Failure mode analysis and optimization of assembled high temperature pressure sensors

Abstract

Thermal-mechanical stresses are a dominant factor limiting the reliability of sensor-systems in harsh automotive environments. Strains and stresses and their effect on the performance and reliability of pressure sensors with operation temperatures up to 500 °C are analyzed with FE-simulations in this study. Platinum based, resistive pressure sensors, fabricated in thin film technology and bulk micro-machining are the subject of this study. The packaging technology combines ceramic substrates with low coefficients of thermal expansion (CTE) and a glass-solder process. The investigated sensor substrates were AlN, Si3N4 and a Low-Temperature-Cofired-Ceramic (LTCC). Two different assembly variants were chosen for the interconnection of the sensors: platinum thin wire bonding and gold micro bump interconnections. 3D FE-models of the sensor-assemblies, including temperature dependent materials properties were developed to analyze the distribution of mechanical stresses in the different assembly components. We measured the global chip-deformation at room temperature for verification of our FE-models. With combination of FE-simulations and metallographic device-cross-sections, cracks in the cavity sealing were identified as major failure mechanism of our sensors. According to the FE-simulations, devices assembled with our flip-chip method combined with LTCC-substrates showed an optimized performance regarding signal-shift and reliability. The sensor-signal drift after the assembly process was reduced from 27% to 3% for the optimized configuration.