Searching Topics
This is a list in chronological order of all the questions I have answered in my thermal design advice column on CoolingZone.com. To find a topic of interest, use the "Find" button of your browser to search the text of this page for keywords. That's as fancy as it gets. When you find a column of interest, click on the date, and you will jump to that column in the CoolingZone archive.Why does my heat sink not perform as well as I expect, based on the increase in surface area?
Is Electronics Cooling a good career choice?
Can the copper in a circuit board be used as a heat sink for a surface mount component?
Can a thermal analyst also do useful work, like drafting?
Do fans need to stay on after electronics are shut off, to prevent “temperature overshoot” due to “thermal inertia”?
How do you convert heat loads from watts to BTU/hour to air conditioning tons?
Can infrared cameras be used to measure electronic component temperatures?
How should you connect a heat sink to a device that is on the opposite side of the printed circuit board? Are vias through the board enough?
Can you recommend some basic textbooks on cooling electronics?
How about a book on how to avoid car accidents?
Isn't thermal engineering of electronics actually getting easier year after year as the gate voltage goes down, reducing power per gate?
Can you give us an anecdote from TV science fiction, illustrating the cyclical nature of power dissipation in electronics over time?
How can I find the flow rate of a fan at reduced speed, and how can I estimate the audible noise of a fan at reduced speed?
Shouldn't CFD tools for electronics cooling include the Fan Laws in their fan models?
Isn't there an error in your Fan Laws article in June 2001 that should be corrected?
I want to measure the temperature at the bottom of a deep drilled hole in a block of aluminum. How to I attach the thermocouple so I know it is really at the bottom of the hole?
Why did you make fun of Professional Engineers in your last column?
Can you use an ohmmeter to help in mounting a thermocouple in a deep drilled hole in a block of aluminum?
Can you recommend a cooling method for overclocking an old 486 laptop, besides shooting CO2 cartridges into the battery slot?
What is proper way to calculate percent error when comparing CFD predictions of temperature to experimental results?
Isn't the error estimate for CFD that you just presented an oversimplification?
Where do you recommend we search for thermal data on products from vendors?
How can I convince my cost-cutting management that thermal engineering contributes to the bottom line of the company?
How does relative humidity affect the air cooling of electronics?
Is there any experimental data on how air cooling of electronics is affected by altitude (or air density)?
If I use Kapton tape to electrically insulate a thermocouple from a live conductor, how can I estimate the error in temperature this introduces?
Is humid air more dense or less dense than dry air? Seems like you made a boo-boo in your November 2001 column.
How does the convective heat transfer coefficient for air change as the air temperature changes?
Which works better for fan cooling, push or pull?
What is the best heat sink coating to help with radiation?
What is the difference between Typical Power and Max Power for an electronic component, and which one should I use in my CFD analysis?
Can you give me a simple, everyday explanation of how a thermocouple, or backwards, a Peltier cooler, works?
I have heard that a thermocouple can act as a heat sink, leading to measurement errors. How does that happen?
Which CFD package is the best?
Which type of thermocouple wire is best for electronics cooling testing?
Why is there such a big difference for the thermal resistance from case to air that I get from the flat plate correlations, and the value for thermal resistance in a component data sheet?
Which temperature scale is more accurate, F or C?
How can you work backwards from component temperature to estimate the component power dissipation?
Where can I find definitions for these confusing terms: thermal resistance, thermal impedance, thermal conductance, thermal intrusion, thermal inductance, etc.?
How can I estimate natural convection coefficients for air at 80,000 feet altitude?
Are there quick and dirty hand calculations that can take the place of CFD analysis, assuming I don't need 100% accuracy? Provide all detailed sources of information.
If not all details, can you at least help me estimate the pressure drop for a box full of circuit cards?
How much of the electrical power drawn by a cooling fan gets converted to heat, and how much into useful air flow?
How can you calculate the useful work done by a fan?
Is there a “Cooling Electronics for Dummies” book or short course?
How can you predict how well a thermal interface material will work in your application? I can't seem to make sense of the vendor data sheets?
Can you tell me how to size a “chimney” for cooling an electronics chassis in natural convection?
Is Theta j-c, the thermal resistance between junction and case, an accurate way to estimate junction temperature? If not accurate, is it always conservative? Does it always give a “safe” estimate?
