Improved electro-migration resistance in nano Ag modified Sn-58Bi solder joints under current stressing

In this study, doped-Sn58Bi solders were prepared by mechanically dispersing Ag nano particles additive in Sn-58Bi solders. The interfacial morphologies of the plain solder and doped Sn-58Bi solders under a direct current (DC) of 2.5 A at 75°C temperature with Cu pads and Au/Ni/Cu on daisy chain type ball grid array (BGA) substrates were characterized. Unlike the plain solder, there is no obvious accumulation of Bi-rich IMC or Sn rich IMC extrusions on the anode side or cathode side, respectively. The Cu-Sn IMCs on the Cu substrate and Ni-Sn IMCs on the Au/Ni substrates were formed near the cathode interface after the first-reflow induced current crowding near the junction. Cluster like Ag-Sn IMCs in the solder matrix prevented the migration of atoms under current stressing and improved the electro-migration resistance. In addition, fracture occurs at the IMC interfacial region during shear testing with a ductile fracture mode. In the solder ball region β-Sn matrix of Sn-58Bi solder joints with a refined microstructure and inter-metallic compound particles Ag were observed, which resulted in an increase in the shear strength, due to a second phase dispersion strengthening mechanism.