What characteristics make a heatsink a good one? There's a number of factors to consider:
High heatsink surface. It's at the surface of the heatsink where the thermal transfer takes place. Therefore, heatsinks should be designed to have a large surface; this goal can be reached by using a large amount of fine fins, or by increasing the size of the heatsink itself.
Good aerodynamics. Heatsinks must be designed in a way that air can easily and quickly float through the cooler, and reach all cooling fins. Especially heatsinks having a very large amount of fine fins, with small distances between the fins may not allow good air flow. A compromise between high surface (many fins with small gaps between them) and good aerodynamics must be found. This also depends on the fan the heatsink is used with: A powerful fan can force air even through a heatsink with lots of fine fins with only small gaps for air flow - whereas on a passive heatsink, there should be fewer cooling fins with more space between them. Therefore, simply adding a fan to a large heatsink designed for fanless usage doesn't necessarily result in a good cooler.
Good thermal transfer within the heatsink. Large cooling fins are pointless if the heat can't reach them, so the heatsink must be designed to allow good thermal transfer from the heat source to the fins. Thicker fins have better thermal conductivity; so again, a compromise between high surface (many thin fins) and good thermal transfer (thicker fins) must be found. Of course, the material used has a major influence on thermal transfer within the heatsink. Sometimes, heat pipes are used to lead the heat from the heat source to the parts of the fins that are further away from the heat source.
Perfect flatness of the contact area. The part of the heatsink that is in contact with the heat source must be perfectly flat. A flat contact area allows you to use a thinner layer of thermal compound, which will reduce the thermal resistance between heatsink and heat source.
Good and bad example for contact area flatnessEdit
Good mounting method. For good thermal transfer, the pressure between heatsink and heat source must be high. Heatsink clips must be designed to provide a strong pressure, while still being reasonably easy to install. Heatsink mountings with screws/springs are often better than regular clips. Thermoconductive glue or sticky tape should only be used in situations where mounting with clips or screws isn't possible. Measuring heatsink performance; thermal resistance θ Heatsink performance is measured in °C/W (or K/W - since we're dealing with temperature differences, there is no difference between Celsius and Kelvin scale here). We refer to this as thermal resistance (θ). An example for what these values mean: if a thermal load of 20W is applied to a heatsink, and this causes the temperature of the heat source to rise by 10°C, the heatsink has a rating of 10°C/20W = 0.5°C/W.
A θ value is valid only for a certain power load and a certain temperature range.
The thermal resistance of standard coolers for PC CPUs is usually not specified by the heatsink manufacturers, and if it is, it's often inaccurate or intentionally skewed for marketing purposes. You cannot judge heatsink performance by comparing θ specifications from different manufacturers.
The θ values specified by manufacturers specialized in heatsinks for industrial applications (especially large passive heatsinks) are usually more accurate, though.
Heatsink testing is not an easy task; many of the heatsink reviews found on the countless cooling-related sites on the net are not done properly.
The thermal conductivity of the heatsink's material has a major impact on cooling performance. Thermal conductivity is measured in W/mK; higher values mean better conductivity. As a rule of thumb, materials with a high electrical conductivity also have a high thermal conductivity. See this Wikipedia article for more information on thermal conductivity.
Alloys have lower thermal conductivity than pure metals, but may have better mechanical or chemical (corrosion) properties.
The following materials are commonly used for heatsinks:
Aluminum. It has a thermal conductivity of 205W/mK, which is good (as a comparison: steel has about 50W/mK). The production of aluminum heatsinks is inexpensive; they can be made using extrusion Due to its softness, aluminum can also be milled quickly; die-casting and even cold forging are also possible (see part 2 of this guide for more information about production methods). Aluminum is also very light (thus, an aluminum heatsink will put less stress on its mounting when the unit is moved around).
Copper's thermal conductivity is about twice as high as aluminum - almost 400W/mK. This makes it an excellent material for heatsinks; but its disadvantages include high weight, high price, and less choice as far as production methods are concerned. Copper heatsinks can be milled, die-cast, or made of copper plates bonded together; extrusion is not possible.
To combine the advantages of aluminum and copper, heatsinks can be made of aluminum and copper bonded together. Here, the area in contact with the heat source is made of copper, which helps lead the heat away to the outer parts of the heatsink. The first heatsink for PC CPUs with an embedded copper piece was the Alpha P7125 (for first-generation Slot A Athlon CPUs). Keep in mind that a copper embedding is only useful if it is tightly bonded to the aluminum part for good thermal transfer. This is not always the case, especially not with inexpensive coolers. If the thermal transfer between the copper and the aluminum is poor, the copper embedding may do more harm than good.
Alpha P7125 base plateEdit
The copper plate helps spread heat across the base plate.
AVC heatsink with copper coreEdit
The copper core helps the heat move to the upper parts of the heatsink.
Thermalright heatsink (prototype) with large heat pipe in the center A heat pipe provides substantially better thermal transfer than a solid piece of copper.
Silver has an even higher thermal conductivity than copper, but only by about 10%. This does not justify the much higher price for heatsink production - however, pulverized silver is a common ingredient in high-end thermal compounds.