Abstract
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 and/or complexes is competitive with 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.
Highlights
Understanding the Failure Mode of Electroless Nickel Immersion Gold Process: In Situ-Raman Spectroscopy and Electrochemical Characterization
Basing on the above considerations, in this paper we provide a comprehensive understanding of the mechanisms involved in the Electroless Nickel Immersion Gold (ENIG) process, adopting an innovative approach coupling Raman in situ analysis and electroanalytical methods
ENIG process can be assumed as a controlled corrosion process, driven by the different nobility of involved metals i.e. Au and Ni, because galvanic displacement reaction occurs on NiP surface during the Au deposition
Summary
Understanding the Failure Mode of Electroless Nickel Immersion Gold Process: In Situ-Raman Spectroscopy and Electrochemical Characterization. The ENIG process is an attractive and interesting finishing, the process often suffers, especially in the real industrial applications, of a problem known as black pad, responsible for mechanical failure of solder joints.[4,10,11] Several efforts have been done in order to understand the causes of this defect and how to prevent it It may be causes by differences of surface potential induced by plating parameters, bath chemical composition and contaminations and pads connection that can induce a non-uniform gold layer deposition affecting the properties of the board both in terms of surface appearance and solderability.[4,10,11,12,13,14,15] This effect, known as galvanic effect, responsible of Ni layer hypercorrosion, is strongly dependent both by the surface preparation, inducing Cu contamination during the IG step, and by chemical species adsorbed on the surface, deeply affecting the gold deposition rate.[4,10,11,16,17] In particular, Au ions tend to be reduced on more cathodic surface with respect to the less cathodic one, resulting in higher deposition rate and more thicker gold layer. The well-known issue of copper contamination in Au deposit was investigated, giving an insight to its origin and relation to the impact on final properties of the electronic components
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