Dry film resist (DFR) functions as a temporary protective layer in various manufacturing processes, most notably in the creation of printed circuit boards (PCBs). Specifically, DFR is a type of photoresist that, when exposed to light and chemically developed, creates a pattern on a substrate for subsequent processing. Here's a breakdown of the process:
The Dry Film Resist Process
The process of using dry film resist typically involves the following steps:
- Attachment: As per the provided reference, the process begins by directly attaching the dry film photoresist to the copper clad board. This is done using a hot-roll lamination process with controlled pressure and temperature. The dry film is supplied in rolls with a protective layer (usually polyethylene) on either side of the photo-sensitive layer. One of these protective layers is removed before lamination onto the copper surface.
- Exposure: A mask containing the desired circuit pattern is then placed over the DFR. The DFR is exposed to ultraviolet (UV) light, using the mask as a guide. The areas of the dry film exposed to UV light undergo a chemical change (polymerization), causing it to harden. Areas shielded by the mask remain unaffected.
- Example: If a positive working DFR is used (the most common type), the area exposed to light becomes the resistant layer during etching. If negative working DFR is used, the non-exposed area becomes resistant during the etching stage.
- Development: The board is then immersed in a developing solution, which selectively removes the unexposed areas of the DFR, thus revealing the desired circuit pattern.
- Etching: The copper that is not covered by the DFR is removed through chemical etching. The resistant layer of DFR protects the copper underneath from the etchant.
- Resist Removal: After etching, the remaining DFR layer is removed, leaving behind the circuit pattern on the copper.
How DFR Works as a Resist
| Step | Action | Result |
|---|---|---|
| Lamination | DFR is attached to the copper clad board | DFR layer adhered to the substrate. |
| Exposure | DFR is exposed to UV light through a mask, chemical change occurs in exposed areas. | Exposed area polymerizes (hardens), becoming resistant to the developer. |
| Development | Board is developed, removing the non-exposed areas of the DFR. | Circuit pattern visible on the copper, areas where the DFR remains are protected. |
| Etching | The exposed copper is chemically removed, while copper beneath the DFR is protected | Desired copper circuit pattern is formed. |
| Stripping | The remaining DFR is removed. | Final patterned copper circuit board is complete. |
Key Features
- Photosensitive: DFR is made of a material that reacts when exposed to UV light, allowing for precise pattern replication.
- Resistant: Once exposed and developed, DFR becomes resistant to various chemicals, including the etchants used to remove copper in PCB fabrication.
- Protective: The DFR acts as a temporary barrier during the etching process, preventing unwanted removal of copper and ensuring accurate circuit patterns.
Advantages of Using Dry Film Resist
- High Resolution: DFR allows for the creation of very fine lines and spacing, making it suitable for complex circuit designs.
- Ease of Use: The lamination process is straightforward and can be automated.
- Consistent Coating Thickness: DFR provides an even layer of resist, which leads to more predictable results.
- Variety: Different types of DFR are available to suit various applications and process requirements.
