Solder mask is primarily made of epoxy resin combined with acrylic compounds, along with photoinitiators, pigments, and mineral fillers. The exact formulation varies by type and manufacturer, but nearly all modern solder masks share this polymer base. The familiar green (or red, blue, black) coating on a circuit board is a carefully engineered material designed to protect copper traces from corrosion, prevent accidental solder bridges, and withstand the extreme heat of assembly.
The Resin Base: Epoxy and Acrylic
The backbone of solder mask is a blend of epoxy and acrylic resins. Epoxy provides strong adhesion to the copper and fiberglass substrate of the circuit board, along with excellent chemical resistance. Acrylic resin makes the material responsive to ultraviolet light, which is how manufacturers pattern it during production. These two resin systems work together in what the industry describes as two types of cross-linking: thermal (heat-driven) and optical (light-driven). In most modern formulations, both mechanisms are at play. UV light triggers the initial hardening in the exposed areas, and a subsequent bake in an oven completes the cure throughout the full thickness of the coating.
Photoinitiators: The UV-Sensitive Trigger
Mixed into the resin are photoinitiators, chemicals that absorb ultraviolet light and kick off the polymerization reaction that hardens the mask. When UV light hits the solder mask through a patterned film, the photoinitiators in the exposed areas generate reactive molecules that cause the resin chains to cross-link into a rigid, insoluble network. The unexposed areas stay soft and wash away during development, leaving openings where components will later be soldered.
The choice of photoinitiator directly affects how long the board needs to sit under the UV lamp and how fine the resulting pattern can be. For boards with tightly spaced pads, manufacturers select photoinitiators that produce sharper edges and need shorter exposure times.
Mineral Fillers for Strength and Heat Resistance
Raw epoxy-acrylic resin alone wouldn’t hold up well under soldering temperatures or provide the right consistency for application. That’s where mineral fillers come in. Silica and alumina are the two most common. Silica particles increase stiffness and help control how the liquid flows during coating. Because silica particles are smaller and have more surface area, they interact more strongly with the surrounding resin, which raises viscosity and improves mechanical strength.
Alumina plays a different role. Its thermal conductivity (around 25 W/mK) is roughly ten times higher than silica’s (0.5 to 2.5 W/mK), so it helps the cured mask dissipate heat more effectively. Some formulations use a hybrid of both fillers. Research on similar epoxy composite systems has shown that combining alumina and silica produces a synergistic effect, boosting thermal conductivity beyond what either filler achieves alone. Filler content can make up a substantial portion of the total formulation by weight, significantly changing the material’s mechanical and thermal behavior.
Pigments and Dyes
The color of solder mask comes from added pigments or dyes. Green is the industry standard largely for historical reasons: it provides good contrast for visual inspection and works well with automated optical inspection systems. But the pigment itself is a small fraction of the total formulation. Red, blue, black, white, and yellow masks use different pigments while keeping the same resin chemistry underneath. Darker colors like black can absorb more heat during reflow soldering, which occasionally matters for temperature-sensitive designs.
How LPI Differs From Dry Film
The most widely used solder mask today is liquid photoimageable (LPI) solder mask, which is the epoxy-acrylic formulation described above. It’s applied as a liquid over the entire board surface (top and bottom), then selectively cured with UV light and developed to remove the uncured portions. LPI masks dominate because they conform well to the uneven topography of a circuit board and can resolve fine features.
Before LPI became standard, dry film solder mask was common. Dry film is a pre-formed sheet of photosensitive polymer laminated onto the board using heat and pressure. It still uses similar resin chemistry, but because it’s a uniform-thickness sheet, it can’t follow the surface contours as closely. On a board with tall copper traces, dry film may tent over gaps rather than filling them. LPI flows into those spaces before curing, giving more consistent coverage. Dry film is still used in some specialty applications, but LPI handles the vast majority of production boards.
Typical Thickness After Curing
Once cured, solder mask thickness varies across the board surface. The general target is at least 0.8 mils (about 20 microns) measured perpendicular to the board in open areas. Over copper traces, you’ll typically find around 0.5 mils (roughly 12.5 microns) of mask. Near the edges of traces, where the coating thins as it transitions from the raised copper to the lower substrate, thickness can drop to 0.3 mils or less. These thin spots are the most vulnerable areas for electrical isolation and chemical protection, which is why solder mask formulation has to deliver reliable performance even in very thin layers.
Halogen-Free Formulations
Traditional solder masks often contained halogenated compounds (bromine or chlorine-based) as flame retardants. Environmental regulations and market demand for greener electronics have pushed manufacturers toward halogen-free alternatives. These formulations replace the halogenated flame retardants with phosphorus-based or nitrogen-based compounds while keeping the same basic epoxy-acrylic resin structure.
Achieving true halogen-free status (typically defined as less than 900 ppm each of bromine and chlorine) without sacrificing performance has been a persistent engineering challenge. Early halogen-free masks sometimes showed differences in surface flatness, curing behavior, and cross-section roughness compared to conventional versions. Newer materials have largely closed that gap. Modern halogen-free solder resists deliver comparable optical and thermal performance to their halogenated predecessors while meeting environmental standards.
Performance Standards
The industry specification that governs solder mask performance is IPC-SM-840, maintained by the IPC standards organization. It defines four classes of requirements (T, FT, H, and FH) based on the end-use environment, from consumer electronics to high-reliability aerospace and military applications. The specification tests for chemical resistance to fabrication solvents, cleaning agents, and fluxes, along with hydrolytic stability (how well the mask holds up in humid conditions over time). Electrical requirements include insulation resistance and moisture resistance, ensuring the cured mask reliably prevents current leakage between adjacent traces. Higher classes demand more stringent performance across all these categories, which often means tighter control over the resin formulation and filler content rather than a fundamentally different chemistry.