We image the outer layers in a clean room to make sure that no dust gets onto the panel surface where it could cause a short or open circuit on the finished PCB.
The panel is first coated with a layer of photosensitive film, the photoresist, which is hot-rolled onto the copper using a cut-sheet laminator.The laminated panels are collected by an automatic rack.The clean room uses yellow lighting as the photoresist is sensitive to UV light.
The bed of the printer has registration pins matching the holes in the phototools and the panel.The operator loads the first film onto the pins, then the laminated panel and finally the second film.The pins ensure that the top and bottom layers are precisely aligned.The printer uses powerful UV lamps to harden the photoresist. So the photomask is clear where we want the resist to harden and black where we don’t want resist.
The Mylar film which protected the photoresist is now removed and the imaged panel is conveyored out of the clean room and through a developer which removes the unhardened resist.For inner layers the copper pattern we want was covered by the resist.For outer layers it is exposed ready to be plated.The operator now checks the panels to make sure that the copper surface is clean and all the unwanted resist has been removed.