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Hot Air Rises and Heat Sinks:  Everything You Know about Cooling Electronics is Wrong

Hot Air Rises and Heat Sinks: Everything You Know about Cooling Electronics is Wrong

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Table of Contents

By Tony Kordyban 

Here is a collection of myths, mistakes, and "lessons learned" from practicing engineers involved in the field of electronic equipment cooling. Through anecdotes and stories based on his experiences at Tellabs Operations, Inc., Tony Kordyban covers the basic dimensions of heat transfer concepts - mostly from real problems that were incorrectly solved at least once before a correct technique was applied.

This book contains simple line drawings to help illustrate the basic concepts, while the text provides accurate and complete technical explanations. Its case-study approach makes it an extremely useful and handy reference - and Kordyban's clear and entertaining writing style, which mixes technical subject matter with humor, is both interesting and instructive.

For the popular sequel to this book, click here:  MORE HOT AIR, by Tony Kordyban.


TABLE OF CONTENTS

Chapter 1: We Don't Sell Air................................................1

Our hero (the author) discovers that his new employer has written some engineering folklore into product design requirements. Should you measure the actual product temperature, or only the temperature of the air coming out of the vents in the back? Lesson: Junction temperature as the source of thermal trouble.

Chapter 2: Every Temperature Tells a Story.........................9
How hot would a resistor have to be to actually glow? Higher or lower than the melting point of solder? Lab legends always mention glowing components or melting solder, but how hot would that actually be? And what is the ideal serving temperature for ice cream? Lesson: Putting some landmarks on the temperature scale.

Chapter 3: Climate Control Isn't Natural.............................15
Herbie learns that an environmental chamber is only good for testing products that will eventually be used only in environmental chambers. Lessons: Natural vs. forced convection; thermal runaway.

Chapter 4: Diamond Is a GAL's Best Friend.........................21
Read the fine print on that thermally conductive epoxy. It may be 50% better than the nonthermal epoxy, but as a conductor it still makes a pretty good insulator. Lesson: Thermal conductivity.

Chapter 5: Lines in the Sand..............................................31
Don't tell a circuit designer which board layout gives the worst thermal performance. He or she will choose it as the only one that will work electrically. Lesson: Introduction to CFD (computational fluid dynamics).

Chapter 6: When Is a Heat Sink Not a Heat Sink?.................39
More folklore from EE-land about how aluminum has the magical ability to absorb heat like a sponge and send it off to a parallel universe. Lessons: Convection and surface area; conduction.

Chapter 7: Trade-Offs.......................................................47
There are trade-offs between electrical performance, cost, and temperature, so it doesn't pay to be TOO cool. Lesson: Junction temperature operating limits.

Chapter 8: Cfmophobia.....................................................53
A whole company is infected with fear of Rotary Gas Acceleration Devices (fans). Lessons: Fans have a lot of drawbacks that justifiably make people afraid of them, so it is best to plan them in from the very beginning.

Chapter 9: Sieve Cooling System.......................................59
A system that stays cool only because of all of the air leaks that were accidentally designed into the chassis. How to predict the performance of a cooling system that is literally full of holes. Lesson: Hand calculation of natural convection flow is nearly impossible to get right.

Chapter 10: Hitting the Wall..............................................67
Natural convection has a limit, because Mother Nature doesn't face much competition and doesn't work hard on process improvement. But computer chips are getting hotter every day. Lessons: Natural vs. forced convection cooling.

Chapter 11: Keeping a Cool Head......................................73
A 25-CFM fan doesn't give 25 CFM of air, and Herbie nearly loses his head over it. Guidance for figuring out fan flow rates is given on the back of a napkin. Lesson: The fan performance curve.

Chapter 12: Tempera-Mental Prototypes...........................79
Cooling an electronic component is different from cooling a power supply is different from cooling a human being. Setting thermal design goals for a project is more than just filling out forms. Lesson: Operating temperature limits.

Chapter 13: Misdata........................................................89
Component data books are chock-full of specs that are only valid when they are not important, just like my temperature-telling watch, which is only accurate when it isn't too hot or too cold outside. Lesson: Temperature limits in terms of air temperature aren't very useful.

Chapter 14: Pessimism: A Tool of Quality..........................95
Herbie and Vlad discover that two fans are not always cooler than one. Lesson: Fans in parallel don't always provide redundancy.

Chapter 15: Blowin' in the Wind......................................101
Junk science and misconceptions in heat transfer. Where does all this baloney come from? Start with the TV weather and the "windchill factor." Lesson: Forced convection heat transfer equation.

Chapter 16: Thermocouples: The Simplest Way to Measure
Temperature Wrong......................................................109

The most reliable and accurate way of measuring component temperature can also literally explode in your face if you follow Herbie's example. Lesson: How a thermocouple works and how it might not work.

Chapter 17: But the Graphics Are Pretty..........................115
Computer simulation can predict the temperature of electronics even before they are built, and can be wrong to eight decimal places. Lesson: More on CFD.

