What is Circuit Design
Circuit Designers try to make connections.
3. Process Overview
3.6. LPI Solder Mask
Liquid Photo-Imagable Solder Mask is the layer of a circuitboard that lies between the outermost copper layers and the parts. Historically it's often been a green film mask with white silkscreen on top.
This circuitboard appears green because of the LPI Mask layer. Image courtesy Wikipedia.com
LPI Mask is applied as a liquid that covers the entire board. Then ultraviolet light is shown on areas of the board where the LPI Mask needs to remain. A cleaning step dissolves the mask on areas that were not hit by UV light revealing lands, testpoints, and other areas of exposed copper.
LPI serves two practical purposes. First, it protects copper traces and vias from corrosion and oxidation. Second, it acts as a solder dam. Without a LPI Mask, solder would bridge traces and lands creating short circuits and leaving an assembled circuitboard full of short circuits and areas of too-little solder.