Electromigration study in the eutectic SnBi solder joint on the Ni/Au metallization

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Microstructural evolution and interfacial reaction in the eutectic SnBi solder joint on the Ni/Au metallization with and without the current stressing of 6.5 × 103 A/cm2 at 70 °C for 5 to 15 days were investigated. Electromigration is found to have significant effects not only on the phase formation at the joint interface but also on the phase coarsening and mass accumulation of Bi in the solder. In the solder joint without the current stressing, only a thin Ni3Sn4 phase was formed at the joint interface. For the solder joint with the current stressing, in addition to the Ni3Sn4 phase, a thick Au–Ni–Bi–Sn phase was formed at the joint interface after 10 days. However, the Au–Ni–Bi–Sn phase was not observed at the anode-side joint interface after 15 days of current stressing. Coarsening of the Bi-rich grain in the solder joint with the current stressing was much faster than that without the current stressing. Mass accumulation of Bi was observed at the anode side of the solder joint with the current stressing and the thickness of the Bi-rich accumulation layer increased with current stressing time. Based on the accumulation rate of the Bi-rich layer and the growth rate of the Bi-rich grain, the product of diffusivity and effective charge number of Bi in the eutectic SnBi solder is calculated to be 4.72 × 10−10 cm2/s.

I. INTRODUCTION

Solder alloys, such as eutectic SnPb and other Pbcontaining alloys, have been widely used as the interconnecting materials in different levels of electronic packaging. However, due to the harmful impact of the element Pb on environments and human health, the usage of Pb-containing materials in electronics has been legislated to decrease and eventually be completely prohibited. Therefore, how to remove Pb from electronics becomes an urgent issue and numerous efforts from industries and academics have been made to search for suitable alternatives. Pb-free solders are recognized as the most potential replacements for Pb-containing solders, and various compositions that perform different properties are addressed for specific applications. Because the eutectic SnBi alloy (42 wt% Sn–58 wt% Bi) can offer a lower melting point and lower coefficient of thermal expansion (CTE) than the eutectic SnPb alloy, it is recommended for use in processes that are sensitive to temperature or thermal damage.1 Electromigration has been a reliability problem in the

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0113 962

J. Mater. Res., Vol. 21, No. 4, Apr 2006 http://journals.cambridge.org Downloaded: 22 Mar 2015

conducting lines of integrated circuits.2,3 In recent years it has been proved that electromigration is also an important concern of the reliabilities in the solder joint systems, and due to the structural and compositional multiplicities, the influences caused by electromigration on the solder joints are even more complicated.4–13 In flip chip solder joints, when electrons flow from the chip-side metallization into the solder bump, the elect

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