No abstract

Power semiconductors and modules are basic components of electrical infrastructure and are currently widely used in applications such as power conversion devices, industrial equipment, railways, and automobiles. Power modules are being developed with the aim of downsizing and increasing power output. With the larger current densities and higher operating temperatures associated with downsizing and increasing power output, degradation of power modules can occur as a result of electromigration. Electromigration is a phenomenon where atoms move due to the momentum transfer between conducting electrons and metal atoms. In addition, atoms are also moved by mechanical stress gradients and temperature gradients, so it is necessary to take into consideration the combined effects of electrical, thermal, and mechanical stress. In this report, we describe an electrical-thermal-mechanical coupled analysis of electromigration in a bonding wire of a power module. First, the analysis is validated under the condition that the displacement of the wire surface is fixed. The distributions of vacancy concentrations and hydrostatic stress are almost equal to those in previous studies. Next, we present the influences of current density, temperature, and the displacement constraint on electromigration in a wire with a simplified shape. The analysis results confirm that the plasticity and creep should be taken into consideration in a bonding wire. This also confirm that vacancy concentration increase more rapidly by changing the displacement of the wire surface from the fixed condition to the free condition. Finally, we present analysis results for a bonding wire with the actual shape found in power modules. In this wire, a local concentration peak appear in the electrode terminal. The analysis results reveal that electromigration may affect not only void formation but also other failure phenomena in the bonding wire of power modules.

The larger current densities accompanying increased output of power modules are expected to degrade solder joints by electromigration. Although previous research has experimentally studied electromigration in solder, die-attach solder joints in Si-based power modules have not been studied because the average current density of the die-attach solder is much smaller than the threshold of electromigration degradation. However, even in die-attach solder, the electromigration degradation may appear where current crowding occurs. This report describes electromigration analysis of die-attach solder joints for Si-based power modules using an electrical-thermal-stress-atomic coupled model. First, we validate our numerical implementation and show that it can reproduce the distributions of vacancy concentrations and hydrostatic stress almost the same as the analytical solutions even at current densities assuming current crowding. We then simulate the die-attach solder joint with a Si-based power device and a substrate. Due to current crowding, the current density at the edge of the solder exceeds the electromigration threshold. Unlike general electromigration phenomena, the vacancy concentration increases at the center and decreases at the edges of the solder joint, regardless of whether it is on the cathode side or anode side, due to the longitudinal driving force in the solder joint generated by the current crowding. Creep strain increased remarkably at the anode edge and the cathode center. The absolute vacancy concentration clearly increased with increasing current density and size ratio. Creep strain significantly increased with increasing current density, size ratio, and temperature.

Prognostic health monitoring technologies for power electronic systems assess their performance degradation, load histories, and degrees of fatigue in order to increase maintenance effectiveness, reliability design methods, and equipment availability under conditions of actual use. To improve reliability and reduce downtime, prediction of reliability in terms of thermal fatigue life under field conditions is important, as is the use of load and health monitoring data from the field in cases of performance degradation during use, maintenance, and field failure. The fatigue life of solder joints is also affected by whether the load history waveform is symmetric or asymmetric. In this paper, we propose a novel health monitoring method for thermal fatigue failure corresponding to time-dependent inelastic strain response, such as in asymmetric cycles, by use of a surrogate model obtained by a finite element method-based thermal stress simulation. We applied this method to an insulated-gate bipolar transistor power module capable of monitoring module temperature, electrical performance, and number of revolutions of the cooling fan. With the proposed method, inelastic strain cycles and thermal fatigue life distribution of solder joints could be estimated from their temperature monitoring history. The method was judged to be useful for assessing thermal load histories and estimating thermal fatigue life in prognostic health monitoring.

In recent years, a fatigue life law based on inelastic strain energy density as proposed by Morrow has been applied to solder materials. In this study, the effectiveness of the fatigue life law based on inelastic strain energy density was compared with the conventional law based on inelastic strain range. First, the fatigue properties of Sn-Ag-Cu solder alloy were investigated by a torsional fatigue test with strain control. It was found that the stress–strain hysteresis loop arising from inelastic deformation occurred even under a low strain load with a fatigue life of about 1 million cycles. Therefore, as an extension of the low-cycle fatigue test, evaluation was performed using inelastic strain range and inelastic strain energy density. Experimental results show that when fatigue life was evaluated using inelastic strain energy density, a single power law was found over a wide range from the low-cycle region to the high-cycle region, and the validity of the fatigue life law based on inelastic strain energy density was confirmed. Next, a simple prediction method for the fatigue life law based on inelastic strain energy density was examined, taking the physical background into account. Two material constants of the fatigue life law based on the inelastic strain energy density were estimated from the stress–strain curve for a monotonic load and shown to be close to the actual fatigue test results.