Research On Electromigration Behavior Of Solder Joints In Coupling Thermal-electric Stress

With the continuous miniaturization of electronic devices, the packaging density is increasing and the current density in solder joints has beyond 104A/cm2. The long-standing reliability problem which was named of electromigration(EM) will exists in micro-electronic packaging technology. As so far, EM has been listed in one of the reasons which restrict the development of high density packaging. Furthermore, the electronic products would assemble with lead-contained PCBs and lead-free solder during the period to lead-free. Currently, the process of bonding and the microstructure has been investigated in composited solder joints. But the EM reliability of composited solder joints is still unknown. Therefore, the investigation of lead-free solder joints and composited solder joints in EM reliability is extremely necessary.In this study, the Sn-Ag-Cu alloy joints with different Ag content were investigated in thermal-electric couple field. The influence of Ag content and temperature on EM behaviors and the change on mechanical properties induced by EM were discussed. In addition, the EM behaviors of composited solder joints which assembled by using Sn-3.0Ag-0.5Cu solder and Sn-37 Pb solder. The interfacial evolution of single composition joints and composited joints during EM was investigated. The model of interfacial IMCs was proposed and the mean time to fail(MTTF) was calculated. The results indicated that in the certain range, the EM reliability of EM was improved when Ag content increasing. High temperature accelerated the EM behaviors. Due to the anisotropy of ?-Sn phase, the atomic diffusion flux was unbalance. ?-Sn phase retained its orientation during EM test then raised at anode, depressed at cathode. The ultimate tensile strength was decreased after EM. The migration of IMCs weakened the bonding strength of IMCs and interface. The fracture position changed from solder fracture to interfacial fracture at cathode. Eutectic phases of Sn-3.0Ag-0.5Cu in structure composited solder joints were coarsened under the effect of thermal-electric couple field and migrated from cathode to anode. The thickness of IMCs at anode increased unconspicuous before 500 hrs. Cu6Sn5 particles of Sn-3.0Ag-0.5Cu migrated from cathode to anode gradually when continue current stressing to 1000 hrs. The thickness of IMCs at cathode was controlled by its own IMCs thickness during EM test. Moreover, the thickness of IMCs at anode was affected by several conditions like solder composition, structure, EM time and temperature. The distribution of current density and temperature was investigated with finite element simulation. The results indicated that real current density in solder joints was affected by the position where current loaded. Joule heat of structure composited solder joints generated by EM was higher than lead-free solder joints and lower than lead-contained solder joints. The calculation of Black equation indicated that the mean time to fail of structure composited solder joints was longer than Sn-37 Pb solder joints and shorter than Sn-3.0Ag-0.5Cu solder joints.