The thermal control of a spacecraft and of instruments on board has traditionally been a passive subsystem and its analysis needs the thermal specifications of other subsystems, which is not usually available up to advance stages of the instrument design. The first approach of the structural design, on conceptual phase, of an instrument aboard on a spacecraft usually takes into consideration the mass budget and mass allocation but not thermal specifications from other subsystems. The intention of the paper is to propose a simplified analytical criterion for sizing spacecraft-instrument junctions by considering thermal and structural requirements simultaneously. To validate the proposed model of this study, a commercial software Nastran/Patran was used to perform the structural design of an instrument within a real spacecraft mission and the results were analyzed with the thermo-mechanical criterion of this paper. The result of this analysis shows some benefits when a thermal condition is included at the first conceptual design of the structural models of instruments on board of a spacecraft.