A study of electromigration in Sn3.5Ag and Sn3.8Ag0.7Cu solder lines

Due to the miniaturization of very-large-scale-integration (VLSI) of circuits, electromigration (EM) has become a reliability concern in flip chip solder joints. Electromigration accelerates the dissolution of under bump metallization (UBM) materials into solder alloys, and the migration of solder matrix to the anode side. This rapid dissolution and migration will finally cause an open circuit failure due to the lose of UBM and voids formation at the cathode. At the anode, EM induces excessive intermetallic compound (IMC) formation at the UBM interface, which will degrade the joint strength and device lifetime. In this study, we investigated the polarity effect of EM in v-groove solder line. Cu/Sn3.8Ag0.7Cu/Cu v-groove structure was stress at 150°C to study the dissolution of copper and IMC at the cathode side. Kinetic analysis provided the calculation of the effective charge number Z* of Cu₆Sn₅, which is 87-68. Single-segment and multisegment of Ni/Sn3.5Ag/Ni v-groove lines were prepared for the study of the IMC growth at the anode side and the length dependence of EM in solder alloy, respectively. The kinetic analysis showed that the time dependence of the Ni₃Sn₄ IMC formation at the anode side was between parabolic and linear. The results from the multi-segment samples showed the existence of back-stress in Sn3.5Ag solder.