What is LPI Solder Mask?

Liquid photoimageable (LPI) solder mask is a photosensitive polymer coating applied to printed circuit boards (PCBs) to protect the copper traces, prevent solder bridges, and provide electrical insulation. LPI solder mask selectively covers the areas of the PCB where components will not be soldered, while leaving the pads and other features exposed for soldering.

The key advantages of LPI solder mask compared to other types include:

Advantage Description
High resolution Allows for finer pitch components and traces
Even coating Provides consistent thickness across the board
Excellent adhesion Bonds well to the copper and laminate surface
Thermal resistance Withstands high temperatures during soldering
Durability Resists scratching and damage during handling

The LPI Solder Mask Application Process

Applying LPI solder mask to a PCB involves several steps to ensure proper coverage and patterning of the coating. The general process flow is as follows:

1. Surface Preparation

Before the solder mask can be applied, the bare PCB must be cleaned and conditioned to promote good adhesion. This typically involves:

  • Scrubbing the copper surfaces to remove oxide
  • Micro-etching to roughen the surface
  • Cleaning to remove any contaminants
  • Pre-baking to drive off moisture

2. Solder Mask Coating

The liquid solder mask is applied to the prepared PCB surface using one of several methods:

Method Description Thickness
Screen printing Mask pushed through a fine mesh screen 0.5-2 mils
Curtain coating PCB passed through a curtain of falling mask 1-4 mils
Spray coating Liquid mask sprayed onto the PCB 0.5-2 mils

The mask must be applied in an even layer of the appropriate thickness, typically 1-2 mils (25-50 microns). Too thin of a coating may not provide adequate coverage, while too thick makes it difficult to properly expose and develop the mask.

3. Pre-baking

After coating, the panels are pre-baked in an oven to partially cure the mask and drive off any remaining solvents. Typical conditions are 80-90°C for 20-30 minutes. The pre-bake solidifies the mask so it can withstand handling during the subsequent steps.

4. Exposure

The coated panels are exposed to UV light through a phototool or photomask. The transparent areas of the phototool allow the UV light to crosslink and harden the solder mask in the desired pattern. The exposure energy, typically 300-500 mJ/cm2, must be carefully controlled to fully cross-link the exposed areas without overcuring.

5. Development

After exposure, the unexposed areas of the mask are selectively removed in an alkaline solution known as the developer. The developer chemically breaks down the non-crosslinked polymer, leaving behind the hardened mask pattern. Spray or immersion developing systems are used, with 1-3% sodium or potassium carbonate solutions at 30-40°C.

Following developing, the panels are rinsed to remove any residual developer and then oven dried.

6. Final Cure

To fully harden the solder mask pattern, the panels undergo a final thermal cure. This can be done in a batch oven or a conveyorized system at temperatures of 130-150°C for 60-90 minutes. The final cure further crosslinks the mask and improves its chemical, heat, and moisture resistance properties.

7. Final Finishing

After cure, the panels may receive additional surface finishing to enhance solderability or prepare them for the next fabrication steps. Options include:

  • HAL (hot air solder leveling) to coat exposed pads with solder
  • ENIG (electroless nickel immersion gold) plating of pads
  • Routing or scoring to remove excess material and singulate boards
  • Electrical test to verify continuity and isolation
  • Visual inspection for coating defects or misregistration

Controlling the LPI Solder Mask Process

Achieving a high-quality and reliable LPI solder mask coating requires careful control of multiple process parameters. Some of the key factors include:

Material Selection

LPI solder masks are formulated from epoxy or acrylic monomers, photoimageable compounds, fillers, and pigments. The exact composition determines properties like:

  • Viscosity and flow behavior
  • UV sensitivity and resolution
  • Adhesion to copper and laminate
  • Thermal, chemical and moisture resistance
  • Appearance and color

Selecting the appropriate mask formulation requires balancing the performance requirements with the processing parameters to achieve the desired results.

Equipment Set-up

The coating, exposure, and developing equipment must be properly set up and calibrated to ensure consistent results. This includes:

  • Controlling the coating viscosity, thickness, and uniformity
  • Setting the exposure energy and time based on the phototool
  • Maintaining the developer concentration, temperature, and spray pressure
  • Verifying the oven profiles for pre-bake and final cure

Regular preventative maintenance, including cleaning and replacement of consumable items, is also critical for consistent performance.

