Solder mask, also called solder resist, is one of the most important functional layers on the outside of a printed circuit board. The green coating visible on most PCBs is solder mask, but changing the board’s appearance is only one small part of its job.
During PCB fabrication, solder mask covers the outer-layer copper traces while leaving openings for component pads, test points, selected vias, and other areas that require electrical contact. This helps reduce the risk of solder bridges between adjacent pads during assembly. It also protects copper from oxidation, contamination, scratches, and accidental contact.
For a conventional double-sided or multilayer PCB, green liquid photoimageable (LPI) solder mask is usually sufficient. When a design includes fine-pitch ICs, BGAs, HDI features, LED optics, special colors, peelable protection, or tightly controlled impedance, however, the mask material, imaging capability, thickness, and opening geometry require separate consideration.
This guide explains solder mask materials, LPI and peelable systems, thickness, color, and common DFM issues from both design and manufacturing perspectives. It also shows what should be specified for a real PCB project.
PCB Solder Mask Structure and Functions
The outer-layer structure of a PCB can be simplified as:
Base material and copper circuitry → solder mask → silkscreen
Solder mask is applied directly over the outer copper circuitry. Silkscreen text is normally printed on the surface of the cured mask.
The Top Solder Mask and Bottom Solder Mask layers in the design data essentially tell the PCB manufacturer:
> Which locations must remain exposed through the solder mask.
Component pads, test points, edge contacts, selected open vias, and specified mechanical or electrical contact areas normally require mask openings. The remaining external copper is covered.
The most direct function of the mask is to control solder. During reflow, wave, or other soldering processes, insufficient mask separation between neighboring copper features makes it easier for molten solder to form a bridge between adjacent pads.
Solder mask also reduces direct exposure of copper traces to air, moisture, and contaminants, while lowering the risk of scratches or accidental electrical contact.
It does not replace the creepage, clearance, or insulation design of the PCB itself. In high-voltage products, electrical spacing must still be designed around the operating voltage, environmental conditions, applicable safety rules, and product standards.
Solder Mask Material: What Is It Made Of?
Modern PCB solder mask is not simply a layer of “green paint.” It is an engineered polymer material system.
A typical formulation contains a resin system, curing components, pigments, fillers, and—when the material is photoimageable—photo-reactive components. Exact formulations vary by supplier, but the main functions of these constituents are broadly consistent.
| Component | Main function |
|---|---|
| Resin system | Provides the primary structure, electrical insulation, adhesion, and chemical resistance |
| Curing components | Participate in the final curing reaction |
| Pigments | Produce green, white, black, and other colors |
| Fillers | Adjust flow, mechanical behavior, thermal behavior, and surface properties |
| Photo-reactive components | Enable exposure and development in an LPI process |
The resin system determines many of the mask’s fundamental properties. Many modern liquid photoimageable solder masks still use an epoxy-based resin system.
For this reason, dividing solder masks into “epoxy versus LPI” is misleading. Epoxy describes the material chemistry more closely, while LPI describes a liquid photoimageable system and its patterning method.
Two products can both be green LPI masks yet behave differently because of differences in resin, pigment, filler, and photochemical formulation. These differences can affect adhesion, exposure latitude, thermal resistance, minimum mask dam capability, and color stability.
Common PCB Solder Mask Systems
Most modern rigid PCBs use a permanent solder mask that remains on the finished board. LPI is the most common process for this purpose.
LPI Liquid Photoimageable Solder Mask
LPI stands for Liquid Photoimageable Solder Mask.
Its typical production sequence is:
Coating → pre-drying → exposure and imaging → development → final cure
Liquid mask first covers the PCB surface. After pre-drying, a phototool or direct imaging system transfers the solder mask pattern to the board. Development then removes material from locations that must be exposed, such as pads, test points, and selected vias.
