Solder Mask is a protective coating applied to the bare printed circuit board. Bare boards are covered with a mask to prevent accidental solder bridging during assembly and to help protect the board from the environment.
Don't confuse solder mask with conformal coating. Conformal coating is also a protective coating but it's applied to the assembled board rather than the unpopulated bare board.
Solder mask is usually green but many other colors are available.
Types of solder mask
At PCB Universe, we use four types of solder mask:
● Top- and Bottom-side Masks
● Epoxy Liquid Solder Masks
● Liquid Photoimageable Solder Masks
● Dry Film Photoimageable Solder Masks
We use LPI more often these days and here is how LPI solder mask applied.
How is LPI Solder Mask Applied?
1. The production panel is thoroughly cleaned to ensure no dust particles are trapped under the mask.
2. The panels are completely covered on both sides with the liquid mask.
3. The coated panels are placed in an oven to tack-dry the mask just enough to finish processing.
4. A film is made of your solder mask Gerber files, one for each side of the board. Where you want a mask to stick to the board, the film will be clear. Where the mask should be removed will be black in the film.
5. The tack-dried boards are placed into a UV developer and the film is precisely aligned over the board. The blacked out locations on the film will prevent the UV light from curing the mask where it is not wanted.
6. After the mask is exposed to the UV lamps, the uncured mask is washed off leaving mask only in the places needed