Power Cycling Lifetime of Shunt Resistors in IGBT Modules

Abstract:
Shunt resistors used for the measurement of phase currents in IGBT modules are subjected to repetitive current stress during inverter operation. Like for the semiconductor dies, the power cycle loading eventually can lead to electrical failures and the lifetime expectancy should be well known and matched to the load requirements of the target application. This paper reports on a comprehensive power cycling study performed on various types of shunt resistors incorporated in IGBT power modules. Depending on the resistor material, the shunt geometry and the used solder material, the number of cycles to failure can vary by several orders of magnitude. Two main failure mechanism could be identified: For shunt types with high legs an increase in resistance occurs after a large number of very high temperature swings. Low-rise shunt types develop cracks in the solder joints already at much lower temperature swings. The power cycling study was supported by coupled thermal-electric and subsequent thermo-mechanical simulation. Already a linear elastic simulation approach yields very good results in terms of predicting the stress in the solder joints and linking it with test results. This simulation approach can help to transfer lifetime curves to similar types of shunt resistors with modified geometry, alternative shunt material or different resistance values without the need for time-consuming test series.