How to Choose the Right Thickness and Type of PCB Soldermask

When studying circuit boards, many people wonder why the top layer of the PCB is green. The answers vary, but everyone agrees on one thing: the soldermask helps with inspection, provides protection for the conductors, and prevents visual fatigue during hand assembly.

When determining the correct type and thickness of soldermask for your board, you need to consider the manufacturer's capabilities and inspection/assembly process. Below are four common types of PCB soldermask. 

1. liquid epoxy soldermask

2. Liquid Photoimageable Soldermask (LPSM)

3. Dry Film Soldermask (DFSM)

4. top and bottom layer foils

What is PCB Soldermask?

Soldermask is a PCB process used to protect metal components on a circuit board from oxidation and to prevent conductive bridges from forming between pads. It is a critical step in PCB manufacturing, especially if reflow or solder baths are used. These techniques don't give you much control over where bits of molten solder fall on the board, but a soldermask gives you a degree of control. Resists are sometimes referred to as "flux resists", which I think is more appropriate as I used to think of them as a whole layer of solder applied to a circuit board.

Types of PCB Solder Resists

All solder masks consist of a polymer layer that is applied to the metal wires on a printed circuit board. There are many types of PCB soldermask and the best choice for your board depends on cost and your application. The most basic soldermask option is to use screen printing, where a liquid epoxy is printed on the conductors, much like spray painting through a stencil. Soldermask can be applied in almost any color.

Liquid Epoxy Soldermask

The most basic soldermask option is to use screen printing to print liquid epoxy on the PCB. This is the least expensive and most popular soldermask option. In this process, a woven mesh is used to support the ink resist pattern. Liquid epoxy is a thermosetting polymer that hardens during the heat curing process. Soldermask dyes are mixed into the liquid epoxy and cured to the desired color.

Liquid Photoimageable Soldermask (LPSM)

More advanced soldermasks use a photolithographic process of dry film or liquid resist, similar to the process used for photoresist exposure in semiconductor manufacturing. LPSM can be screen printed like epoxy or sprayed onto a surface, which is often a cheaper method of application. A more accurate method is to use a photolithography process to define soldermask openings for pads, vias, and mounting holes.

In this process, a photolithographic foil is made from your Gerber file that matches your desired soldermask. The panelized board is then thoroughly cleaned to ensure that there are no dust particles under the hardened soldermask. Both sides of the panel are completely covered with liquid LPSM.

When using LPSM, you will notice that the black portion of the photolithography foil defines the area where you want the conductor to be exposed, and the area of the board where you want the soldermask to be covered will be clear.

After covering the board with LPSM, the board is dried in an oven and placed in a UV developer. The photolithographic film is carefully aligned on the dried board and the board is then exposed to UV light. the exposed areas of the LPSM material are cured by the UV light and the unexposed areas are washed away with solvent, leaving a hard soldermask.

Dry Film Soldermask (DFSM)

DFSM soldermask uses a similar process to LPSM. Both PCB soldermask types are exposed during the lithographic process. Instead of a liquid coating, dry film uses a vacuum lamination process to apply the coating in the form of a soldermask foil. This vacuum lamination step forces the unexposed soldermask to adhere to the board and removes air bubbles from the film. After exposure, the unexposed areas of the soldermask are removed with a solvent, allowing the remaining film to cure in a thermal process.

Top and Bottom Film Sheets

If you look at other guides on PCB soldermask types, the two types of soldermasks that are often mentioned are top and bottom layers. These simply refer to specific soldermask layers placed on the top or bottom of the board; they do not involve a specific manufacturing process or a specific type of soldermask material.

Final Steps: Curing and Surface Preparation

Applying the media shown above requires that the boards be cleaned to remove all dust. Afterwards, they will also undergo a final hardening and curing process. Since there is no UV exposure, the liquid epoxy soldermask is heat cured, and the LPSM and DFSM films are cured by UV exposure during the lithography process. After exposure, these films are cured and hardened by heat treatment.

