The junction temperature of the quad flat nonlead (QFN) electronic devices equipping embarked assemblies may be controlled so that it does not exceed the maximum value recommended by the manufacturer. The packaging design is then important to ensure correct operation and high reliability, given the significant power generated during operation and the inclination angle of the packages during the flight. It is particularly important when thermoregulation is achieved by means of natural convection. The objective of this study is to examine the influence of the adhesive paste used to connect the Die of the QFN with its base. The study deals with three devices among the most used in the conventional arrangements: the QFN16, 32, and 64. A three-dimensional (3D) numerical solution based on the control volume formulation allows to determine their thermal behavior for generated power ranging from 0.1 to 1.0 W by steps of 0.1 W and inclination angle varying between 0 deg (horizontal position) and 90 deg (vertical position) by steps of 15 deg. A wide range of the paste’s thermal conductivity has been considered, varying between −80% and +100% of its average value, measured by means of the transient plane source (TPS) method. The numerical results confirmed by measurements show that the junction temperature strongly increases when the conductivity of the paste decreases. The temperature is moderately reduced when the paste is thermally more conductive. Relationships are proposed to calculate the junction temperature for the three considered devices, according to the generated power, the inclination angle, and the relative paste’s thermal conductivity.

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