| The EU has issued the WEEE mandate that aimed to prohibit the use of Sn-Pb solders in year 2006. Under the pressure of trade competitions, large electronic manufacturers and research institutes started to make wide investigations in the substitutes for Sn-Pb solders. It is suggested that the new lead-free solders should not only have good process performance, but have higher mechanical properties, especially the creep resistance of the solder joint. Due to the excellent wetting and mechanical properties, SnAgCu solder alloys have been regarded as the most promising Pb-free substitutes for the Sn-Pb solders. The Sn-3.8Ag-0.7Cu solder has also garnered great attentions because of its high creep resistance. However, under the drives of increasingly finer pitch design and severe service conditions, novel lead-free solders with higher creep performance are needed. Adding surface-active element to alloy is considered as an effective way to improve the high temperature performance of solders.Taking the most promising substitute of Sn-3.8Ag-0.7Cu solder as the research base, investigations were made to explore the rules of effect of different rare earth contents on the general properties of Sn-3.8Ag-0.7Cu solder. Creep rupture life tests were conducted to examine the effect of different content of rare earth on the creep rupture life of Sn-3.8Ag-0.7Cu solder joint. Creep strain tests were performed to acquire the relevant creep parameters, such as stress exponent,activation energy and constant A, which reflects the creep mechanism. The final creep constitutive equations for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints were established. Optical and SEM observations were made to examine the variation of microstructures of Sn-3.8Ag-0.7Cu solder alloy and its solder joint with the addition of RE.In the examined the series of SnAgCuRE solders, no low-temperature eutectic peak was observed on the DSC curves, which implicated that no low-temperature eutectic phase was produced by adding small amount of rare earth. Rare earth doesn't affect the melting temperature of Sn-3.8Ag-0.7Cu solder greatly. The liquid temperatures of SnAgCuRE alloy lie in the scope of 220℃~225℃. The tests ofphysical properties,prcocessability and regular mechanical properties of SnAgCuRE show that the content of RE added into the Sn-3.8Ag-0.7Cu is not suggested to exceed 1.0wt.%.Minute amounts of RE can increase the creep rupture life of solder joint at room temperature, up to 7 times more than that of Sn-3.8Ag-0.7Cu when RE is 0.1wt.%. As RE is beyond 0.1wt.%, creep rupture life decreases, especially when RE adds up to 1.0wt.%, the lifetime of SnAgCuRE solder joint is below that of Sn-3.8Ag-0.7Cu, but is similar to Sn-37Pb. Hence, the content of RE in the SnAgCu solder alloy should not be higher than 1.0wt.%. At 65℃, the creep rupture life of joints increases not too much, only 2 times as that of SnAgCu at most. In general, the best range of content of rare earth in the Sn-3.8Ag-0.7Cu solder alloy is 0.05wt.%-0.25wt.%, and should not be beyond 1.0wt.%.Measurements were conducted on the Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints for confirming the creep parameters, i.e., stress exponent,activation energy and constant A. The final constitutive equations for these two solder joints were established, which demonstrated the dependence of steady-state creep rate on the stress and temperature. By correcting the apparent creep activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints from the experiments, the true creep activation energy is obtained. Results indicated that under low stress, the true creep activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the lattice self-diffusion activation energy, so the steady creep strain rates are dominated by the rate of lattice self-diffusion under low stress. Under high stress, the true creep activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the dislocation pipe diffusion activation energy, so the steady creep strain rates...
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