Specifying Solder Mask

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Introduction to Solder Mask

Solder mask, also known as solder resist, is a thin lacquer-like layer of polymer that is usually green in color and covers the bare copper traces on a printed circuit board (PCB). The main purposes of solder mask are:

  • To prevent solder from bridging between conductors during assembly. This helps prevent short circuits.
  • To provide insulation around conductors, preventing accidental contact with other metal items.
  • To mark the boundaries between conductors and the board substrate. This helps identify conductors during debugging and repair.
  • To prevent corrosion and dendritic growth on the conductors.
  • To provide mechanical support by coating over the fiberglass.
  • To provide markings for orientation and identification.

The solder mask is one of the final layers applied during PCB fabrication. It is applied by screen printing, photoimaging, electrophoresis, or spray coating. Openings in the solder mask, called solder pads, provide access to the underlying PCB traces for placement and soldering of components.

When designing a PCB, the solder mask must be carefully specified to ensure proper application and performance. Key specifications include material type, thickness, color, coverage extent, and requirements for solder pad openings. This article provides guidance on specifying solder mask for optimal results.

Selecting Solder Mask Materials

Several material types and formulations are available for solder mask. The most common options are:

Liquid Photoimageable Solder Mask (LPISM)

  • Most widely used solder mask today
  • Applied by screen printing then exposed/developed with artwork patterns
  • Acrylic-based with epoxy and photoinitiators
  • Can achieve fine features down to 100 μm

Dry Film Solder Mask

  • Solid sheet laminated onto PCB then imaged/developed
  • Epoxy-based film with acrylic adhesives
  • Lower cost but lower resolution than LPISM

UV Curable Solder Mask Ink

  • Direct inkjet printing of mask with UV curing
  • High mix of acrylic monomers and oligomers
  • Requires special equipment but offers customization

Silicone-Based and Flexible Solder Masks

  • Used for flexible PCBs
  • Withstand bending and flexing
  • Polydimethylsiloxane or polyimide chemistry

High Reliability / High Temperature Solder Masks

  • Withstand repeated thermal cycling with no cracking
  • Withstand soldering up to 288°C
  • Usually polyimide-based

Electrophoretic Solder Mask

  • Charged polymer particles deposited by electrophoresis
  • Provides conformal coating over complex topography
  • Can coat recessed areas unreachable by screening

When selecting the solder mask material, key considerations include:

  • Temperature tolerance – The mask must withstand soldering processes and any high-temperature operations.
  • Flexibility – If used on flex PCBs, the mask must flex without cracking.
  • Resolution – Finer masks can produce smaller openings and traces.
  • Conformality – The ability to coat uneven topography.
  • Moisture resistance – Prevent delamination or electrical leakage.
  • Abrasion/scratch resistance – Withstand handling and assembly.
  • Chemical resistance – Withstand cleaning agents, fluxes, and assembly process chemicals.
  • Dielectric properties – High dielectric breakdown voltage.
  • Thermal cycling performance – Withstand repeated heating/cooling without cracking.
  • Process compatibility – Compatible with fabrication processes and equipment.
  • Cost – Balancing performance against material and processing costs.

The majority of rigid PCBs use standard LPISM or dry film masks. Flexible circuits may require polyimide or silicone masks. Discuss material options with your PCB manufacturer early in the design process.

Solder Mask Thickness

Solder mask thickness is typically in the range of 25-75 μm (1-3 mils). Thicker masks can provide better insulation, abrasion resistance, and mechanical support. However, they increase thermal resistance, dielectric loss, and processing difficulties. Common thickness specifications include:

  • Standard (1 mil) – 25-38 μm – Default for most PCBs. Balances properties and processing.
  • Intermediate (2 mil) – 50-75 μm – When increased insulation or durability needed.
  • Thick (3+ mil) – >75 μm – Only when exceptional insulation or coating thickness is required. Significantly increases material usage and cost.

Mask thickness can be reduced to allow conformal coating over high density interconnects:

  • Thin (0.5 mil or less) – <12 μm – Allows coating into tight spaces but reduces durability.

A mask thickness of 1 mil (25 μm) is suitable for the majority of PCB designs. Increase to 2 mil if the design requires maximum insulating capability or ruggedness. Thicknesses above 2 mil are typically excessive. Discuss thickness requirements with your PCB manufacturer and material vendor.

