Solder masks are referred to as solder stop masks or solder resist masks. They are thin lacquer-like layer of polymers, which are usually applied to the copper traces of a printed circuit board (PCB) for protection against oxidation. This lacquer-like layer lads to the formation of solder bridges between closely spaced solder pads. Liquid Photoimageable Solder Mask (LPISM or LPSM or LPI) is an electronic material of alkali type. LIPSM is a dual-cured system used in UV-curing and thermal curing.
LPISM is used for the application of printed circuit boards (PCB) in almost all electronics products. A solder bridge is an unintended electrical connection between two conductors by means of a small blob of solder. Liquid imageable solder mask is not always used for hand soldered assemblies, but is essentially used for mass-produced boards that are soldered automatically using reflow or solder bath techniques. Solder mask comes in different media depending upon the demands of the application. LPSM can be silkscreened or sprayed on the PCB, exposed to the connectivity mesh pattern, and developed to provide openings in the pattern for parts to be soldered to the copper pads. DFSM is vacuum laminated on the PCB and then exposed and developed.In electronic design automation, the solder mask is treated as a layer of the printed circuit board, and is described as a Gerber file akin to any other layer such as copper and silkscreen layers.
In order to cope with the environmental requirements, there is a trend to dehalogenate phthalocyanine pigments and dry films instead of inks in liquid state. As an alternative to epoxy resin, the possibility of oxetane resin is being currently investigated. Other issue concerning the environmental requirements is the increased soldering temperature associated the with usage lead-free solder.
An opening must be made in the solder mask wherever components are soldered. This is accomplished using photolithography. Solder mask is traditionally green, but is now available in many colors. The lowest-cost solder mask is epoxy liquid that is silkscreened through the pattern onto the PCB. Other types are liquid photoimageable solder mask inks and dry film photoimageable solder mask (DFSM). Currently, module circuit boards for substrate use of CPU and even flexible circuit boards to be incorporated in the liquid crystal driver are available. Flexible circuit boards are expected to be used extensively in the near future.
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