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For power devices, the reliability of thermal fatigue induced by power cycling has been prioritized as an important concern. Because, the power device like the converter system is the key in vehicle applications. So, high reliability is demanded by reduction in the size and high power capacity. Therefore shortening reliability evaluation is demanded using finite element method (FEM). Since power cycling produces non-uniform temperature distribution in the device, coupled thermal-mechanical analysis is required to evaluate thermal fatigue mechanism. The thermal expansion difference between a package and a substrate causes thermal fatigue. Many studies on thermal fatigue in electronics devices have been reported but temperature is often assumed to be uniform under thermal cycling environment. In this study, thermal fatigue of solder joints on power device was evaluated. Joule heating produces temperature distribution, which affects the behavior of thermal fatigue. The FEM was used to evaluate temperature distribution induced by joule heating. Higher temperature appears below the bonded wire because the electric current flow through the bonding wire. Coupled thermal-mechanical analysis was also performed to evaluate the inelastic strain distribution. The damage of each element can be calculated from equivalent inelastic strain range and crack propagation was simulated by deleting damaged elements step by step. The crack initiates below the bonded wire and propagates concentrically under power cycling. There is the difference from environmental thermal cycling where the crack initiates at the edge of solder. And power cycle switching frequency was compared. It was clarified that power cycle fatigue life depends on power cycle switching frequency. As a result, the failure mechanism of thermal fatigue by joule heating was clarified. Therefore the evaluation method could propose best experimental conditions in power cycle experiment.
Date of Conference: 28-31 May 2008