LPI provides good image resolution and registration, making it well suited to features commonly found on modern boards:
- Fine-pitch SMT pads
- QFN packages
- BGA packages
- High-density pad fields
- Relatively narrow solder mask dams
For a standard commercial rigid PCB, green LPI solder mask is normally the most mature and widely available default.
LPI and screen printing are not mutually exclusive. Some LPI materials are screen-coated onto a panel, then exposed and developed to create the final pattern.
Non-Photoimageable or Direct-Pattern Liquid Mask
Unlike LPI, which is coated first and patterned through exposure and development, some non-photoimageable liquid masks can be printed directly only where the coating is intended to remain.
The process can be relatively simple, but its resolution and registration capability are generally lower than those of a modern LPI process. It is therefore better suited to simple PCB layouts with less demanding mask geometry.
Fine-pitch SMT, BGA, QFN, and high-density designs normally require LPI or another high-resolution solder mask system.
Dry Film Solder Mask
Dry film solder mask is laminated onto a PCB as a thin film and then exposed, developed, and cured to form the final pattern.
Film thickness can be controlled comparatively well, and the process can support fine features. Dry film, however, is not the default choice for an ordinary rigid PCB.
It is more often considered when a project has particular thickness-control, fine-feature, or substrate requirements. Selection depends on the board structure, material system, and factory process. A board does not automatically require dry film merely because it is described as advanced or uses HDI technology.
Peelable Solder Mask vs. Permanent Solder Mask
Peelable mask is often discussed alongside conventional solder mask, but the two serve different purposes.
Permanent LPI remains on the completed PCB. Peelable mask is a temporary protective material.
During wave soldering, selective soldering, or another localized process, certain pads, holes, edge contacts, or connector areas may need temporary protection from solder or processing media. Peelable material can be applied to those areas and removed after the operation.
| Item | Permanent LPI | Peelable mask |
|---|---|---|
| Remains on the finished PCB | Yes | No |
| Primary function | Protects copper and defines solderable areas | Provides temporary process protection |
| Typical application | Most rigid PCBs | Selected local process areas |
| Removed after production | No | Yes |
Peelable mask should therefore not be treated as an alternative type of permanent solder resist. If temporary protection is required, its location and purpose should be clearly identified in the fabrication drawing.
How the Solder Mask Process Affects the Finished PCB
A typical LPI process includes surface preparation, coating, pre-drying, exposure, development, curing, and inspection.
Design engineers do not need to control every production parameter, but understanding the critical steps helps explain why the mask design must match the factory’s actual capability.
Surface Preparation Determines Adhesion
Before coating, the copper surface must be cleaned and conditioned appropriately.
Oxidation, oil, or process residue can interfere with adhesion between the mask and copper, leading to weak bonding, local lifting, or other surface defects.
Exposure and Development Determine Opening Accuracy
Modern solder mask patterns are principally formed by exposure and development.
Registration becomes especially important around fine-pitch pads. Misregistration can cover part of a pad or expose copper that should remain protected. Around BGA, QFN, and dense SMT areas, solder mask is therefore a routine DFM checkpoint.
Final Cure Determines Service Performance
After development, the mask must be fully cured to achieve its specified adhesion, mechanical properties, chemical resistance, and heat resistance.
Final inspection commonly checks for mask misregistration, pinholes, missing coating, unintended exposed copper, absent dams, scratches, and contamination.
Solder Mask Thickness Is Not One Fixed Number
“Solder mask thickness” is a common search term, but it is also one of the most oversimplified PCB parameters.
Many sources reduce the answer to:
> PCB solder mask is XX μm thick.
In reality, liquid solder mask is coated over a PCB surface with significant topography. Outer-layer traces and copper areas stand above the exposed laminate. The coating also flows and levels before curing, so the final thickness is not identical across the entire board.
One PCB may include different thickness conditions over:
- The laminate surface
- The top of a copper trace
- A trace edge
- Areas with heavy or dense copper topography
As a general example, some common LPI materials may produce a cured thickness of approximately 10–20 μm over the top of a copper trace, but this is not a universal PCB standard.
