This paper presents an application of thermal analysis methods for the investigation of the internal structure of electronic device packages. The problem is illustrated based on the example of two silicon carbide power diodes. These diodes provided by different manufacturers have the same ratings and package type but one of the diodes exhibits oscillatory behaviour when used in a power converter. The presented results of thermal tests and analyses confirmed that there exist important differences between the two devices in their internal structures, possibly indicating the presence of some imperfections in the die attach or the wire bonds. These faults, in turn, have negative impact on their electrical performance in the investigated circuit.