Introduction
When it comes to Printed Circuit Boards (PCBs), solder mask application is a fundamental step in the fabrication process. It protects copper traces from oxidation, minimizing the risk of electrical shorts and avoiding the wrong placement of components. In this blog post, we will compare two popular solder mask application methods: Liquid Photo-Imageable (LPI) and Direct Photo-Imageable (DPI).
LPI Solder Mask
LPI solder mask application uses a liquid ink that hardens when exposed to ultraviolet light to protect areas of the PCB. The process includes these steps:
- Clean the board to remove contaminants
- Apply the LPI solder mask to the board
- Cure the mask under UV lights
- Remove the excess solder mask using a chemical solution
LPI solder mask application is less expensive and more commonly used. However, it has some disadvantages. It can be a challenge to mask fine-pitch components, and the final result might have some bleed due to inaccurate resolution.
DPI Solder Mask
DPI solder mask application uses a dry film that impresses a layer of solder mask onto the board by printing it through a photomask. The process includes these steps:
- Apply the dry film to the panel
- Expose the copper lands and the solder mask areas
- Develop the photoresist and remove unexposed areas
- Etch the copper and strip the dry film
DPI solder mask application has some benefits. It provides a higher resolution and accuracy, making it suitable for fine-pitch components. It also reduces the risk of bleed and misalignment.
Comparison
| Parameter | LPI | DPI |
|---|---|---|
| Resolution | 50 to 75 microns | 25 to 50 microns |
| Minimum solder mask clearance | 0.05 mm | 0.025 to 0.035 mm |
| Registration accuracy | ± 75 microns | ± 25 microns |
| Cost | Lower | Higher |
Conclusion
Both LPI and DPI solder mask application methods have their own advantages and disadvantages depending on the particular PCB layout and requirements. While LPI is more cost-effective and widely used, DPI provides a higher resolution and accuracy, making it more suitable for finer-pitch components. Therefore, selecting the most suitable method for a particular design depends on a balanced consideration of cost and accuracy.
References
- M. Papesch, "A beginner's guide to comparing PCB Solder Mask types," Medium, Jan. 29, 2019.
- "Solder mask - Liquid Photo Imaginable (LPI)," Eurocircuits Blog, Dec. 17, 2014.
- "Solder mask - Direct Photo Imaginable (DPI)," Eurocircuits Blog, Sept. 23, 2014.