To predict component reliability for active safety devices under automotive application

Abstract

The new generation active safety control units has enhanced features like pedestrian detection, tunnel detection, night vision etc. These driver assistance functions support the driver by triggering warnings in critical driving situations. These devices mounted on the vehicles has to sustain these additional loads and harsh environmental conditions. Reliability of such devices are very critical to human safety, hence needs to be designed with a very critical development process. Various design iterations are to be evaluated and optimized with the help of advanced design practices and finite element analysis followed by rigorous measurement and testing. One of the most common and critical environment load comes from thermal loading which accounts for maximum number of failures of electronic devices in automotive application. With this objective a coupled thermal and thermo-mechanical simulation was carried out considering active temperature cycle loads on the device at system-level. As the thermomechanical fatigue of solder joints on the system level is more complex to predict than on the board level. We used a two-step submodel approach. In the first step the electronic device with BGA package was included in the global device, though the creep behavior of solder joints was omitted, by considering the populated PCB with all relevant components (i.e. capacitors, inductors, connector etc.) which influence the strain on the PCB during thermal loading conditions, it is observed that the tendency of strain over temperature is in very good agreement, leading to more realistic PCB strains to co-relate with the measurement data. In the second step the simulation of the solder joints fatigue was carried out with the help of the submodel. The submodel technique allowed to reduce the simulation model of a system to the electronic device model with a piece of the PCB underneath and at the same time maintain realistic PCB deformations and the realistic temperature field in the entire submodel during the temperature cycle. The warpage of the component in not soldered state was used to obtain the proper material properties as well as to account also for the form change (cry, smile) during thermal cycling. This shape change leads to additional loading on the solder balls. Additional, the warpage in the soldered state on free PCB and in the housing was measured and compared with simulation results. By combining FE simulation and measurements at the early stage of product development, it is possible to estimate the risk factor to meet the design specifications.