It depends. As for metal base material, both copper and aluminum are used in PCB but with various application;
However, if we discuss this question from PCB Plating perspective, then we’ll get another totally different answer:
PCB plating mainly includes copper plating, tin plating, nickel plating and gold plating. Copper plating is a significant processing technology in the fabrication of PCB.
You know, copper plating has couples of merits like terrific conductivity, thermal conductivity, mechanical ductibility, etc, which refers to Immersion copper after drillling + Plating the whole PCB+ Thicken the circuit with copper, all of which are to guarantee the conductivity; Besides, the common plating solution is CuSO4;
In short, the whole process is like this: Drilling—Immersion Copper—Plating—Drawing—Etching