The dry film technique is a fundamental process, often used in the manufacturing of printed circuit boards (PCBs), that utilizes a thin, solid sheet of light-sensitive material to transfer a pattern onto a substrate. Unlike liquid resists, this method employs a dry film photoresist, which simplifies handling and provides uniform thickness.
At its core, the technique involves applying this special film to a surface, exposing it to ultraviolet (UV) light through a patterned mask, and then developing it to create a resist pattern that protects specific areas during subsequent processing steps, such as etching.
Understanding the Dry Film
The dry film itself is ingeniously constructed in layers. According to the reference, the film is typically comprised of three distinct parts:
- Middle Layer: This is the crucial layer, often blue in color. It consists of a polymer designed to harden or become insoluble when exposed to ultraviolet (UV) light.
- Protective Layers: On each side of the polymer layer is a transparent, protective film. These layers safeguard the polymer during handling and storage and are removed just before the film is used in the process.
This multi-layer structure allows for easy application and robust protection of the active photoresist material until needed.
The Dry Film Process Steps
Using the dry film technique involves a series of steps to transfer the desired pattern onto the substrate (like a copper-clad PCB):
- Surface Preparation: The substrate surface must be clean and dry to ensure proper adhesion of the dry film.
- Film Lamination: The dry film is laminated onto the substrate, usually with heat and pressure, after removing one of the transparent protective layers.
- Second Protective Layer Removal: The remaining transparent protective film is removed from the top surface of the laminated dry film.
- UV Exposure: The substrate with the dry film is exposed to UV light through a phototool or mask, which contains the desired pattern. The areas of the blue polymer layer exposed to UV light undergo a chemical change, causing them to harden or cross-link. Unexposed areas remain soluble.
- Development: The reference highlights that the PCB is developed in a solution of washing soda (Sodium Carbonate). This developing solution dissolves and washes away the areas of the dry film that were not exposed to UV light (the unhardened areas). This leaves the hardened film pattern on the substrate, corresponding to the desired design.
- Etching: Then the PCB is etched. This involves immersing the substrate in an etchant solution (like Ferric Chloride for copper). The etchant removes the material (e.g., copper) from the areas not protected by the hardened dry film pattern. The areas covered by the hardened film resist the etchant.
- Striping (Optional): After etching, the remaining hardened dry film resist is typically removed using a different chemical solution, revealing the final patterned substrate (e.g., copper traces on a PCB).
Practical Applications and Benefits
The dry film technique is widely used in manufacturing processes requiring precise pattern transfer, most notably:
- Printed Circuit Board (PCB) Fabrication: Creating the intricate copper traces, pads, and features that make up electronic circuits.
- Chemical Milling: Etching patterns into metal parts.
- Microfluidics: Creating channels and features in microfluidic devices.
Benefits of using dry film photoresist:
- Uniform Thickness: Provides a consistent resist layer, leading to more predictable etching results.
- Clean Handling: Easier and cleaner to handle compared to liquid photoresists.
- High Resolution: Capable of resolving fine lines and spaces, crucial for modern electronics.
- Good Adhesion: Adheres well to various substrates.
This technique is a staple in industries where precise patterning is essential, offering a reliable and efficient method for transferring complex designs onto different materials.
