Table Of ContentAIR COOLING
TECHNOLOGY
ELECTRONIC
for
EQUIPMENT
About the cover
The inset photograph shows the temperature fields around four
one-inch-square simulated electronic components flush mounted
on a conducting board cooled by air flowing
from left to right. Isotherms are made visible
by using thermochromic liquid crystal
thermography. (Photo courtesy of Professor
Alfonso Ortego of the Heat Transfer
Laboratory at the University of Arizona.)
AIR COOLING
TECHNOLOGY
for ELECTRONIC
EQUIPMENT
Edited by
Sung Jin Kim
SangWoo Lee
CRC PRESS
Boca Raton London New York Washington, D.C.
Library of Congress Cataloging-in-Publication Data
Air cooling technology for electronic equipment I Sung Jin Kim, Sang Woo Lee, editors
p. em.
Includes bibliographical references and index.
ISBN 0-8493-9447-3 (alk. paper)
1. Electronic apparatus and appliances-Cooling. 2. Electronic packaging-Cooling.
3. Heat-Convection. 4. Air flow I. Kim, Sung Jin. II. Lee, Sang Woo.
TK7870.25.A43 1996
621.381 '046-dc20 94-45903
CIP
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FOREWORD
Cooling technology has been a vital prerequisite for the rapid, if not explo
sive, growth of the electronic equipment industry. This has been especially true
during the last 15 years with the advent of integrated circuit chips and their ap
plications in computers and related electronic products. Today, cooling tech
nology is well recognized as a gating factor for the development of future
electronic equipment. There are many reasons why cooling technology always
will be pivotal in the design of electronic equipment, but the following two are
most fundamental:
• Thermodynamically, all electronic devices are undergoing an irreversible
process with the net result being the generation of heat which must be
removed in order to maintain continuous operation.
• Thermophysically, the reliability and performance of all electronic de
vices are temperature dependent with various degrees of sensitivity, but
generally the lower the temperature, the better.
With these in mind, the search for new and improved cooling technologies
has been unabated for the last three decades as evidenced by both industrial de
velopment efforts and university research activities. IBM has been involved ac
tively with sponsored electronic cooling research at many universities across
the country since 1967 with equal emphasis on both air cooling and liquid cool
ing. Almost without exception, all research findings sponsored by IBM have
been published, contributing substantially to the advancement of cooling tech
nology for the industry.
Recent developments since 1992 have resulted in adapting CMOS technol
ogy for pervasive applications in almost all commercial electronics, including
computers. This development, coupled with the increasingly competitive na
ture of the industry has made air cooling much more enticing. Therefore, a book
on "Air Cooling Technology for Electronic Equipment" is both natural and log
ical, and I endorse this timely idea wholeheartedly. Dr. Sung Kim and Dr. Sang
Lee, co-editors of this book, are my colleagues at IBM and all chapter authors
are my professional friends of many years. I have every reason to believe that
their joint effort will be a successful one.
Richard C. Chu
IBM Fellow
PREFACE
Improvements in microelectronics and large scale integration technologies
result in power densities as high as 10 W/cm2 for semiconductor devices. This
continuous increase in power densities has placed an increased emphasis on the
thermal control of electronic equipment. Convection air cooling is still the most
common method for removing heat from the heat generating elements. This is
because air is readily available and the air moving devices are relatively inex
pensive. Even though there are a lot of research activities on "exotic" and "ex
pensive" cooling technologies, more and more emphasis is placed on extending
the limits of air cooling capabilities because of the competitive nature of the
computer industry.
The following questions arise with regard to the thermal design of electronic
systems. What is the optimal spacing between the printed circuit boards? What
is a good estimate of the heat transfer coefficient and the associated pressure
drop for forced convection over package arrays? How are heat transfer and fluid
flow characteristics in the entrance region different from those in the fully de
veloped region? What is the effect of substrate conduction on convection cool
ing? How can heat transfer be enhanced to push the limit of air cooling? What
is the eventual upper limit of air cooling? These questions are frequently asked
by thermal engineers and packaging practitioners.
This book addresses all these questions in detail. Six chapters are designated
to answer the above-mentioned questions systematically. They are based on re
search projects funded primarily by IBM. In addition, we have tried to include
the most recent information on air cooling technologies performed outside of
IBM. Hence, this book will serve as a handy technical source of information
for thermal and packaging engineers who would like to get the most out of air
cooling. The book in its present state reflects the latest development in cooling
techniques and thermal design guides with air as a cooling medium.
