The outer layers of our multi-layer consist of sheets of glass cloth pre-impregnated with uncured epoxy resin (prepreg) and a thin copper foil.
The lay-up operator has already placed a copper foil and 2 sheets of prepreg on the heavy steel baseplate. Now he places the pre-treated core carefully over the alignment pins. Then he adds 2 more sheets of prepreg, another copper foil and an aluminium press plate
He builds up 3 panels on the baseplate in the same way. Then he rolls the heavy stack under a press which lowers down the steel top plate. He pins the stack together and rolls the finished stack out of the clean room into a rack.
The press operator collects 3 stacks on a loader and loads them into the bonding press. This press uses heated press plates and pressure to bond the layers of the PCB together. The heat melts and cures the epoxy resin in the prepreg while the pressure bonds the PCB together. The process is computer controlled to build up the heat and the pressure correctly, hold it and then to cool the press down. In this way we ensure a permanent bond that will last the lifetime of the PCB. Our board has 4 layers but complex PCBs for defence, avionic and telecommunications applications can have more than 50. These may include sub-assemblies of cores, prepregs and foils drilled and plated before being assembled into the final PCB.
Once the cycle is completed the press operator unloads the press and carefully rolls the heavy stacks into the clean room. Here the lay-up operator de-pins the stack and removes the top plate. He unloads each of the panels from the stack, removing the aluminium press plates used to ensure a smooth copper finish. The copper foil is now bonded in place to form the outer layers of the PCB.