Fracture mechanism and strength-influencing factors of Cu/Sn–4Ag solder joints aged for different times

https://doi.org/10.1016/j.jallcom.2009.06.108Get rights and content

Abstract

Tensile properties and fracture mechanisms of Cu/Sn–4Ag solder joints aged at 180 °C for different times were investigated at the strain rates of 1.25 × 10−4 s−1 and 1.25 × 10−1 s−1. At the low strain rate, it was found that the tensile strength of the solder joints decreased with increasing aging time in principle, though the tendency was not monotonously at the early stage of aging; and all the solder joints had similar tensile curves but different fracture morphologies and fracture processes. At the high strain rate, tensile strength of the solder joints was much higher and decreased monotonously with increasing aging time, with identical fracture process and fractographies. Evolution of the Cu/Sn–4Ag interfacial morphology during aging process and the effect of aging on tensile property of the Sn–4Ag alloy were also involved for further analysis. Based on the experimental results and observations, the fracture processes were revealed and some factors controlling the tensile strength of solder joints were discussed qualitatively.

Introduction

Because the toxicity of Pb element in conventional Sn–37Pb solder presents a major health hazard, many lead-free solder alloys have been proposed as alternatives in electronic package. Since the solders used in electronic packaging field serve not only to provide the electronic connection but also to ensure the mechanical reliability of solder joints under the service conditions [1], it has been recognized that one of the major concerns for the integrity of the solder interconnection is the strength and damage mechanisms at the solder/substrate interfaces under various practical service conditions.

In recent years, there have been many investigations on various series of lead-free solder joints from this viewpoint [2], [3], [4], [5], [6], [7], [8], while tensile behaviors of as-soldered and isothermal aged solder joints were widely investigated. It has been reported that tensile strength of the Cu/lead-free solder joints decreased with increasing interfacial IMC thickness in principle and crack usually occurred at the Cu/solder interface [2], [3], [4], [5], [6]. Nevertheless, most of these studies only involved the experiments performed to evaluate the tensile strength of the solder joints; only a few reports are concerning the fracture mechanisms. Lee et al. [3] suggested that the fracture mechanism of the Sn–Ag/Cu solder joints changed from dimple-ductile fracture to transcrystalline-brittle fracture with increasing IMC thickness at the strain rate of about 2 × 10−4 s−1. Kikuchi et al. [9] reported that the as-soldered Sn–Ag/Cu solder joints exhibited a dimple-ductile fracture with a high elongation at the strain rate of 8.3 × 10−4 s−1. However, both of the fracture mechanisms were simply predicated based on only some special region of fracture surfaces. In addition, effect of tensile strain rate on adhesive strength of the solder joints has also been investigated [4], [9]. It has been recognized that the tensile strength increased with increasing strain rate, but influence of strain rate on fracture mechanism of solder joints was less informed. Meanwhile, although influence of aging on mechanical properties of the lead-free solder has been reported [10], [11], [12], its effect on the adhesive strength of solder joints was not ever proposed. The evolution regularity of the tensile strength of solder joints with increasing aging time was not well explained, too.

The discussion above indicates that a comprehensive understanding on fracture mechanism of the solder joints and a conclusion of strength-influencing factors are required. In this study, the Cu/Sn–4Ag solder joints were used; tensile fracture behaviors of the solder joints aged for different times were investigated. The aging time was chosen with a short time step to better reveal the evolution of interfacial morphology and transition of fracture mechanism during aging process. In addition, effect of strain rate on deformation and fracture behaviors of the solder joint was also considered for comparison. By comprehensively comparing and analyzing the fracture surfaces of solder joints, both the intrinsic and the external factors influencing tensile strength were considered to understand the fracture mechanism. Furthermore, it is expected that the current research will provide a new understanding on fracture mechanism of the lead-free solder joints.

Section snippets

Experimental procedures

As the most widely used substrate material in electronic package and scientific experiment, Cu was chosen as substrate, prepared from oxygen-free-high conductivity (OFHC) Cu of 99.999% purity by the Bridgman method in a horizontal furnace, in this study. Moreover, the Sn–Ag alloy is considered as one of the most potential lead-free solder candidates for its well soldering ability and excellent mechanical properties [1], [13], [14], thus, Sn–4Ag (wt%) solder, prepared by melting high purity

Evolution of interfacial morphology

Fig. 2 shows the microscopic morphologies of the as-soldered and aged Sn–4Ag/Cu interfaces. For the as-soldered interface, an IMC layer with scallop-like morphology was formed along the interface, as shown in Fig. 2(a). EDX analysis indicates that the IMC layer with average thickness of ∼2 μm is the Cu6Sn5 phase. After isothermally aging at 180 °C for 4 h, the IMC/solder interface became evidently flat (see Fig. 2(b)). Meanwhile, careful examination on the interface revealed that the scallop-type

Conclusions

Tensile behaviors of Cu/Sn–4Ag solder joints aged at 180 °C for different times and tested at the strain rates of 1.25 × 10−1 s−1 and 1.25 × 10−4 s−1 were investigated. Based on the experimental results and analysis, the following conclusions can be drawn:

  • (1)

    Due to coarsening of the eutectic structure and the needle-like Ag3Sn particles during early aging process, tensile strength of the aged Sn–4Ag solder is relatively lower than that of the as-cast one, while the elongation is significantly improved.

Acknowledgements

The authors would like to acknowledge X.H.An, H.F. Zou, Q.Q. Duan, Q.S. Zhu, P. Zhang, X.X. Zhang, and W. Gao for sample preparation, tensile tests and SEM observations. This work was financially supported by National Basic Research Program of China under grant No. 2004CB619306, the National Natural Science Foundation of China (NSFC) under grant No. 50625103.

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