PCB Solder mask

Posted by

A solder mask is a thin layer of polymer that is applied to the surface of a printed circuit board (PCB) to protect the copper traces from oxidation and prevent solder bridges from forming between closely spaced solder pads during the soldering process. Solder mask defines the solderable and non-solderable areas on the PCB surface. It is one of the final steps in manufacturing a printed circuit board.

The solder mask serves several important functions:

  • Prevents solder bridging – The solder mask acts as a physical barrier that prevents solder from bridging between adjacent pads during assembly. This is critical for densely packed surface mount components.
  • Protects against oxidation – The solder mask protects the exposed copper from oxidizing. Oxidation of the copper traces would reduce solderability.
  • Insulates conductors – The solder mask provides electrical insulation between adjacent conductors. This helps prevent short circuits.
  • Improves abrasion resistance – The mask protects the PCB traces from mechanical wear and abrasion.
  • Improves chemical resistance – It shields the PCB from corrosion and contamination during assembly and use.
  • Aesthetics – Provides a consistent color and appearance to the PCB. The solder mask color is often used to denote the circuit board function or identify the manufacturer.

Solder Mask Materials

The most common materials used for PCB solder mask include:

  • Liquid photoimageable solder mask (LPSM) – The predominant solder mask technology, LPSMs are UV sensitive polymers that are photo-patterned to create the solder mask openings. Popular LPSM materials include epoxy, polyimide, and silicone based formulations.
  • Dry film solder mask – Dry film masks consist of a photoimageable polymer laminated onto a polyester carrier film. The mask artwork is transferred by photocuring. Dry films offer high resolution but tend to be more expensive.
  • Screen printed solder mask – A liquid ink is screen printed through a stencil to form the solder mask pattern. Screen printing allows lower volume custom masks but offers lower resolution than photolithographic methods.
  • UV curable inkjet solder mask – Solder mask layers can be printed using inkjet deposition of UV-curable polymers. Provides design flexibility but lower throughput than traditional methods.

Key properties that influence solder mask selection include:

  • Resolution and ability to produce fine openings and gaps
  • Thermal stability and UL ratings
  • Adhesion to PCB substrate
  • Chemical resistance
  • Abrasion and scratch resistance
  • Dielectric constant and insulation properties
  • CTE match to substrate
  • Ease and cost of application

Solder Mask Application Process

The main steps in applying a solder mask layer are:

1. Surface Preparation

The PCB surface is cleaned to remove any oil, grease, or particulates. A chemical process such as plasma etching may be used to oxidize the copper and improve mask adhesion.

2. Liquid Photoimageable Mask Application

The board is coated with the liquid solder mask material, typically by curtain coating, spray coating, or screen printing. The mask layer is deposited at a target thickness of around 25-75 μm.

3. Mask Exposure

The solder mask coating is exposed to UV light through a phototool film or mask plate. The mask openings solidify while the unexposed areas remain solvent sensitive.

4. Developing

The unpolymerized mask material is washed away in a chemical developer, leaving behind the cured mask pattern. A secondary plasma etch may be used.

5. Curing

The PCB undergoes a final UV or thermal curing process to fully harden the solder mask material. Common cure temperatures range from 140°C to over 200°C.

6. Automated Optical Inspection

Cameras and image processing algorithms inspect the solder mask for any defects or irregularities. This helps ensure mask quality and avoid costly rework.

7. Touch-up and Repair

Minor solder mask defects can be manually touched up using ink pens or brushes. Larger defects may require mask stripping and reapplication.

Solder Mask Design Guidelines

Certain design rules should be followed to successfully implement a solder mask layer:

  • Minimum web width – Small openings in the solder mask require a minimum web width (overhang) of mask material between the pad edge and opening. This prevents mask undercutting.
  • Mask expansion – Pads are typically smaller than the mask openings, with a designed expansion distance or pullback margin. This ensures the pads are fully exposed after mask processing.
  • Clearance from pads – Adequate mask clearance must exist from pad peripheries to prevent mask slivers which impair solderability. Clearances vary based on mask material.
  • Via protection – Vias can optionally be tented (covered) by the mask to prevent solder wicking into the hole.
  • Fiducials – Alignment marks should be placed in the mask layer to facilitate solder mask alignment.
  • Panelization tabs – Special mask openings may be required to expose tab routing connections.

By following these guidelines, PCB designers can ensure their boards are compatible with the solder masking process.

