Size effect behavior of microstructure and mechanical properties in Sn-58Bi micro solder joints

The effect of reducing the size of reflow solder joints on the microstructure and morphology of Sn-58Bi solder joints was studied. The effect of reducing the size of solder joints at different aging times on the evolution of interfacial microstructure and shear properties of solder joints was also analyzed. The results showed that under the same cooling condition, the reduction of solder joint size resulted in a greater undercooling of solder joints during the solidification stage, the growth of β-Sn primary phases, and the differences in the grain size distribution of Sn and Bi phases within the solder joints. Due to the differences in the diffusion rates of Cu atoms, the reduction in the size of Sn-58Bi solder joint was beneficial to the growth of interfacial metallic compounds (IMCs) after reflowing. Meanwhile, under aging condition, the interfacial growth rate of IMC at the solder joint with size of 300 μm was greater than that with size of 400 μm and 760 μm. At the interface of solder joints, the joints with size of 300 μm needed a shorter time to grow from a scallop shape to a flatten shape. Due to constrain effect, the joint with size of 300 μm presented a higher shear strength compared with the joints with size of 400 μm and 760 μm.