How Heat Sinks Work

Heat can be transferred in three different ways: convection, radiation and conduction. Conduction is the way heat is transferred in a solid, and therefore is the way it is transferred in a heat sink. Conduction occurs when two objects with different temperatures come into contact with one another. At the point where the two objects meet, the faster moving molecules of the warmer object crash into the slower moving molecules of the cooler object. When this happens, the faster moving molecules from the warmer object give energy to the slower moving molecules, which in turn heats the cooler object. This process is known as thermal conductivity, which is how heat sinks transfer heat away from the computer's processor.

Heat sinks are usually made of metal, which serves as the thermal conductor that carries heat away from the CPU. However, there are pros and cons to using every type of metal. First, each metal has a different level of thermal conductivity. The higher the thermal conductivity of the metal, the more efficient it is at transferring heat.

One of the most common metals used in heat sinks is aluminum. Aluminum has a thermal conductivity of 235 watts per Kelvin per meter (W/mK). (The thermal conductivity number, in this case 235, refers to the metal's ability to conduct heat. Simply put, the higher the thermal conductivity number of a metal, the more heat that metal can conduct.) Aluminum is also cheap to produce and is lightweight. When a heat sink is attached, its weight puts a certain level of stress on the motherboard, which the motherboard is designed to accommodate. Yet the lightweight make up of aluminum is beneficial because it adds little weight and stress to the motherboard.

One of the best and most common materials used to make heat sinks is copper. Copper has a very high thermal conductivity of 400 W/mK. It is, however, heavier than aluminum and more expensive. But for operating systems that require an extensive amount of heat dissipation, copper is frequently used.

So where does the heat go once it's been conducted from the processor through the heat sink? A fan inside the computer moves air across the heat sink and out the computer. Most computers also have an additional fan installed directly above the heat sink to help properly cool the processor. Heat sinks with these additional fans are called active heat sinks, while those with the single fan are called passive heat sinks. The most common fan is the case fan, which draws cool air from outside the computer and blows it through the computer, expelling the hot air out of the rear.

Heat sinks are like any other product in the always-advancing field of computers. Companies are constantly striving to find lighter, more conductive materials to make efficient heat sinks. They don't need to be made from just one material. For example, some heat sink producers are bonding copper and aluminum together. The design consists mainly of aluminum (for its lightweight properties) surrounded by a copper plate (for its high rate of thermal conductivity). These are great -- in theory -- but if the copper doesn't bonded tightly with the aluminum, which is often the case in inexpensive heat sinks, the copper plate can do more harm than good.

In October 2008, the firm Applied Nanotech announced that the future of heat sinks is an isotropic material called CarbAl. CarbAl is made up of 20 percent aluminum and 80 percent of two different carbon-derived materials with excellent thermal conductivity. Applied Nanotech was excited about the material because it has a thermal conductivity of 425 W/mK (higher than both aluminum and copper) and has a density similar to aluminum. Basically, CarbAl is more conductive than copper and weighs the same as aluminum, making it the best of both worlds.

Another material that's gaining popularity with heat sink producers is a natural graphite composite material. It's not as conductive as copper, but it's close with a thermal conductivity of 370 W/mK. But the real advantage of graphite is its weight -- it weighs just 70 percent of the weight of aluminum.

Whatever the material, there is one rule of thumb for heat sinks: inexpensive ones cost more in the long run. Many of the cheaper heat sinks on the market contain fans that use sleeve bearings. Sleeve bearings often break down after a very short period of time due to problems with their lubrication. While heat sinks with fans using ball bearings are more expensive, they last far longer than sleeve bearings and are cheaper in the long run. We'll talk more about how to choose a heat seat on the next page.

When you buy a computer, whether it's a Dell, Sony or an HP, the heat sink is already installed. However, if you're building your own computer, there are certain factors you must consider when choosing the right heat sink. Since you have so many options when choosing a processor, just be certain that the thermal output of the processor you buy matches the thermal handling capability of the heat sink you buy. To determine the heat sink performance required by the processor you buy, you need to know three things:

These figures should be in the owner's manual. Once you have these, they can be plugged into a mathematical formula to determine the heat sink required by your processor. The rate at which the heat sink transfers heat from the processor to the air is known as thermal resistance. To find the thermal resistance needed for the processor, you subtract the maximum inlet temperature from the maximum case temperature and divide that number by the maximum power dissipation of the processor. The thermal resistance is measured in degrees Celsius per watt (C/W).

Let's say, for example, that you buy a processor with a maximum allowable case temperature of 70 degrees Celsius (158 degrees Fahrenheit). Its maximum allowable inlet temperature is 36 degrees Celsius (96.8 degree Fahrenheit) and it allows for a maximum power output of 110. The formula would look like this:

R = (70-36) / 110

R = 0.31 C/W

Therefore, when purchasing a heat sink for this processor, you should be sure that its thermal resistance is no higher than 0.31. It's fine to use a heat sink with this processor that has a lower thermal resistance than 0.31. This will only improve the cooling. However, as is the case with any heat sink, you should never buy one with a higher thermal resistance than is required by your processor.