Summer cooling for your CPU

If you notice your server or workstations melting, it is probably time to check out some cooling solutions. In this Daily Drill Down, James McPherson describes what to look for when trying to keep your machines chilled.

Ah, summer. Sitting in a hammock with a good book and a glass of lemonade. If only our CPUs had it so good. They swelter in their cases at temperatures hotter than a Turkish sauna even while sitting in an air-conditioned office. So, if you are worried about them overheating, it is time to look at cooling solutions.

Cooling units have two major components: the heat sink and the fan. While you rarely purchase the two separately, a good fan is pointless on an inefficient heat sink and vice versa if your fan doesn’t move enough air.

A history lesson in CPU heat
A good fan and heat sink combination is a necessity on today’s chips. Not that it’s anything new; heat sinks have been common for decades, and fans didn’t take long to follow. However, for a long time, the very presence of a heat sink and/or fan was sufficient. Then came the Pentium II and the CPU slot. The slot-compatible packaging allowed the CPU and cache memory to be on separate silicon dies and still be a single component. The package included a minimalist type of heat sink (a heat spreader, really). However, the downside was the extra layers between the CPU and the cooling system, not to mention the vertical chip placement that further hindered thermal distribution. Heat rises in open air and with a vertical chip, the heat rose vertically across the chip, or rather across the sink, reducing the overall effectiveness of the sink.

The AMD Athlon adopted the slot-compatible design to make things easier on motherboard manufacturers, who were ramped up to build that type of board. However, it was hampered even more by the poor thermal conditions due to its increased power consumption. Decent cooling became a necessity on virtually all chips.

The switch back to chips using sockets without the heat spreaders made the leap from virtual necessity to absolute necessity. A high-end Athlon running without a heat sink or fan will burn to a cinder in less than a minute. Pentium’s flip-chip design will last a little longer. However, nothing stands between you and a scorched piece of silicon other than a cooling solution you’ve never thought about and probably spent less than $10 on.

Today, every major market desktop CPU comes in the retail package with a heat sink and fan (one of those $10 jobs) from the manufacturer. These devices are usually more than adequate for typical usage. Remember that the manufacturer determines “typical usage,” so if they feel like using cheap fans, they can reduce the “nominal” expected case temperature to get by. Trusting that fan seems less and less comforting.

Which poses the question: What if you have nontypical usage? Perhaps your case suffers from extreme internal temperatures due to multiprocessors or RAID arrays? Maybe you have a workstation that needs a little more power than you can get on the market and you’ve taken the overclocking plunge. Or maybe you need to replace a broken fan or need a new cooling solution for a system you’re building. What are the available cooling solutions for these situations?

Heat sink
Heat-sink design is a bit more esoteric than fans. Ironically, the heat sink rarely gets the attention the fan does by marketing types. And the thing is, it is the heat sink's job to get the heat off the processor and out to flowing air. If that isn’t done right, it really doesn’t matter how much air the fan is moving. Doing it right requires balancing thermal conductivity, aerodynamics, and cost. Cost is an important factor, since all too often, the heat sink is an afterthought, if it ever even gets a thought at all.

The material
The material the sink is made of defines much of the design. Thin vanes keep the surface area to mass ratio low, which helps to vent heat but can jeopardize the structural integrity. Furthermore, harder materials are more difficult to machine, which increases cost.

Aluminum is good at transmitting heat, but copper is better—about 30 percent better. The trouble is, copper corrodes quickly, acquiring that greenish patina, which cuts down the thermal conductivity. Manufacturers are forced to make alloys or somehow seal the metal, increasing the cost. Aluminum is therefore the most common material, with copper used only on the most expensive, high-performance models. The cheapest use iron and nickel. They are pretty much crap, performing half as well as aluminum and should be avoided at all cost.

Surface area
Surface area is how the heat gets out of the metal and into the airflow. The goal is to have the most exposed surface area within the useful airflow stream. You’ll see that the classic square fins and corrugations are on the way out, as aerodynamics is being applied to the sink’s design. Spiral-cut protrusions and radial fins are now being introduced to maximize airflows and increase the amount of air to which the heat sink is exposed. A cool design shouldn’t be the only thing you base your decision on. After all, there aren’t many aeronautics engineers working in the heat sink industry, so many visually appealing but technically devoid designs are out there.

The fan gets most of the attention because it has the most specifications, making it easier to sell. Marketing loves that fact, but it does make it harder to find what you need.

