It is to “stack ”inner layers,prepreg (which is for insulation purpose) and copper foil of out layers (top and bottom) together.
It come out by using a combination of specific temperature(heat and cooling), pressure for a specific time to allow the resin within the pre-preg to flow and bond the layers together to form a solid multilayer panel.
Normally,there are many times of lamination process for some multi-stacked via and blind holes.
PCB layers stack up is critical to multilayer PCB,WELLER will not change customer’s stack up unless get official approval from customer. This action are far more stringent than those of other suppliers.