Strain Softening Behavior and Impact of Microalloying in SnBi Solder Alloys

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The impact of microalloying additions to SnBi solder alloys’ mechanical behavior is studied in this paper. We examine the microstructure and mechanical behavior of ten SnBi-based low-temperature solder (LTS) alloys against 96.5Sn-3Ag-0.5Cu (SAC305) high-temperature solder (HTS) alloy, focusing on the effects of microalloying additions of Ag, Sb, and Ni on solder alloy performance and the underlying shear deformation mechanisms when soldered onto Cu pads. We specifically compare eutectic and near-eutectic alloys: Sn58Bi, Sn57Bi-1Ag, Sn57.5Bi-0.5Sb, Sn56.5Bi-0.5Sb-1Ag, Sn57Bi-1Sb, Sn56Bi-1Sb-1Ag, and four proprietary and commercially available Bi-containing solder alloys identified here as Alloys 1–4. Since assemblies consisting of solder joints contain intermetallic compounds of Sn with Cu, Ag, Sb, and Ni, which influence mechanical behavior significantly, it is critical to test solder joints in size scale similar to those used in practice. Therefore, the test specimens were microscale solder assemblies on circuit board coupons containing Cu pads with an organic solderability preservative (OSP) surface finish. Extensive monotonic and creep tests of the specimen were carried out using a precision mechanical tester under closed-loop strain rate or load-controlled tests. Post-test scanning electron microscopy (SEM) revealed microcracks in the bulk solder joints of the SnBi alloys, which is consistent with damage accumulation during mechanical deformation and the observed strain-softening behavior. The extent and distribution of microcracking varied with alloy composition. Binary Sn58Bi and Ag-containing Sn57Bi-1Ag exhibited extensive microcrack networks, consistent with their larger load drops during monotonic shear testing. In contrast, Sb-containing solder alloys such as Sn57.5Bi-0.5Sb and Sn57Bi-1Sb showed fewer and smaller microcracks, suggesting that Sb addition is associated with improved ductility, possibly through its influence on microstructural stability and crack propagation behavior. When Sb and Ag were combined, as in Sn56.5Bi-0.5Sb-1Ag and Sn56Bi-1Sb-1Ag, microcracks were still present but appeared more uniformly distributed, suggesting that the combined presence of SnSb and Ag₃ Ag 3 Sn precipitates may influence crack path development.