Fan speed
Fan speed (measured in revolutions per minute or RPM) is the most common measurement for fans. It is also something of a null attribute. Much like comparing the speed of different processors solely by their clock cycle in MHz, RPM doesn't reflect overall performance. A perfectly flat disk used as the rotor moves virtually no air, regardless of RPM. It takes a fan blade to move air, and when moving air equals moving heat, you want to pay attention to more than speed. Sure, if both fans used the same blades, RPM would be a good indicator, but the odds of that happening are pretty slim across manufacturers.

Airflow (cubic feet per minute) is the real key quality of a good fan. The amount of air it can move directly relates to the heat it can dissipate. Not all fan manufacturers will list this value, but a good one won’t be ashamed to.

Noise is more of an aesthetic quality than performance specification, but much can be said for keeping the sound from your PC down to a dull roar. Noise is also a good indicator of vibration, which can cut the fan's life span. The sound generated by a fan is measured in decibels (dB), a logarithmic scale. To get a perspective on how loud a small PC fan should be, understand that 60 dB is a normal conversation, home theaters top out at 85 dB, and a jet engine can reach upwards to 150 dB.

Thermal grease
The silent, unseen part of the cooling system is the bond between heat sink and processor. If the sink doesn’t have decent contact with the processor, you won’t be able to pull off the heat. Thermal expansion and the general fragility of the processors make it unwise to bond the heat sink and processor together tightly by force. A material is needed that transmits heat as well (if not better than) the heat sink and that is capable of getting into every nook and cranny of the CPU’s cap and the heat sink.

Enter thermal grease. Thermal grease has a very long life span when exposed to great heat or physical stress. Naturally, this grease doesn’t need to endure as much physical or thermal abuse, yet it does need to be more conductive. The addition of metal particles provides the main thermal conduction.

The best and most readily available thermal grease is silver paste. Silver has better conductive properties than copper and when coated in grease, it does not corrode. Silver is far too expensive and vulnerable to corrosion to use as a heat sink. However, if you use it in quantities of a quarter of a gram or less, it becomes more than cheap enough. The basic rule of thumb is any thermal grease is better than none at all. As long as the tube says “thermal conductive grease” on it somewhere, you should be okay. You don’t want to find some strange insulating silicon grease and seal your processor in, so go to an electronics store; they’ll know what you’re after. If you spend more than $2, you’ll probably have enough for two or three processors.

One last note; you won’t have a problem if you get a fan that’s more powerful than you need or a heat sink with extreme thermal conductivity. However, if you use too much thermal grease, you’ll cause problems no matter what cooling solution you have in place. You don’t want it to get into your socket and short out a processor. In addition, it will get so thick that there are too many metal particles between the CPU and the sink. That’s bad.

Nontraditional cooling
A metal heat sink and an attached fan are pretty normal. There are some exotic cooling options that you might want to consider—or might be forced to consider—in extreme situations.

Refrigeration has made the leap from food preservation to processor preservation. These systems are integrated into the case and are very, very expensive. Their performance is spectacular, far beyond that of virtually any other system. Of course, they also draw a lot of power, being a mini-fridge and all.

The systems place a thermal conduction plate with vapor seals around the processor that are connected to the main compressor unit by an insulated tube filled with the coolant hoses. The seals are necessary to prevent moisture forming around the processor, shorting out your board. No, I’m not making this up: If you don’t properly seal the cooling unit to the processor, you’ll have to defrost the CPU.

Part of the incredible performance comes from the fact that refrigeration systems vent the heat external to the case, while standard cooling dumps the heat within the case. Then there’s the fact that refrigeration can easily create subzero temperatures and you have an incredible overclocking rig. Or you could run your computer at the bottom of Death Valley.

Peltier coolers
Peltier coolers are an active heat sink. The application of electricity to these particular materials causes incredible thermal flows, almost as good as refrigeration units. The downside is that they can’t be more than a few millimeters thick, making them reliant on a fan to get the heat out of the case. Their ability to draw heat off of the CPU is nothing short of amazing, and their low form factor enables them to fit in almost any case. Also, their power requirements aren’t as intimidating as a refrigeration unit. As a result, they are the preferred choice for hardcore overclockers who aren’t insane enough to use a refrigeration unit.

The last venting
Not just a cute way to wrap up this article—I should remind you not to neglect the fans in your case. If you aren’t using a refrigeration unit, you are dumping the heat from the CPU to the case. You had better be able to get that heat out. Most cases are laid out well enough to allow the heat to escape. However, be aware that you should have two fans in your case: a front inlet fan and an exhaust fan at the rear of the case. If you can wing it, the inlet fan and exhaust fan should be in line with the CPU and/or graphic card to create the most airflow possible across those tiny furnaces. Finally, take a chill pill. As you can see, there are many options to help both you and your computer avoid the heat.

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