Nickel
Nickel is a silvery white metal with the atomic number 28. Since nickel dissolves very slowly in liquid solder while at the same time having good wetting characteristic, a nickel is used in the electronic industry preferentially as a barrier (keep-out metallization) for soldering. As nickel oxidizes very easily, and as the oxide layer is very durable, the nickel coating on components and on the boards need to be protected from oxidization. This is achieved with an additional coating of, for example, gold or silver- palladium.