Solder Mask Materials
Solder mask comes in different media depending upon the demands of the application. The lowest-cost solder mask is an epoxy liquid that its silkscreen through the pattern onto the PCB.
Other types are the liquid photo-imageable solder mask (LPSM or LPI) inks and dry film photo-imageable solder mask (DFSM).
LPSM can be silkscreened or sprayed on the PCB, exposed to the pattern and developed to provide openings in the design for parts to solder on the copper pads.
DFSM is vacuum laminated on the PCB then presented and developed. All three processes typically go through a thermal cure of some type after the pattern is defined although LPI solder masks are also available in Ultra Violet (UV) cure.
Here I will introduce solder mask materials in three types of media – Epoxy liquid, Liquid Photoimageable, and Dry-film Photoimageable masks. The best option for your PCB depends on your application, as well as the amount of money you intend to spend.
1. Epoxy Liquid
Epoxy liquid, the cheapest type around, is silkscreened onto the PCB pattern. Silk-screening is a printing technique that involves the use of a woven mesh to support ink-blocking patterns. The mesh creates open spaces for ink transfer.
While artists often use silk, synthetic fibers with similar texture have become commonplace in electronic applications.
2. The Liquid Photo-Imageable Solder Mask (LPSM)
LPSM is delivered as an ink formulation that can be silkscreened or sprayed onto a board before it is exposed and developed. Hot air surface leveling (HASL) is one particular process that is common in LPSM masking.
HASL involves using high-pressure water or vapor sprays called developers to make the mask uniform on the surface.
How is LPSM 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 cover 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 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.
3. Dry-Film Photo-Imageable Solder Mask (DPSM)
DPSM has to be vacuum-laminated on the PCB before it is exposed and developed to avoid trapping air bubbles. After developing then need to create openings in the mask, and copper is layered inside the holes and on the traces using electrochemical processing.
Tin is sometimes also applied to protect the circuitry.
Photoimageable solder resists are generally the acceptable mask types for modern PCB designs. The surface topography of your board will dictate whether to use liquid (LPSM) or dry (DPSM) application.
Applying a dry-film mask can give you a uniform thickness across the surface, but it only adheres best if a board is exceptionally flat.
LPSM won’t result in an entirely consistent layer, but if the PCB has complex surface features, it provides better contact with the copper traces than DPSM.
Solder masks typically go through a thermal curing stage after the PCB pattern is defined, but LPSM masks can also be hardened in UV light.