PC Hacks 100 Industrial-Strength Tips and Tools

< Day Day Up >

Hack 24 Keep It Cool

Better to overcool than undercool. A CPU survives best with adequate cooling to keep it stable.

Hackability of the CPU and system board is not the only consideration for a CPU speed tweak. As the CPU goes faster, the internal temperature rises, stressing the incredibly small wires and component structures inside. With excessive heat comes random lockups of the system and possibly catastrophic failures, with some spectacular but short-lived fireworks as the CPU melts down. To counteract excessive heat requires significant cooling capability attached to the CPU chip, so you will see a lot of heat-sink and cooling fan gimmicks and gadgets for sale with CPUs. Check the documentation that comes with the CPU chips, and you will find recommendations and warnings about ensuring proper CPU-to-heat-sink contact and adequate ventilation. Figure 3-10 shows an example of a specially milled heavy-duty supercooling heat sink from an HP server with an integrated fan. HP engineers lay claim to inventing this style of cooling device, and it either works very well or just looks cool as heck! This design has been cloned by many aftermarket vendors.

Figure 3-10. This bolt-down heavy-duty heat sink from an HP server keeps the CPU quite cool

Never run your CPU without a heat sink, especially the ultra-hot AMD processors!

Anyone who has run an AMD Athlon or Duron CPU—any version at any speed, overclocked or not—will tell you that the chip must be fitted with a decent heat sink and fan before any power is turned on, or the CPU will almost certainly fail. Figure 3-11 shows two CPU chips that have suffered catastrophic thermal failure when operated without a heat sink. Try as you might, you cannot put the "magic smoke" back in the chip and have it work again.

Figure 3-11. Fried CPUs with evidence of explosive damage along the near edges of the cover over the CPU

Avoid inhaling the smoke or fumes from a "flamed-out" chip. When the internal elements of a CPU or other semiconductor melt or burn, they give off very foul-smelling and possibly toxic fumes. If a CPU does burn up, ventilate the area well to clear the air and be wary of nausea, dizziness, or other ill effects of toxic contamination.

The stock heat sink that comes with your CPU is adequate for operating the CPU at its rated speed, but overclocking and voltage adjustments can raise CPU temperature dramatically. In most cases of moderate (10-20%) clock or voltage (5-10%) increase, a slightly bigger heat sink and better ventilation will suffice to keep the chip temperature within safe operating range. In the rare cases when you can kick the CPU speed up by 25-200% or more, you need to provide some serious heat removal.

Current CPU types provide internal temperature sensors that can be read by the system BIOS and by some utility programs like SiSoft Sandra. Reading the temperature of your system running normally will give you a baseline operating temperature to compare with as you overclock. You must avoid reaching or exceeding the thermal limits of your CPU.

Although the maximum idle temperature on many AMD CPUs can be as high as 95 degrees Centigrade, the actual running temperature is 30-40 degrees C. Many BIOS versions provide CPU temperature alarms at 60, 65, and 70 degrees C. Your heat sink and ventilation should keep the CPU's running temperature well below 60 degrees C, and you should overclock your CPU no more than 25%.

The secret to heat removal is to have a large mass of material with low thermal resistance to conduct heat away from the chip into the surrounding cooler air. Alternatively, you can attach a device with a circulating coolant that draws the excess heat away quickly and dumps away from the system components, like the radiator in your car or home air conditioner does.

Aluminum is the ideal metal for most heat sinks. It has low thermal resistance, so it can accept and dissipate thermal energy very efficiently. It is inexpensive and easily manufactured into a variety of shapes that provide fast thermal dissipation and contact with almost any surface that needs cooling. Copper, also used in some heat sinks, is more expensive but is the material of choice for water-cooled devices.

In addition to using a highly thermal-conductive material, that material must be as tightly attached to the CPU as possible. It is not adequate to merely place the material next to the CPU: the bond must be as close to being a part of the CPU as possible. The bond is usually made with a very thin layer of thermally conductive grease or epoxy adhesive specifically for heat-sink bonding.

The layer must be very thin because the compound or adhesive is intended to improve the metal-to-metal contact by filling in minute imperfections in both surfaces to provide optimal contact and thermal transfer. If the layer can be made thin enough, it will only cover the imperfections and leave metal-to-metal contact at the high spots common to both surfaces.

An often neglected attribute of using a thin layer of thermal compound is that it eliminates air bubbles between the surfaces that may trap a small amount of moisture. Moisture trapped between a hot device and its heat sink does not constitute water cooling; instead it could be a small water bomb waiting to go off. See Step 11 in the instructions that follow. If the temperature of the moisture bubble exceeds 100 degrees Celsius or 212 degrees Fahrenheit (the boiling point of water)—and it can with a souped-up CPU—the water will expand to 2,700 times its volume as steam and demand to go someplace. That will likely be in the direction of destroying the CPU chip or at least weakening the overall thermal bond, causing the CPU to overheat and self-destruct. If you've removed a heat sink from a CPU (best done with either a slight twisting motion to separate heat sink and CPU or very light prying between the two), beware that some are glued on with high-temp epoxy and cannot be removed without destroying the CPU. You've probably experienced this thermal grease or heat-sink compound—a tenacious white material that looks and feels like toothpaste but stains like red wine in the middle of a new white carpet. Thermal compound is typically a mixture of aluminum oxide for thermal conduction and a silicon paste to hold the aluminum oxide together (see Figure 3-12).

Figure 3-12. Thermal compound fills the gaps between heat sink and CPU

Two new compound mixtures have emerged: one containing aluminum oxide in a fine ceramic form, the other silver and silver oxide. According to product documentation at http://www.articsilver.com, the typical aluminum-oxide-based white paste provides the lowest thermal conductivity and the least CPU temperature drop (2-7 degrees), the ceramic compound is next in the order of effectiveness (2-10 degree drop), and the silver-based compound the most efficient, providing a 3-12 degree drop in CPU temperature. The effectiveness is also represented in the cost of the compound—between $4 and $9 per tube. Unless you see your CPU temperature rising towards its maximum limits, the typical aluminum oxide, and certainly the ceramic paste, are more than adequate for the task.

For a time, Intel prebonded heat sinks to some versions of their Pentium I CPUs using thermal epoxy, making it impossible to separate the two if you wanted to add a larger heat sink.

To speed production processes and make applying thermal bonding cleaner, many vendors have chosen to use thermal pads, as shown in Figure 3-13. Thermal pads are fine in lower-temperature applications, but, while they certainly fills gaps between surfaces, they do not give way to allow direct surface contact between high spots. If you separate a CPU and heat sink that were bonded with a thermal pad, it is acceptable to replace the pad with thermal paste instead, unless the warranty on your CPU requires the use of the supplied thermal pad and heat sink.

Figure 3-13. Two forms of thermal pads used on CPU heat sinks

No matter which compound you choose, the technique for properly applying thermal compound to obtain optimal thermal bonding between a cooling device and a CPU involves a few very simple items and steps.

What you will need (see Figure 3-14):

• Thermal compound

• A clean, dry cloth, something as lint-free as possible

• Isopropyl (rubbing) alcohol

• A vinyl glove or piece of plastic wrap

• A straightedge, such as a single-edged razor blade or used plastic card

• An antistatic pad or chip storage bag to pad the CPU pins and reduce the chance of static damage

Figure 3-14. Basic items needed to bond CPU and heat sink

Use these items to install your heat sink as follows:

Follow the directions carefully for heat-sink fastening. The mechanics and fastening system for your heat sink, CPU, and system-board socket may be different than the one shown.

< Day Day Up >