Microstructural evolution and thermal fatigue reliability of Sn-Ag-Cu/Sn-Bi-Ag hybrid BGA solder joints assembled by low-temperature soldering

The Sn-3.0Ag-0.5Cu/Sn-49Bi-1Ag hybrid BGA solder joints were assembled by low-temperature soldering at 190°C and 210°C, respectively. The microstructural evolution and thermal fatigue reliability (-55~100°C) of hybrid solder joints were investigated. In the as-soldered complete melting solder joint (210°C), the chemical composition was uniform and identified as Sn-8.3Bi-2.7Ag-0.4Cu (wt.%); while in the partial hybrid solder joint (190°C), an inhomogeneous structure of un-melted Sn-3.0Ag-0.5Cu solder ball was partially wrapped by Bi-rich zone, which was identified as Sn-14.0Bi-2.4Ag-0.4Cu (wt.%). The Bi phases in the partial hybrid solder joint was mostly irregular reticular. The higher Bi content and irregular reticular Bi phases in partial hybrid solder joints led to a better thermal fatigue performance. However, the thick interfacial IMC layer and the large difference in thermal expansion coefficient among the materials in complete melting solder joints caused worser thermal fatigue performance of the complete melting solder joints than that of partial hybrid solder joints.