To produce the inner layers of our multilayer PCB, we start with a panel of laminate. Laminate is an epoxy resin and glass-fibre core with copper foil pre-bonded onto each side.
The first step is to clean the copper.
We print the panels in a clean room to make sure that no dust gets onto the surface where it could cause a short or open circuit on the finished PCB.
The cleaned panel is coated with a layer of photosensitive film, the photoresist.
The bed of the printer has registration pins matching the holes in the phototools and in the panel. The operator loads the first film onto the pins, then the coated panel then the second film. The pins ensure that the top and bottom layers are precisely aligned. The printer uses powerful UV lamps which harden the photoresist through the clear film to define the copper pattern.
Under the black areas the resist remains unhardened. The clean room uses yellow lighting as the photoresist is sensitive to UV light.
Outside the yellow room the panel is sprayed with a powerful alkali solution which removes the unhardened resist ( etching) . The panel is pressure-washed and dried. The copper pattern we want is now covered by the hardened resist. The operator checks a sample of the panels to make sure that the copper surface is clean and all the unwanted resist has been removed. You can now see in the blue resist what will be the copper on our inner layer panel.