Mechanistic study of electroless nickel immersion gold (ENIG) deposition

Electroless nickel immersion gold is one of the widely used free-lead final finish in the electronic industry and, even though it is highly expensive due both to the use of precious metal i.e. gold and to process working conditions, its use in PCB pro-duction has been growing significantly because it is compatible with a wide range of component assembly methods. ENIG is a very versatile surface finish: it is sol-derable, aluminium wire bondable and provides an excellent electrical contacting surface. Nevertheless, even if this selective finishing is one of the most convenient process for PCB, some problems related to hypercorrosion phenomena during the ENIG process cause an effective loss of solderability. These problems are known as black pad and pinholes: they weaken the adhesion of the gold film to the surface and can lead to severe failures in solder joints and low shear forces after assembly. Since the ENIG process can be considered as a controlled corrosion of NiP during a displacement reaction driven by the potential difference between Ni and Au, the purpose of this work is to mechanistic understand the effects of each complexing agent present into the real gold bath. The effects are studied in terms of Ni corrosion both from the electro-chemical and from the morphological point of view. To do this the samples are treated in the same way and directly immersed into different electrolytes for the electrochemical characterization by means of OCP and LSV experiments. The effect on the process of different parameters i.e. working conditions, temperature, pH of the solution and presence of contaminants have been evaluated. Regarding morphological and microstructural studies, each sample electrochemically tested has been analysed by means of scanning electron microscope (SEM) and x-ray diffraction (XRD) To assess the different species adsorbed on the NiP surface and on the Au layer during the electrochemical process, an innovative study using the in situ-Raman spectroscopy has been done: the spectra are collected in full swing of the Au deposition at the process temperature using at the beginning the single complexing agents, the mixtures of them and at the end the complete solution. In order to understand the effect of Cu contamination, accountable of reddish gold issue, from the morphological point of view, a study on nucleation and growth of the gold deposit, both from pure and contaminated solution, has been done: samples at different immersion times have been analysed using atomic force micros-copy. The Cu effect has been investigated from the applicative point of view performing some solderability tests like edge dip solder, solder spread, ball shear and cold pull ball tests.