Since Theta j-c is no good, how should we get the junction temperature of a component?
I saw a “heat sink pen” in an electronics supply catalog. What does it do?
How fast does heat conduction go in a solid? Is there some time scale at which conduction becomes negligible?
When are going to need to abandon air cooling for electronics and switch to liquid cooling? Have all the bugs been worked out of liquid cooling, since the technology is so new?
What type of adhesive is best for attaching thermocouples to components? Do you need to use thermal epoxy?
Why do we get large differences in component temperature when the Product Safety guys measure the same circuit boards as we do in Environmental Testing?
When you anodize a heat sink to improve radiation, don't you mess up the convection to the air by introducing a layer of insulation?
Can I learn anything useful at any of these thermal conferences, symposia, and/or expositions? Which do you recommend?
When I add a heat sink, my component gets 20 degrees C cooler. When I add thermal grease between them, the component temperature goes down another 10 degrees? If I only need a 10 degree improvement, can I just use the grease and no heat sink?
Did a man really get burned on his lap by a laptop computer? What implications does this have for cooling electronics?
My board runs in the vacuum of space. Can I use Theta j-c to estimate the junction temperature of a component, assuming I know the temperature of the printed circuit board right under it?
How can I learn to use simple spreadsheet
tools to do thermal analysis, as you do in your column?
Can I approximate the effect of solar loading on my enclosure by just increasing the ambient temperature in my test chamber a few degrees?
How can you calculate the heat spread angle
in a piece of metal?
What analogy can you use to explain the difference between heat and temperature to your mother-in-law?
Does the amount of heat radiated by my
enclosure to the surroundings change much as the ambient increases, or
is this effect negligible?
How should I estimate the value of "h", the convective heat transfer coefficient for forced convection, for the pins of a pin fin heat sink? I think I should use the textbook correlation for banks of tubes, but it always gives a much bigger value than the flat plate or duct values of "h". Which one is right?
What is a "push-pull" fan? Does it
reverse the air flow direction on each cycle, pushing it one way, and then
pulling it the other, like a Japanese hand fan?
Can a Phase Change Material (PCM) used as an interface between a component and a heat sink act as a "thermal diode"? Can it be used to regulate the temperature of the component by changing its thermal resistance?
Can I use an old appliance cardboard
box as a natural convection environmental chamber, providing I use enough
duct tape to seal it up properly? How do you recommend I maintain
a constant ambient temperature in such a test chamber?
You wrote about the possibility of a fan that could stir up the air in an enclosure by flapping back and forth like a traditional Japanese fan. What you thought was a joke is already being developed as real technology. I found so-called "Miniature and Ultrasonic Piezoelectric Fans" developed by a research group at Purdue University. Tell us more about it, and if it can really help cool electronics.
You wrote an article that said we shouldn't
use Theta j-c (the thermal resistance from junction to case)
to calculate junction temperature. You claimed that it might give
an estimate of the junction temperature that was lower than the actual
junction temperature. One justification you gave was that Theta j-c
was measured in a stirred liquid bath. The latest test method for
Theta j-c is the Top Cold Plate Method. In this test the bottom of
the printed circuit board is insulated and a liquid-cooled cold plate is
clamped against the top of the component, forcing nearly all of the heat
from the die through the top of the package. Doesn't that version
of Theta-jc give you a safe estimate of junction temperature?
In your discussion of the piezoelectric fan in a cell phone you stated that fixed total power and a sealed phone with no holes prohibit the fan from lowering the case temperature. If the piezoelectric fan perfectly mixes the internal air to its average temperature, so that no hot spots remain, and the average temperature difference between the internal air near the case wall to ambient air would thus would be greater. With a constant external convection coefficient, the heat transfer from inside the case to ambient air should then increase, therefore lowering the case temperature. Are there are any 'holes' in this theory?
You wrote an article that said we shouldn't
use Theta j-c (the thermal resistance from junction to case)
to calculate junction temperature. You claimed that it might give
an estimate of the junction temperature that was lower than the actual
junction temperature. One justification you gave was that Theta j-c
was measured in a stirred liquid bath. The latest test method for
Theta j-c is the Top Cold Plate Method. In this test the bottom of
the printed circuit board is insulated and a liquid-cooled cold plate is
clamped against the top of the component, forcing nearly all of the heat
from the die through the top of the package. Doesn't that version
of Theta-jc give you a safe estimate of junction temperature?
