In Situ-Raman Spectroscopy and Electrochemical Characterization on the Enig Process
Authors: Accogli Alessandra , Lucotti Andrea , Magagnin Luca 
Published: February 2020

Electroless nickel immersion gold (ENIG) 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 production 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 solderable, aluminium wire bondable and provides an excellent electrical contacting surface. [1,2] 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.[3-6] 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 understand the effects of each complexing agent used for this surface finishing treatment by means of electrochemical and spectroscopic investigations. The effects are studied in terms of Ni corrosion from the electrochemical 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. For 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 different electrolytes. The same procedure will be applied on Cu-contaminated solution in order to understand if Cu presence, accountable of reddish gold issue, can affect the adsorption phenomena on the electrode surface. Moreover, a study on gold deposit nucleation and growth, both for pure and contaminated solution, has been done: samples at different immersion times have been analysed using atomic force microscopy. The Cu effect has been investigated from the applicative point of view performing solderability tests which results will be presented.

References

[1] QV Bui, ND Nam, JW Yoon, DH Choi, A Kar, JG Kim, et al. Effect of Gold on the Corrosion Behaviour of an Electroless Nickel/Immersion Gold Surface Finish, Journal of ELECTRONIC MATERIALS. 40 (2011) 1937-1942.
[2] DE Walsh, G Milad, D Gudeczauscas. Final Finsh: Printed Circuit Boards, Metal Finishing. (2003) 25-26.
[3] R Ramanauskas, A Selskis, J Juodkazyte, V Jasulaitiene. PCB failure analysis related to the ENIG black pad problem, Circuit World. 39, issue 3 (2013) 124-132.
[4] N Biunno, M Barbetta. A root Cause Failure Mechanism for Solder Joint Integrity of Electroless Nickel/Immersion Gold Surface Finishes, IPC EXPO Conference Proceeging. (1999) S18-5-1-S18-5-8.
[5] M Goosey. Factors influencing the formation of “black pad” in electroless nickel‐immersion gold solderable finishes—a processing perspective, Circuit World. 28 (2002) 36-39.
[6] G Milad. Is “black pad” still an issue for ENIG? Circuit World. 36 (2010) 10-13.