In addition to the wiring on the substrate, the metal coating is where the substrate wires are welded to the electronic components.In addition, different metals also have different
prices, different will directly affect the cost of production;Different metals also have different weldability, contact, and resistance values, which will directly affect the performance of components. 

Common metal coatings are:
Copper；
Tin；

The thickness is usually between 5 and 15 cm Lead-tin alloy (or tin-copper alloy).
That is, solder, usually 5 to 25 m thick, with a tin content of about 63%.

gold； Generally will only be plated on the interface.

silver；Generally will only be plated on the interface, or the whole is also an alloy of silver.