General Heat Transfer Guide

Getting Rid of Heat

This content was originally featured on Amdmb.com and has been converted to PC Perspective’s website. Some color changes and flaws may appear.

This really needs to cover a minimum of two different topics. The first is specific heat sources like the CPU while the second is overall heat transfer through the case.

Heat Transfer from the CPU

The first step is to determine how much heat you need to eliminate. The AMD technical documents provide the stock power and the guidelines above allow you to calculate the impact of overclocking. Now the question is, “What HSF do I need to get the job done?” This is not a simple question to answer due to all of the variables involved. Let’s discuss these variables and their impact on final temperature. Note, if you plan to use water-cooling, you may wish to skip ahead. If you plan to use a Peltier, read on because it will still need a HSF to get rid of the heat.

Important notes on temperatures:

There is a difference between actual temperature and measured temperature. No measurement device is entirely accurate 100% of the time. Each motherboard model has its own sensor calibration and this calibration often differs from one BIOS revision to the next. Each individual motherboard may vary ever so slightly from the next. For these reasons, it is best to compare your temperatures against others using the same motherboard and BIOS.

The best test of whether or not your temperature is acceptable is the stability of your computer. No, temperature is not the only thing that determines stability, but one motherboard’s 55°C reported temperature may be fine while another may choke when it sees temperatures higher that 50°C. You need to look around at the motherboards you are considering to find the “typical” good temperature range.

The variables that affect the final actual (not affected by measurement errors) CPU temperature include:

1. CPU power consumption
2. Interface thermal resistance
3. Heat sink material and geometry
4. Fan flow rate and direction
5. Air temperature feeding the fan

We’ve already discussed how to determine the CPU power consumption, so let’s take a look at the remaining four items.

Interface thermal resistance

As mentioned earlier, the contact between the heat sink and die is not perfect. In the microscopic world, both surfaces look like mountainous regions. While most people are well advised not to touch the surface of the die, many heat sinks can have their surface improved by lapping. Unless the heat sink surface is rough enough to file your fingernails, lapping will not dramatically improve the best temperature you’ll see by very much. If you place a heat sink directly onto the die with no interface material, the “peaks” of the heat sink and die will contact one another while the “valleys” will be separated by very small pockets of air. Compared to the copper or aluminum of the common heat sink, air is an extremely effective insulator. Without question, the die temperature will be quite high in the absence of an interface material.

There is a range of interface materials available. The three primary classes include generic greases/pastes, high-performance greases/pastes, and PCMs. You can purchase generic thermal grease at Radio Shack for less than $2, US currency. You can purchase a high-performance paste like Arctic Silver II for about $8. Many heat sinks ship with a small square of PCM.

PCM should not be used for any processor that’s going to exceed about 50 watts. Even if you will be using a relatively low-power processor, I still advise against using PCM. The primary reason is that it’s a one-time material. In its native state, the material has poor conductivity. The first time you turn on your machine, the processor will heat up to the point that the PCM melts. As it melts, it forms to the voids in the interface. As it forms to the voids, its thickness and hence its thermal resistance decreases. This reduces the chip temperature to the point where the PCM again solidifies. From this point on, the PCM remains a solid and goes about its job of transferring heat from the die to the heat sink. Even after this transition, the thermal conductivity of PCM remains much worse than thermal grease or paste.

The problem comes if (almost certainly when) you need to remove the heat sink. The PCM can’t be reused. It also leaves behind a gooey residue on both the chip and heat sink that you need to remove before either can be reused. Finally, there have been isolated reports of the PCM chemically attacking some heat sink materials resulting in pitting of the heat sink surface. Do yourself a favor and throw away any PCM that may have arrived with your heat sink. This leaves generic thermal greases and the high-performance alternatives. Without question, the thermal conductivity of the high-performance alternatives does exceed that of the generic options. For those that want the absolute best, feel free to spend the little bit extra. A word of caution, however, is warranted. The quality of the installation job matters more than the choice of interface material.

Joe User1 that applies the proper amount of generic compound will get better results that Joe User2 that applies far too much of a high-performance compound. The reason is simple. The only purpose of the compound is to fill the voids that exist between the heat sink and die. Any extra material applied that doesn’t get squeeze out the sides is unnecessary thermal resistance. Not also that some materials have low electrical resistance. Applying too much may result in compound covering some of the CPU’s bridges. This could cause problems for the CPU.

Best advice is to apply the thinnest layer possible and install the heat sink. Remove the heat sink. See if there is thermal compound the size and shape of the die stuck to the heat sink. If so, you have enough thermal compound. If not, either lap the heat sink or reapply a slightly thicker coat of compound. As an option for heat sinks with a rough finish, you may also apply some compound to the heat sink. Rub this compound into the heat sink and wipe off any excess. Some of the compound will remain in the voids covering the heat sink surface.