Doctoral Dissertations

Date of Award

5-2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Majid Keyhani

Committee Members

Masood Parang, William Johnson, Marion Hansen

Abstract

The cooling of electronic components is one of the most important tasks in the design packaging of electronic equipments as insufficient thermal control can lead to low reliability, short life, and failure of the electronic components. According to a study conducted by Philips Corporation, every ten degree Celsius increase in temperature causes a fifty percent decrease in the operating life of many integrated circuits.

With the increasing design complexities and reliability requirements, today’s electronic design engineers rely significantly on software packages (often based on methods of Computational Fluid Dynamics, or CFD) for the prediction of electronics operational temperature. In the early to intermediate product design phase, numerical analysis is used to select a cooling strategy and refine a thermal design by parametric analysis. In the final design phase, detailed analysis of product thermal performance is carried out for performance and reliability predictions. However, it is recognized that, for a large class of electronics applications, progress in reliability prediction is currently hampered by the lack of accurate prediction methods. This is especially true for problems in which there is significant heat generation due to the flow of electrical currents in traces and conductors. The main difficulty comes from the fact that the majority of the heat is dissipated as a result of the flow of electrical current in various conductors. In order to predict temperatures accurately, the important mechanisms for heat generation and heat transfer must be adequately considered. The issue that complicates matters is that the amount of heat generated by the electric current flowing through the board is itself dependent on temperature, thus requiring an approach that considers the coupling between the electrical and thermal aspects of the model.

This study is aimed at this class of electronics cooling applications. Its objective is to present a novel approach to the thermal management of electronic systems that focuses on the multi-physics nature of these problems by coupling electrical, thermal and CFD effects.

Following the presentation of relevant, governing, partial differential equations, a numerical approach is presented that centrally addresses various coupled mechanisms involved, namely:

Conjugate convection and conduction throughout the region of interest

Surface-to-surface thermal radiation

Calculation of the coupled flow field using the techniques of computational fluid dynamics.

Solution of electrical field and the resulting current densities for determination of heat dissipation in electrical connectors.

The software package ElectroFlo is developed by the author for the thermal management of electronic devices using this methodology.

In Chapter six, the software is used to predict the temperature rise resulting from the flow of electrical current in an embedded trace. The predicted temperature rise is shown to be in excellent agreement with the experimental data reported by the IPC researchers. In Chapter seven, a far more complex transient problem involving electrical current flow in a multi-layer board with connecting vias is studied. This type of board is frequently encountered in aircraft electronics boxes and it requires the solution of the voltage field to obtain the power dissipation distribution. The results of these simulations demonstrate the effect of coupling the electrical and thermal solutions. It is shown that failure to recognize this effect would result in an inaccurate model prediction; the coupled analysis predicts failure, whereas the uncoupled analysis predicts acceptable temperature levels.

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