Abstract
In this dissertation a simplified nondimensional approach for the thermal
analysis of power hybrid circuits is presented. The new technique uses only the
metallization and the substrate as layers and represents everything below the
substrate by an external thermal resistance (expressed as an equivalent convective
heat transfer coefficient, h). In this study, the impact on thermal management
of thick film metallization and copper cladding on alumina, aluminum nitride,
and beryllia ceramic substrates is compared. The thermal conductivity of the
substrate material, the thickness of the copper layer, the thermal resistance of the
heat sink system, the size of the device, and the spacing between two heat
dissipating devices are considered. The model results show that increasing the
thickness of the copper layer can significantly decrease the device temperatures
on alumina but may increase temperatures on high thermal conductivity
substrates. Moreover, the model results show that increasing the thickness of the
copper layer requires that the devices be placed farther apart to prevent thermal
interaction. The results also demonstrate that the external heat sink resistance
can have a significant impact on the heat flow paths and temperatures in the
substrate. As the external resistance increases, the spacing required to prevent
thermal interaction also increases.
In addition to the above, a series of experiments were conducted on various
hybrid circuits samples for a low and high heat sink external resistance, i.e., large
and small convective heat transfer coefficients, respectively. These samples were
constructed using thick film resistors as heat sources on alumina and beryllia
substrates. The temperature rise was measured using infrared thermal imaging
technique. These experimental results were compared to results predicted by the
thermal model. In general, the model underpredicts or overpredicts the
experimental temperature rise by 0-2 ·C and the agreement is within the
experimental uncertainty of ±2°C.
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