Solder Mask Defects

Common solder mask defects include:

  • Voids – Empty areas or holes in the solder mask expose unprotected copper. Voids can lead to traces oxidizing or solder bridges forming.
  • Mask slivers – Small remnants of mask material partially covering component pads, which impair solderability. Caused by inadequate clearance between pads and mask openings.
  • Scratches – Abrasion damage to the mask surface which impacts insulation and leaves copper exposed. Scratches occur in handling or through contact with abrasive components.
  • Delamination – Detachment or lifting of the mask layer from the underlying PCB laminate. Results from poor adhesion or thermal stress.
  • Tenting – Improper formation of the mask over pad openings. Partial tenting leaves a thin web over pads while complete tenting fully covers the pad.
  • Misregistration – Incorrect alignment between the solder mask openings and PCB pads, resulting in partial pad coverage or mask shifting.
  • Pinholes – Tiny holes through the mask exposing narrow regions of copper. Pinholes can lead to dendritic plating growth during electroless nickel/immersion gold (ENIG) processing.

Solder Mask Repair Techniques

Several methods exist for reworking defective solder mask areas:

  • Manual touch-up – Mask pens or brushes can fill voids and scratches. Touch-up provides a quick repair solution but may lack durability.
  • Scraping – A knife or razor blade is used to scrape away mask slivers covering component pads, restoring solderability.
  • Sanding – Light abrasion with fine sandpaper can remove mask tenting and surface defects.
  • Mask stripping – Complete removal of the old mask using chemical or plasma stripping, followed by re-application. Provides the most robust repair result.
  • Inkjet printing – Scanning the PCB and then inkjet printing new mask material provides selective repair capability.

The chosen repair technique depends on the mask defect type, location, and required quality. Many minor mask issues can be rectified through selective touch-up.

Solder Mask Color Options

Solder mask is available in a spectrum of colors beyond the traditional green. Common colors include:

  • Green – The most prevalent mask color. Provides good contrast against copper.
  • Blue – Alternative high contrast color preferred by some manufacturers. Blue solder mask is becoming increasingly common.
  • Red – Used to designate high voltage boards or identify power circuitry.
  • Yellow – Improves solder mask visibility against white substrates. Often used for RF/microwave boards.
  • Black – Provides an elegant appearance, but low contrast against PCB substrate.
  • White – Gives high visual contrast for automated optical inspection.
  • Clear – Transparent or translucent mask allows the copper to show through. More difficult to inspect visually.
  • Custom colors – Special colors and gradients can be used for branding or unique cosmetic effects.

The mask color is typically specified by the PCB designer based on circuit function, inspection considerations, cosmetic preference, and manufacturing capabilities.

Solder Mask Colorshift

A phenomenon known as colorshift can occur with some solder mask types exposed to high temperatures. The mask material undergoes a chemical change, causing the color to fade or shift to a different hue.

For example, green solder mask may turn more bluish or yellowish after repeated thermal cycling to high temperatures. This is often observed on boards with large ground planes acting as heat sinks.

Colorshift occurs because most mask pigments are organic compounds that break down above 150°C. Inorganic pigments can provide more thermally stable colors.

To avoid colorshift, the mask material and processing should be selected appropriately for the application temperature range. When color stability up to 150°C is needed, high temperature solder masks or ceramic filled formulations are preferred.

Solder Mask and Thermal Management

Since solder mask is an electrical insulator, it affects heat transfer on printed circuit boards. This impacts the thermal management design.

Key considerations when using solder mask for thermal control include:

  • Low mask thickness – Thinner mask layers have lower thermal resistance allowing better heat spreading from components. Typical mask thickness is 25-75μm.
  • High dielectric strength – A mask with higher breakdown voltage withstands greater potential differences between conductors. This allows minimizing mask web widths.
  • Thermal conductivity – Formulations with ceramic filler particles exhibit higher thermal conductivity than unfilled polymers. This improves in-plane conduction.
  • Unmasked ground planes – Leaving internal ground planes largely unmasked enhances heat spreading and dissipation through the board laminate.
  • Selective mask openings – Strategic openings in the mask over ground planes or thermal pads reduces interface thermal resistance.
  • Mask-defined outlines – The solder mask can be used to outline heat dissipating copper shapes instead of etching away unneeded copper.

With careful design, the solder mask layer can be optimized to help remove heat from circuit boards in end applications demanding high power or thermal cycling.

Solder Mask for RF and High Frequency PCBs

For radio frequency, microwave, and other high speed PCBs, the solder mask dielectric properties become especially important.

Key considerations for solder mask materials on RF/microwave boards include:

  • Low dielectric constant – Minimizes delay and signal loss for traces passing under solder mask. Values under 3.5 are desirable.
  • Tight dielectric constant tolerance – Consistent ER allows matching controlled impedances across the board during layout.
  • Low loss tangent – Important for preventing leakage and attenuation of high frequency signals. Look for loss tangent under 0.02.
  • High resistivity – Prevents conductance losses from the mask layer. Aims for resistivity above 10^15 ohm-cm.
  • Low moisture absorption – Limits variation in dielectric constant from humidity influence. Target moisture absorption below 2%.
  • Thermal stability – Maintains stable electrical parameters through thermal excursions and avoids colorshift.
  • Smooth surface – Reduces scattering losses. Liquid photoimageable masks offer flatter surface profiles than dry films.