Regardless of the type of PCB soldermask you use, the resulting soldermask will leave exposed copper areas on the board. These exposed areas must be coated with a surface treatment to prevent oxidation. The most common surface finish is Hot Air Solder Leveling (HASL), although other popular finishes are Electroless Nickel Immersion Gold (ENIG) and Electroless Nickel Electroless Chemical Palladium Immersion Gold (ENEPIG). If applicable, additional holes are left in the diaphragm layer for the flux layer. The flux layer is used to attach pads or other components to the printed circuit board and is treated differently depending on the manufacturing process.

What is the standard thickness of the flux layer?

The thickness of the soldermask depends mainly on the thickness of the copper traces on the board. The thickness of LPSM and DPSM soldermask in the blank areas of the board generally varies with location. Typical soldermask thickness (perpendicular to the board) is at least 0.8 mils. Near the wire edges, the soldermask thins out to 0.3 mils or less. Generally, you will need about 0.5 mils of soldermask over the traces. Sprayed epoxy soldermask provides a more uniform thickness across the PCB.

In addition to preventing corrosion of copper traces, the soldermask is used to place a barrier between adjacent pads on the board. For component pads, this is accomplished by defining a small gap between the soldermask and the exposed pad, called soldermask relief. This creates a barrier that prevents molten solder from one pad from flowing to neighboring pads. It is important for fine-pitch BGAs and other components with high pin densities. Such a small relief around the edge of the pad allows the solder droplet to fully wet the pad and hold itself in place, which prevents bridging during the soldering process.

What color should the soldermask be?

The color of the soldermask is determined by the dyes used in the soldermask material, and the chemical properties of the dyes affect the thickness of the cured soldermask. One reason green soldermask is widely used is that it can be used to create thin soldermask barriers (~0.1 mm). The dyes used in other colors of soldermask tend to create thicker soldermask barriers. Regardless of which dye you choose to use, the thickness of the solder resist on a PCB for a specific industry or application is defined in the IPC-SM-840D.

The soldermask color is an essential part of an automated or manual visual inspection. Black soldermask provides the lowest contrast between the board and trace, which can make automated inspection difficult. This is another reason why green film is preferred. The color of the silkscreen you use also affects visual contrast and can affect visual fatigue during manual inspection.

As with any other manufacturing parameter or process, you should consider the sensitivity of your end application and plan your design accordingly. It is always important to discuss manufacturing options with your manufacturer. They may even be able to suggest better options based on their capabilities.

What soldermask should I use?

It depends on the physical dimensions of the board, holes, components and conductors, the surface layout, and the final application of the product to determine the appropriate soldermask.

First, if your PCB soldermask will be used in the aerospace, telecom, medical, or other "high-reliability" industries, check the industry standards for soldermasks and your general intended application. There are specific requirements that supersede any others you may have learned on the Internet.

For most modern printed circuit board designs, you will need a photolithographic soldermask. The surface graphics will determine whether to use a liquid or dry application. A dry application will create a uniform thickness of soldermask across the entire surface. However, if the surface of your board is very flat, a dry film will adhere best. If the surface features are complex, you're better off with the liquid (LPSM) option for better access to the copper of the traces and the laminate. The disadvantage of liquid application is that the thickness is not completely uniform across the board.

You can also get different finishes on the laminate layers. Talk to your manufacturer about what they have available and how this will affect production. For example, if you are using a solder layer reflow process, a matte finish may reduce solder balls.

How do I include soldermask in my PCB layout?

When designing a printed circuit board, the soldermask layer should be its own layer (top and bottom) in your PCB layout and Gerber file. This is not defined in your Layer Stack Manager. Instead, it is usually defined as an additional layer by default in your CAD tool. If the soldermask layer is not fully centered, you usually need to leave a 2 mil border around the element. The minimum distance between pads is usually 8 mils to ensure that the diaphragm prevents the formation of solder bridges.

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