Introduction to Spray Photoresist

Spray photoresist is a crucial step in the process of creating printed circuit boards (PCBs). Photoresist is a light-sensitive material that is applied to the surface of a PCB before the etching process. When exposed to light, the photoresist hardens and protects the covered areas of the PCB from being etched away. This allows for the creation of precise circuit patterns on the board.

Spray photoresist offers several advantages over other methods of applying photoresist, such as dip coating or spin coating. It provides a more even and consistent coating, and it is faster and easier to apply. Additionally, spray photoresist can be used on larger PCBs that may not fit in a dip tank or spin coater.

In this article, we will dive into the details of how to spray photoresist on a PCB, including the necessary materials and equipment, the step-by-step process, and tips for achieving the best results.

Materials and Equipment Needed

Before you begin spraying photoresist on your PCB, you will need to gather the following materials and equipment:

• PCB board
• Spray photoresist
• Compressed air source (such as an air compressor)
• Spray gun or airbrush
• Protective gear (gloves, safety glasses, respirator)
• Cleaning supplies (isopropyl alcohol, lint-free wipes)
• UV exposure unit
• Developing solution
• Etching solution

It’s important to choose a high-quality spray photoresist that is compatible with your PCB Material and the etching process you will be using. Some common types of spray photoresist include positive and negative photoresists, as well as liquid and dry film photoresists.

Preparing the PCB for Spraying

Before you can spray the photoresist onto your PCB, you need to ensure that the board is clean and free of any contaminants that could affect the adhesion of the photoresist. Here are the steps to prepare your PCB:

1. Clean the PCB with isopropyl alcohol and a lint-free wipe to remove any dirt, grease, or oils from the surface.
2. Dry the PCB thoroughly with compressed air or let it air dry completely.
3. Inspect the PCB for any defects or irregularities that could affect the spraying process. If necessary, sand down any rough spots or fill in any holes or gaps with a PCB Repair kit.
4. If your PCB has any components or solder mask already in place, protect them with tape or a stencil before spraying the photoresist.

Setting Up the Spraying Equipment

Once your PCB is prepared, it’s time to set up your spraying equipment. Here are the steps to follow:

1. Connect your spray gun or airbrush to your compressed air source, making sure that the air pressure is set to the recommended level for your specific photoresist and equipment.
2. Pour the photoresist into the spray gun or airbrush, following the manufacturer’s instructions for filling and handling.
3. Adjust the nozzle and flow settings on your spray gun or airbrush to achieve the desired spray pattern and coverage.
4. Test the spray on a piece of scrap material to ensure that the photoresist is flowing smoothly and evenly.

Spraying the Photoresist

Now that your equipment is set up and your PCB is prepared, you’re ready to start spraying the photoresist. Follow these steps:

1. Put on your protective gear, including gloves, safety glasses, and a respirator.
2. Place your PCB on a flat, stable surface in a well-ventilated area.
3. Hold the spray gun or airbrush perpendicular to the PCB surface, about 6-8 inches away.
4. Begin spraying the photoresist onto the PCB in smooth, even strokes, overlapping each pass slightly to ensure complete coverage.
5. Apply multiple thin coats rather than one thick coat to avoid drips or pooling of the photoresist.
6. Allow the photoresist to dry completely according to the manufacturer’s instructions, which may take several minutes to an hour depending on the specific product.

Exposing and Developing the Photoresist

After the photoresist has dried, the next step is to expose it to UV light through a patterned mask, which will harden the areas of the photoresist that are not covered by the mask. Here’s how to do it:

1. Create a transparency mask of your circuit design using a laser printer or photoplotting service.
2. Place the PCB with the dried photoresist in your UV exposure unit, with the mask placed on top of the photoresist surface.
3. Expose the photoresist to UV light for the recommended time based on your specific photoresist and exposure unit.
4. After exposure, remove the PCB from the unit and place it in a tray with developing solution.
5. Gently agitate the tray to ensure even development of the photoresist, following the manufacturer’s instructions for development time and temperature.
6. Rinse the PCB with water to stop the development process and remove any excess developing solution.

