The principle of ENEPIG: Compared to ENIG, there is one more layer palladium(Pd) between Nickel (Ni) and Gold (Au).

As you probably know, deposition thickness of Ni is 120～240μin (about 3～6μm), Pd is 4～20μin( about 0.1～0.5μm) and Au is 1～4μin (about 0.02～0.1μm). In addition, Pd can prevent the board from displacement reaction (this is easy to lead to corrosion) so that it makes full preparation for the coming immersion gold. That’s to say, Au will cover the Pd more closely in order to provide a terrific contact surface.

PS: ENEPIG— Electroless nickel immersion gold.

Electroless nickel immersion gold - Wikipedia