Material formulation, coating method, copper weight, and surface topography all influence the result. Specialized mask systems may use substantially different coating thicknesses.
If thickness is functionally important—for example, as part of a defined insulation structure or mechanical-protection requirement—state the requirement in the RFQ and fabrication drawing. Do not apply a single assumed industry value.
Why Green Became the Default PCB Solder Mask Color
Green has been the most common PCB solder mask color for many years. It is not preferred because it conducts electricity better, dissipates heat more effectively, or inherently provides higher electrical reliability.
The practical reason is manufacturing maturity. Green solder mask has a long history of high-volume use, so its material supply, exposure, development, inspection, and process-control windows are well established.
Green also generally provides useful visual contrast during inspection. For a PCB with no special appearance or optical requirement, green LPI normally offers a mature process, broad material availability, and relatively low manufacturing risk.
Solder Mask Color Is More Than an Appearance Choice
PCBs can also use red, blue, black, white, and other custom mask colors.
Color alone does not determine whether the board will function electrically. Different pigments can, however, change the material’s optical absorption and exposure response. Each color may therefore have a different manufacturing window.
| Color | Common reason for selection | Manufacturing factors to confirm |
|---|---|---|
| Green | General-purpose PCB | Mature process and broad material supply |
| Red | Product identification or appearance | Material availability and factory capability |
| Blue | Appearance or product differentiation | Potentially fewer material choices |
| Black | Appearance or light blocking | Inspection contrast and exposure capability |
| White | LED, lighting, or optical applications | Reflectivity, color stability, and yellowing |
When a PCB includes very fine mask dams, BGAs, or dense SMT features, color should not be selected on appearance alone. First confirm the manufacturer’s minimum mask dam, registration tolerance, and stable production capability for the requested color. SAYFU’s published rigid PCB manufacturing capabilities can be used as an initial reference, but final manufacturability depends on the complete design.
White Solder Mask for LED PCBs Is an Optical Choice
White solder mask is common on LED lighting boards, light modules, and other optical PCBs.
Its main value is high visible-light reflectance—not higher thermal conductivity.
For an LED PCB, the relevant properties may include reflectivity, whiteness, color consistency, and resistance to thermal yellowing. LEDs and drivers can keep the board in a warm environment for long periods. If the mask yellows noticeably, it can affect both appearance and optical consistency.
For an optically demanding project, writing only “Solder Mask: White” may not be enough. If reflectivity or long-term color stability is a functional requirement, confirm the applicable material data and acceptance criteria.
Solder Mask Openings and Dams Are Critical DFM Features
Manufacturability is usually influenced less by the color of the mask than by whether the pattern provides a realistic process margin.
Solder Mask Opening and Clearance
The solder mask layers in the design data define the locations that must be opened.
Because fabrication always includes some registration tolerance, the mask opening is normally larger than the exact copper-pad geometry. If it is too small, registration error can cover part of the pad. If it is too large, adjacent pads may lose the mask separation intended to control solder.
Minimum solder mask clearance should not be copied mechanically from a different project. It should be selected around the current board geometry and the actual capability of the chosen manufacturer.
Solder Mask Dam or Web
The narrow strip of mask remaining between adjacent openings is called a solder mask dam or solder mask web.
Around fine-pitch ICs, QFNs, and BGAs, these dams can help separate neighboring pads. Narrower is not always better. If the designed width is below what the factory can consistently expose, develop, and retain, dams may be missing, broken, or unstable in production.
The manufacturer may recommend increasing spacing, modifying individual openings, or using a gang opening in selected areas. Minimum solder mask dam is therefore a key parameter to confirm during high-density PCB DFM.
Via Tenting and Solder Mask Openings
Not every via must remain open.
When solder mask covers the via opening, it is usually described as a tented via. When the hole or annular pad remains exposed, it is an open via.
Ordinary vias are often covered to reduce exposed copper and lower the risk of unnecessary solder entering the hole. Vias required for testing, soldering, electrical contact, or a special process may need to remain open.
