Copper - not just good for pennies
Copper is a versatile metal that works very well for the conduction of electrical current. In fact, it has the highest electrical conductivity rating of all non-precious metals, making it highly effective for performance and cost. For a vast majority of PCB designs, copper is the conductor of choice. But how do you get copper to line up in a nice orderly manner exactly where the design calls for it?
For PCB manufacturing, copper is integrated into the board in three different ways. The raw laminate material (dielectric) is copper clad, meaning there is a consistent thickness of copper adhered to both sides of the dielectric material. This copper has either a plating mask or an etching mask applied that defines all of the circuitry and pads for that layer. Eventually all of the copper that is between and surrounding the desired circuitry and pads will be chemically etched away, defining all of the needed copper components.
For the external and other plated layers (where there are plated holes), the second application of copper is needed and added after the holes are drilled. This copper is a very thin layer of catalytically deposited copper used primarily to metalize the plated through holes to the point that they will conduct current for electroplating. This deposition of copper is a critical step to insure that all of the holes and vias will successfully plate with copper.
Once these holes are metalized and the plating mask is applied, it is time to electroplate all of the circuitry, pads and through holes with copper. The goal here is to increase the thickness of the base copper foil by approximately 1.0 mil and deposit a total thickness of 0.8 mils of copper into the holes (for typical class 2 designs). This gives the circuitry the current carrying capacity needed and makes the plated through holes robust enough to withstand thermal expansion during operation - as well as maintain electrical connection to all of the necessary internal layers.