Cooling Techniques for Electronic Equipment Second Edition Dave S. Steinberg A WILEY-INTERSCIENCE PUBLICATION John Wiley & Sons, Inc. NEW YORK I CHICHESTER / BRISBANE I TORONTO / SINGAPORE In recognition of the importance of preserving what has been written, it is a policy of John Wiley & Sons, Inc., to have books of enduring value published in the United States printed on acid-free paper, and we exert our best efforts to that end. Copyright 0 1991 by John Wiley & Sons, Inc. All rights reserved. Published simultaneously in Canada. Reproduction or translation of any part of this work beyond that permitted by Section 107 and 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. Library of Congress Cataloging in Publication Data: Steinberg, Dave S., 1923- Cooling techniques for electronic equipmentiDave. S. Steinberg. -2nd ed. p. cm. “A Wiley-Interscience Publication.” Includes bibliographical references and index. 1. Electronic apparatus and appliances-Cooling. I. Title. TK7870.25.S73 1991 62 1.381 -dc20 91-16009 ISBN 0-47 1-5245 1-4 CIP Printed in the United States of America 20 19 18 17 16 15 14 13 Preface For many years the electronics industry has been striving to improve the reli- ability of electronics systems by reducing the operating temperature and the junc- tion temperatures of electronic components. During this same period there has been a strong drive to reduce the size and cost of these electronic assemblies. Problems have resulted from these trends because the power densities have in- creased rapidly as the volume decreased. As a result, many electronic systems now require more exotic cooling techniques with special fans, liquid cooling, heat pipes, and thermoelectric cooling to keep junction temperatures under 100°C. MIL-HDBK-217 shows that the failure rates of many types of electronic com- ponents can double when there is a 20°C rise in the hot spot temperature for com- ponents that are operating at about 50% of their rated power. Electronic equipment manufacturers have therefore reduced the hot spot temperatures by improving the cooling techniques, to increase the reliability of these systems. However, reducing component hot spot and junction temperatures have not reduced the failure rates as much as had been expected. There appear to be other forces and mechanisms at work that are affecting the reliability of these electronic assemblies. An examination of numerous failures in different electronic systems over sev- eral years has shown that other major factors besides high temperatures often cause rapid failures in these systems. Some of these factors are a poor understanding of how thermal coefficients of expansion can produce high forces and stresses in elec- tronic assemblies, along with a poor understanding of thermal cycling stress and fatigue and vibration cycling stress and fatigue, and their effects on electronic components, electrical lead wires, solder joints, and plated throughholes. Many failures in electronic systems have been traced to large differences in the thermal coefficients of expansion and the lack of proper restraints or strain relief in the electrical lead wires. Many failures have also been traced to large dynamic xv X Vi PREFACE displacements developed in circuit boards when their resonant frequencies are ex- cited during operation in severe vibration environments. These subjects therefore have been added to this textbook to promote a better understanding of other failure mechanisms that affect the reliability of sophisticated electronic systems. The con- cept of damage accumulated in thermal cycling and vibration cycling environments is introduced to show how the total damage and the fatigue life of many types of systems operating in different combined environments can be approximated with the use of Miner’s cumulative fatigue damage criteria. A chapter on finite element methods has been included to show how these meth- ods can be used to design and analyze electronic equipment to improve its reli- ability in severe thermal and vibration environments. An added chapter on envi- ronmental stress screening (sometimes called “shake and bake” tests) shows how to expose electronic equipment to thermal cycling and vibration to improve the reliability of electronic hardware, without using up too much of the useful life of the equipment. DAVES . STEINBERG Wrstlukr Villugr, Culiforniu