One of the reasons for a reduction of production quality is the so-called ‘hidden faults’, i.e. defects which will not be detected by the available testing equipment in spite of measures for ensuring a good quality. These testing methods, which are mostly electrical, can be supplemented in a suitable way by non-contacting temperature measuring techniques, since the variation of temperature as a function of operating conditions and environment constitutes one of the most important problems in the design of electronic components and sub-assemblies, and of their reliability.In this paper a relation is presented between a qualitative diagnostic model, reserve stability and other characteristics, on the one hand, and the permissible thermal energy that can be radiated, on the other hand, which facilitates the detection of hidden faults.