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Abstract: A simple thermal resistance model for conjugate natural convection heat transfer from an isothermal cubic heat source attached to a vertical plate is developed to predict heat source temperature and the thermal performance of the plate. The proposed model is useful in applications related to microelectronic cooling, as it can provide circuit designers with a quick estimate of the junction temperature and the effect of common design parameters due to natural convection cooling of a single chip package on printed circuit boards (PCB).
The model is based on a novel analytical-numerical method to determine various heat transfer resistances. The convection and radiation resistances from the heat source to the ambient and the contact resistance between the heat source and the plate are determined analytically. The resistances of the plate center section are obtained from an analytical solution of the 3D heat conduction equation, and the resistances of the plate fin section are obtained numerically from the solution of the discretized conduction equation. The effect of some parameters such as the plate conductivity, thickness and emissivity, and the contact resistance between the heat source and the plate are examined in detail to determine the relative merit of each as a means of improving cooling of IC packages on PCBs.
The validity and accuracy of the model is demonstrated by comparisons with experiment data and a MicroElectronic Thermal Analyser (META). An experiment is conducted in which the heat transfer from an aluminum heated cube attached to a vertical plate with or without copper layer is examined by monitoring the temperature excess of the cube. It is shown that good agreement is obtained between the experimental data and META and the prediction of the thermal resistance model.
It is found that adding high conductivity copper land on one side of the board around the heat source can reduce the heat source temperature up to 20%. The most significant effect of the copper land is attributed to a very small region closest to the heat source, with increases in the size of land contributing very little to the overall cooling of the heat source. It is also shown that the effect of radiation should be included, as the radiation from the heat source surface and the entire circuit board surface may account for up to 33% and 46% of the total heat dissipation, respectively.
International Electronic Packaging Conference, San Diego, CA, Sept. 12-15
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