It may be difficult to fathom but each PCB layer is created one at a time. These individual pieces are then bonded together to form your final PCB stackup. The most common PCB building block is 2-layer stock material. This material is two copper sheets laminated to either side of a fixed thickness substrate. This may sound complicated, but it’s just a fancy way of saying: a nonconductive material sandwiched between two pieces of copper.
The easiest way to understand the construction process is to work through an example. So we will build on the 4-layer PCB example used in the layers and stackup articles. Since there are two layers on each piece of stock material, 2 pieces of this material are required to build our 4-layer board (4 total layers required divided by 2 layers per piece of stock material). Instead of using two separate pieces though, the manufacturer will etch your layers and those of many other customers onto very large sheets of stock material. Your individual layer pieces are then cut out of the larger material and separated from those that belong to other customers. Given our example, our layers would be etched as follows: Piece #1 – Top Routing Layer and the Internal Ground Plane, Piece #2 – Internal Power Plane and Bottom Routing Layer.
Now that our individual layers are etched and cut out, the manufacturer will apply a nonconductive material known as prepreg between them and subject the whole thing to pressure and high temperature while the prepreg cures. At the end of the cure, we’re left with our custom 4-layer PCB. This process is called lamination.
Now that we have our 4-layer PCB, any holes specified in the design are drilled into the board. This includes mounting holes, through holes for components and via holes. After the holes have been drilled, the board then goes through a plating process which accomplishes two things: provides a protective layer to the exposed copper on the top and bottom layers and fills in the designated holes (that were just drilled) to make through-hole connections between the top and bottom layers. Once plating has been completed, the manufacturer puts the final touches on your PCB by applying solder mask and silkscreening your reference designators onto the top and bottom layers.