Solder is widely used to connect chips to their packaging substrates (e.g., print circuit board) in electronic industry. For a long time, Pb-containing Sn-based solder alloys were predominant in electronic packaging because of their excellent wetting property, high reliability and low cost. However, Pb presents potential health hazards so that their use in electronics must be minimized. To limit Pb use in electronics, US, Japan and EU have taken various legislative steps, including a complete ban of Pb-bearing solders in electronic products. In preparation for a complete ban, the electronic packaging industry has been actively searching for Pb-free replacement of Pb-bearing solders. Among many candidates, the eutectic Sn-Ag-Cu is emerging as the most promising replacement.
For the selection of a suitable replacement, the reliability of the solder joint is a critical issue. The reliability concerns may arise from creep and electromigration of the solder alloy because of low melting temperature of Sn-based alloys. However, both the creep resistance and electromigration resistance of solder alloys were not well understood.
In this study, deformation and rupture behavior of the eutectic Sn-Ag-Cu solder alloy was investigated under a range of mechanical and electromechanical loading conditions. Under monotonic loading, the strain rate sensitivity of ultimate tensile strength (UTS) was measured by changing the loading rates. The strain rate sensitivity was found to be relatedto micro-necking process. Under both static and cyclic loadings, the alloy exhibited higher creep stress exponents, which can be explained by the introduction of a threshold stress. The threshold stress was shown to arise from the interaction between dislocations and second phase particles. From the values of the creep activation energy, it was concluded that more than one creep mechanisms operated and the dominant creep mechanism in this solder alloy depended on applied stress and temperature. In the static creep, the dominant creep mechanism went through a transition in the low stress region. However, in the cyclic creep, the transition was independent of the applied stress and occurred at a lower temperature. The transitions were explained by athermal vacancy model, which also provided a good explanation for cyclic creep acceleration and retardation. Analysis of the creep rupture data indicated that the rupture time could be best analyzed by the Manson-Haferd method.
Under electromechanical loading, the solder alloy showed severe softening and faster deformation rates. Application of electric loading before tensile tests resulted in significant weakening of the solder alloy and solder interface. Such a current-induced softening of the solder alloy was related to the excess vacancies generated by electromigration. With prolonged electrical loading, the rupture mode changed from ductile to brittle failure. The relaxation experiments showed that the stress relaxation rate after electromigration was higher than that of as-cast sample, but the creep activation energy was the same. Even faster deformation rates were found under combined electrical and mechanical loads.
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