Che Ni/Au selective or ENIG after solder mask
To protect the solder pads for oxidation and to make the copper better solder able we can apply ENIG, Electro less Nickel Immersion Gold or also called Che Ni/Au, chemical nickel gold or also called soft gold.
ENIG is a chemical process, applied after solder mask that adds 3-6µm Nickel before adding between 0.06µm and 0.10µm Gold onto the copper..
Che Ni/Au overall or ENIG before solder mask
ENIG can also be applied before solder mask. In that case it resides over the entire copper surfaces of the outer layers of the PCB and we call it, Che Ni/Au overall. This of course has a rather important consequence on the price of the board as the surface covered with gold is many times larger than when applied after solder mask.