Heat Transfer and Thermal Interface
~ Why You Want Them, And Why More Is Definitely Not Better ~
by David Forster
Maybe you've seen those pictures of someone cooking an egg on a CPU
– and if you're the curious type, just maybe you wondered if there was
a new cooling product idea somewhere in that experiment. More likely
you just thought Hoo Doggie, I knew those things were hot! (Okay. Maybe
you didn't think Hoo Doggie. But I bet there was something exclamatory.)
Then you nodded and thought, Yep, proper cooling. Mucho importante.
|Forgive the fairy
tale analogy, but with Thermal Compound or Paste that's the watchword:
too much and too little are both bad. You want always to use just
the right amount.
So now what? If a Grade A Large isn't the best way to keep your CPU
running (and take our word for it, it isn't), then what is?
Well, frankly, I'm not going to answer that.
There are too many options, and since issues like how much noise you
are willing to put up with, or how much you're willing to pay, are very
personal, there's no way I could make a recommendation that would fit
everyone's requirements anyway.
Instead, let me address one part of the problem, heat transfer, with
two ends in mind: 1) to help you make a more informed decision for yourself,
and 2) to help you avoid a certain error we've seen too often (and if
the error kills your CPU, then even once is too often), even if you've
already decided and purchased a product.
Heat – don't like it; gotta get it outta here...
The basic idea, of course, is to transfer heat out of your CPU core.
Since we want to simplify matters, we will assume the core as a whole
is running at the same temperature. And that means that what we're really
talking about is transferring heat off the die surface – which is the
only place on the chip we can really access anyway.
Now that the obvious has been made clear, there are three mechanisms
we can use to transfer heat: radiation, convection, and conductance.
And in order to avoid too much detail, just remember this: radiation
and convection are your CPU core's enemy, or at least they don't help;
conduction on the other hand is its friend.
Here's why (but feel free to skip ahead):
Radiation is direct heat loss through output across the electromagnetic
spectra – or, in other words, all bodies with a temperature, which is
to say ALL bodies, glow. Most of us glow in the infra-red, but left
unprotected your CPU core would quickly glow in the optical wavelengths
like a stovetop burner, first red, then yellow-white just before it
melts into a molten glob no faster than, at best, an old 80286 chip.
Convection is what you see just above a Texas blacktop in August – all
those wavy lines looking like a mirage as the oven-like air carries
heat off the surface. Unfortunately for our purposes, the air in your
computer case can't carry the heat off your CPU die-top as fast as it's
produced, quickly resulting in the molten scenario described just above.
Conduction is the transfer of heat from one location or part of a system
to another by the kinetic activity (bumping together) of the molecules
– in our case, from the die-top to the heatsink because they are touching
and the one heats the other up. It's the old frypan on the burner syndrome
– the cast-iron skillet keeps the burner from glowing red until it too
gets red-hot. (And because the heatsink is a lot larger and heavier
than the CPU core, it takes a lot more heat to heat it up as far.)
Now that we've established that the heatsink helps cool the CPU because
it's sitting on it (no duh), we can get to the point: improving the
thermal transfer at the interface between the die top and the heatsink
At the Interface
If you think about it, there is a potential problem here. Unless the
bottom of the heatsink and the top of the die are mirror-smooth, or
better yet smoother than a mirror-finish, then contact between the heatsink
and the die-top will be less than optimal.
If you could look at a sufficiently magnified picture of the bottom
of your heatsink, you'd find the smooth metal surface to be anything
but. Instead it's full of rifts and peaks and valleys – and only the
'peaks' will touch the surface of the CPU. The 'valleys' will sit up
off the die top, filled with stagnant air.
|Thermal Pad No-nos
- Never re-use a pad. Ever.
- Never use thermal compound with a thermal pad.
- Don't misplace or misalign the pad.
- Don't forget to remove the plastic wrap (if any).
- Never mar or touch the surface of the pad.
