Thermal stability of electroless nickel/immersion gold surface finish for direct bond copper

We focus on the electroless nickel/immersion gold (ENIG) plating on a Cu disk and evaluate the initial characteristics of Ni(P) layer and the thermal stability of ENIG during the thermal storage test at 250°C in air condition. The initial plated Ni(P) contains about 6.5 wt.% phosphorus and shows the mixture of amorphous and small amount of nanocrystalline Ni. After the thermal storage up to 720 h at 250°C, the crystallization of Ni and phase transformation into Ni₃P are detected.