Solder Mask Color

The most common solder mask colors are:

  • Green – This is the default color and can be formulated to a wide range of green hues and intensities. The eye is most sensitive to the green wavelength, providing high visibility and contrast.
  • Red – Used when a dark color is needed. Provides good contrast on red boards.
  • Blue – Gives increased contrast against tin-lead and some lead-free solders. Blue solder mask is less common than green.
  • Black – Used when electromagnetic shielding is needed. Also provides a dark background for components and markings. Can reduce visibility.
  • White/Translucent – Uncommon, but allows backlighting of PCBs.
  • Custom colors – Any color can be specially formulated, but minimum order quantities may apply.

Green solder mask is used on most PCBs, with the exact shade often being inconsequential. Red mask provides contrast for red boards. Blue, black, white, and other colors have specific applications but are less widely used. Discuss color requirements with your PCB manufacturer.

Solder Mask Coverage Area

The solder mask opening design, or “solder mask defined pads”, determine the regions where the mask covers the conductors and where solder mask openings expose the pads. The design rules specify the minimum mask web width around conductors and the minimum overlap onto pads.

Typical design guidelines for mask coverage include:

  • Conductor masks – 25-50 μm mask web from edge of traces
  • SMD pads – 25-75 μm mask overlap onto pad
  • PTH pads – 50-100 μm mask overlap onto pad
  • Exposed copper – < 25% of board area recommended

The mask web reduces the risk of solder bridges between closely spaced traces. The overlap onto pads improves alignment tolerance and reduces pad lifting. Excessive exposed copper increases mask delamination risk.

Work with your PCB manufacturer on specific clearance and overlap values during design rule development. Take into account factors such as circuit density, trace pitch, pad sizes, and other board complexities when tailoring the solder mask coverage margins.

Solder Pad Opening Design

The openings in the solder mask expose the component pads for soldering. These are sometimes called solder stealthing. The pad openings should be designed with:

  • Sufficient clearance – Account for mask misalignment and component tolerances.
  • Thermal relief – Avoid acute mask angles around pads to prevent cracking.
  • Opening shape – Adjust to match component termination geometries.
  • Size – Larger openings if more solder coverage needed.
  • Annular rings – Define minimum web width around pad openings.

Insufficient clearance risks the pad being partially covered by mask, preventing soldering. Excess clearance wastes space and reduces masking effectiveness. Thermal relief cutouts improve cracking resistance during temperature cycling.

The optimum pad opening design depends on factors such as land pattern geometry, component lead design, pitch and spacing, and manufacturing capabilities. Coordinate with your PCB fabricator and assembler to determine appropriate opening dimensions and shapes.

Recommended Solder Pad Opening Dimensions

Although opening designs are customized for each PCB, here are some typical clearance values:

  • SMD pads – 75-150 μm clearance from pad edge
  • PTH pads – 100-200 μm clearance from pad edge
  • Thermal relief – Pad openings to have ≥ 75 μm uniform web around pad, with rounded corners

This provides 50-100 μm overlap of mask onto pad, accounting for alignment tolerances. Openings may need to be enlarged on dense boards and for challenging component geometries. Talk with your PCB manufacturer about your specific design needs.

Specifying Solder Mask Registration

The registration, or alignment accuracy, of the solder mask layer relative to the pads and traces is critical. Tight registration ensures proper overlap and clearance. Common registration capabilities include:

  • Standard – ±50 μm
  • Middle – ±25 μm
  • High accuracy – ±15 μm

Standard registration is suitable for most consumer PCBs. Precision applications may require high accuracy mask alignment. This increases manufacturing costs.

Defining pad and opening tolerances in your design rules ensures functionality within the specified registration. If particularly tight clearances are needed, requiring high precision alignment, inform your PCB manufacturer upfront during discussions.

Solder Mask Design Rules Overview

Some key solder mask parameters to specify in your PCB design rules:

  • Minimum web width – Mask overlap on traces
  • Mask overlap – Overlap onto component pads
  • Solder pad clearance – Clearance around openings
  • Minimum web width – Annular ring around openings
  • Registration – Alignment tolerance relative to pads

Work iteratively with your PCB manufacturer on design rules, balancing manufacturability against design shrinkage and pad density. Perform assembly trials to confirm functionality within specified tolerances.

