Presensitized PCB: An Ultimate Guide

What is a Presensitized PCB?

Presensitized Printed Circuit Boards (PCBs) are copper clad boards that have been pre-coated with a light-sensitive resist material, also known as photoresist. This coating allows for the creation of intricate circuit patterns on the PCB through a process called photolithography. Presensitized PCBs offer several advantages over traditional PCB Fabrication methods, making them a popular choice for both hobbyists and professionals in the electronics industry.

Advantages of Presensitized PCBs

How Presensitized PCBs are Made

PCB Substrate

The foundation of a presensitized PCB is the substrate, which is typically made of a glass-reinforced epoxy material known as FR-4. This material offers excellent electrical insulation properties, mechanical strength, and thermal stability. The substrate is coated with a thin layer of copper on one or both sides, depending on the desired PCB design.

Photoresist Application

The copper-clad substrate is then coated with a light-sensitive photoresist material. This coating is usually applied through a process called roller coating or Curtain Coating, which ensures an even and consistent layer of photoresist across the entire surface of the PCB.

Types of Photoresist

There are two main types of photoresist used in presensitized PCBs:

1. Positive Photoresist: When exposed to light, the areas of positive photoresist that are exposed become more soluble in the developer solution. This results in the exposed areas being removed during the development process, leaving behind the desired circuit pattern.

2. Negative Photoresist: In contrast to positive photoresist, the areas of negative photoresist that are exposed to light become less soluble in the developer solution. This means that the unexposed areas are removed during development, leaving behind the circuit pattern.

Protective Film

After the photoresist is applied, a protective film is placed over the PCB to prevent any damage or contamination to the photoresist layer. This film is removed just before the exposure process.

Designing a Circuit Pattern

PCB Design Software

To create a circuit pattern for a presensitized PCB, you’ll need to use PCB design software. Some popular options include:

• KiCad
• Eagle
• Altium Designer
• Autodesk Fusion 360

These software packages allow you to create schematic diagrams, assign footprints to components, and design the physical layout of your PCB.

Design Considerations

When designing your circuit pattern, consider the following factors:

• Component placement: Ensure that components are placed in a logical manner, minimizing the distance between connected components to reduce signal interference and improve overall performance.
• Trace width: The width of your traces should be appropriate for the amount of current they will carry. Wider traces are needed for higher current applications.
• Via size and placement: Vias are used to connect traces on different layers of the PCB. Ensure that vias are sized appropriately and placed in a way that doesn’t interfere with other components or traces.
• Clearance: Maintain proper clearance between components, traces, and the edge of the board to prevent short circuits and other issues.

Output Files

Once your design is complete, you’ll need to generate the necessary output files for the photolithography process. The most common file formats are Gerber files (for the circuit pattern) and drill files (for the holes and vias). Make sure to double-check your output files before proceeding to the next step.

Exposing the Presensitized PCB

Artwork Printing

The first step in exposing a presensitized PCB is to print the artwork onto a transparent film. This film will be used as a mask during the exposure process. The artwork should be printed with a high-resolution printer to ensure the best possible results.

Exposure Setup

To expose the presensitized PCB, you’ll need an exposure unit, which typically consists of a UV light source and a vacuum table. The vacuum table holds the PCB and artwork in place during the exposure process, ensuring tight contact between the artwork and the photoresist layer.

Exposure Process

1. Remove the protective film from the presensitized PCB.
2. Place the printed artwork on top of the PCB, with the printed side facing down.
3. Secure the PCB and artwork to the vacuum table.
4. Turn on the vacuum to hold the PCB and artwork in place.
5. Expose the PCB to UV light for the recommended time (usually a few minutes, depending on the photoresist and exposure unit).
6. Turn off the vacuum and remove the PCB and artwork.

Developing the PCB

Developer Solution

After exposing the presensitized PCB, you’ll need to develop the photoresist to reveal the circuit pattern. This is done using a developer solution specific to the type of photoresist used (positive or negative).

Development Process

1. Prepare the developer solution according to the manufacturer’s instructions.
2. Immerse the exposed PCB in the developer solution for the recommended time (usually a few minutes).
3. Gently agitate the solution to ensure even development.
4. Remove the PCB from the solution and rinse it with water.
5. Inspect the PCB to ensure that the circuit pattern has been completely developed. If necessary, repeat the development process.

Etching the PCB

Etching Solution

To remove the unwanted copper from the PCB, you’ll need to use an etching solution. The most common etching solution is ferric chloride, which is available in both liquid and powder form.

Etching Process

1. Prepare the etching solution according to the manufacturer’s instructions.
2. Immerse the developed PCB in the etching solution.
3. Agitate the solution to ensure even etching.
4. Monitor the etching progress, removing the PCB from the solution when all the unwanted copper has been removed.
5. Rinse the PCB thoroughly with water.

Stripping the Photoresist

Stripping Solution

After etching, you’ll need to remove the remaining photoresist from the PCB. This is done using a stripping solution, which is typically a strong alkaline solution.

Stripping Process

1. Prepare the stripping solution according to the manufacturer’s instructions.
2. Immerse the etched PCB in the stripping solution for the recommended time.
3. Agitate the solution to ensure even stripping.
4. Remove the PCB from the solution and rinse it with water.
5. Inspect the PCB to ensure that all the photoresist has been removed.

Drilling and Finishing

Drilling

After etching and stripping, you’ll need to drill any necessary holes and vias in the PCB. This is typically done using a CNC drilling machine or a manual drill press. Use the drill file generated earlier to guide the drilling process.

Finishing

To protect the copper traces and improve the PCB’s appearance, you may want to apply a surface finish. Some common surface finishes include:

• HASL (Hot Air Solder Leveling)
• ENIG (Electroless Nickel Immersion Gold)
• OSP (Organic Solderability Preservative)

Choose a surface finish that suits your specific application and budget.

Frequently Asked Questions (FAQ)

1. What is the shelf life of presensitized PCBs?

Presensitized PCBs typically have a shelf life of 6 to 12 months when stored properly in a cool, dark, and dry environment. Always check the manufacturer’s recommendations for specific storage guidelines.

2. Can I use presensitized PCBs for multilayer designs?

Yes, presensitized PCBs can be used for multilayer designs. However, the process is more complex and requires careful alignment of the layers during the exposure and lamination stages.

3. What safety precautions should I take when working with photoresist and etching solutions?

Always wear appropriate personal protective equipment (PPE) when working with photoresist and etching solutions. This includes gloves, safety glasses, and a respirator if working with powdered chemicals. Work in a well-ventilated area and follow the manufacturer’s safety guidelines.

4. Can I reuse the etching solution?

Etching solutions can be reused multiple times, but their effectiveness will decrease over time. Monitor the etching speed and replace the solution when it becomes too slow or loses its effectiveness.

5. How do I dispose of used photoresist, developer, and etching solutions?

Used photoresist, developer, and etching solutions should be disposed of in accordance with local environmental regulations. Contact your local waste management authority for guidance on proper disposal procedures.

Conclusion

Presensitized PCBs offer a precise, consistent, and cost-effective solution for creating custom circuit boards. By understanding the process of designing, exposing, developing, etching, and finishing presensitized PCBs, you can create high-quality PCBs for your projects. Always prioritize safety and follow the manufacturer’s guidelines when working with the various chemicals and materials involved in the process. With practice and attention to detail, you can master the art of presensitized PCB fabrication and bring your electronic designs to life.