Solder mask is a protective polymer coating applied to printed circuit boards that serves multiple purposes. Besides being an aid in the PCB manufacturing process, it also serves a reliability purpose.

Solder mask is a protective epoxy coating that is translucent but tinted, depending on what additives are used. It comes as a liquid, and then hardens with exposure to UV light or a thermal cure of some type. The additives don’t just serve a cosmetic purpose. It turns out that matte green is the best color for spotting manufacturing defects, but also the mixture that gives the green color is also optimal for manufacturing. That said, other colors, such as red and matte black, are available. But green is the most common color for good reason.

The solder mask is the second to last layer of a PCB, after the last layer of copper traces, but before the silkscreen that indicates the position and function of components. The ink from the screen printing process can interfere with conductivity, so it is important for the silkscreen to be above the mask.

Purposes of solder mask

what is solder mask

For DIY PCB repairs, hobbyists can buy liquid solder mask to apply to the repaired area. It cures under UV light.

During manufacturing, the mask provides solder resistance to help prevent solder bridges between contacts on components that shouldn’t be connected. When placing components, the component pins go into the open areas of the mask, which form what is known as solder mask dams. This prevents unintentional bridges from the soldering process, which cause short circuits and malfunctions. Accidental solder bridges are an easy mistake to make, especially on small parts like SMT components.

After manufacture, the solder mask layer continues to serve a purpose. It protects the traces on the board from corrosion, oxidation, and other damage.

What solder mask is and how it works

Solder mask is a special formulation of epoxy liquid that cures when exposed to ultraviolet light. The mask is durable and under normal conditions holds up for decades. Generally, when you find damage on a vintage printed circuit board, it is because a battery, capacitor, or something else containing corrosive chemicals leaked onto the board and wasn’t cleaned up quickly enough. Quickly enough is relative. We’re not talking hours, we’re generally talking years.

Excessive heat can also damage the solder mask, as well as the copper traces or solder pads underneath.

It is possible to repair damaged traces, copper pads, and solder mask on a damaged circuit board. The methods people use can vary. The easiest repair is to simply run a wire between two undamaged spots on the board to bypass the damaged areas. A next-level repair is to use enameled wire or bare copper wire to complete the repair. For a full-on restoration, and experienced technician can create new pads from copper foil, epoxy them into place, then connect the old copper with new with a minute and barely visible solder joint.

A professional repair will then coat the area with fresh liquid mask and cure it with UV exposure. Due to subtle changes in solder mask formulas over the years, they may or may not get an exact match, but a well-executed result can be extremely difficult to distinguish from factory work.

Hobbyists will frequently substitute colored nail polish for solder mask to protect their repairs. The enameled nail polish isn’t as strong as the original, and the color match won’t fool anyone, but it provides reasonable protection under normal storage conditions.