Scptrmher 1991 Contents Preface xv Nomenclature xvii 1. Evaluating the Cooling Requirements 1 1.1 Heat Sources I 1 1.2 Heat Transmission I 2 1.3 Steady State Heat Transfer I 4 1.4 Transient Heat Transfer I 5 1.5 Electronic Equipment for Airplanes, Missiles, Satellites, and Spacecraft I 6 1.6 Electronic Equipment for Ships and Submarines I 8 1.7 Electronic Equipment for Communication Systems and Ground Support Systems I 9 1.8 Personal Computers, Microcomputers, and Microprocessors I 10 1.9 Cooling Specifications for Electronics I 11 1.10 Specifying the Power Dissipation I 12 1.11 Dimensional Units and Conversion Factors I 13 2. Designing the Electronic Chassis 21 2.1 Formed Sheet Metal Electronic Assemblies I 2 1 2.2 Dip-Brazed Boxes with Integral Cold Plates I 22 V Vi CONTENTS 2.3 Plaster Mold and Investment Castings with Cooling Fins / 24 2.4 Die Cast Housings / 25 2 2.5 Large Sand Castings / 25 2.6 Extruded Sections for Large Cabinets / 26 2.7 Humidity Considerations in Electronic Boxes / 26 2.8 Conformal Coatings / 27 2.9 Sealed Electronic Boxes I 28 2.10 Standard Electronic Box Sizes 1 33 3. Conduction Cooling for Chassis and Circuit Boards 35 3.1 Concentrated Heat Sources, Steady State Conduction / 35 3.2 Mounting Electronic Components on Brackets / 36 3.3 Sample Problem-Transistor Mounted on a Bracket / 37 3.4 Uniformly Distributed Heat Sources, Steady State Conduction / 41 3.5 Sample Problem-Cooling Integrated Circuits on a PCB I 44 3.6 Circuit Board with an Aluminum Heat Sink Core / 45 3.7 Sample Problem-Temperature Rise along a PCB Heat Sink Plate / 46 3.8 How to Avoid Warping on PCBs with Metal Heat Sinks / 47 3.9 Chassis with Nonuniform Wall Sections / 48 3.10 Sample Problem-Heat Flow along Nonuniform Bulkhead / 49 3.11 Two-Dimensional Analog Resistor Networks / 53 3.12 Sample Problem-Two-Dimensional Conduction on a Power Supply Heat Sink / 54 3.13 Heat Conduction across Interfaces in Air / 60 3.14 Sample Problem-Temperature Rise across a Bolted Interface / 64 3.15 Sample Problem-Temperature Rise across a Small Air Gap / 65 3.16 Heat Conduction across Interfaces at High Altitudes / 66 3.17 Outgassing at High Altitudes I 69 3.18 Circuit Board Edge Guides / 70 3.19 Sample Problem-Temperature Rise across a PCB Edge Guide / 71 3.20 Heat Conduction through Sheet Metal Covers / 72 CONTENTS Vii 3.21 Radial Heat Flow / 73 3.22 Sample Problem-Temperature Rise through a Cylindrical Shell I 74 4. Mounting and Cooling Techniques for Electronic Components 77 4.1 Various Types of Electronic Components / 77 4.2 Mounting Components on PCBs I 78 4.3 Sample Problem-Hot Spot Temperature of an Integrated Circuit on a Plug-in PCB I 82 4.4 How to Mount High-Power Components I 88 4.5 Sample Problem-Mounting High-Power Transistors on a Heat Sink Plate I 90 4.6 Electrically Isolating High-Power Components I 9 1 4.7 Sample Problem-Mounting a Transistor on a Heat Sink Bracket I 93 4.8 Component Lead Wire Strain Relief / 94 5. Practical Guides for Natural Convection and Radiation Cooling 101 5.1 How Natural Convection Is Developed / 101 5.2 Natural Convection for Flat Vertical Plates I 104 5.3 Natural Convection for Flat Horizontal Plates I 104 5.4 Heat Transferred by Natural Convection I 105 5.5 Sample Problem-Vertical Plate Natural Convection / 106 5.6 Turbulent Flow with Natural Convection I 108 5.7 Sample Problem-Heat Lost from an Electronic Box I 109 5.8 Finned Surfaces for Natural Convection Cooling I 112 5.9 Sample Problem-Cooling Fins on an Electronic Box I 113 5.10 Natural Convection Analog Resistor Networks I 116 5.11 Natural Convection Cooling for PCBs I 118 5.12 Natural Convection Coefficient for Enclosed Airspace I 119 5.13 Sample Problem-PCB Adjacent to a Chassis Wall I 120 5.14 High-Altitude Effects on Natural Convection I 123 5.15 Sample Problem-PCB Cooling at High Altitudes / 124 5.16 Radiation Cooling of Electronics I 126 5.17 Radiation View Factor I 129 5.18 Sample Problem-Radiation Heat Transfer from a Hybrid I 136 Viii CONTENTS 5.19 Sample Problem-Junction Temperature of a Dual FET Switch 1 138 5.20 Radiation Heat Transfer in Space / 140 5.21 Effects of cy/e on Temperatures in Space / 142 5.22 Sample Problem-Temperatures of an Electronic Box in Space / 143 5.23 Simplified Radiation Heat Transfer Equation / 144 5.24 Sample Problem-Radiation Heat Loss from an Electronic Box / 145 5.25 Combining Convection and Radiation Heat