Mask materials for RF designs are specially engineered for electrical performance rather than just physical protection. Working closely with the PCB laminate supplier ensures a compatible system.

Laser Ablation of Solder Masks

Laser direct structuring (LDS) is a process for selectively ablating or removing the solder mask layer to create openings through laser micromachining.

This allows adding exposed metalized areas on the PCB after fabrication. Applications include:

  • Prototyping – Rapidly iterate on a board by laser machining new openings to test component placements or routing changes.
  • Late stage circuit modifications – Laser ablation can remove solder mask in specific locations to patch or optimize a design.
  • Rework and edit – Removing mask tenting or fixing registration issues by ablating misplaced mask openings.
  • LDS circuit formation – Using laser ablation to physically construct conductive pathways.
  • Depaneling – Lasers cut through tab routings to singulate boards.
  • RF tuning – Fine tune microwave filters and antennas by selective mask removal over resonators and ground planes.
  • Thermal dissipation – Opening windows in the mask over heated components improves heat transfer.

Laser mask ablation provides a versatile tool for rapid PCB editing and modification from prototype through production. The process is mask material dependent and works best with polymer-based solder masks.

Conductive Solder Masks

Electrically functional solder masks represent an emerging technology trend. Conductive masks act as an extension of the PCB trace layer.

Types of conductive solder mask include:

  • Intrinsically conductive polymers (ICP) – Doping the mask material with conductive fillers like carbon or silver nanoparticles produces uniform sheet resistance across the mask layer.
  • Patterned conductive masks – Use a subtractive process to selectively remove insulation and form circuit traces in the mask layer.
  • Embedded conductive particle masks – Localized placement of conductive particles creates discrete conductive traces surrounded by insulating mask material.

Key applications for conductive solder masks include:

  • RF grounding – Extend ground planes seamlessly covering both board and components.
  • EMI shielding – Replace separate EMI gaskets or clips with integrated shielding.
  • Thermal dissipation – Expand heat spreading ground planes beneath components.
  • High density routing – Embedded horizontal traces supplement multilayer boards.
  • Fine pitch components – Allow tighter pitch than possible on surface laminate.
  • Repair – Bridge broken or damaged traces.
  • Sensors – Print force, temperature, or strain sensitive traces.

Continued development of novel functional mask materials will enable integration of more electrical functionality into this robust top layer coating.

Solder Mask Trends and Developments

Key trends shaping solder mask technology evolution include:

  • Laser direct imaging (LDI) – Mask exposure using laser light sources improves resolution and enables rapid changeovers.
  • Light emitting solder masks – Adding luminescent materials creates masks that glow when illuminated with UV light. Assists automated optical inspection.
  • LED curing – Next generation UV LED light sources provide faster mask curing with lower energy consumption.
  • Low temperature cure – Enable solder masking sensitive substrates like aluminum or paper-based flex circuits.
  • Improved thermal performance – Higher temperature ratings and thermal conductivity to manage heat dissipation from high power PCBs.
  • Halogen-free formulations – More environmentally friendly masks through eliminating brominated flame retardants while maintaining thermal and electrical performance.
  • Flexible solder masks – Better elongation, bend radius, and adhesion for conformal coating onto flexible PCBs.
  • Integrated processing – Solder mask applied in combo with other functional coatings like legend, EMI shielding, or conformal coating.

Frequently Asked Questions

1. What is the typical thickness of a solder mask layer?

The standard thickness for solder mask layers is around 25-75 microns (1-3 mils). Thicker mask layers give better insulation and protection but increase manufacturing cost. Thinner masks down to 15 microns are sometimes used for high density PCBs where minimizing web widths is critical.

2. How are openings formed in the solder mask layer?

Openings in photosensitive liquid solder masks are defined using photolithography. A phototool film or mask plate blocks UV light during exposure to leave uncured areas that are washed away during developing. For screen printed masks, the openings are formed directly by the masking stencil.

3. What are common reasons for solder mask peeling or delamination?

Delamination of the solder mask is typically caused by poor adhesion between the mask layer and the PCB surface, often due to inadequate surface preparation. Other factors include thermal stress, chemical contamination, mask cure issues, and moisture ingress weakening the interface over time.

4. How does solder mask misregistration occur and how is it avoided?

Misregistration happens when the applied solder mask layer is incorrectly aligned to the underlying PCB pads, often due to inaccurate fiducials or equipment errors during masking. Tight process controls, proper fixturing, accurate fiducials, and inspection testing helps prevent pattern misalignment.

5. Can solder mask be applied to only one side of a PCB?

Yes, it is possible to solder mask just a single side of a PCB. This is often done to reduce cost on double sided boards where only the bottom side containing SMT lands needs solder mask definition. The top side copper is then left exposed. Selective single sided masking requires special fixturing during the coating process.