Understanding the Failure Mode of Electroless Nickel Immersion Gold Process: In Situ-Raman Spectroscopy and Electrochemical Characterization

Since the early 70s, printed circuit boards (PCBs) are firmly entrenched in all electronic branches, from the consumer electronics to scientific and medical equipment, culminating later in the personal computer industry. Electroless nickel immersion gold (ENIG) process is one of the most used selective finishing in PCBs production. It involves two different electroless deposition mechanisms: (1) NiP autocatalytic deposition and (2) gold galvanic immersion plating in which displacement reactions are involved. Because during ENIG process, NiP is dissolved from the electrode surface into the solution, it can be considered as a controlled corrosion process of the metal substrate: selection of the complexing agents in solution is thus crucial. Since they are the most used complexing agents, EDTA and citric acid have been investigated. In order to understand their effects on the NiP surface during the immersion plating, open circuit potential (Voc) and linear sweep voltammetry (LSV) measurements have been performed. Furthermore, to establish which complexes are adsorbed on the electrode surface during the plating process, in situ Raman spectroscopy was carried out showing EDTA to be the most effective Ni cations chelating agent. To assess the effect of Cu contamination, accountable of reddish gold failure mode, the electrochemical and in situ spectroscopic analyses on both immersion gold solution and polluted revealed that the adsorption of Cu(CN)-2 and/or Cu(CN)32- complexes is competitive with Au(CN)-2. To evaluate the effects of copper contamination on gold nucleation and growth FE-SEM and AFM were carried out, while gold layer solderability has been evaluated according to NF-A-89 400 standard.