Reliability and failure analysis of Sn-Ag-Cu solder interconnections for PSGA packages on Ni/Au surface finish

The reliability and failure modes of eutectic Sn-Ag-Cu solder joints were studied and compared to eutectic Sn-Pb-Ag ones. Two different failure modes occur: brittle fracture and fatigue. The results show that with a Ni/Au surface finish the reliability of Sn-Ag-Cu solder is much better than that of Sn-Pb-Ag solder. First, when the joint is deformed at high thermomechanical strain, the chance of brittle fracture at the Ni-Au interface is significantly reduced when using Sn-Ag-Cu solder. The reason is a reduced formation of the brittle (Au, Ni)Sn₄ intermetallic at the UBM interface, responsible for the brittle fracture mode. Second, after deformation at low thermomechanical strain, both solders fail due to solder fatigue, but the Sn-Ag-Cu shows a better lifetime. This better reliability of the Sn-Ag-Cu solder is attributed to a new solder fatigue mechanism: the crack propagates through the bulk of the solder in a web-fashion way, linking the Au-containing particles formed in the volume. This is beneficial for the joint reliability as it hinders the crack propagation.