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Solder Mask is a product that goes on the outside of a PCB to protect it from oxidation of the copper and to define the areas in which components will be attached by solder. There are mainly 2 styles of solder mask. The most popular style is the liquid epoxy-polymer version that can be applied in many ways from spraying to screening to curtain coating. The second type is one that is based off of a dry-film that is extruded and needs to be applied by pressure and a little bit of heat.
Dry-film solder mask is basically only used in one application nowadays and that is flex boards. There are few other rigid applications, but it is usually only seen on legacy parts.
Liquid solder mask comes in a variety of colors and types. The most common color is green, and the most common type is an acrylic based polymer. They all contain a 2-part system that requires the product to be catalyzed. Solder mask can be cross-linked by either the energy from heat or by energy from UV light. Solder mask is photo-definable by partially cross-linking the product with UV light and developing the areas that were protected from the UV light. After this step you final cure the product using UV or by temperature, with temperature being the most common method. The only thing that a PCB manufacturer has to pay close attention to is the solder mask specification. This is overlooked by many manufacturers because of them being uninformed by the LPI manufacturers. There are two main classes for solder mask recognized by IPC. They are Class H and Class T. Very few solder mask manufacturers certify their materials for both classes, so this is something that as a buyer you need to pay close attention too.