Power modules are packages that contain various semiconductor devices, such as MOSFETs, IGBTs, and PiN diodes, and are widely used as a basic component in electrical infrastructure, including power conversion devices, industrial equipment, railways, and automobiles. However, power modules can be degraded by electromigration, a phenomenon where atoms move due to the transfer of momentum from interatomic interactions and from electrons passing through conductors. The atoms are also moved by stress gradients and temperature gradients. The risk of electromigration increases with increasing current density and operating temperature. One of the most effective methods for designing power modules with improved reliability is to develop integrated analytical technology for electromigration that takes into consideration the mutual influences of factors such as electrical, thermal, and mechanical stress. In this paper, we describe an electric-thermal-stress coupled analysis for electromigration in solder joints for power modules. First, the analysis was validated by comparing the results with the literature. Without consideration of the governing equations for heat transfer, stress equilibrium, and vacancy strain, the distribution of vacancy concentrations at each time point was almost the same as the results reported in the literature. When governing equations other than for heat conduction were considered, the steady-state hydrostatic stress and concentration distribution were almost the same as the results in the literature. Next, to evaluate the effect of the solder joint in power modules, the analysis was performed on a single solder joint with consideration of all governing equations. By using an elastic model for stress analysis, the time length to reach the steady state and vacancy concentration in the steady state were evaluated with respect to analysis parameters of thickness, terminal temperature, and current density. As the thickness was increased, the time length and vacancy concentration also increased. With increasing terminal temperature, the time length increased and the vacancy concentration remained almost completely unchanged. As the current density was increased, the time length remained almost completely unchanged and the vacancy concentration increased. However, when an elasto-plastic creep model was used in the stress analysis and the current density or the thickness exceeded a certain threshold, the rate of increase in vacancy concentration was very slow. These results indicated that inelastic properties may have a strong effect on the electromigration degradation in solder joints for power modules. Next, to consider the effects of peripheral components in the power module, the analysis was performed using a SiC device and a substrate in addition to a solder joint. Because the current was concentrated in the substrate, the local peak of current density appeared at the bottom edge of the solder joint, and the absolute value of vacancy concentration increased significantly at the edge. Furthermore, due to the appearance of the current peak, a vacancy concentration gradient was generated not only in the thickness direction of the solder joint but also in its longitudinal direction. Because of the effects of plasticity, creep, and longitudinal gradient, the vacancy concentration did not reach a steady state even after a long time. These results indicate that peripheral components may have a strong effect on the electromigration degradation of solder joints in power modules. In addition, we simulated the behavior of metal atoms passing through the interface of the solder joint and investigated the effects of electromigration on intermetallic compounds.


Electromigration Analysis of Solder Joints for Power Modules Using an Electrical-Thermal-Stress Coupled Model

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

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