The board designer has prepared his layout on a Computer Aided Design or CAD system. Each CAD system uses its own internal data format, so the PCB industry has developed a standard output format to transfer the layout data to the manufacturer. This is Extended Gerber or RS274X. The Gerber files define the copper tracking layers (4 in the job we are following) as well as the soldermasks and component notations.
First we check that data meets our manufacturing requirements. These checks are mostly done automatically. We check the track widths, the space between tracks, the pads around the holes, the smallest hole size etc. The engineer can also check and measure individual areas where he wishes. Once the data is verified as good he will output all the tool files needed to drive the machines that will make and test the PCB.
We use laser photoplotters in a temperature and humidity-controlled darkroom to make the films we will use later to image the PCBs. The photoplotter takes the board data and converts it into a pixel image. A laser writes this onto the film. The exposed film is automatically developed and unloaded for the operator. The films are ready now for the PCB fabrication process.
We have generated one film or phototool per PCB layer. Now the films are registered with each other so that the different layers of the PCB will be perfectly aligned. We do this by punching precise registration holes in each sheet of film. The operator puts the film on the table of the punch and then micro-adjusts the table until the targets on the film are exactly lined up with the targets on the film punch. She then punches each sheet of film with the registration holes which will fit onto the registration pins in our imaging equipment.
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.
2. Etching inner layer in PCB production
We remove the unwanted copper using a powerful alkaline solution to dissolve (or etch away) the exposed copper from the inner layer. The process is carefully controlled to ensure that the finished conductor widths are exactly as designed. But designers should be aware that thicker copper foils need wider spaces between the tracks. The operator checks carefully that all the unwanted copper has been etched away.
Next we strip off the blue photoresist which protected the copper image. So now we have the exact pattern required. The operator checks that all the photo-resist has been removed. You can see that Eurocircuits put several different designs on one production panel. That way we can make small numbers of PCBs cost-effectively.