It goes without saying that this book would not be possible without the ded
ication of the chapter authors. We would like to extend our thanks to Bob Stem
and Norm Stanton of CRC Press for their enthusiastic support. Our greatest
thanks go to our wives, Yeon S. Kim and Ann H. Lee, for their continuous
prayers and full support during the last two years. In addition, we would par
ticularly like to thank Mohinder Grewal and Ron Russell of IBM Corporation
for providing a working environment that makes a fruitful result such as this
possible. Special thanks to Mr. Richard Chu, IBM Fellow, for the insightful
foreword.
SungJ. Kim
SangW. Lee
CONTRIBUTORS
Yutaka Asako SungJin Kim
Professor of Mechanical Advisory Engineer
Engineering Thermal Engineering Center
Tokyo Metropolitan University IBM Storage Systems Division
Tokyo, Japan Tucson,AZ
KavehAzar Sang Woo Lee
Member of Technical Staff Adjunct Professor
AT&T Bell Laboratories of Mechanical Engineering
North Andover, MA Duke University
Durham,NC
Adrian Bejan
J.A. Jones Professor Majid Molki
of Mechanical Engineering Professor of Mechanical
Duke University Engineering
Durham,NC Esfahan University of Technology
Esfahan, Iran
Mohammad Faghri
Professor of Mechanical Alfonso Ortega
Engineering Associate Professor
University of Rhode Island Department of Aerospace
Kingston, RI & Mechanical Engineering
University of Arizona
Suresh Garimella Tucson,AZ
Cray Research Associate Professor
of Mechanical Engineering Richard Wirtz
University of Professor of Mechanical Engineering
Wisconsin-Milwaukee University of Nevada
Milwaukee, WI Reno, NV
TABLE OF CONTENTS
Chapter 1
Geometric Optimization of Cooling Techniques 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 00 0 ol
Adrian Bejan
Chapter 2
Entrance Design Correlations for Circuit Boards
in Forced-Air Cooling 00 0 00 0 00 00 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 .47
o o o o o o o o o
M. Faghri, M. Molki, andY. Asako
Chapter 3
Forced Air Cooling of Low-Profile Package Arrays 0 0 0 00 0 00 0 00 0 0 0 0 00 0 81
R.A. Wirtz
Chapter4
Conjugate Heat Transfer in Forced Air Cooling
of Electronic Components 00 0 00 0 00 0 0 0 0 0 0 0 0 0 0 0 0 00 0 00 0 0 0 0 0 0 0 0 00 103
o o
Alfonso Ortega
Chapter 5
Enhanced Air Cooling of Electronic Equipment 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 00 173
Suresh V. Garimella
Chapter 6
Limits of Air Cooling-A Methodical Approach 0 0 0 0 0 00 0 0 00 0 0 0 0 0 0 0 00 203
KavehAzar
Index 0 0 00 00 0 00 00 0 0 00 00 00 0 00 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 00 245
Chapter 1
GEOMETRIC OPTIMIZATION
OF COOLING TECHNIQUES
Adrian Bejan
CONTENTS
Introduction ............ ........... .... ...... .. .. .. .... .....2
Stack of Vertical Plates Cooledb y Natural Convection .. . .. ... .... .. . . 2
The Small-D Asymptote ................ ... .. ... .... .. ..... .4
The Large-D Asymptote ........ ... .... ................ .....5
The Optimal Plate-to-Plate Spacing .. ... .. ... ..... .. .. ........ 5.
Bundle of Horizontal Cylinders (Pin Fins) Cooled
by Natural Convection ... ................ . ...... ..... .........6
Stack of Parallel Plates Cooled by Laminar Forced Convection .........8
Stack of Parallel Plates with Flush-Mounted
and Protruding Heat Surfaces .. .... ........ ... ... . .. .. .. ..... .. 12
Stack of Parallel Plates Cooled by Turbulent Forced Convection .... .. . 15
Stack of Parallel Plates Immersed in a Free Stream ..... .... .. .... .. 17
Bundle of Cylinders Cooled by Forced Convection ... ......... ... . . 25
Heat Generating Plate Cooled Inside a Parallel Plate Channel ...... ...2 7
Board with Large Thermal Conductance
in the Transversal Direction .. ......... .. . .... .. ... .... .. ....2 7
Board with Finite Thermal Conductance
in the Transversal Direction .. ............ ... ............. .. . 30
Stacks of Plates Shielded by Porous Screens .. .. ....... .. ... .... . . 32
Forced Convection .. ...... .. .. ...... . . .. .. ...... ... ...... . 33
Natural Convection .... .... ................. ...... . ..... .. 34
Plate Fins with Variable Thickness and Height .. .... ...... ... .... .. 35
Heat Sinks with Pin Fins and Plate Fins ....... ........... ..... ...3 7
Pin Fin Arrays .... ... . ...... .... . .. .. .. ....... .. ... .. ....3 8
Plate Fin Arrays .. ..... . .. .. .. ... .... ..... ... .... .... .... .40
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