Solder resist is a coating applied to a printed circuit board after the board has been etched and any through holes plated. The purpose of the mask is to prevent solder paste and melted solder from bridging areas that should not be electrically connected and to guide the solder into the areas where it is needed to attach the component leads. It is primarily used with surface mount assemblies or where wave/reflow or pot soldering will be used. The material used to form the mask must withstand the heat used to melt the solder as well as any chemical effects of the solder flux and cleaners used to remove excess flux after soldering. It must also be compatible with the adhesives used to attach surface mount components, and with conformal coatings which are sometimes used to protect the completed circuit after assembly.
Removable/Peelable types: Another type of solder resist is the removable or "peelable" type used to prevent through holes and edge connectors from being filled with solder when using wave soldering. This type is removed after the solder bath to allow component placement and solder flux application before re-flow of the solder to complete the connections.
Hobby/Prototype application: Solder Masks are typically not needed in hobby or prototype production. When hand assembling SMT devices, experience and care will avoid solder bridges and even if they do occur, inspection and reheating or removal of excess solder with a solder wick braid will correct the problem.
Methods:
Silkscreen: Traditionally, solder resist is applied via a silkscreen type method using a thin metal stencil and a paint which is chemically, UV, or heat curable to form a solid coating.
Photo: Another system which is becomming more popular is photosensitive dry film or photo imageable ink which are exposed and developed, removing the non-exposed areas, and then hardened by baking or IR exposure. The process and materials are very much the same as used in echant resists except that the coating is often thicker and the resolution typically does not need to be as high. Solder resist is typically 20-30 microns thick.
InkJet: Certain heat curable inkjet inks can be used to form a minimal solder mask layer. See Direct InkJet Resist Print,
Marker: Pebeo Porcelaine 150 or Vitrea paints in a fine point marker can be used to apply solder resist manually or with a plotter.
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