Most boards have a epoxy-ink soldermask printed onto each side to protect the copper surface and prevent solder shorting between components during assembly.
The panels are first cleaned and brushed to remove any surface tarnish and then are conveyored into the yellow room.
Each panel is given a final clean to remove any dust from the surface and loaded into the vertical coater. The coating machine simultaneously covers both sides of the panel with the epoxy soldermask ink. The double action ensures that the ink completely encapsulates the copper tracking, typically now 35 – 40 microns higher than the surface of the panel.
The panels are now racked and put through a conveyorised drier which hardens the resist just enough to allow it to be printed (“tack-dried”). The operator checks for a complete and even coating.
Next the coated panels are imaged. For this we use a two drawer UV printer. The operator mounts the phototool films on the machine and then places the panel onto the registration pins. She checks that the film and the copper layer are precisely aligned. Mask alignment will be better than 50 microns. As with the etch and plating resists used earlier in the process, the UV lamps in the machine harden the ink where the film is clear, that is where we need soldermask on the finished board.
The imaged panels are put on a conveyor out of the clean room and into the developer which strips off the unhardened and unwanted resist. Later the required resist will be further hardened or “cured” to provide a robust and permanent coat. For this we use a conveyorised oven in the same way in which the boards were previously tack-dried. But first the operator checks the alignment of the soldermask on the panel and makes sure that there are no traces of ink on the pads or through the holes. Even slight traces will compromise the solderability of the finished PCB.