When studying circuit boards, especially as a beginner in the electronics industry, I always wondered why PCBs are green on the top layer. Answers vary, but everyone agrees on one thing: solder mask aids inspection, protects conductors, and reduces eye fatigue during manual assembly. Different PCB solder mask types vary in application method, composition, and cost.
By Zachariah Peterson
When studying circuit boards, especially as a beginner in the electronics industry, I always wondered why PCBs are green on the top layer. Answers vary, but everyone agrees on one thing: solder mask aids inspection, protects conductors, and reduces eye fatigue during manual assembly. Different PCB solder mask types vary in application method, composition, and cost.
To determine the right type and thickness of solder mask for your board, you need to consider your manufacturer’s capabilities as well as inspection and assembly processes. Below are four common PCB solder mask types:
- Liquid Epoxy Solder Mask
- Liquid Photo-Imaging Solder Mask (LPSM)
- Dry Film Solder Mask (DFSM)
- Top and Bottom Solder Mask Layers
What is a PCB Solder Mask?
Solder mask is a PCB process used to
protect metal features on the board from oxidation and
prevent conductive bridging between pads. It is a critical step in PCB manufacturing, especially with reflow or wave soldering—processes that offer little control over molten solder placement, while solder mask provides precise control.
Solder mask is sometimes called “solder resist,” which I find more accurate, as I once thought solder mask was a full layer of solder applied to the board.
Types of PCB Solder Mask
All solder masks consist of a polymer layer applied over the metal traces on a printed circuit board. Many types exist, and the best choice depends on cost and your application. The most basic option is screen‑printed liquid epoxy, like stenciling paint onto conductors. Solder mask can be produced in nearly any color.
1. Liquid Epoxy Solder Mask
The most basic and lowest‑cost popular option. A woven mesh is used to define the solder mask pattern. Liquid epoxy is a thermoset polymer that hardens during heat curing. Dye is mixed into the liquid epoxy to produce the desired color after curing.
2. Liquid Photo-Imaging Solder Mask (LPSM)
A more advanced solder mask using photolithography similar to photoresist processes in semiconductor manufacturing. LPSM can be screen‑printed like epoxy or sprayed (often cheaper). The more precise method uses photolithography to define openings for pads, vias, and mounting holes.
- A photolithography film is produced from your Gerber files to match your required solder mask pattern.
- The panelized board is thoroughly cleaned to remove dust under the cured solder mask.
- Both sides of the panel are fully coated with liquid LPSM.
- The board is baked and placed in a UV developer.
- The photolithography film is aligned carefully over the dried board.
- The board is exposed to UV light; exposed areas cure, unexposed areas are washed away with solvent, leaving a hard solder mask.
3. Dry Film Solder Mask (DFSM)
DFSM uses a process similar to LPSM, both exposed in a photolithographic process. Instead of a liquid coating, DFSM is applied as a dry film via vacuum lamination, which removes air bubbles and ensures adhesion. After exposure, unexposed areas are removed with solvent, and the remaining film is cured thermally.
4. Top and Bottom Solder Mask Layers
These simply refer to solder mask applied to the top or bottom side of the board; they do not specify a manufacturing process or material type.
Final Steps: Curing and Surface Finish
After applying the dielectric layers above, boards are cleaned to remove dust, then undergo final hardening and curing:
- Liquid epoxy: thermally cured (no UV exposure).
- LPSM and DFSM: UV‑cured during photolithography, then thermally cured.
Exposed copper areas require surface finishing to prevent oxidation. Common finishes:
- HASL (Hot Air Solder Leveling)
- ENIG (Electroless Nickel Immersion Gold)
- ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)
Additional openings may be included in the solder mask for solder paste layers, used to attach components during assembly.
What is the Standard Solder Mask Thickness?
Solder mask thickness mainly depends on copper trace thickness. For LPSM and DFSM:
- Typical thickness (perpendicular to the board): at least 0.8 mil in blank areas.
- Near trace edges: thinner, down to 0.3 mil or less.
- Recommended coverage over traces: ~0.5 mil.
Sprayed epoxy provides more uniform thickness across the board.
Solder mask creates a barrier between adjacent pads using solder mask relief—a small gap between the mask and exposed pad. This is critical for fine‑pitch BGAs and high‑density components, preventing solder bridging while allowing full wetting.
What Color Should the Solder Mask Be?
Solder mask color is determined by dye, whose chemistry affects cured thickness.
- Green is widely used partly because it supports thin barrier layers (~0.1 mm).
- Other colors often result in thicker barriers.
All thicknesses must comply with IPC‑SM‑840D.
Color strongly impacts visual and automated optical inspection (AOI):
- Black has the lowest contrast between the board and traces, complicating AOI.
- Green offers high contrast and low inspector eye fatigue.
Silkscreen color also affects visibility and inspection efficiency.
Which Solder Mask Should I Use?
Choose based on board geometry, hole size, components, layout, and end application.
- For high‑reliability industries (aerospace, telecom, medical), follow industry standards first.
- Most modern designs use photolithographic solder mask (LPSM or DFSM).
- DFSM: uniform thickness; best adhesion on very flat surfaces.
- LPSM: better coverage on complex topographies; thickness less uniform across the board.
Discuss available options with your manufacturer, as surface finish (e.g., matte) can reduce solder balls during reflow.
How to Include Solder Mask in My PCB Layout?
Solder mask must be its own layer (Top and Bottom Solder Mask) in your layout and Gerber files—usually enabled by default in CAD tools, not in the layer stack manager.
- Allow a 2‑mil border around features if mask registration is imperfect.
- Minimum pad‑to‑pad spacing: typically 8 mil to ensure solder bridging is prevented.
About Maxipcb
Maxipcb empowers innovators to turn cutting-edge technologies into reality.
We offer one-stop solutions for design, simulation, testing, PCB manufacturing, component procurement and SMT assembly, enabling efficient development, rapid deployment and risk control across the full product lifecycle.Serving the world in communications, industrial automation, aerospace, automotive, semiconductor and beyond, we build a safer, more connected future together.
