Electro-migration study of nano Al doped lead- free Sn-58Bi on Cu and Au/Ni/Cu ball grid array (BGA) packages

In order to study the effect of nano particles addition to solders on the electro-migration properties, Al doped-Sn58Bi solders were prepared by mechanically dispersing Al 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 pads on daisy chain type ball grid array (BGA) substrates were analyzed. Unlike the plain solder, the doped solder does not have obvious formation of Bi-rich or Sn-rich IMC extrusions on the anode or cathode side, respectively. The Sn-Al-Cu ternary phase particles in Cu BGA and Sn-Al binary phase particles in Au BGA are formed in the solder matrix in addition to the Cu-Sn and Ni-Sn phases which were formed near the cathode interface after the first-reflow, blocked the movement of metal atoms/ions, but then induced current crowding. 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.