Transfer / 148 5.26 Sample Problem-Electronic Box in an Airplane Cockpit Area / 148 5.27 Equivalent Ambient Temperature for Reliability Predictions / 150 5.28 Sample Problem-Equivalent Ambient Temperature of an RC07 Resistor / 152 5.29 Increase in Effective Emittance on Extended Surfaces / 153 6. Forced-Air Cooling for Electronics 157 6.1 Forced Cooling Methods 1 157 6.2 Cooling Airflow Direction for Fans I 158 6.3 Static Pressure and Velocity Pressure / 160 6.4 Losses Expressed in Terms of Velocity Heads I 163 6.5 Sample Problem-Airflow Loss at a Fan Entrance / 163 6.6 Establishing the Flow Impedance Curve for an Electronic Box I 164 6.7 Sample Problem-Fan-Cooled Electronic Box I 166 6.8 Hollow Core PCBs / 182 6.9 Cooling Air Fans for Electronic Equipment 1 184 6.10 Air Filters / 187 6.11 Cutoff Switches / 187 6.12 Static Pressure Loss Tables and Charts / 187 6.13 High-Altitude Conditions I 188 6.14 Sample Problem-Fan-Cooled Box at 30,000 Feet / 190 6.15 Other Convection Coefficients / 195 6.16 Sample Problem-Cooling A TO-5 Transistor / 197 6.17 Conditioned Cooling Air from an External Source / 199 6.18 Sample Problem-Generating a Cooling Airflow Curve / 199 CONTENTS iX 6.19 Static Pressure Losses for Various Altitude Conditions I 20 1 6.20 Sample Problem-Static Pressure Drop at 65,000 Feet I 204 6.21 Total Pressure Drop for Various Altitude Conditions I 2 10 6.22 Sample Problem-Total Pressure Loss through an Electronic Box I 211 6.23 Finned Cold Plates and Heat Exchangers I 21 1 6.24 Pressure Losses in Multiple-Fin Heat Exchangers I 2 13 6.25 Fin Efficiency Factor I 214 6.26 Sample Problem-Hollow Core PCB with a Finned Heat Exchanger I 217 6.27 Undesirable Airflow Reversals I 230 6.28 Direct Air Impingement Cooling I 233 6.29 Sample Problem-Direct Air Impingement Cooling of a High-Power Cabinet I 235 6.30 Effects of Altitude on Heat Exchanger Performance I 243 6.31 Sample Problem-Heat Exchanger Temperatures for Different Altitude and Power Conditions I 244 7. Thermal Stresses in Lead Wires, Solder Joints, and Plated Throughholes 249 7.1 Introduction I 249 7.2 Avionics Integrity Program I 249 7.3 Thermal Expansion Effects in Electronic Equipment I 250 7.4 Sample Problem-Thermal Cycling Stresses in the Lead Wires and Solder Joints of a Surface Mounted Transformer I 25 1 7.5 Reducing the Thermal Expansion Forces and Stresses I 256 7.6 X-Y Thermal Expansion Stresses for Throughhole Mounting I 258 7.7 Sample Problem-Thermal Stresses in a Throughhole Mounted Resistor I 259 7.8 Throughhole Mounting of a Small Axial Leaded Component I 263 7.9 Sample Problem-Axial Force Induced in a Small Glass Diode I 264 7.10 Effects of PCB Bending Stiffness on Lead Wire Stress I 266 7.11 Sample Problem-How PCB Bending Reduces Lead Wire Forces I 267 X CONTENTS 7.12 2 Axis Expansion Effects on Plated Throughhole Reliability / 268 7. I3 Sample Problem-Thermal Expansion Stresses in Copper PTH I 270 7.14 Surface Mounting Techniques for Chip Carriers I 271 7.15 Sample Problem-Solder Joint Stresses in a Surface Mounted Ceramic Chip Carrier 1 274 7.16 Bending Stresses in the Chip Carrier Lead Wires / 282 7.17 Crowbar Effects on DIP Lead Wires due to Thermal Expansion / 283 7.18 Effects of Z Axis Thermal Expansion on Component Lead Wires and Solder Joints for Throughhole Mounted Components / 284 7.19 Sample Problem-Throughhole Mounted Transformer on a PCB I 285 7.20 Reducing Solder Joint Shear Stresses I 288 8. Predicting the Fatigue Life in Thermal Cycling and Vibration Environments 291 8.1 Fatigue Generation / 291 8.2 Physical Properties of Solder I 292 8.3 Slow Cycle Fatigue and Rapid Cycle Fatigue / 294 8.4 Estimating the Thermal Cycle Fatigue Life / 297 8.5 Sample Problem-Fatigue Life of Surface Mounted Transformer Solder Joints / 297 8.6 Vibration Fatigue in Lead Wires and Solder Joints / 299 8.7 PCB Resonant Frequency / 299 8.8 Sample Problem-Resonant Frequency of a Plug-In PCB / 300 8.9 Desired PCB Resonant Frequency for Sinusoidal Vibration I 302 8.10 Sample Problem-Desired PCB Resonant Frequency for Sine Vibration I 303 8.11 Random Vibration Fatigue Life / 303 8.12 Sample Problem-Desired PCB Resonant Frequency for Random Vibration I 304 8.13 Miner’s Cumulative Damage Fatigue Ratio / 305 8.14 Sample Problem-Damage Accumulated in Several Different Thermal Cycling Environments I 306 8.15 Electronic Systems Operating in Combined Environments / 309