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The Difference and Role of PCB Paste Mask and Solder Mask


Printed circuit boards integrate a wide array of materials and coatings beyond just traces and laminates. Two of the most important additional PCB layers are paste mask and solder mask. While their names sound similar, these materials serve distinct roles in the PCB fabrication and assembly processes.

This article provides an in-depth look at PCB paste mask and solder mask including:

  • The composition and properties of each material type
  • Key differences between paste mask and solder mask
  • The roles and purposes they serve in PCB manufacturing
  • Typical application and patterning methods
  • New developments in these materials
  • Examples illustrating paste mask and solder mask usage
  • Guidelines for designing and applying these layers
  • Common defects to avoid
  • FAQs about these critical PCB coatings

Developing a strong understanding of paste mask and solder mask enables electrical engineers to apply them optimally during design and production to improve manufacturing yields, long-term reliability and product quality.

PCB Paste Mask Overview

Low Temperature Solder Paste
Low Temperature Solder Paste

Paste mask, also referred to as solder paste mask or solder resist, is a temporary coating used during the SMT assembly process to facilitate solder paste application. Key properties include:


  • Polymer materials like epoxy or acrylic resins
  • Solvent carriers for deposition
  • Filler particles for rheology

Key Characteristics

  • Excellent solder paste release and wetting
  • Solder bleed resistance during reflow
  • Easy stripping and cleaning after soldering

Patterning Methods

  • Liquid photoimageable mask exposed via lithography
  • Laser direct imaging of dry film masks
  • Screen printing of liquid masks

Paste masks provide a low-cost, processing-friendly material optimized for the demands of high-yield SMT manufacturing.

Solder Mask Overview

Solder mask serves as a permanent protective coating on PCBs. Typical properties:


  • Epoxy or acrylic polymers for adhesion, toughness
  • Solvents carriers to enable coating
  • Fillers like silica for rheological properties

Key Characteristics

  • Electrical insulation and corrosion resistance
  • Repairability and chemical compatibility
  • Soldering heat resistance
  • Color options from green to black to white

Patterning Methods

  • Liquid photoimageable solder mask is dominant
  • Also dry film laminates and screen printed masks

Solder masks safeguard PCBs throughout long-term use across demanding operating environments.

Key Differences Between Paste Mask and Solder Mask

Solder paste stencil frame

While both materials facilitate soldering, there are significant differences:

ParameterPaste MaskSolder Mask
PurposeDefine solder paste regionsLong-term protective coating
LocationsOnly on pads/landsAcross conductors and board surface
Typical MaterialsWater soluble epoxiesSolvent resistant epoxies
Deposition MethodLamination, screen printingLiquid coating, curtain coating
Patterning ProcessPhoto, laser imagingPhotoimaging
Soldering Process RoleConfine pasteProtect underlying features
After SolderingRemoved by cleaningRemains as permanent coating
Reliability ConsiderationsMinimize solder ballsWithstand environment; prevent corrosion and dendrites

These distinct roles mandate different material properties and processes for optimal results.

The Role and Purpose of PCB Paste Mask

Paste masks provide several key functions:

Defines Solder Paste Regions

  • Mask openings expose pads for paste printing
  • Eliminates solder beads between pads

Facilitates Consistent Paste Deposit

  • Apertures act as stencil for uniform paste release
  • First article inspection confirms coverage

Confines Paste During Reflow

  • Prevents solder spreading across board
  • Reduces bridging and solder balls

Enables Solder Paste Recovery

  • Easily wipe and clean after reflow
  • Retrieving unused paste minimizes waste

Protects Board During Soldering

  • Mask prevents solder adhering where unwanted
  • Guards against pad etching or lifting during reflow process

Thoughtful paste mask design is crucial for defect-free SMT assembly.

Solder Mask Key Roles and Functions

Conversely, solder masks provide long-term protection:

Electrical Insulation

  • Isolates conductors from unintended connections
  • Prevents short circuits across board surface

Corrosion Resistance

  • Barrier against environmental contaminants
  • Guards against tin whiskers, dendritic growth

Mechanical Protection

  • Cushions board against impacts
  • Stabilizes conductors against vibration loads

Soldering Heat Resistance

  • Withstands repeated soldering and desoldering
  • Prevents pad lifting or separation


  • Mask color contrasts with metal
  • Allows component designators and identifiers


  • Color coats board
  • Branding or camouflage options

Robust solder masks are integral for PCB durability across product lifetimes.

Typical Paste Mask Application and Patterning

Applying paste mask requires compatible processes:

Liquid Photoimageable Mask

  • Mask deposited by curtain coating
  • Dried then exposed through lithography artwork
  • Developed to reveal solder paste regions

Laser Direct Imaging (LDI)

  • Dry film laminate applied
  • Laser scans image directly based on CAD
  • Etchant dissolves exposed mask

Screen Printing

  • Screens transfer mask material
  • Print, dry, clean, inspect steps
  • Well-suited for high volume

Tenting Vias

  • Mask coats over vias
  • Prevents solder wicking into holes

Automated optical inspection after patterning validates paste mask registration and expected openings.

Typical Solder Mask Application and Patterning

Solder mask requires similar steps:

Liquid Photoimageable Mask

  • Deposited by curtain coating
  • Dried then exposed through artwork
  • Developed then cured at elevated temperature

Laser Direct Imaging (LDI)

  • Same dry film process but with different dedicated material
  • Laser defined openings based on CAD data

Screen Printing

  • Screens transfer solder mask ink
  • Used for high volume or simple boards

Covering Copper

  • Mask coats over remaining exposed copper
  • Windows opened over connectors, testpoints etc.

