Failure mechanisms of Ni/Sn3.0Ag0.5Cu/OSP flip chip solder under high current stressing

The electromigration-induced failure of Ni/Sn-3.0Ag-0.5Cu/ organic solderability preservatives (OSP) flip chip solder joints was investigated under a current density of 1 × 10⁴ A/cm² at 150 °C for 1000 h. Three-dimensional (3-D) finite element simulations on current density and temperature distribution in the test structure were carried out. Current density simulation implied that current crowding effect obviously existed at the electron-entry point and electron-exit point. While temperature distribution simulation implied that the temperature was quite uniform inside the entire solder bump. During EM, when electrons flowed from the PCB to the chip, i.e., the Cu pad on the PCB was the cathode, the Cu pad on the PCB was almost completely consumed and the voids extended across the entire cathode interface, which induced the failure of the solder joint. The dissolved Cu atoms were driven toward the anode side and precipitated a large amount of Cu₆Sn₅ in the solder matrix near the electron-exit corner. When electron flowed from the chip to the PCB, i.e., Ni UBM on the chip was the cathode, no serious consumption of Ni UBM and underneath Cu pad occurred, and no large voids formed at the cathode interface. Furthermore, no large numbers of Cu₆Sn₅ IMCs formed in the solder matrix. The growth of (Cu,Ni)₆Sn₅ IMCs at the Ni/solder interface (the cathode) was retarded and the growth of Cu₆Sn₅ IMCs at the Cu/solder interface (the anode) was enhanced.