A: A PCB solder mask, also known as solder resist or solder stop mask, is a protective layer applied to the surface of a PCB to insulate and prevent solder from unintentionally bridging or short-circuiting the electrical connections on the board during the soldering process. It’s a thin, usually green-colored layer that covers the copper traces, pads, and vias on the PCB’s surface.

The primary purpose of the PCB solder mask is to provide isolation between the conductive elements on the PCB, such as traces and pads, and prevent the solder from flowing where it shouldn’t during soldering. This helps to ensure proper electrical connections while minimizing the risk of short circuits or unintended connections that could lead to circuit malfunction or failure.

Solder masks are typically made from a liquid photo-imageable polymer applied to the PCB’s surface in the desired pattern. After application, the PCB is exposed to UV light through a mask or stencil that defines the areas where the solder mask should be left in place. UV light exposure cures the polymer, hardening it in the desired pattern. The uncured mask areas are removed, leaving only the solder mask covering the designated areas.

PCB solder masks can come in various colors, with green being the most common for historical reasons. However, other colors like red, blue, black, and white are also used for specific applications or design aesthetics.

In addition to preventing solder bridges, solder masks offer some other benefits:

  1. Protection: Solder masks protect the copper traces and pads from environmental factors like moisture, dust, and oxidation, which can deteriorate the performance and reliability of the PCB over time.
  2. Soldering Accuracy: A solder mask helps guide the solder onto the correct pads during the soldering process, increasing the accuracy of solder placement.
  3. Aesthetics: Solder masks can enhance the visual appeal of the PCB by giving it a uniform and professional appearance.

Overall, a solder mask is essential to a PCB’s manufacturing process, contributing to the board’s reliability, performance, and longevity.

Read our 10-page whitepaper, ANZERs to Understanding Printed Circuit Board Assembly, to learn about the PCB assembly process.

Four-panel PCB with Green Solder Mask
Four-panel PCB with Green Solder Mask