Microstructure evolution and deformation behaviour of Sn-xBi-1Ag Solder alloys: Influences of Bi content

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The study of microstructural evolution in ternary Sn–Bi–Ag solder alloys is critical due to its direct impact on the mechanical properties of these low-temperature solder materials. This research compares the solidification and deformation behaviors of two such alloys, Sn–10Bi–1Ag and Sn–57Bi–1Ag, to shed light on the influences of bismuth (Bi) content variations. Experimental results indicate that increasing Bi content alters the solidification process and the degree of undercooling. In the Sn–10Bi–1Ag alloy, the eutectic reaction produces plate-like Ag3Sn with a predominant with {101‾0}hcp facet, considering Ag3Sn as a pseudo-hexagonal structure. Conversely, in the Sn–57Bi–1Ag alloy, Ag3Sn nucleates as the primary phase, exhibiting blocky or extensively branched morphologies, enclosed by various facets including {0001}hcp and {101‾0}hcp. Additionally, two distinct Bi particle sizes were identified in the Sn–10Bi–1Ag alloy: eutectic lamellar Bi (∼200 nm) and Bi precipitates (∼15 nm), when cooled at 10 °C/min, in comparison to typical lamellar β-Sn/Bi eutectics in Sn–57Bi–1Ag. In-situ tensile test revealed that the Sn–10Bi–1Ag demonstrates superior strength and reduced ductility. This is attributed to the reinforcement provided by the smaller-sized Bi particles and the eutectic Ag3Sn structures. In contrast, for the Sn–57Bi–1Ag, the primary Ag3Sn phase tends to fracture during deformation, and the deformation mechanism is predominantly controlled by grain boundary and phase boundary sliding. This study elucidates the significance of Bi precipitates and Ag3Sn in Sn–Bi–Ag solder, providing valuable insights for the alloy design of future low-temperature solders.