And that might not be so bad, if it weren't for a principle of stagnant
air: namely, that it's a fairly effective insulator. That's the same
principle on which fiberglass insulation, foam insulation, and even
that shredded-newspaper insulation is based. The air in your uninsulated
house wall can move, and therefore can carry (convect) the heat from
one surface to the other. By trapping the air, insulation greatly reduces
the heat transfer properties of your wall.
But a 'semi-insulating' layer is not what you want between your CPU
and heatsink. Increased conduction is. So you've got to get rid of the
air, as much as possible. And since conduction occurs where there is
material contact, increasing the total area of contact is also what
Enter Thermal Pads and Thermal Compounds
Thermal Pads and Thermal Compounds both function to replace the air
and, as much as possible, create total surface contact between your
CPU die top and heatsink. And so both aid the conduction of heat from
where you don't want it to where you do. But an interesting feature
that needs to be taken into account is that neither is as thermally
conductive as bare metal-to-metal contact, so both can be considered
to be slightly insulative. They are far less insulative than a layer
of air, nevertheless forgetting this fact can potentially get you into
It is this fact that leads us to the Three Bears Principle for Thermal
Compounds: always use just the right amount. Too little won't replace
all of the air or create full surface contact. Too much will prevent
what metal-to-metal contact you could otherwise achieve. Either will
be less effective than the right amount.
| Thermal compound is not some magical
elixir that somehow causes heat to just disappear.
How much is the right amount? Read and follow the directions that come
with your selected product, but generally scraping a thin layer onto
the surfaces in question with the edge of something stiff and flat,
and only where they will mate, is effective – largely because a good
compound will flow away from the pressure points between the surfaces.
BUT, and this is extremely important, don't put so much on that it flows
out all over the sides of the die core, and NEVER get it on the other
(non-core) components on your CPU chip. Beyond the unwanted insulating
properties of a huge glob of compound, some compounds are also electrically
conductive and present a risk of shorting out your components.
Now, if you've been paying attention you might be wondering about Thermal
Pads. If one of the good properties of a Thermal Compound is that it
can flow out of the way and allow metal-to-metal contact where it would
have already existed, aren't thermal pads a problem?
Probably not, unless you are overclocking or doing something else very
hot or otherwise unusual. Most thermal pads will flow away from pressure
to some extent when first heated, which is a good thing, and for most
standard purposes they are thermally-conductive enough to do the job.
Thermal Pads do offer some attractive features over compounds: namely,
that they are easy to use, quick and neat. On the downside, if you want
to call it that, they are one-shot solutions only. You MUST replace
the thermal pad if you ever remove the heatsink from its mounted position,
because the heat of the operating CPU will have caused the thermal pad
to conform to your die top. So once you move the heatsink – and I don't
care whether it is to replace the CPU or just to look at it for the
fun of it – there will be airgaps between the surfaces if you try to
re-use the thermal pad. If you dismount the heatsink, replace the thermal
And of course, just as with compounds, more is not better. Stacking
two or three pads on top of each other between your CPU and heatsink
might well be enough to kill your CPU.
The same goes for using thermal compounds and thermal pads together
- don't do it. (This, by the way, is the error I mentioned earlier that
we had seen too much of.) Thermal compound is not some magical elixir
that somehow causes heat to just disappear, so don't add it just for
the heck of it. If you've understood what I've written here you already
know why, but let's be clear: adding thermal compound on top of a thermal
pad actually reduces the ability of heat to flow to the heatsink. It
unnecessarily increases the thermal resistance between the two surfaces,
and it almost guarantees you won't have the metal-to-metal contact that
is always preferred.
So In Conclusion...
That's it. No eggs (of any size or grade); just the right amount of
thermal compound (if you use it); if thermal pads are the way you go,
use one and only one, and replace as necessary (never re-use).
I could probably go on, but at this point I doubt any of us wants that.
Use this knowledge in good health and to good effect. Happy computing.