What is a Passive Heatsink?
Heatsinks are devices used to keep computer processing units (CPUs) and chipsets cool. Most heatsinks are active, meaning the design includes a small fan powered by a connector on the motherboard, or a lead from the power supply unit. A passive heatsink does not include a fan in the design, and is typically larger than a standard model, using the extra surface area of the device to improve thermal cooling in compensation for lack of a fan. Its purpose is to reduce system noise and eliminate the possibility of catastrophic overheating due to fan failure.
When chipsets and CPUs are in operation, considerable heat is generated from electrical activity. These chips would quickly become damaged and inoperable without cooling. A heatsink sits on top of the CPU or chipset, creating a path for heat to rise from the chip into the heatsink where it can dissipate. A passive heatsink accomplishes this without benefit of an incorporated fan.
Many elements factor into the efficiency of a heatsink. The first consideration is the material used. Aluminum is an extremely light and inexpensive material with a high degree of thermal conductivity. Copper is three times heavier and quite a bit more expensive than aluminum, but is also twice as efficient in conducting heat. A passive heatsink might be made of one or of both of these materials in combination.
A heatsink has a flat base made to interface with the chip face. Extending upwards from the base are rows of pins or “fins” that supply surface area for heat dissipation. A passive heatsink typically has more surface area and the pins or fins are often made of aluminum alloys to keep weight down. Copper might be used strategically in the base and in heat pipes or other design elements. Heat pipes are often used to more efficiently funnel thermal buildup from the base of the heatsink into the fins or pins where circulating air inside the computer case can carry the heat away.
Heatsinks attach to chips by locking mechanisms that differ depending on the model. Some locking mechanisms are easier to work with than others, but the CPU socket type will determine which heatsink models the motherboard can accommodate. A passive heatsink that is large and heavy might require removal of the motherboard for installation of a special bracket or locking mechanism.
As always, thermal compound must be used between the base of the heatsink and the chip. Imperfections in these surfaces create voids that introduce resistance along the thermal conduction path. Applying a thermal compound will fill these gaps to improve the efficiency of the heatsink and ensure a cooler running chip. Thermal tape is the least expensive type of compound, but in general, thermal pads or thermal grease is considered more efficient and is quite affordable.
While a passive heatsink can be large, it has advantages over an active heatsink. Active heatsinks — or those that rely on an incorporated fan — can get away with a smaller surface area, but if the fan fails the heatsink will not be able to keep the chip cool and damage could result. A passive heatsink, properly installed and rated for the chip it is cooling, cannot fail under normal operating conditions.
Another advantage of a passive heatsink is lack of noise. Every system must include fans, but eliminating the chipset or CPU fan can help to keep the overall decibels lower. A passive heatsink also does not require power.
The main disadvantage is size. Due to the larger surface area normally incorporated into a passive heatsink, the footprint can be quite tall and might not fit into all computer cases. Installation can also be more challenging in some instances. Nevertheless, the payoff is a quieter system with no chance for heatsink failure, and these two factors are appealing to many enthusiasts.
It is important to choose a heatsink that is rated to cool the CPU or chipset desired. In some cases, chip manufacturers recommend particular heatsinks and even compounds, and using another model or compound might void the chip’s warranty. Check with the manufacturer’s website for details as required.
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