Multilayer PCB fabrication is a process that follows the computer design, pre-engineering, prototyping and testing. It gives you the ready product, usually in mass-scale production, ready to be populated by components.
Methods of multilayer PCB fabrication are divided into subtractive and additive. Subtractive methods are the most common in PCB fabrication and involve apllying a thin copper foil over both sides of the board completely, thus making a blank PCB and then removing the extra copper forming the traces that will provide the connection among the elements.
In PCB fabrication for removing extra copper with subtractive methods one of the following mechanisms is used: Silk screen printing with etch-resistant chemicals, photoengraving and PCB milling by removing copper mechanically. All these methods resemble plotter printing in one way or another and are conducted by a computer.
Multilayer PCB fabrication may require using an additive method because it makes possible to achieve a precise plating-through of the vias that will eventually provide a connection between the layers.
The most popular and inexpensive method of producing such boards cannot be attributed entirely to one of the common methods. That's why it is called semi-additive.
The first stage of multilayer PCB fabrication using the semi-additive method will be applying an extra-thin copper foil on the substrate material. Then a mask exposing the future traces is applied on the board and extra copper is added to the exposed traces. Newly formed copper traces are then plated with tin, gold or other plating material. Afterwards the initial copper foil is removed by etching, which is a subtractive method.
