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Stage 5: Solder Resist

Solder resist is available in a wide variety of colours and whilst adding a pleasing aesthetic to a finished PCB it has a much more functional role to play

Due to the excellent chemical resistance and dielectric properties in solder resist applied to the surface of the PCB tighter track spacings and features can be utilised without compromising signal integrity. The other primary function is to allow bare PCB’s to be produced with various surface finishes and soldered through wave soldering equipment. As the name suggests it resists solder ensuring only exposed component holes and pads are soldered during the assembly process.

Solder resist is a liquid photo-imageable ink which is mainly applied to the PCB surface by a screen printing process. Both sides of the PCB are flood coated with ink and tack dried in a large thermal curing oven for a short period of time. A photo-tool mask of all the areas that require an opening in the solder resist is aligned to both sides of the board using pre drilled tooling holes and then exposed in a high intensity UV exposure unit. The UV light polymerises any exposed ink, hardening it, and leaves all other unexposed areas unaffected. Once exposure is complete the panel can be developed in a horizontal processor which efficiently and cleanly removes all unexposed ink from the surface of the board leaving clear solderable areas for components and wires. To finalise the process the developed panel is returned to a thermal cure batch oven for an hour to ensure the ink is hardened and cured correctly before the board is passed on to the next stage of processing – surface finishing.