Where is the dividing line between conduction
and convection in an air gap? I understand that when you have a trapped
pocket of air that is very small, currents can't form, and heat can only
cross the air gap by conduction. And air is not a good conductor.
But how small is small?
I am using a thermal simulation program to calculate the air temperature inside an outdoor electronics hut. It asks for something called "T sky" in the section about radiation. My guess is that this is the temperature of the sky. Supposing my guess is right, how do I find T sky? I don't see it listed in the weather data for my site.
There are consultants in electronics cooling
that could help us. Can thermal problems really be solved long distance,
by working over the phone and through e-mail? There are no local
thermal experts in our area -- we would have to work with somebody hundreds
of miles away, and the expense of bringing somebody here for days
or weeks at a time would break our budget. Is it practical to do
electronics cooling by "remote control?"
In one of your previous columns, you suggested that component power could be estimated by measuring case temperature, then comparing that to a CFD model of the component on its board. You modify the component power in the CFD software until the case temperature in CFD matches the measurement. Isn't there another way of getting to the power number? Here is an idea I have: Put on top of the component package a layer of material of known thermal conductivity and known thickness, and measure the temperature on top of that layer. Then take that off, and do the same thing with another, different material with different thermal conductivity.
To cool something off you blow air on it
with a fan, right? If you put your hand in the air blowing
out of a fan, it feels cooler than the surrounding air. But if you
put a temperature probe at the inlet of the fan and in the exhaust, they
read exactly the same temperature. How do I explain how the fan can
cool something down without making the temperature lower?
My liquid-to-air heat exchanger doesn't
always chill the air below ambient as much as I need. Could the problem
be the relative humidity of the air that is being chilled?
Which is better, to burn in the products at a steady 50 degrees C for 24 hours, or to do temperature cycling, between -20 degrees and 50 degrees C? I have heard that temperature cycling is more effective, but the chamber to do that is much more expensive.
Aren't there some component failure mechanisms
that actually slow down with higher temperature? Maybe we should
do Freeze-Out instead of Burn-In to get rid of infant mortality.
How many grid cells do you use? Jerry
says his typical thermal analysis uses between 50,000 and 100,000 grids,
but Kim insists that if you don't have at least a million cells in your
CFD simulation, you might as well have just done a hand calculation.
Since disk space, RAM, and even processor speed are so cheap and abundant
these days, shouldn't I be using as much grid as I can afford on every
single problem and get the maximum possible accuracy?
Is there still a good reason to use a coarse grid for anything? Is
there such a thing a too much grid?
Can you tell us about a rack-mount air
conditioners? We design and build custom entertainment centers for
celebrities and sports figures -- you know, people with too much money
and too much time on their hands. We mount a lot of off-the-shelf
entertainment electronics like CD players, amplifiers, HDTV video receivers
and other junk like that in racks that are hidden away in a cabinet or
small closet. The air temperature in those closets can get as high
as 50 degrees C (I know, because I often have to install upgrades in them
myself). The problem is that most of the really trendy, high-profit-margin
stuff is built only for the consumer market, and is rated to operate no
higher than about 35C. My electronics need 35 degrees and my room
air is 50 degrees. Seems to me there is no choice but air conditioning.
But my gut tells me it is not such a good idea to just toss in a standard
window air conditioning unit. Can you point me in a more professional
direction?
I'd like to use a thermostatically controlled
fan, so that it only turns on when it is needed, or runs at the slowest
speed possible. That would reduce fan noise for the customer, and make
the fan live longer. Where is the best place to put the temperature
sensor? Inlet air? Exhaust air? And is there a formula
that tells me how many degrees C I get for every RPM of fan speed?
What is the current carrying capacity for
different wire gauges? Also, what copper trace thickness is required
in a printed circuit board for each level of electrical current?
Component power keeps going up, while the voltage levels are going down.
That means higher and higher current flowing in the copper traces.
I'm starting to worry the board is going to burn up before the components.
I used to have an old chart of the current capacity of wire gauges.
It was a copy of a fax of a page torn out of a somebody's lab notebook,
but I can't lay my hand to it anymore. Can you give some up-to-date
guidance, or at least help find the original source for that current/gauge
chart?
