After all the layers that comprise the PCB have been cleaned, they’re ready for layer alignment, optical inspection, and lamination.
In order to have proper alignment of all the layers, registration holes are accurately punched using an optical punch. Subsequently, another machine performs an optical inspection to make sure there are no imperfections or defects. This inspection stage is very important because once the layers are laminated together, errors cannot be corrected. The AOI machine scans the layers using a laser sensor and electronically compares the digital image of the PCB layers to the gerber files. If the AOI finds an inconsistency, the issue is displayed on a monitor for the operator to assess. Once the layers pass the inspection they are ready to be laminated together.
The PCB laminating process
There are 2 stages in this process: lay-up and bonding.
The outer layers of a multi-layer PCB consist of sheets of glass cloth pre-impregnated with uncured epoxy resin (prepreg) and a thin copper foil.
The lay-up occurs on a heavy steel table with metal clamps. The layers are stacked tightly into alignment pins attached to the table.
The operator lays-up the layers, alternating the etched inner layer substrates with layers of prepreg finishing with a press plate used to ensure a smooth copper finish.
Once the copper press plate is in place, the stack is ready to be pressed. The laminating press uses a pair of heated plates to apply both heat and pressure to the stack of layers. The heat of the plates melts the epoxy inside the prepeg and the pressure bonds all the layers together. The copper foil is now bonded in place to form the outer layers of the PCB.