Understanding Solder Masks: Types, Processes, and When to Use Them

In PCB manufacturing, solder mask isn’t just about making a board look green—it’s a functional layer that protects copper traces, prevents solder bridging, and improves overall assembly yield. But not all solder masks are created equal.

From rigid PCBs to flexible circuits and high-density designs, the type of solder mask used—and how it’s applied—can impact everything from durability to manufacturability. This guide covers the most common solder mask types, their processing methods, and how to choose the right one for your design.

What Is a Solder Mask?

Solder mask is a thin protective layer—usually green, but also available in black, white, red, blue, orange, and more—that covers the non-pad areas of a PCB. It prevents unintended solder from flowing onto traces during reflow and protects the copper from oxidation, moisture, and contaminants.

It’s applied after imaging and etching, but before surface finish and component assembly.

Common Types of Solder Mask

There are several solder mask formulations and application methods, each with trade-offs in performance, process complexity, and cost.

1. Liquid Photoimageable (LPI) Solder Mask

Most common in modern PCB manufacturing.
• Process: LPI is screen- or spray-applied as a liquid, then imaged using UV light through a photomask. The exposed areas are hardened and the rest is developed away, leaving openings for pads.
• Finish: Smooth, thin, and consistent coverage. Great for fine-pitch components.
• Best for: Most rigid PCBs, HDI boards, and any design requiring tight tolerances.

Pros:
• Excellent resolution
• Compatible with high-volume production
• Can be applied to both rigid and flex PCBs (with formulation changes)

Considerations:
• Requires controlled UV imaging and cleanroom conditions
• Not ideal for very thick copper or extremely uneven surfaces

2. Epoxy Screen-Printed Solder Mask

Older, lower-cost method, used less frequently today.
• Process: Thermally cured epoxy ink is pushed through a stencil onto the PCB.
• Finish: Thicker, more textured, lower resolution.
• Best for: Simple, low-density PCBs or non-critical consumer applications.

Pros:
• Low cost
• Simple process, no photoimaging required

Considerations:
• Poor registration accuracy
• Not suitable for fine-pitch components
• Slower to apply in large volumes

3. Dry Film Solder Mask

Used in specialty applications, especially when consistent thickness is critical.
• Process: A dry film (like a solder mask “sticker”) is laminated onto the PCB, exposed with UV, and developed.
• Finish: Uniform thickness, excellent edge definition.
• Best for: High-reliability applications, high-voltage designs, or very fine-line spacing where precise clearance is required.

Pros:
• Clean application with good definition
• Excellent dielectric strength

Considerations:
• Higher material cost
• Less flexible than LPI or epoxy masks
• Can be labor-intensive