Sn-Ag-x solders were used as the interfacial layers between SnBi solder and Cu substrate. The effects of Sn-Ag-x layers on the solderability, microstructure, and mechanical properties of SnBi solder joint were investigated. Experimental results indicate that all the barrier layers have positive effects on improving the wettability of SnBi solder. The relative area and grain size of β-Sn was enlarged due to the addition of Sn-Ag-x layers. Meanwhile, the addition of the interfacial layers decreased the hardness of the SnBi solder joint. The addition of Sn-Ag-x layers increased the thickness of the interfacial intermetallic compound (IMC) but had limited effects on the shear force of the SnBi solder joint. Due to the addition of the interfacial layers, the brittleness of the SnBi/Cu solder joints during the shear test was slightly suppressed.

The effects of porous copper (P-Cu) on the microstructure, hardness, and shear strength of SnBi, SnBi-0.4Ag, and SnBi-1Ag solder joints were investigated in this paper. The experimental results show that P-Cu frames distribute in the solder bulks and form triangular areas. The addition of P-Cu leads to the microstructural refinement in the enclosed areas by the P-Cu frames in the solder bulks. The average hardness here is increased due to the fine grain strengthening mechanism. The SnBi-1Ag@P-Cu solder bulk shows smaller Bi-rich grains but larger β-Sn dendrites than the other two P-Cu-enhanced solder bulks. Porous Cu exists as a frame structure in the solder joints, which hinders the initiation and propagation of cracks and has a positive effect on the improvement of joint strength. Compared to the SnBi and SnBiAg solder joints, the shear strength of the P-Cu-enhanced solder joints is increased by 15%. The average shear strength of the SnBi-0.4Ag@P-Cu solder joint is 79.34 MPa, which is the highest among all the solder joints investigated in this study.