Analysis of field loads in automotive ECUs and MEMS sensors

In order to optimize the reliability of automotive electronics, comprehensive knowledge of the acting field loads is an indispensable prerequisite. Automotive electronics are exposed to variable environmental conditions with different intensities, frequencies and durations, depending on a car's usage. This paper describes a mobile acquisition system for measuring all relevant field loads including temperature, humidity, vibration and stresses and strains in an automotive electronic control unit (ECU) mounted under the hood. For the measurements, a combination of commercial sensors and a novel strain-sensitive test-chip, which was specifically developed, were used. The investigation presented in this paper, focuses on the thermo-mechanical analysis of a commercial LFBGA-package, which is soldered on the printed circuit board (PCB) of the ECU. For comparison to standardized environmental tests, the ECU was also investigated with two different temperature profiles in laboratory tests. It is shown, that the developed measurement set-up makes it possible to characterize the stress and strain loads of the device. The data were used for verification of a simulation model of the LFBGA assembly. Furthermore, the FE-model was used to establish a relation between the measured package deformation and the global shear strain in the critical solder ball. Load cycle of the average shear deformation time history could be computed using this relation in combination with the Rainflow counting algorithm and measured field data.