How to Spray Photo Resist on PCB

Photo resist application is a critical process in printed circuit board (PCB) fabrication. The photo resist layer masks portions of the copper layer during etching, defining the conductive traces and features that form the circuit board layout. Applying a high quality photo resist coating is essential for achieving the dimensional accuracy and reliability required of PCBs in electronic products.

Spray coating methods provide an efficient means of rapidly and uniformly coating photo resist films onto PCB panels. This article will overview key considerations when setting up a spray resist process, guiding readers through best practices from lab-scale development to high volume manufacturing.

Photo Resist Coating Methods

There are three primary approaches for applying photo resist:

Spray Coating - Liquid resist formulation is atomized into an aerosol spray and deposited onto the board surface.

Electrodeposition - An electric field moves resist molecules suspended in a liquid solution onto the board.

Laminating Films - Dry film resist laminates are applied under hot rollers.

Spraying using specially designed equipment provides strong production advantages:

• Fast, continuous processing of panels
• Uniform resist films within tight tolerances
• Adjustable coating thickness capability
• High material transfer efficiency minimizes waste
• Lower initial capital costs than electrodeposition

For these reasons, spray coating warrants consideration as resist application technique.

Photo Resist Formulations

Modern sprayable photo resists leverage advanced chemical formulations to best meet PCB manufacturing needs:

Solvent Systems - The base resist polymer is dissolved into a solvent carrier, allowing low viscosity flow through the spray nozzle.

Functional Additives - Compounds are added to tune resist properties like adhesion strength, flexibility, etch speed, stripping ease, and shelf life.

Sensitizers - Photo-active ingredients absorb light during exposure to initiate the chemical reactions rendering resist soluble or insoluble.

Particulate Filters - Micron-scale filters remove resist particulates to prevent nozzle clogging.

Environmental Requirements - Modern systems focus on water-based, non-flammable materials with low emissions and waste.

Fine tuning resist chemistry enables balancing the capillary flow needed for thin coatings with the viscosity required for uniform leveling without defects.

Spray Process Design Considerations

When designing a spray coating process, key aspects to consider are:

System Setup

• Spray Nozzle - The nozzle determines droplet size distribution and spray area. Fine atomization is key.
• Fluid Handling - Pumps, valves, regulators, and filters feed resist from drums into lines leading up to spray heads.
• Fume Extractor - A fume hood with exhaust duct pulls out resist volatile organics evaporated in spray booths.
• Panel Fixtures - Boards must be held flat in the spray zone allowing resist to evenly coat top and bottom sides.

Programmable Film Thickness Capability

• Resist thickness directly sets copper undercut during etching, determining finer etched feature sizes achievable.
• Electric sensors provide real-time measurement enabling automatic feedback control of thickness.
• Target thickness capabilities down to 8 mils with tolerances of +/- 10%.

Continuous In-Line Processing

• Pre-clean, spray resist coating, drying, and post apply bake stations compose an automated cell.
• Indexing conveyors seamlessly move panels between stations minimizing human involvement.
• For high volume facilities, multiple replicated cells enable round-the-clock production.

Environmental Controls

• Humidity control prevents absorbed surface moisture preventing uniform resist spread.
• Air flow management ensures cleanliness and directs overspray away from critical machine areas.
• Waste disposal processes collect used thinner and dissolved resist for treatment.
• Emergency response systems protect against chemical spills and fire hazards.

Carefully integrating and optimizing these capabilities enables creating scalable, high yield spray coating processes for applying high performance resist films.

Facility Infrastructure Requirements

Constructing a capable, robust spray coating facility demands significant infrastructure:

1. Clean Room Construction

PCB fabrication facilities require Class 1000 (ISO Class 6) or better clean rooms:

Maintaining cleanliness is crucial for defect-free resist application onto boards.

2. Chemical Storage and Supply

Drums of liquid photo resist are stored in a vented cabinet inside a bermed room, providing containment against spills and fumes release. Automatic pumping and monitoring systems deliver resist on demand to the coating machines.

Solvent and waste storage tanks also require strict protocols to limit environmental and explosive risks.

3. Exhaust Ventilation

Organic solvent fumes from open baths warrant exhaust ventilation 4x stronger than the supply air volume in coating process rooms. Belt and exhaust fans avoid spark hazards while ducting air outside.

Makeup air intakes compensate for exhausted volumes, enabling steady pressurization.

4. Emergency Preparedness

Facilities necessitate:

• Chemical spill kits
• Eye wash stations
• Safety showers
• Solvent and acid storage cabinet spill containment
• Fire detection and suppression systems

Together with established hazard response protocols, these measures minimize safety risks.

Key Process Steps and Parameters

Executing a precision photo resist coating on a panel involves carefully controlled process steps:

1. Panel Preparation

• Degreasing - Alkaline cleaners strip organic and flux residues from prior processing.
• Etching - Microetch to roughen surface promoting mechanical resist adhesion.
• Desmear - Swell and etch resin smear from drilling to open coated through holes for plating conductivity.

2. Resist Application

• Preheat – 60-100°C bake evaporates moisture from boards critical for adhesion.
• Coating - Automatic horizontal bottom-board transport through enclosed spray tunnels ensures even resist distribution.
• Drying - 140-160°C convection ovens remove solvent carriers leaving solid resist film.
• Cool Down - Ambient air cooling solidifies resist prior to handling to prevent board warpage.
• Post Apply Bake - 190-200°C bakes further drive out residual solvents while enhancing even resist distribution across board surface.

3. Quality Testing

• Record Panel Details – operator logs of machine parameters and resist lot provides traceability.
• Visual Inspection – experts examine boards under bright lighting for spray uniformity.
• Coating Weight - gravimetric analysis confirms applied thickness vs. formula.
• Photo Microscope Review - SEM cross-sectioning validates resist penetrated through the densest through holes for subsequent plating.

4. Exposure and Developing – With resin coating completed, subsequent processes transfer the desired circuit pattern.

Maintaining consistency across these processing steps enables achieving the stringent quality and dimensional accuracy PCB fabricators require.

Troubleshooting and Defect Mitigation

Despite best efforts, photo resist spray processes still incur defects requiring remediation:

Addressing these common issues through tuning of the overall process enables maximizing production yield and product quality.

Conformal Coating Integration

For boards that require protective conformal coating after assembly, the choice of photo resist has implications on compatibility and process integrity. Conformal coatings options include:

• Acrylic
• Silicone
• Urethane resin
• Parylene

Key considerations for integration include:

• Temperature resistance of photo resist formulations
• Outgassing and surface cure inhibition
• Relative surface energy affecting