Automotive Electronic Control Unit (ECU) has semiconductor devices performing various time dependent functions. It is essential to understand the transient thermal behavior of these devices for designing a reliable system. Detailed thermal model (DTM) is the need of the hour to understand the characteristics by performing a transient system level simulation. The information to build DTM is not readily available in the public domain due to intellectual property protection from the device suppliers. To overcome this, the present work showcases the procedure to develop Transient Thermal Network Model (TTNM) using resistance and capacitance values extracted from the impedance curve provided by the semiconductor manufacturer. TTNM model is developed for a typical DPAK and D2PAK device and validated by comparing impedance curve derived from simulation with datasheet. Thermal characterization studies are also carried out to compare the behavior of the traditional lumped model and TTNM to that of the DTM. The results indicate in the absence of DTM, the TTNM model is the best candidate considering the accuracy with limited information available. Further a transient thermal qualification of an automotive ECU is also presented using transient thermal network based models for critical devices.