A novel in situ reliability testing method and the prototype of an industrially applicable automated system (the Viking Reliability Tester) was described in this paper for the characterization of TIM aging effects in semiconductor devices. The introduced reliability testing hardware-software framework utilizes Mentor Graphics T3Ster thermal transient tester to analyze the stress-induced deviations in the heat flow path from the pre-stress reference results. Measurement results collected during the active or passive cycling of the DUTs at regular intervals serve as basis for the comparison. The details of the degradation process can be used as input for life-time prediction and for physics of failure modeling. The practical application of the concept was demonstrated on power LEDs, including details about pre-stress measurement results and aging test-plan.