flow diagram]
- A photolithography film is produced from your Gerber files to match your required solder mask pattern.
- The panelized board is thoroughly cleaned to remove dust that could get trapped under the cured solder mask.
- Both sides of the panel are fully coated with liquid LPSM.
- The board is baked in an oven and then placed in a UV developer.
- The photolithography film is carefully aligned over the dried board.
- The board is exposed to UV light; exposed areas of the LPSM cure, while unexposed areas are washed away with solvent, leaving a hard solder mask layer.
Solder mask is coated with epoxy resin or photosensitive polymer.
3. Dry Film Solder Mask (DFSM)
DFSM uses a process similar to LPSM, as both are exposed via photolithography. Instead of a liquid coating, DFSM is applied as a dry film through vacuum lamination, which forces the unexposed solder mask film to adhere to the board and removes air bubbles from the film. After exposure, unexposed areas of the solder mask are removed with solvent, and the remaining film is cured through a thermal process.
[Illustration: DFSM vacuum lamination process diagram]
4. Top and Bottom Solder Mask Layers
These simply refer to solder mask applied to the top or bottom side of the board; they do not specify a manufacturing process or material type.
Final Steps: Curing and Surface Finish
After applying the dielectric layers above, the boards are cleaned to remove all dust, then undergo a final hardening and curing process:
- Liquid epoxy solder mask: Thermally cured (no UV exposure required).
- LPSM and DFSM films: Cured via UV light during photolithography, then further hardened through heat treatment.
Regardless of the solder mask type used, exposed copper areas will remain on the board. These exposed areas must undergo surface finishing to prevent oxidation. The most common surface finishes are:
- HASL (Hot Air Solder Leveling)
- ENIG (Electroless Nickel Immersion Gold)
- ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)
If applicable, additional openings are left in the solder mask for solder paste layers, which are used to attach components to the printed circuit board and are processed differently depending on the manufacturing workflow.
What is the Standard Solder Mask Thickness?
Solder mask thickness mainly depends on the thickness of the copper traces on the board. For LPSM and DFSM:
- Typical thickness (perpendicular to the board surface): at least 0.8 mil in blank areas of the board.
- Near trace edges: Thinner, down to 0.3 mil or less.
- Recommended coverage over traces: Approximately 0.5 mil.
Sprayed epoxy solder mask provides more uniform thickness across the entire PCB.
In addition to preventing copper trace corrosion, solder mask acts as a barrier between adjacent pads on the board. For component pads, this is achieved by defining a small gap between the solder mask and the exposed pad, called solder mask relief. This creates a barrier that prevents molten solder from flowing from one pad to an adjacent pad—especially critical for fine-pitch BGAs and other high-pin-density components. The small relief around the pad edges allows solder droplets to fully wet the pad and secure themselves in place, preventing bridging during soldering.
[Illustration: Diagram of solder mask relief around a component pad]
What Color Should the Solder Mask Be?
Solder mask color is determined by the dye used in the solder mask material, and the chemical properties of the dye affect the cured thickness of the solder mask.
- Green is widely used partly because it can form thin barrier layers (~0.1 mm).
- Dyes for other colors often result in thicker solder mask barriers.
Regardless of the dye chosen, solder mask thickness for specific industries or applications is defined in IPC-SM-840D.
Solder mask color is a key part of automated or manual visual inspection:
- Black solder mask provides the lowest contrast between the board and traces, making automated detection difficult.
- This is another reason green is the preferred solder mask color—it offers good contrast and reduces inspector eye fatigue.
The color of the silkscreen also affects visual contrast and eye fatigue during manual inspection.
Like any other manufacturing parameter or process, you should consider the sensitivity of the end application and plan your design accordingly. It is always important to discuss manufacturing options with your manufacturer—they may even recommend better choices based on their capabilities.
Which Solder Mask Should I Use?
Choosing the right solder mask depends on the physical dimensions of the board, holes, components, and conductors, the surface layout, and the final application of the product.
- First, if your PCB solder mask will be used in aerospace, telecommunications, medical, or other “high-reliability” industries, check industry standards for solder mask as well as the requirements of your specific application. Some industry-specific requirements may override general guidelines.
- For most modern printed circuit board designs, you will need a photolithographic solder mask. The surface pattern will determine whether to use liquid (LPSM) or dry (DFSM) application:
- Dry film (DFSM): Provides uniform solder mask thickness across the entire surface; best adhesion on very flat board surfaces.
- Liquid (LPSM): Better coverage on complex surface features, ensuring full contact with copper traces and laminates; downside is slightly uneven thickness across the board.
- Different surface finishes can be applied to the solder mask layer. Discuss with your manufacturer the available options and how they will impact production—for example, a matte finish can reduce solder balling during reflow soldering.
How to Include Solder Mask in My PCB Layout?
When designing a printed circuit board, solder mask should be defined as its own layers (Top Solder Mask and Bottom Solder Mask) in your PCB layout and Gerber files. These layers are not defined in the layer stack manager; instead, they are usually enabled by default as additional layers in your CAD tool.
- If solder mask alignment is not perfectly centered, you typically need to leave a 2-mil border around components and features.
- The minimum distance between pads is usually 8 mil to ensure the solder mask can effectively prevent bridging.
If you are in the business of designing or manufacturing PCBs, it is essential to choose PCB design software that supports different types of PCB solder mask. Altium Designer provides a complete set of PCB layout and manufacturing tools, giving you control over all aspects of the design.
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