Chapter 18: Too Much of a Good Thing.............................121
Pin fin heat sinks look like they have a lot more surface area in those magazine pictures. How come they don't work any better? Lesson: Convection works with surface area parallel to fluid flow.

Chapter 19: Computer Simulation Software As Test
Equipment?..................................................................127
Nobody trusts a computer simulation except the guy who did it, and everybody trusts experimental data except the guy who did it. Why not combine the two and get results everybody can mistrust a little? Lesson: CFD as a way of interpreting temperature test results.

Chapter 20: Thermotriples.............................................133
Thermocouple folklore and the ongoing debate: Should you weld or solder your thermocouple junctions? It doesn't matter if you're going to measure in the wrong place anyway. Lesson: More details of why thermocouples work.

Chapter 21: Mixed-Up Convection..................................143
Natural convection and forced convection should be friends. Why make them fight each other, unless fans of the Chicago Cubs are involved, and then nothing needs to make sense anyway. Lesson: What happens when natural and forced convection work in opposite directions.

Chapter 22: A Dependable Answer.................................149
How many watts can a 64-lead component safely handle? How big do the vents in my box have to be? What percentage of the heat comes out of the solder side of the printed circuit board? The answer to these and most other common electronics cooling questions is "It depends." Lessons: Component package power limits and their limitations.

Chapter 23: Sunscreen or Smokescreen?.......................155
A university study claims that sunscreen keeps skin 20% cooler than bare skin. This is so obviously wrong that even an EE can spot it. Lesson: Temperature is not an absolute scale.

Chapter 24: When 70°C Is Less Than 50°C......................161
Is a thermal test done at 70°C and 1000 ft/min air velocity more severe than a test at 50°C and 0 ft/min? Not always. Lesson: Convective heat transfer depends on a combination of air velocity and temperature difference, not just air temperature.

Chapter 25: Even a Watched Pot Boils Eventually...........165
Roxanne the Intern hasn't learned her cooling folklore. Instead of following the traditional lab procedure of waiting an hour, then recording a temperature, she actually waits until the temperature reaches a maximum, and all heck breaks loose. Lesson: Thermal time constraints and transient convection.

Chapter 26: The Latest Hot CD......................................175
When you get a fever, the nurse doesn't have you put some ice under your tongue and then take your temperature again. Herbie wants to put a heat sink only on the components that have been measured as being too hot. Lesson: A complex assembly may have more than a single operating temperature limit, and the limit may change under different environmental conditions.

Chapter 27: What Is a Watt?.........................................181
How hot does a component get that dissipates 1 watt? Like real estate, it depends on location, location, and location. Lesson: Convection + conduction = conjugate heat transfer, a tricky problem that can baffle your intuition.

Chapter 28: Resistance Mythology................................187
Finding the junction temperature is the key to everything. But it turns out that the only way of calculating it is based on ancient mythology instead of physics. "But all legends have some basis in fact," as Captain Kirk says, so maybe you just stick with the myth until something better comes along. Lessons: Conduction; definition of the thermal resistance between junction and case.

Chapter 29: Thermoelectric Coolers Are Hot...................195
Electrical Engineers love these all-electronic refrigerators. Herbie proposes them for use in a new system until he learns that not only do they cost a lot, but they still require fans and heat sinks, and leave the components hotter than they would be without them. Why are they so bad if they do everything manufacturers claim? Lesson: Peltier-effect cooling.

Chapter 30: The House of Cards.....................................201
Even the experts resort to a little mythologizing once in a while. A late night confession reveals that this business of controlling the temperature of electronics to improve performance and reliability isn't nearly as precise as is claimed. There is hope that someday soon the advance of technology might be able to slip a solid foundation under this house of cards without toppling the whole thing. Why isn't anybody worried? Lesson: The not-so-scientific relationship of temperature and reliability in electronics.

Herbie's Homework Helpers..........................................209
If I have whetted your interest in heat transfer and cooling of electronics, or on the topic that everything you know in general is probably wrong, please go to these sources for lots more detail.

  • Publisher: ASME
  • Publish Date: 1998
  • Pages: 240
  • Language: English
  • ISBN: 9780791800744

