In PCB (Printed Circuit Board) manufacturing, photoresist is used to transfer circuit patterns onto a substrate, a crucial step in defining the electronic pathways and components. The two primary types of photoresist are dry film and wet film, each with distinct properties, advantages, and applications. Understanding these differences helps in choosing the right technology for specific PCB production needs.
Table of Contents
- Introduction to PCB Photoresist film
- Difference between dry film and wet film
- Dry Film Photoresist
- Wet Film Photoresist
- Conclusion
Introduction to PCB Photoresist film
Photoresist film is a light-sensitive polymer that changes its chemical properties when exposed to ultraviolet (UV) light. This change allows selective removal of the material during the development process, thereby defining the patterns required for subsequent manufacturing steps.
Types of PCB Photoresist Film and Their Functions
Description: Photoresist films are light-sensitive materials used to create precise patterns on PCBs. They are applied in liquid or dry-film form and undergo a photolithography process to define circuit patterns.
Types:
Dry Film Photoresist: A pre-sensitized, solid film that is laminated onto the PCB substrate. It is exposed to UV light through a mask to define the circuit pattern, then developed to remove the unexposed areas.
Liquid Photoresist: Applied as a liquid and spun onto the substrate. It is then exposed to light through a mask and developed similarly to dry film.
Applications: Creating the copper patterns on the PCB, defining component locations, and forming vias and traces.
Difference between dry film and wet film
In PCB manufacturing, both dry film and wet film processes are used for photoresist application, which is a critical step in defining the circuit patterns on the board. Here’s a detailed comparison of dry film and wet film photoresist processes:
Dry Film Photoresist
1. Definition:
– Dry film photoresist is a photosensitive material that is pre-coated onto a thin film. It comes as a roll or sheet and is laminated onto the PCB substrate.
2. Application Process:
– Lamination: The dry film is laminated onto the PCB using heat and pressure, adhering the photoresist to the surface.
– Exposure: The PCB with the dry film is exposed to UV light through a photomask, which transfers the circuit pattern onto the photoresist.
– Development: The exposed PCB is then developed in a chemical solution that removes either the exposed or unexposed areas, depending on whether it is a positive or negative photoresist.
– Etching: The PCB is then etched to remove the unwanted copper, leaving the desired circuit pattern.
3. Advantages:
– Consistency: Provides a consistent thickness and uniform coverage, which is important for high-density designs.
– Resolution: Capable of achieving high resolution and fine lines, suitable for complex and high-density PCBs.
– Ease of Use: The dry film is easier to handle and apply compared to liquid resists, reducing the risk of contamination and application errors.
4. Disadvantages:
– Cost: Generally more expensive than wet film processes due to the cost of the pre-coated film and lamination equipment.
– Handling: Requires precise lamination equipment and techniques, which can be more complex.
Wet Film Photoresist
1. Definition:
– Wet film photoresist is a liquid photoresist that is coated onto the PCB substrate in a liquid form.
2. Application Process:
– Coating: The liquid photoresist is applied to the PCB substrate using techniques such as spin coating, spray coating, or curtain coating.
– Baking: The coated PCB is then baked or cured to solidify the photoresist layer.
– Exposure: The PCB with the wet film photoresist is exposed to UV light through a photomask.
– Development: The exposed PCB is developed using a chemical solution to remove either the exposed or unexposed areas, similar to the dry film process.
– Etching: The PCB is etched to remove the unwanted copper, leaving the circuit pattern.
3. Advantages:
– Cost: Generally less expensive than dry film photoresist, with lower material and equipment costs.
– Flexibility: Can be easily applied to various sizes and shapes of PCBs, and is suitable for large-scale production.
4. Disadvantages:
– Thickness Control: Achieving consistent thickness can be challenging, potentially affecting the resolution and quality of the PCB.
– Resolution: Typically less capable of achieving the same fine resolution as dry film, which can be a limitation for very high-density designs.
Conclusion
Both dry film and wet film photoresists are essential in the PCB manufacturing process, each offering unique benefits suited to different production requirements. Dry film is preferred for high-resolution and high-density applications due to its uniform thickness and ease of handling. Wet film is valued for its cost-effectiveness and flexibility, making it suitable for a range of production scales and applications. Understanding the characteristics and advantages of each type helps in selecting the appropriate technology for achieving optimal PCB performance and quality.
