Enhanced electromigration reliability via Ag nanoparticles modified eutectic Sn–58Bi solder joint

Abstract

A comprehensive study on the effect of Ag nanoparticles doping on the electromigration related microstructure evolution in Sn---58Bi solders is reported. Doped Sn---58Bi solders were prepared by mechanically dispersing Ag nanoparticles additive in Sn---58Bi solders. The interfacial morphologies of the undoped and doped Sn---58Bi solders under a direct current of 2.5 A (103 A/cm2) at 75 °C temperature with Cu pads and Au/Ni/Cu pad on daisy chain type ball grid array substrates for up to 20 days, were analysed. Unlike the undoped solder, there is no obvious formation of Bi-rich IMC growth on the anode side. The Cu---Sn and Au---Sn intermetallic compounds were formed on the Cu and Au pad, respectively, near the cathode and anode interface after the first reflow. Ag3Sn particles were found dispersed in the solder matrix, which behaved like barriers, blocking the diffusion of metal atoms during current stressing. In addition, during the shear test, fracture occurred at the intermetallic compound interface showing a ductile fracture mode. In the solder ball region of Sn---58Bi solder joint, β-Sn matrix with a refined microstructure and nano Ag intermetallic compound were observed, which resulted in an increase in the shear strength, due to a second phase dispersion strengthening mechanism.