Etching and Finishing the PCB

With the photoresist developed, you can now etch away the unwanted copper from your PCB using an etching solution. Here are the basic steps:

1. Place your PCB in a tray with etching solution, following the manufacturer’s instructions for dilution and temperature.
2. Agitate the tray gently to ensure even etching, and monitor the progress of the etching process closely.
3. Once the etching is complete, remove the PCB from the solution and rinse it thoroughly with water.
4. Remove the remaining photoresist from the PCB using a stripping solution or by sanding it off.
5. Clean the PCB with isopropyl alcohol and inspect it for any defects or areas that need touch-up.
6. Apply a solder mask and silkscreen if desired, and drill any necessary holes for components.

Tips for Success

Spraying photoresist on a PCB can be a tricky process, but with practice and attention to detail, you can achieve excellent results. Here are some tips to keep in mind:

• Always work in a clean, well-ventilated area and wear appropriate protective gear.
• Make sure your PCB is thoroughly cleaned and dried before spraying the photoresist.
• Use a high-quality photoresist and follow the manufacturer’s instructions carefully.
• Apply multiple thin coats of photoresist rather than one thick coat to avoid drips or pooling.
• Allow adequate drying time between coats and before exposure.
• Use a high-resolution transparency mask and ensure good contact between the mask and the photoresist surface during exposure.
• Monitor the development and etching processes closely and adjust as needed based on your specific materials and equipment.

Troubleshooting Common Issues

Even with careful preparation and technique, problems can sometimes arise when spraying photoresist on a PCB. Here are some common issues and how to troubleshoot them:

Issue	Possible Causes	Solutions
Photoresist not adhering to PCB	– PCB not clean – Photoresist expired or contaminated	– Clean PCB thoroughly – Use fresh photoresist
Uneven or incomplete coverage	– Spray gun settings incorrect – Spraying too far away	– Adjust spray gun settings – Move spray gun closer to PCB
Drips or pooling of photoresist	– Applying too much photoresist at once	– Apply thinner coats and allow drying time between coats
Poor resolution or edge definition	– Low-quality transparency mask – Poor contact during exposure	– Use high-resolution mask – Ensure good contact during exposure
Photoresist not developing or etching properly	– Exposure time too short/long – Developer or etchant expired or contaminated	– Adjust exposure time – Use fresh developer and etchant

Frequently Asked Questions

1. What is the difference between positive and negative photoresist?
   Positive photoresist becomes soluble in developer solution when exposed to UV light, while negative photoresist becomes insoluble when exposed. Positive photoresist is more common for PCB Fabrication.

2. Can I use a regular spray paint gun to apply photoresist?
   While it may be possible, a dedicated spray gun or airbrush designed for photoresist application will give you better results and more control over the process.

3. How long does the photoresist take to dry?
   Drying time can vary depending on the specific photoresist product and the thickness of the applied coat, but it typically ranges from several minutes to an hour.

4. What UV exposure time do I need for my photoresist?
   The exposure time will depend on your specific photoresist, exposure unit, and PCB material. Consult the photoresist manufacturer’s instructions or do some test exposures to determine the optimal time for your setup.

5. Can I reuse the developer and etching solutions?
   Developer and etching solutions can often be reused multiple times, but they will eventually become exhausted and need to be replaced. Keep an eye on the performance of the solutions and replace them when necessary to ensure consistent results.

Conclusion

Spraying photoresist on a PCB is a critical step in the fabrication process that requires careful preparation, technique, and attention to detail. By following the steps outlined in this article and keeping the tips and troubleshooting advice in mind, you can achieve high-quality results and create PCBs with precise, reliable circuit patterns. With practice and experimentation, you’ll be able to fine-tune your process and produce professional-grade PCBs using spray photoresist.