Process Control

Multiple process variables must be monitored and controlled to achieve the desired solder mask properties. Some of the key parameters include:

Parameter Typical Range Effect of Variation
Coating thickness 1-2 mils Resolution, coverage, adhesion
Pre-bake conditions 80-90°C, 20-30 min Solvent removal, handling strength
Exposure energy 300-500 mJ/cm2 Resolution, surface tackiness
Developer concentration 1-3% Developing speed, residue
Developer temperature 30-40°C Developing speed, surface roughness
Final cure temperature 130-150°C Chemical & heat resistance, adhesion
Final cure time 60-90 min Degree of crosslinking, brittleness

Establishing the appropriate operating ranges and monitoring the process variables with SPC (statistical process control) methods allows for early detection and correction of issues.

Troubleshooting Common Defects

Even with a well-controlled process, various defects may occur that impact the quality and reliability of the solder mask. Some common issues and their potential causes include:

Defect Potential Causes
Poor adhesion Inadequate surface prep, underexposure, undercure
Incomplete developing Low exposure, weak or cold developer, insufficient developing time
Rough or pitted surface High exposure energy, strong or hot developer
Poor resolution Thick coating, low exposure, phototool issues
Discoloration or yellowing Overcure, UV degradation, chemical attack

Identifying the root cause of defects often requires a systematic approach of isolating the variables, performing experiments, and analyzing the results. Corrective actions may involve adjusting the process settings, modifying the materials, or improving the maintenance procedures.

Evaluating LPI Solder Mask Performance

The quality and reliability of an LPI solder mask coating are ultimately determined by its ability to meet the performance requirements in the end-use environment. Some of the key properties and test methods used to evaluate LPI masks include:


Solder mask adhesion to the copper and laminate is critical for long-term reliability, particularly under thermal cycling and mechanical stress. Adhesion is typically evaluated by cross-hatch tape testing per IPC-TM-650 2.4.1 or peel testing per IPC-TM-650 2.4.8.

Insulation Resistance

The solder mask must provide adequate electrical insulation between conductors, even under high humidity conditions. Insulation resistance is measured per IPC-TM-650 2.6.3, with a minimum value of 500 MΩ required after exposure to 90% RH at 35°C for 7 days.


The exposed pads and holes must be solderable without dewetting or non-wetting. Solderability is evaluated per IPC J-STD-003, with a minimum of 95% coverage required after aging for 8 hours at 155°C.


The cured solder mask must be hard enough to resist scratching and abrasion during handling and assembly. Hardness is typically measured using the pencil hardness test per IPC-TM-650 2.4.27, with a minimum value of 6H required.


For applications requiring UL listing, the solder mask must meet the flammability requirements of UL 94V-0. This involves self-extinguishing within 10 seconds after removal of the flame, with no flaming drips allowed.


The solder mask must be compatible with the other materials and processes used in PCB fabrication and assembly. This includes resistance to molten solder, fluxes, cleaning agents, and conformal coatings. Compatibility testing is often performed using production processes and environments to verify performance.

By carefully controlling the application process and verifying the performance through testing, LPI solder mask can provide reliable protection and insulation for a wide range of PCB applications.


What is the difference between LPI and dry film solder mask?

LPI solder mask is applied as a liquid and then patterned using photolithography, while dry film is a solid sheet that is laminated to the PCB and then patterned. LPI offers higher resolution and finer pitch capability compared to dry film.

Can LPI solder mask be applied over HASL or OSP coatings?

No, LPI solder mask must be applied directly to the bare copper surfaces of the PCB. HASL or OSP coatings are applied after the solder mask to protect the exposed pads from oxidation.

What is the typical curing temperature and time for LPI solder mask?

LPI solder mask is typically cured at 130-150°C for 60-90 minutes, depending on the specific formulation and performance requirements. The curing step crosslinks the polymer and enhances its final properties.

How does the solder mask color affect the processing parameters?

Different pigments and dyes used to create colored solder masks can affect the UV sensitivity and exposure time. In general, darker colors like blue, black, and green require longer exposure times compared to lighter colors like white, yellow, and clear.

What is the shelf life of LPI solder mask?

Most LPI solder mask formulations have a shelf life of 6-12 months when stored at room temperature in sealed containers. The mask may start to gel or increase in viscosity over time, which can affect its processing and final properties. It is important to use the mask before its expiration date and to follow the manufacturer’s storage and handling guidelines.

Categories: PCBA


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