How to Spray Photo Resist on PCB

What is Photoresist and Why Use It?

Photoresist is a light-sensitive material used in the fabrication of printed circuit boards (PCBs). It is applied to the copper surface of the PCB and then exposed to light through a photomask, which selectively hardens the resist in the desired pattern. The unexposed areas can then be dissolved away, leaving the copper exposed for etching.

Using photoresist is an essential step in the PCB Manufacturing process as it allows for the creation of precise and detailed circuit patterns. It provides a protective layer over the copper that will not be etched away, ensuring the integrity of the designed circuit.

Types of Photoresist

There are two main types of photoresist:

1. Positive Photoresist: In this type, the exposed areas become soluble and can be removed by the developer solution, while the unexposed areas remain intact.

2. Negative Photoresist: Contrary to positive photoresist, the exposed areas of negative photoresist become insoluble and remain on the surface after development, while the unexposed areas are dissolved away.

The choice between positive and negative photoresist depends on the specific requirements of the PCB design and the manufacturing process being used.

Preparing the PCB Surface

Before applying the photoresist, it is crucial to ensure that the PCB surface is clean and free from any contaminants. Follow these steps to prepare the PCB:

1. Clean the PCB with a mild detergent solution to remove any dirt or grease.
2. Rinse the board thoroughly with deionized water and dry it with compressed air or a lint-free cloth.
3. If necessary, use a chemical cleaner specifically designed for PCBs to remove any stubborn residues.
4. Inspect the surface for any defects or irregularities and address them before proceeding.

Safety Precautions

When working with photoresist and other chemicals, always prioritize safety:

• Wear appropriate personal protective equipment (PPE) such as gloves, safety glasses, and a lab coat.
• Work in a well-ventilated area or under a fume hood to avoid inhaling fumes.
• Follow the manufacturer’s guidelines for handling and storage of the photoresist and related chemicals.
• Dispose of any waste materials according to local regulations and guidelines.

Spray Photoresist Application

Spray photoresist application is a popular method for covering larger PCBs or those with complex topographies. This method ensures an even distribution of the resist across the surface.

Equipment and Materials

To spray photoresist, you will need the following:

• Spray photoresist
• Compressed air source
• Spray gun or airbrush
• Pressure regulator
• Ventilated spray booth or fume hood
• PPE (gloves, safety glasses, lab coat)

Spray Photoresist Techniques

1. Fill the spray gun or airbrush with the photoresist according to the manufacturer’s instructions.
2. Adjust the pressure regulator to the recommended pressure for the specific photoresist being used.
3. Place the PCB in the spray booth or fume hood.
4. Hold the spray gun perpendicular to the PCB surface at a distance of 6-8 inches (15-20 cm).
5. Begin spraying the photoresist using smooth, even strokes. Overlap each pass by approximately 50% to ensure complete coverage.
6. Apply multiple thin layers rather than one thick layer to achieve a uniform coating without drips or runs.
7. Allow the photoresist to dry according to the manufacturer’s guidelines before proceeding with exposure and development.

Troubleshooting Spray Photoresist Application

Exposing and Developing the Photoresist

After applying the photoresist, the next steps are exposure and development.

Exposure

1. Place the photomask on top of the photoresist-coated PCB, ensuring proper alignment.
2. Expose the PCB to UV light for the recommended time based on the photoresist type and thickness.
3. Follow the manufacturer’s guidelines for exposure time and light intensity.

Development

1. Prepare the developer solution according to the manufacturer’s instructions.
2. Immerse the exposed PCB in the developer solution and gently agitate it.
3. Monitor the development process closely and remove the PCB once the unexposed areas have dissolved away.
4. Rinse the PCB thoroughly with deionized water to stop the development process.
5. Inspect the PCB for any defects or incomplete development and repeat the process if necessary.

Etching and Stripping the Photoresist

After exposure and development, the PCB is ready for etching.

Etching

1. Prepare the etching solution according to the manufacturer’s guidelines.
2. Immerse the developed PCB in the etching solution and agitate it gently.
3. Monitor the etching process and remove the PCB once all the exposed copper has been etched away.
4. Rinse the PCB thoroughly with deionized water to stop the etching process.

Stripping the Photoresist

1. Use a photoresist stripper solution to remove the remaining hardened photoresist from the PCB.
2. Follow the manufacturer’s instructions for the stripping process.
3. Rinse the PCB with deionized water and dry it with compressed air or a lint-free cloth.

Frequently Asked Questions (FAQ)

1. What is the shelf life of photoresist?

2. The shelf life of photoresist varies depending on the specific product and storage conditions. Generally, most photoresists have a shelf life of 6-12 months when stored properly in a cool, dark place.

3. Can I reuse photoresist that has already been applied to a PCB?

4. No, photoresist is a single-use material and cannot be reused once it has been applied and exposed. Attempting to reuse photoresist can lead to poor results and defects in the final PCB.

5. How can I tell if the photoresist has been exposed for the correct amount of time?

6. The correct exposure time depends on factors such as the photoresist type, thickness, and light intensity. Consult the manufacturer’s guidelines and conduct exposure tests to determine the optimal time for your specific setup.

7. What should I do if the photoresist is not developing properly?

8. If the photoresist is not developing as expected, first check that the exposure time and developer concentration are correct. If the issue persists, the photoresist may be too old or contaminated, or the PCB surface may not have been cleaned properly.

9. How should I dispose of photoresist and other chemical waste?

10. Always dispose of photoresist and related chemical waste in accordance with local regulations and guidelines. Contact your local waste management authority for specific instructions on proper disposal procedures.

By following these steps and guidelines, you can successfully apply spray photoresist to your PCB and create precise, high-quality circuit patterns. Remember to prioritize safety, adhere to the manufacturer’s instructions, and practice proper technique to achieve the best results.