When vias are placed next to BGA pads—or directly in the pads—the issue also involves:
- Solder wicking
- Via filling
- Via-in-pad construction
- Planarization
In this situation, “tent or open” is not enough to define the requirement. Fabrication and assembly requirements must be considered together.
SMD vs. NSMD Pads
BGA packages commonly use one of two pad definitions:
- Solder Mask Defined (SMD)
- Non-Solder Mask Defined (NSMD)
In an NSMD design, the mask opening is larger than the copper pad, exposing the entire edge of the copper land. In an SMD design, the mask overlaps part of the copper-pad edge, so the opening defines the final solderable area.
Neither structure is universally superior. The correct choice depends on the component manufacturer’s recommendation, package design, assembly process, and PCB fabrication capability.
For a BGA in particular, do not change the recommended pad structure merely to retain a narrower solder mask dam.
Does Solder Mask Affect Controlled Impedance?
For outer-layer impedance-controlled traces, the answer is yes: solder mask can have a measurable effect.
Solder mask is itself a dielectric material. Covering a microstrip or coplanar trace changes the dielectric environment around the conductor and can therefore change its impedance.
For an ordinary signal trace, this change may not be important. For a strict controlled-impedance or high-frequency PCB, the manufacturer’s final impedance model should account for:
- FR-4 laminate dielectric constant (Dk)
- Dielectric thickness
- Copper thickness
- Trace geometry
- Solder mask coverage
The coverage condition of a critical outer-layer trace should not be changed casually after the stack-up and impedance calculation have been established.
Solder Mask vs. Silkscreen, Carbon Ink, and Conformal Coating
Several inks and coatings may appear on a PCB or PCBA, so their functions are easily confused.
| Material or coating | Primary function | Typical production stage |
|---|---|---|
| Solder mask | Protects copper and defines solderable areas | Bare PCB fabrication |
| Silkscreen ink | Prints text, marks, and component references | Bare PCB fabrication |
| Carbon ink | Creates conductive, contact, or resistive functional areas | Specialized PCB fabrication |
| Conformal coating | Provides environmental protection for an assembled PCBA | After PCB assembly |
Silkscreen Ink Is Primarily for Identification
Silkscreen, also called legend, is used for reference designators, polarity marks, connector names, warning text, and other production information.
It is normally printed on top of the solder mask. Its purpose is identification; it does not protect copper or control solder. Silkscreen ink is therefore not solder mask.
Carbon Ink Is a Functional Material
Carbon ink can be used for keypad contacts, selected resistor structures, conductive jumpers, or wear-resistant contact areas.
These carbon features perform a defined electrical or contact function and are not conventional solder mask.
Conformal Coating Protects the Assembled PCBA
Solder mask is completed during bare-board fabrication. Conformal coating is normally applied after components have been assembled and tested.
Its role is to improve the assembled electronics’ resistance to moisture, contaminants, and corrosive environments. Solder mask and conformal coating are different layers and cannot replace one another.
Solder Mask Standards and Customer Specifications
IPC-SM-840 is a key industry specification covering qualification and performance requirements for permanent solder mask and related cover materials.
Completed rigid PCBs may also be manufactured and accepted with reference to IPC-6012, IPC-A-600, and the customer’s own fabrication specification. See this overview of PCB IPC standards for related material and board classifications. Always verify the current document revision and the order-of-precedence rules for the project.
A conventional commercial PCB does not usually benefit from a long, indiscriminate list of standards in the RFQ. If the project requires a particular mask grade, UL recognition, halogen-free formulation, special heat or chemical resistance, or a mandatory brand, state that requirement explicitly in the fabrication data.
