Electrochemical corrosion study of Pb-free solders
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W. Jillek Department of EFI, Georg Simon Ohm University of Applied Science, 90489 Nürnberg, Germany (Received 1 April 2005; accepted 22 June 2005)
This paper presents an investigation on the corrosion behavior of five solders by means of polarization and electrochemical impedance spectroscopy (EIS) measurements. The Sn–9Zn and Sn–8Zn–3Bi solder, in comparison with the Sn–3.5Ag–0.5Cu and Sn–3.5Ag–0.5Cu–9In solder, were tested in 3.5 wt% NaCl solution and 0.1 wt% adipic acid solution, respectively. The Sn–37Pb solder was for reference in this work. The polarization curves indicated that the Sn–9Zn and Sn–8Zn–3Bi solder showed the worst corrosion resistance both in the salt and acid solutions, in terms of corrosion-current density, corrosion potential, linear polarization resistance, and passivation-current density. Meanwhile, the Sn–3.5Ag–0.5Cu solder remained the best corrosion characteristics in both solutions. It was found that due to microstructure alteration, Bi additive to the Sn–9Zn solder improved the corrosion behavior in the salt solution, whereas decreased that in the acid solution. However, the additive of In degraded the Sn–3.5Ag–0.5Cu solder in both solutions. The EIS results agreed well with the noble sequence of the five solders subjected to the two solutions with polarization. The equivalent circuits were also determined. Nevertheless, the four Pb-free solders exhibited acceptable corrosion properties since there was not much difference of key corrosion parameters between them and the Sn–37Pb solder.
I. INTRODUCTION
Due to recent legislative and market driving forces around the world, the use of traditional Pb-containing solders in the electronic industry are either severely restricted or completely prohibited.1 Many Sn-based alloy series have been developed as candidates of Pb-free solders. The Sn–9Zn and Sn–8Zn–3Bi solder, with combination of low cost and mechanical attributes, are possible replacements of the Sn–37Pb solder without markedly increasing conventional soldering temperature.2 Although this Sn–Zn system may be an industrial choice, especially in Asia, it gives rise to reliability concerns of poor wetting, oxidation and corrosion behaviors because the element Zn is believed to be quite active. So far a lot of work has been performed on the former two properties, with relatively little effort on their corrosion behaviors.3–5 However, they are susceptible to corrosion should they have long-time contact with corrosive
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0035 62
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 1, Jan 2006 Downloaded: 04 Feb 2015
vehicles such as salt or acid matter combined with moisture. For the corrosion researches on Pb-free solders, Mori et al. found that the presence of Bi in Sn–Bi solders slightly increased the preferential dissolution of Sn in H2SO4 solution and dramatically accelerated the dissolution in HNO3 solution, compared with that of pure tin.6 Oulfajrite et al. claimed that increase
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