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Table of Contents

Chapter 1: We Don't Sell Air................................................1 Our hero (the author) discovers that his new employer has written some engineering folklore into product design requirements. Should you measure the actual product temperature, or only the temperature of the air coming out of the vents in the back? Lesson: Junction temperature as the source of thermal trouble. Chapter 2: Every Temperature Tells a Story.........................9 How hot would a resistor have to be to actually glow? Higher or lower than the melting point of solder? Lab legends always mention glowing components or melting solder, but how hot would that actually be? And what is the ideal serving temperature for ice cream? Lesson: Putting some landmarks on the temperature scale. Chapter 3: Climate Control Isn't Natural.............................15 Herbie learns that an environmental chamber is only good for testing products that will eventually be used only in environmental chambers. Lessons: Natural vs. forced convection; thermal runaway. Chapter 4: Diamond Is a GAL's Best Friend.........................21 Read the fine print on that thermally conductive epoxy. It may be 50% better than the nonthermal epoxy, but as a conductor it still makes a pretty good insulator. Lesson: Thermal conductivity. Chapter 5: Lines in the Sand..............................................31 Don't tell a circuit designer which board layout gives the worst thermal performance. He or she will choose it as the only one that will work electrically. Lesson: Introduction to CFD (computational fluid dynamics). Chapter 6: When Is a Heat Sink Not a Heat Sink?.................39 More folklore from EE-land about how aluminum has the magical ability to absorb heat like a sponge and send it off to a parallel universe. Lessons: Convection and surface area; conduction. top of page Chapter 7: Trade-Offs.......................................................47 There are trade-offs between electrical performance, cost, and temperature, so it doesn't pay to be TOO cool. Lesson: Junction temperature operating limits. Chapter 8: Cfmophobia.....................................................53 A whole company is infected with fear of Rotary Gas Acceleration Devices (fans). Lessons: Fans have a lot of drawbacks that justifiably make people afraid of them, so it is best to plan them in from the very beginning. Chapter 9: Sieve Cooling System.......................................59 A system that stays cool only because of all of the air leaks that were accidentally designed into the chassis. How to predict the performance of a cooling system that is literally full of holes. Lesson: Hand calculation of natural convection flow is nearly impossible to get right. Chapter 10: Hitting the Wall..............................................67 Natural convection has a limit, because Mother Nature doesn't face much competition and doesn't work hard on process improvement. But computer chips are getting hotter every day. Lessons: Natural vs. forced convection cooling. Chapter 11: Keeping a Cool Head......................................73 A 25-CFM fan doesn't give 25 CFM of air, and Herbie nearly loses his head over it. Guidance for figuring out fan flow rates is given on the back of a napkin. Lesson: The fan performance curve. Chapter 12: Tempera-Mental Prototypes...........................79 Cooling an electronic component is different from cooling a power supply is different from cooling a human being. Setting thermal design goals for a project is more than just filling out forms. Lesson: Operating temperature limits. Chapter 13: Misdata........................................................89 Component data books are chock-full of specs that are only valid when they are not important, just like my temperature-telling watch, which is only accurate when it isn't too hot or too cold outside. Lesson: Temperature limits in terms of air temperature aren't very useful. top of page Chapter 14: Pessimism: A Tool of Quality..........................95 Herbie and Vlad discover that two fans are not always cooler than one. Lesson: Fans in parallel don't always provide redundancy. Chapter 15: Blowin' in the Wind......................................101 Junk science and misconceptions in heat transfer. Where does all this baloney come from? Start with the TV weather and the "windchill factor." Lesson: Forced convection heat transfer equation. Chapter 16: Thermocouples: The Simplest Way to Measure Temperature Wrong......................................................109 The most reliable and accurate way of measuring component temperature can also literally explode in your face if you follow Herbie's example. Lesson: How a thermocouple works and how it might not work. Chapter 17: But the Graphics Are Pretty..........................115 Computer simulation can predict the temperature of electronics even before they are built, and can be wrong to eight decimal places. Lesson: More on CFD. Chapter 18: Too Much of a Good Thing.............................121 Pin fin heat sinks look like they have a lot more surface area in those magazine pictures. How come they don't work any better? Lesson: Convection works with surface area parallel to fluid flow. Chapter 19: Computer Simulation Software As Test Equipment?..................................................................127 Nobody trusts a computer simulation except the guy who did it, and everybody trusts experimental data except the guy who did it. Why not combine the two and get results everybody can mistrust a little? Lesson: CFD as a way of interpreting temperature test results. Chapter 20: Thermotriples.............................................133 Thermocouple folklore and the ongoing debate: Should you weld or solder your thermocouple junctions? It doesn't matter if you're going to measure in the wrong place anyway. Lesson: More details of why thermocouples work. top of page Chapter 21: Mixed-Up Convection..................................143 Natural convection and forced convection should be friends. Why make them fight each other, unless fans of the Chicago Cubs are involved, and then nothing needs to make sense anyway. Lesson: What happens when natural and forced convection work in opposite directions. Chapter 22: A Dependable Answer.................................149 How many watts can a 64-lead component safely handle? How big do the vents in my box have to be? What percentage of the heat comes out of the solder side of the printed circuit board? The answer to these and most other common electronics cooling questions is "It depends." Lessons: Component package power limits and their limitations. Chapter 23: Sunscreen or Smokescreen?.......................155 A university study claims that sunscreen keeps skin 20% cooler than bare skin. This is so obviously wrong that even an EE can spot it. Lesson: Temperature is not an absolute scale. Chapter 24: When 70°C Is Less Than 50°C......................161 Is a thermal test done at 70°C and 1000 ft/min air velocity more severe than a test at 50°C and 0 ft/min? Not always. Lesson: Convective heat transfer depends on a combination of air velocity and temperature difference, not just air temperature. Chapter 25: Even a Watched Pot Boils Eventually...........165 Roxanne the Intern hasn't learned her cooling folklor
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