Interfacial reactions between Sn–8Zn–3Bi–xNi lead-free solders and Cu substrate during isothermal aging

https://doi.org/10.1016/j.matchemphys.2010.05.028Get rights and content

Abstract

In this work, interfacial reactions of Sn–8Zn–3Bi–xNi (x = 0, 1) lead-free solders with Cu substrate and the growth of intermetallic compounds (IMCs) during isothermal aging were investigated. After soldering at 250 °C for 90 s, the Cu5Zn8 and CuZn5 phases formed at the Sn–8Zn–3Bi/Cu interface and only the Cu5Zn8 phase was found at the interface of the solder with addition of Ni. During aging treatment at 150 °C for 100, 400 and 900 h, the CuZn5 IMC at the Sn–8Zn–3Bi/Cu interface transformed to the Cu5Zn8 due to Cu atoms diffusing from Cu substrate. The Cu5Zn8 IMC layer at solder/Cu interfaces grew thicker with increasing the aging time and the growth was diffusion controlled. Moreover, the thickness of the IMC layer at the Sn–8Zn–3Bi/Cu interface was thicker than that at the Sn–8Zn–3Bi–1Ni/Cu interface. The reduction effect of Ni addition to the solder on the interfacial reaction might be attributed to the formation of the Ni5Zn21 IMC in the solder bulk, which effectively suppressed the diffusion of Zn atoms to the interface to react with Cu.

Introduction

With the legislation banning the use of lead-containing substances in electronics due to increasing environmental and health concerns, traditional Sn–Pb solder alloys are being urgently required to be replaced by lead-free solders in electronic industry [1]. Recent years, a lot of Sn-based lead-free solders have been developed, such as Sn–Ag, Sn–Ag–Cu, Sn–Cu, Sn–Zn. Among the new lead-free candidates, the Sn–9Zn eutectic alloy is widely recommended due to its low cost, excellent mechanical properties and especially because of the significant benefit of its low melting point (197 °C), which is closer to the traditional eutectic Sn–Pb (183 °C) solder than any other lead-free solders [2]. Nevertheless, Sn–Zn alloys have some disadvantages, such as poor wetting properties, easy oxidation [3]. Therefore, extensive studies have been performed to improve these features by adding some foreign elements such as In, Bi, Al, Ag into the solder alloys [4], [5], [6], [7]. It has been found minor Bi elements addition to the Sn–Zn alloy could lower the melting temperature and increase the wettability [5]. Also, Yu et al. reported that the mechanical properties of Sn–Zn alloy could be improved by adding Al [6]. And adding Ni to the Sn–9Zn solder alloy could effectively improve the poor oxidation resistance as well as the mechanical properties by forming Ni–Zn IMCs [8].

As we all know, one of the major concerns of electronic packaging is the reliability of the solder joints. And during soldering process of electronic interconnections, the solder alloy reacts with the substrate to form IMCs at the interface. Then the growth of IMCs during solid-state aging at the interface can affect the solder joint reliability seriously. The excessive IMCs growth between the solder and substrate can significantly degrade the mechanical properties and the performance of solder joints due to the brittle nature of IMCs and the dissimilar coefficients of thermal expansion of solders and metallizations [9], [10]. Thus, it is necessary to understand the kinetics of IMC growth in the interfacial reaction during thermal aging. It has been reported that the interfacial reaction was suppressed by adding some strong compound forming elements with Zn such as Cu, Ni, Cr, Ag to the Sn–Zn solder [7], [11], [12]. As Cu is most commonly used for the under bump metallization in the flip chip technique in electronics assemblies, considerable investigations of the interfacial reactions between the solder and Cu substrate have been made in recent years [13], [14], [15]. However, the IMCs formation and microstructure evolution in the reaction between Sn–Zn–Bi solder with addition of Ni and Cu substrate during isothermal aging have not been sufficiently studied yet.

The purpose of this paper is to investigate the interfacial reactions between Sn–8Zn–3Bi–xNi (x = 0, 1 wt.%) lead-free solders and Cu substrate during aging at 150 °C for different durations.

Section snippets

Experiment

Sn–8Zn–3Bi and Sn–8Zn–3Bi–1Ni solder alloys were employed in this study. The Cu plate with 99.99% purity was used as the substrate. Before the experiment, the Cu surface was ground with fine sandpaper and then was deoxidized in 3% HCl solution for 1 min, and finally ultrasonically cleaned by deionized water and then by ethanol. A piece of solder was placed on the Cu substrate after the above treatment with rosin mildly activated flux. Then it was put in a furnace for 90 s at a temperature of 250 

Interfacial reactions of Sn–8Zn–3Bi–xNi/Cu during soldering

Fig. 1a and b show the back-scattered electron (BSE) micrographs of the microstructure in the as-soldered Sn–8Zn–3Bi and Sn–8Zn–3Bi–1Ni solders respectively. In both Sn–8Zn–3Bi and Sn–8Zn–3Bi–1Ni solders, needle-shaped Zn-rich phase and Bi phase, as indicated by arrowheads, distributed in the β-Sn matrix. However, in the Ni-containing solder, some Ni–Zn IMCs formed inside the bulk solder. As analyzed by EPMA, the composition of the IMC is Ni:Zn:Sn = 15.8:81.2:3.0 at.%. Based on the compositions

Conclusions

The Cu5Zn8 and CuZn5 IMC layers formed at the Sn–8Zn–3Bi/Cu interface, while only the Cu5Zn8 IMC layer formed at Sn–8Zn–3Bi–1Ni/Cu interface after soldering at 250 °C for 90 s. With increasing the aging time, the CuZn5 phase transformed to the Cu5Zn8 phase. The Cu5Zn8 IMC layer at the solder/Cu interfaces grew thicker with increasing the aging time and the growth was diffusion controlled. It is noticeable that thickness of the Cu5Zn8 IMC layer at the Sn–8Zn–3Bi–1Ni/Cu interface was much thinner

Acknowledgements

The authors would like to acknowledge the support of the National Natural Science Foundation of China (50674071), the Tianjin Natural Science Foundation (06YFJZJC01300) and the Program for New Century Excellent Talents in University (NCET-06-0245).

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