Why ENIG PCB need to plate Nickel?

Do you know what ENIG means for PCB?It is an acronym for Electroless Nickel/Immersion Gold,but some customers ask us why PCB finishing need nickel when they decide to do PCB surface as gold.Best Tech think lots of PCB  Practitioner don't understand or cannot explain clearly,but PCB designers and electronic company should know more information about PCB ENIG,so we offer some skill information for anyone who would like.But don't hesitate to contact us if you would like to know more.

1.plating nickel is to protect base copper as the base copper is very easy oxidation

2.The weldability of nickel is better than gold,and more stable

3.If to plate gold on the copper,the coper ions will migrate to the gold surface，Cause the mottled oxidation，Affect both the function and appearance