What is a heatsink?
A heatsink is a passive heat exchanger that transfers heat. The heatsink is typically a metallic part which can be attached to a device releasing energy in the form of heat, with the aim of dissipating that heat to a surrounding fluid in order to prevent the device overheating.
In many applications, the device is an electronic component (e.g. CPU, GPU, ASIC, FET etc.) and the surrounding fluid is air. The device transfers heat to the heatsink by conduction. The primary mechanism of heat transfer from the heatsink is convection, although radiation also has a minor influence.
There are two distinct types of convection:
- Natural convection – where the movement of the fluid particles is caused by the local changes in density due to transfer of heat from the surface of a solid to the fluid particles in close proximity.
- Forced convection – where the movement of the fluid particles is caused by an additional device such as a fan or blower.
Heatsinks are designed to significantly increase the contact surface area between solid and fluid, thereby increasing the opportunity for heat transfer. A typical ASIC may have a surface area in contact with air of only 1600mm2. The surface area of a typical heatsink used to cool that device may be 10 or 20 times that value.
There are many designs for heatsinks, but they typically comprise a base and a number of protrusions attached to this base. The base is the feature that interfaces with the device to be cooled. Heat is conducted through the base into the protrusions. The protrusions can take several forms, including:
Heatsinks are usually constructed from copper or aluminum. Copper has a very high thermal conductivity, which means the rate of heat transfer through copper heatsinks is also very high. Whilst lower than that of copper, aluminum’s thermal conductivity is still high and it has the added benefits of lower cost and lower density, making it useful for applications where weight is a major concern.
The performance of a heatsink is a consequence of many parameters, including:
- Surface treatment
- Air velocity
- Interface with device
The last item is very important. Whilst electronic components and heatsink bases are manufactured to be very flat and smooth, at a microscopic level their surfaces are rough. This results in very few points of contact and many tiny air gaps between the component and its heatsink. Air has a low thermal conductivity, resulting in poor conduction of heat from the device to the heatsink. To combat this, a thermal interface material (TIM) can be applied to the base of the heatsink to fill these gaps and provide more conduction paths between device and heatsink.
Air gaps between device and heatsink base negatively impact the conduction path
Thermal interface material (TIM) fills the air gaps, significantly improving the conduction path
Heatsink performance is characterized by its thermal resistance. This parameter can be thought of as the difference in temperature between the air around the heatsink and the device surface in contact with the heat sink per unit of input power. Thermal resistance is denoted by the symbol θ and has the unit °C/W.
The junction temperature (TJ) is the temperature of the hottest part of the device. This is the critical temperature for its correct operation. The case temperature (TC) is the temperature of the surface of the device which is in contact with the heatsink assembly. TC is lower than TJ due to the junction-to-case thermal resistance (θJC). The performance of the heatsink assembly is defined by the case-to-ambient thermal resistance (θCA). This is the difference in temperature between the device surface (TC) and the surrounding air (TA) for a unit of input power.
The increase in TJ over TA for each Watt of thermal power the device generates is the sum of θJC and θCA.
HEATSINK MANUFACTURING METHODS
Heatsinks can be manufactured in a variety of ways depending on the required performance, cost and volume. These include:
- Machining – where a CNC machine is used to cut the metal
- Extrusion – where metal is heated and pushed through a mold
- Forging – where metal is heated and shaped by pressurization
- Stamping – where the metal fin is cut, and then soldered onto the base
- Skiving – where a blade is used to slice, and push up the single block of metal
For more complex thermal issues, heatpipes and vapor chambers may be employed within the heatsink assembly. These devices are sealed objects containing a fluid (typically water) and utilize the release of heat during fluid phase changes to vastly increase their conductivity when compared to a solid metallic object of the same geometry.
Radian Thermal Products offers all these manufacturing techniques and we can therefore provide an unbiased approach to each thermal requirement.