Discoloration related failure mechanism and its root cause in electroless nickel immersion gold (ENIG) pad metallurgical surface finish

Electroless nickel and immersion gold (ENIG) plating is one of the common pad metallurgical surface finish for area array packages and printed circuit boards. ENIG provides a uniform coating on copper pads for excellent solderability and enhanced resistance to corrosion and wear. Nickel plays an important role as a diffusion barrier for Cu while the thin immersion gold layer over nickel protects the Ni from oxidation. This paper describes a discoloration related failure mechanism in ENIG pads. The discoloration is caused by severe oxidation of the Ni, which in extreme severity leads to electrical fails due to increased contact resistance. This paper explains the root cause for the Ni oxidation as due to the porosity of the immersion gold and the synergistic degradation of the pores during the down-stream backend and assembly's thermal and wet treatment. Optimization of the IG plating solution recipe and IG bath control parameters is necessary to mitigate the degrading impact of the downstream processes on ENIG surface finish and for elimination of the discoloration related ENIG quality issue.