Additional Solder Mask Guidelines

Beyond the basics covered so far, here are some other solder mask guidelines and best practices:

Leave Dam Bars Along Board Edges

Dam bars are unmasked strips along the board perimeter. They help contain the mask during application and prevent wicking onto fixture edges. Specify 25-50 mm wide dam bars in the design rules.

Specify Any Keep-Out Areas

If certain board sections should be free of solder mask, such as for connectors, testpoints or conformity masking, specify keep-out regions in the design data.

Mark Component Polarity

Apply small solder mask dots, arrows, or shapes to indicate component orientation or polarity. This avoids reliance on silkscreen.

Ensure Mask Adhesion With Surface Finishes

Inform your PCB manufacturer if immersion silver, ENIG, or other surface finishes are used. Adhesion processes may need adjustment.

Allow Sufficient Curing and Drying Time

Solder mask requires adequate soft and full cure times to ensure correct formation and stability. Discuss process requirements with your PCB manufacturer.

Perform Adhesion Testing

Do peel strength and cross-hatch testing to qualify solder mask adhesion. This identifies any risk of delamination during assembly or field use.

Confirm Temperature and Bend Resistance

Subject boards to thermal shock preconditioning and bend tests before assembly. Ensure the solder mask performs reliably under thermal and mechanical stresses.

Problems From Poor Solder Mask Implementation

Deficiencies in the solder mask can lead to many problems, including:

  • Short circuits from inadequate clearance between traces
  • Open circuits from mask sealing component pads
  • Disturbed soldering process due to insufficient opening sizes
  • Cracking or delamination under thermal cycling or flexing
  • Electromigration or dendritic growth without insulation
  • Reduced creeping distances and voltage isolation

Careful solder mask specification during the design stage is essential to avoiding these issues. Work closely with your PCB manufacturer to ensure full understanding of their capabilities and process limitations.

Solder Mask Design Review Checklist

When reviewing a PCB layout, inspect the solder mask design with respect to:

  • Solder mask clearance from traces and spacing between traces
  • Mask overlap onto component pads
  • Adequate pad opening dimensions for soldering
  • Sufficient web width around pad openings
  • Inclusion of dam bars along board perimeter
  • Solder mask extent markers on silkscreen layer
  • Presence of thermal relief cutouts around pads
  • Allowance for registration tolerance variations
  • Specification of any solder mask keep-out zones

Run design rule checks and visually inspect the board layout. Confirm all openings are defined and no pads are completely covered. Review with your PCB manufacturer prior to mask application.


Solder mask is one of the most critical PCB fabrication processes. The material, thickness, clearance, registration, and other parameters must be designed properly for a functioning board. Work closely with your PCB manufacturer to define appropriate design rules and fabrication requirements. Allow enough time in the development schedule for design reviews and assembly prototyping to validate solder mask implementation. With careful specification upfront, solder mask will provide reliable performance, protect PCB traces, and facilitate robust component soldering.


What are the main purposes of solder mask on PCBs?

The main purposes of solder mask are to prevent solder bridging between pads, provide insulation around conductors, mark conductor boundaries, prevent corrosion, provide mechanical support, and enable board markings for orientation.

What are the most widely used solder mask materials?

The most widely used solder mask materials are liquid photoimageable solder mask (LPISM) and dry film solder mask. LPISM provides higher resolution while dry film is lower cost.

What is a typical solder mask thickness used on PCBs?

A typical solder mask thickness is 25-38 μm (1 mil). This provides a good balance of properties and processability. Thicker masks are sometimes used when more insulation or ruggedness is needed.

Should solder mask overlap onto component pads?

Yes, it is recommended for solder mask to overlap slightly onto component pads, typically 25-75 μm. This overlap improves alignment tolerance and reduces pad lifting during soldering.

How are the pad openings in solder mask designed?

Pad openings require sufficient clearance from pad edges to account for misalignment. Thermal relief cutouts help avoid mask cracking. Annular web rings around openings prevent solder wicking.

What problems can occur from poor solder mask implementation?

Poor mask can cause short circuits, open circuits, disturbed soldering, mask cracking/delamination, insufficient insulation, and reduced creepage distances. Careful specification is needed to avoid these issues.