Part1: What is PCB Solder Mask Opening
Solder mask is a PCB oil layer that covers the copper wires and PCB materials to insulate and protect the PCB external from shorts and the environments. On the solder mask layer, you need to create openings to allow PCB pads to solder or connect, and some vias without PCB ink covered for thermal dissipation. These openings on the solder mask layer are called solder mask openings.
During PCB design, you should take note that the solder mask is those empty zones. When it comes to PCB manufacturing , those empty zones are printed with PCB ink in green, black, white, or another color. Instead, the shapes on the solder mask layer in your PCB design are solder mask openings, where the copper is exposed.