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Heat Transfer and Thermal Interface Products
~ 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.

Three Bears Principle
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 bottom surface.

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 you want.

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 trouble.

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 pad.

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.