Immersion gold pcb board

Multilayer FR4 High-TG with Immersion gold circuit board. Surface treatment according to IPC-4552, EING AU: 2-4U",NI: 118-236u".  Inner layer 2oz and outer layer 2.5oz. Min hole copper thickness according to IPC-III, min 25um. FR4 S-1000-2, TG170. board thickness 2.5mm. Dark green soldermask and white silkscreen.
Through chemical reaction, the gold particles crystallize and attach to the PCB's pad because of weak adhesion, also known as soft gold.
Like immersion Tin, We do immersion gold after make the soldermask. Copper that not covered with solder resist ink will be covered with gold.
The colour of the immersion gold surface is golden yellow.