High temperature die-attach materials for aerospace power electronics: Lifetime tests and modeling

Increasing the temperature in power electronic applications usually causes a decreasing lifetime and reliability. This study shows that packaging materials and technologies, such as silver-sintering or gold germanium solders combined with silicon-carbide devices, can easily deal with temperatures above 200 °C/392 °F. Furthermore, lifetime tests (active power cycling) with 300 devices offered more cycles to failure at 120 °C/248 °F heat sink temperature than at 40 °C/104 °F (same ΔT) for silver-sintered samples and gold-germanium solders. SAC305 and tin-lead solders were also tested for comparison but could not withstand the harsh conditions. The samples were silicon carbide diodes attached to copper-ceramic-substrates (DBCs). For testing, the devices were heated up by current to achieve a 130 K temperature swing at different coolant temperatures (250 °C/482 °F maximum temperature). The reason behind the higher lifetime at elevated temperatures is the increasing ductility over temperature. The materials capability against thermo-mechanical stress is better at higher temperatures, while creep effects are not dominating. This effect can be used especially for high temperature application with extraordinary requirements on lifetime and reliability. Analytical models based on stress strain calculations can explain this material behavior. Together with an in-situ measurement of the thermal impedance the models can predict the lifetime consumption of the application and thereby upcoming maintenance.