it is just a 8 layers PCB with min line 4mil ,6mil holes in a 10 mil pads, Can somebody tell me why we need gold plating for each inner layer while immerison nickel gold for all out layers.
Both inner and outer layers require gold as an etchant mask. The outer layers require nickel because the etchant chemistry is different. Difference of chemistry for inner and outer layers is caused by involvement of electroless step for holes walls plating. Typically, palladium is the starter for walls. I read this on the EPA site.
The dominating requirement after all is environmental impact of etchants.