Four common PCB solder mask layers
What is PCB Solder Mask?
Solder mask is a PCB process used to protect metal components on a board from oxidation and to prevent conductive bridges from forming between pads. This is a critical step in PCB manufacturing, especially if reflow or solder baths are used. These techniques do not provide much control over where the molten solder lands on the board, but solder mask allows for some control. Solder mask is sometimes referred to as "solder resist," which I think is more appropriate, as I used to think of solder mask as a full layer of solder applied to a board.
PCB Solder Mask Types
All solder masks consist of a polymer layer that is applied over the metal conductors on a printed circuit board. There are many types of PCB solder mask, and the best choice for your board depends on cost and application. The most basic solder mask option is to use screen printing, printing a liquid epoxy over the conductors, much like spraying paint through a stencil. Solder mask can be applied in almost any color.
The most basic solder mask option is to use screen printing, printing a liquid epoxy over the PCB. This is the lowest cost and most popular solder mask option. In this process, a woven mesh is used to support the ink barrier pattern. Liquid epoxy is a thermosetting polymer that hardens during a heat curing process. The solder mask dye is mixed into the liquid epoxy and cured to the desired color.
More advanced solder masks use a photolithography process of dry film or liquid solder mask, similar to the process used for photoresist exposure in semiconductor manufacturing. LPSM can be screen printed like epoxy or sprayed on the surface, which is generally a cheaper application method. A more advanced (and accurate) method uses a photolithography process to define solder mask openings for pads, vias, and mounting holes.
In this process, a photoresist film is made that matches the desired solder mask based on the Gerber file. The panelized board is then thoroughly cleaned to ensure that there are no dust particles under the hardened solder mask. Both sides of the panel are completely covered with liquid LPSM.
When using LPSM, you will notice that the black portion of the photoresist film defines the areas where you want the conductors to be exposed, while the areas of the board where you want the assembly solder layer to be covered will be clear.
After covering the board with LPSM, the board is dried in an oven and placed in a UV developer. A photoresist film sheet is carefully aligned over the dried board and the board is then exposed to UV light. The exposed areas of the LPSM material are cured by UV light, while the unexposed areas are washed away with solvent, leaving behind a hard layer of solder mask.
DFSM solder mask uses a similar process to LPSM. Both PCB solder mask types are exposed in a photoresist-type process. Rather than a liquid coating, dry film uses a vacuum lamination process to apply the coating in the form of a solder mask film sheet. This vacuum lamination step forces the unexposed solder mask to adhere to the board and removes air bubbles from the film. After exposure, the unexposed areas of the solder mask are removed with a solvent, leaving the remaining film to cure in a thermal process.
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If you look at other guides on PCB solder mask types, the two types of solder mask that are often mentioned are top and bottom layers. These simply refer to specific solder mask layers placed on the top or bottom of the board; they do not refer to a specific manufacturing process or a specific type of solder mask material.
Final Steps: Curing and Surface Treatment
After applying the media shown above, the boards need to be cleaned to remove any dust. Afterwards, they will also undergo a final hardening and curing process. Liquid epoxy solder mask layers are thermally cured due to the absence of UV exposure. LPSM and DFSM films are cured by UV exposure during the photolithography process. After exposure, these films are cured and hardened by heat treatment.
Regardless of the type of PCB solder mask used, the resulting solder mask will leave exposed copper areas on the board. These exposed areas must be plated with a surface treatment to prevent oxidation. The most common surface treatment is hot air solder leveling (HASL), although other popular surface treatments are electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG). If applicable, additional holes are left in the film layer for the solder paste layer. The solder paste layer is used to connect pads or other components to the printed circuit board and is treated differently depending on the manufacturing process.
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