Kirkendall voids formation in the reaction between Ni-doped SnAg lead-free solders and different Cu substrates

In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t The reactions between Sn2.5Ag solder doped with different levels of Ni (0–0.1 wt.%) and two different types of Cu substrates, electroplated Cu and high-purity Cu substrates, were studied. The main objective was to investigate the effect of Cu substrate and the effect of Ni additions on the formation of Kirkendall voids within the Cu 3 Sn phase. Reaction conditions included one reflow and subsequent aging at 160 °C for up to 2000 h. After reflow, Cu 6 Sn 5 was the only reaction product observed for all the different solders and substrates used. During aging, both Cu 6 Sn 5 and Cu 3 Sn formed. Nevertheless, Kirkendall voids were observed only when the electroplated Cu was used, and was not observed when high-purity Cu was used. It was proposed that impurities in electroplated Cu helped the nucleation of these voids. The Ni additions made the Cu 3 Sn layer thinner. For the case of the electroplated Cu substrates, the amount of Kirkendall voids decreased correspondingly with the Ni additions. At temperatures greater than 50–60 °C, the solid-state reactions between Cu and many Sn-based solders produce two reaction products, Cu 6 Sn 5 and Cu 3 Sn [1]. The growth of Cu 3 Sn tends to induce the formation of Kirkendall voids, while the growth of Cu 6 Sn 5 does not [1–4]. Zeng et al. [4] reported that a large number of Kirkendall voids formed when electroplated Cu was reacted with the eutectic PbSn solder at 100–150 °C. These voids located not only at the Cu/Cu 3 Sn interface but also within the Cu 3 Sn layer. The formation of Kirkendall voids is not limited to the eutectic PbSn solder. Kirkendall voids were also reported to form when electroplated Cu was reacted with SnAgCu solders [5], or pure Sn [1]. The relatively rapid diffusion of Cu out of the Cu 3 Sn layer could be the major contributing factor for the formation of these voids. It was indeed shown that although both Cu and Sn were mobile within Cu 3 Sn, the Cu flux was somewhat greater than that of the Sn …