The solder mask process is refined for smooth, complete coverage and adhesion.

Recent Advances in Paste Masks and Solder Masks

Developments in materials and processing aim to enhance performance:

Laser Ablatable Solder Masks

  • Excimer laser removes mask in precise locations
  • No additional coating/imaging steps

Flexible Solder Masks

  • Withstand repeated bending and flexing motions
  • Enable flexible PCBs

Reworkable Masks

  • Designed for selective removal
  • Replace components without full mask strip

Thermally Conductive Masks

  • Filled epoxies dissipate heat
  • Aid thermal management

Hydrophobic Masks

  • Repel water, moisture and fluids
  • Improve reliability

High Aspect Ratio Masks

  • Allow coating high topography and cavities
  • Protect complex surface mount parts

Electrically Insulating Anisotropic Pastes

  • Prevent solder bridging
  • Redirects current flow from paste

Innovation continues expanding capabilities.

Paste Mask Design Guidelines

To maximize manufacturing yield and quality solder joints, engineers should:

  • Provide sufficient registration margins between paste mask and pads
  • Account for potential mask misalignment and smearing
  • Surround pads with mask to limit solder spreading
  • Tent vias to prevent solder wicking
  • Include generous fillets spacing pads
  • Keep openings large enough for even paste release
  • Follow manufacturer design rules for minimum apertures
  • Verify adequate paste opening coverage through inspection
  • Test stripability to avoid pad lifting

Thoughtful paste mask layout prevents defects for optimized SMT assembly.

Solder Mask Design Guidelines

For robust solder mask performance:

  • Maintain adequate overlap over traces and spacing from pads
  • Account for misalignment margins in design rules
  • Include generous fillets spacing between traces
  • Surround exposed copper with mask to prevent oxidation
  • Cover all unused board surface area
  • Mask bottom side if components mounted on both sides
  • Leave openings only where required like connectors
  • Follow minimum trace/space rules for coating coverage
  • Test final adhesion, hardness, and dielectric strength

Careful solder mask design ensures complete insulation and protection.

Common Paste Mask Defects

Some potential paste mask flaws to avoid:


  • Apertures shift from pads
  • Causes missing or blocked solder paste deposition

Undersized Openings


  • Mask material partially covers pads
  • Hinders solder wetting and adhesion


  • Mask lifts from board during soldering
  • Allows solder leaching under mask

Poor Strippability

  • Mask leaves residue after cleaning
  • Contaminates pads prior to next process steps

Following design guidelines and inspection helps prevent defects.

Common Solder Mask Defects

And some potential solder mask flaws:

Insufficient Overlap

  • Exposes copper traces to corrosion and contamination

Excessive Spacing

  • Allows solder to bridge between features
  • Reduces insulation resistance


  • Opens up keepout regions to solder leaching


  • Creates uncoated regions without insulation
  • Allows traces to lift during soldering

Cracking or Peeling

  • Permits moisture ingress degrading insulation


  • Aesthetic issue suggesting material degradation

Proper process controlsCoupled with design rule checks minimizes defects.

Paste Mask and Solder Mask Example Applications

Here are some examples highlighting use cases:

Sensors Product

  • Epoxy-based black solder mask provides electrical insulation and water resistance for reliability
  • Acrylic paste mask on pads prevents solder bridging between fine pitch leads

Automotive Control Module

  • High temperature solder mask withstands heat cycling in engine bay
  • LPI paste printing on top side opens pads for component placement

HDI Telecom PCB

  • Photoimageable solder mask coats 6 mil traces spacing microvias
  • Tented vias prevent solder wicking into holes

Large LED Video Display

  • Screen printed solder mask quickly coats boards in high volume
  • matching black paste mask provides cosmetic surface

Medical Diagnostic Kit

  • Flexible, biocompatible solder mask enables repeated bending
  • Water-soluble paste mask simplifies post-reflow cleaning

Proper selection and integration secures performance.

Frequently Asked Questions

Here are some common questions regarding solder mask and paste mask:

Q: What are some typical minimum clearance gaps for paste mask apertures?

A minimum 2-3 mil overlap onto pads or clearance between openings is typical. High tolerance processes allow smaller 1-2 mil gaps.

Q: How do you determine the right solder mask overlap over traces?

The overlap margin is dictated by design rules, with 3-5 mils typical. Mask misalignment tolerance must also be considered.

Q: What are key parameters used to specify solder mask properties?

Adhesion strength, dielectric breakdown voltage, surface insulation resistance, thermal conductivity, and flammability are typical specifications.

Q: What are some methods to improve solder mask adhesion?

Surface treatments, cleanliness, proper curing, thermal cycling testing, choosing compatible mask and substrate materials, and roughening surfaces help adhesion.

Q: How does solder mask color impact manufacturing?

Light backgrounds like white make inspection easier. Dark masks can require longer exposure times and risks lower cure depth.


Although their names sound similar, paste mask and solder mask provide distinct capabilities essential to PCB fabrication and reliability. Leveraging the in-depth overview provided in this article, PCB designers can apply these materials optimally to secure manufacturing yields while enhancing circuit protection, insulation and product lifetimes.




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