Solder mask (or solder resist) is a thin, protective polymer layer applied over the copper traces of a PCB. It exposes only selected areas such as component pads, through-holes, and test points. It is typically applied after copper patterning and before silkscreen and surface finish.
Functions of solder mask:
- Prevents solder bridges between adjacent pads during reflow or wave soldering.
- Confines solder to designated pads during wave soldering or reflow processes.
- Provides electrical insulation, improving creepage/clearance and high‑voltage reliability.
- Shields the underlying copper from oxidation and environmental damage.
- Improves the appearance of silkscreen for inspection.
Types of solder mask:
- Liquid photoimageable (LPI): It is made of industry‑standard UV‑imageable liquid ink and is suitable for modern SMT and high‑density boards. It supports precise patterning and narrow solder dams (typically down to about 2–3 mil, depending on the fabricator).
- Dry film: A vacuum-laminated photoimageable sheet that ensures a perfectly uniform thickness across the entire board surface. It is the preferred choice for high-frequency RF and aerospace applications where consistent dielectric height is critical for impedance control.
- Inkjet/direct-imaged: A digital-grade acrylic or epoxy ink deposited directly from CAD data through inkjet heads. It eliminates physical phototools, significantly improving registration accuracy and reducing chemical waste.
- Epoxy liquid: A thermosetting epoxy resin applied through traditional screen printing. It is a low-cost solution best suited for simple, low-density power boards where fine-pitch precision is not required.
- Peelable mask: A temporary, flexible synthetic rubber or polymer coating applied to thick layers. It is used to shield gold contacts or through-holes during soldering and can be removed by hand after processing.