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The thermomechanical fatigue of solder joints on the system level is more complex to predict than on the board level. The damage of the solder joints of an electronic device in an ECU (electronic control unit) depends on the thermal expansion mismatch between the materials of the device and the PCB (printed circuit board), so called local mismatch, as well as on the global deformation of the PCB induced by the casing of the ECU. Therefore we have performed a transient sequentially coupled thermal-mechanical simulation on the system level. We used a two-step submodel approach. In the first step the electronic device with a PG-LQFP package was included in the entire ECU model, though the creep behaviour of its SnAgCu solder joints was omitted. 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 simulation is validated with the temperature and strain gage measurement. The simulation predicts a loss of the life time by a factor of 0,5 for the solder joints in the ECU compared to the the board level tests.