How to Select Solder Mask for Different PCBs
Most conventional rigid PCBs do not require a complicated mask-selection exercise. Green LPI is usually a sensible default. The design conditions determine when more detailed evaluation is necessary.
| PCB condition | Solder mask requirements that deserve attention |
|---|---|
| Standard double-sided or multilayer PCB | Green LPI is normally sufficient |
| Fine-pitch, BGA, or QFN | Opening, registration, and minimum mask dam |
| HDI | Imaging resolution and retained mask-dam capability |
| LED or optical PCB | White mask, reflectivity, and yellowing resistance |
| Temporary process protection | Peelable mask |
| Controlled impedance | Include outer-layer mask coverage in the impedance model |
| Special reliability requirement | Material grade, IPC requirements, heat resistance, and chemical resistance |
For a dense PCB, the first question should be:
> Can the selected PCB factory produce this solder mask geometry consistently?
That is normally more important than choosing a nonstandard color first.
How to Specify Solder Mask in a PCB Drawing or RFQ
For a standard PCB, a practical default callout is:
> Solder Mask: Green LPI
This allows the manufacturer to select a standard product from its qualified material system.
When the project has special requirements, specify the applicable items:
- Color
- Matte or gloss finish
- Halogen-free material
- Peelable-mask areas
- Via tenting requirements
- SMD or NSMD requirements
- Minimum solder mask dam, when design-critical
- A specific material or brand, when mandatory
For a fine-pitch, HDI, or special-color board, do not simply copy values such as “3 mil solder mask dam” or “2 mil clearance” from another manufacturer’s capability table.
Mask capability depends on the material, pigment, imaging equipment, copper weight, board topography, and the factory’s process. The production requirement must fit the stable process window of the current supplier.
PCB Solder Mask FAQ
What Is PCB Solder Mask Material?
Modern PCB solder mask is an engineered polymer system containing resin, curing components, pigments, fillers, and—in photoimageable products—photo-reactive components.
Many modern LPI masks use an epoxy-based resin system, but the exact formulation depends on the supplier and product family.
What Is LPI Solder Mask?
LPI means Liquid Photoimageable Solder Mask. The liquid material is coated onto a PCB, then exposed, developed, and finally cured to produce an accurate pattern.
Because LPI supports fine-pitch openings and dense SMT layouts, it is a standard process for modern rigid PCBs.
What Is the Typical PCB Solder Mask Thickness?
As a general reference, some common LPI materials may produce approximately 10–20 μm of cured mask over the top of a copper trace. This is not a universal value.
Thickness can differ over the top of a trace, along a trace edge, and above bare laminate. The actual result depends on the mask material, copper weight, topography, and factory process.
Why Do Most PCBs Use Green Solder Mask?
Green mask has a mature material supply and a long-established exposure, development, inspection, and mass-production process window. It also offers useful visual contrast during inspection.
This does not mean that green mask has special electrical properties.
Is White Solder Mask Suitable for an LED PCB?
Yes. White solder mask is widely used on LED and optical PCBs because it can provide high visible-light reflectance.
For products with long-term optical requirements, also evaluate reflectivity, whiteness, color consistency, and resistance to thermal yellowing.
What Is the Difference Between Solder Mask and Conformal Coating?
Solder mask is formed during bare PCB fabrication to protect outer-layer copper and define solderable openings.
Conformal coating is normally applied after the PCBA has been assembled. It protects the electronic assembly from moisture, contamination, and corrosive environments. The two layers perform different functions and are not interchangeable.
Conclusion
PCB solder mask may look like a thin colored film, but it influences bare-board fabrication, soldering, fine-pitch design, impedance control, and surface reliability.
For most conventional boards, a mature green LPI system provides good manufacturing stability and cost performance. Selecting a more complex material simply because it appears to be a higher specification is rarely necessary.
For BGA, QFN, HDI, fine-pitch SMT, controlled-impedance, LED optical, or specialized process applications, the important factors are the mask material system, opening accuracy, minimum dam, registration capability, via treatment, and the manufacturer’s stable production window.
Solder mask should not be treated merely as a final color applied to the PCB. In a complex design, it should be reviewed together with the stack-up, pad geometry, via structure, and overall DFM before the production data are released.