Thermal Computations for Electronics

Conductive, Radiative, and Convective Air Cooling

Gordon N. Ellison

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000047462
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

The first edition of Thermal Computations for Electronics: Conductive, Radiative, and Convective Air Cooling was based on the author's lecture notes that he developed over the course of nearly 40 years of thermal design and analysis activity, the last 15 years of which included teaching a university course at the senior undergraduate and graduate levels. The subject material was developed from publications of respected researchers and includes topics and methods original to this author. Numerous students have contributed to both the first and second editions, the latter corrected, sections rewritten (e.g., radiation spatial effects, Green's function properties for thermal spreading, 1-D FEA theory and application), and some new material added.

The flavor and organization of the first edition have been retained, whereby the reader is guided through the analysis process for systems and then components. Important new material has been added regarding altitude effects on forced and buoyancy driven airflow and heat transfer. The first 20% of the book is devoted to the prediction of airflow and well-mixed air temperatures in systems, circuit board channels, and heat sinks, followed by convective ( PCB -mounted components included), radiative, and conductive heat transfer and the resultant temperatures in electronic equipment. Detailed application examples illustrate a variety of problems.

Downloads (from the CRC website) include: MathcadTM text examples, exercise solutions (adopting professors only) plus PDF lecture aids (professors only), and a tutorial (Chapter 14) using free FEA software to solve a thermal spreading problem.

This book is a valuable professional resource for self-study and is ideal for use in a course on electronics cooling. It is well-suited for a first course in heat transfer where applications are as important as theory.

CHAPTER 1

Introduction

This chapter might be described as a very brief survey of modeling air cooled electronics, but little detail will be included to show you how to make quantitative predictions. If you have limited electronics thermal analysis experience, then hopefully the following sections will encourage you to read more of this book. If you do have some experience in analyzing electronics cooling designs, then you will at least get a preview of the author’s inclinations in this field.

Not surprisingly, we will begin with the conventional basics, i.e., conduction, convection, and radiation, and include a few illustrative examples. We will consider these topics in greater detail in succeeding chapters. Finally, this book would seem incomplete without a couple of examples that require digital computer analysis, namely using the thermal network and finite element methods, for which software is sufficiently affordable for individuals desiring to conduct their own consulting activities.

1.1 PRIMARY MECHANISMS OF HEAT FLOW

Engineering thermal analyses of electronic systems are based on any or all of three methods of thermal-energy transport: conduction, convection, and radiation. Conduction takes place within a medium, but without obvious transport of the medium itself.

Convective heat transfer also requires a medium for energy flow, but in this instance there is mass transport of the medium. One of the most visible examples of material transport is water heated in an open kettle. The warm water rises in the center of the kettle and falls to the bottom as it becomes cooled upon transferring heat to the kettle walls. The liquid flow not only mixes the fluid, but actually aids in the rate of heat transfer in the vicinity of the walls. A very careful examination of the fluid immediately adjacent to the wall would show negligible fluid flow. In this very thin layer, heat is transferred by conduction.

In a manner similar to the heated water in a kettle, circulating convective air currents in the interior of sealed electronic enclosures aid the transfer of heat energy to the cabinet wall. Upon conduction through the metal or plastic enclosure, external convective heat transfer aids removal of thermal energy from the system.

Radiation heat transfer is totally unique when compared with conduction and convection in that no transport medium is required because radiation energy transport occurs via the propagation of an electromagnetic radiation field through space. Although this field typically covers the entire electromagnetic spectrum, most of the thermal radiation encountered from conventional microelectronic components and systems is located in the infrared region.

All three heat transfer mechanisms obey the second law of thermodynamics in the sense that there is a net energy flow only from a higher temperature to a lower temperature region.

1.2 CONDUCTION

Heat conduction in a one-dimensional bar is illustrated in Figure 1.1, where the heat flow Q_k at the location x is in the positive x-direction. The cross-sectional area at x is A_k, where there is a temperature gradient dT/dx. The manner in which different materials conduct heat is represented by a “constant” of proportionality, the thermal conductivity k. The conductive heat transfer is, with few exceptions, quantified by Fourier’s law:

$\begin{array}{cc}\boxed{Q_k=-k{A}_{k}dT/dx{|}_x}& \text{Fourier}’\text{s}\text{law}\end{array}$

(1.1)

where

$\begin{array}{lll}{Q}_k\hfill & \equiv \hfill & \text{heat}\text{trans...}\hfill \end{array}$

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface to the Second Edition
• Preface to the First Edition
• About the Author
• Acknowledgments
• Chapter 1 Introduction
• Chapter 2 Thermodynamics of airflow
• Chapter 3 Airflow I: Forced flow in systems
• Chapter 4 Airflow II: Forced flow in ducts, extrusions, and pin fin arrays
• Chapter 5 Airflow III: Buoyancy driven draft
• Chapter 6 Forced convective heat transfer I: Components
• Chapter 7 Forced convective heat transfer II: Ducts, extrusions, and pin fin arrays
• Chapter 8 Natural convection heat transfer I: Plates
• Chapter 9 Natural convection heat transfer II: Heat sinks
• Chapter 10 Thermal radiation heat transfer
• Chapter 11 Conduction I: Basics
• Chapter 12 Conduction II: Spreading resistance
• Chapter 13 Additional mathematical methods
• Appendix i: Physical properties of dry air at atmospheric pressure
• Appendix ii: Radiation emissivity at room temperature
• Appendix iii: Thermal conductivity of some common electronic packaging materials
• Appendix iv: Some properties of Bessel functions
• Appendix v: Some properties of the Dirac delta function
• Appendix vi: Fourier coefficients for a rectangular source
• Appendix vii: Derivation of the Green’s function properties for the spreading problem of a rectangular source and substrate - method A
• Appendix viii: Derivation of the Green’s function properties for the spreading problem of a rectangular source and substrate - method B
• Appendix ix: Proof of reciprocity for the steady-state Green’s function;
• Appendix x: Finned surface to flat plate h conversion
• Appendix xi: Some conversion factors
• Appendix xii: Altitude effects for fan driven airflow and forced convection cooled enclosures
• Appendix xiii: Altitude effects for buoyancy driven airflow and natural convection cooled enclosures
• Bibliography
• Index