Heat sink
A heat sink is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device's temperature. In computers, heat sinks are used to cool CPUs, GPUs, and some chipsets and RAM modules. Heat sinks are used with other high-power semiconductor devices such as power transistors and optoelectronics such as lasers and light-emitting diodes, where the heat dissipation ability of the component itself is insufficient to moderate its temperature.
A heat sink is designed to maximize its surface area in contact with the cooling medium surrounding it, such as the air. Air velocity, choice of material, protrusion design and surface treatment are factors that affect the performance of a heat sink. Heat sink attachment methods and thermal interface materials also affect the die temperature of the integrated circuit. Thermal adhesive or thermal paste improve the heat sink's performance by filling air gaps between the heat sink and the heat spreader on the device. A heat sink is usually made out of a material with a high thermal conductivity, such as aluminium or copper.
Heat transfer principle
A heat sink transfers thermal energy from a higher-temperature device to a lower-temperature fluid medium. The fluid medium is frequently air, but can also be water, refrigerants, or even oil. If the fluid medium is water, the heat sink is frequently called a cold plate. In thermodynamics a heat sink is a heat reservoir that can absorb an arbitrary amount of heat without significantly changing temperature. Practical heat sinks for electronic devices must have a temperature higher than the surroundings to transfer heat by convection, radiation, and conduction. The power supplies of electronics are not absolutely efficient, so extra heat is produced that may be detrimental to the function of the device. As such, a heat sink is included in the design to disperse heat.Fourier's law of heat conduction shows that when there is a temperature gradient in a body, heat will be transferred from the higher-temperature region to the lower-temperature region. The rate at which heat is transferred by conduction,, is proportional to the product of the temperature gradient and the cross-sectional area through which heat is transferred. When it is simplified to a one-dimensional form in the x direction, it can be expressed as:
For a heat sink in a duct, where air flows through the duct, the heat-sink base will usually be hotter than the air flowing through the duct. Applying the conservation of energy, for steady-state conditions, and Newton's law of cooling to the temperature nodes shown in the diagram gives the following set of equations:
where
Using the mean air temperature is an assumption that is valid for relatively short heat sinks. When compact heat exchangers are calculated, the logarithmic mean air temperature is used.
The above equations show that:
- When the air flow through the heat sink decreases, this results in an increase in the average air temperature. This in turn increases the heat-sink base temperature. And additionally, the thermal resistance of the heat sink will also increase. The net result is a higher heat-sink base temperature.
- * The increase in heat-sink thermal resistance with decrease in flow rate will be shown later in this article.
- The inlet air temperature relates strongly with the heat-sink base temperature. For example, if there is recirculation of air in a product, the inlet air temperature is not the ambient air temperature. The inlet air temperature of the heat sink is therefore higher, which also results in a higher heat-sink base temperature.
- If there is no air flow around the heat sink, energy cannot be transferred.
- A heat sink is not a device with the "magical ability to absorb heat like a sponge and send it off to a parallel universe".
Design factors
Thermal resistance
For semiconductor devices used in a variety of consumer and industrial electronics, the idea of thermal resistance simplifies the selection of heat sinks. The heat flow between the semiconductor die and ambient air is modeled as a series of resistances to heat flow; there is a resistance from the die to the device case, from the case to the heat sink, and from the heat sink to the ambient air. The sum of these resistances is the total thermal resistance from the die to the ambient air. Thermal resistance is defined as temperature rise per unit of power, analogous to electrical resistance, and is expressed in units of degrees Celsius per watt. If the device dissipation in watts is known, and the total thermal resistance is calculated, the temperature rise of the die over the ambient air can be calculated.The idea of thermal resistance of a semiconductor heat sink is an approximation. It does not take into account non-uniform distribution of heat over a device or heat sink. It only models a system in thermal equilibrium and does not take into account the change in temperatures with time. Nor does it reflect the non-linearity of radiation and convection with respect to temperature rise. However, manufacturers tabulate typical values of thermal resistance for heat sinks and semiconductor devices, which allows selection of commercially manufactured heat sinks to be simplified.
Commercial extruded aluminium heat sinks have a thermal resistance ranging from for a large sink meant for TO-3 devices, up to as high as for a clip-on heat sink for a TO-92 small plastic case. The popular 2N3055 power transistor in a TO-3 case has an internal thermal resistance from junction to case of. The contact between the device case and heat sink may have a thermal resistance between, depending on the case size and use of grease or insulating mica washer.
Material
The materials for heat sink applications should have high heat capacity and thermal conductivity in order to absorb more heat energy without shifting towards a very high temperature and transmit it to the environment for efficient cooling. The most common heat sink materials are aluminium alloys. Aluminium alloy 1050 has one of the higher thermal conductivity values at 229 W/ and heat capacity of 922 J/, but is mechanically soft. Aluminium alloys 6060, 6061, and 6063 are commonly used, with thermal conductivity values of 166 and 201 W/ respectively. The values depend on the temper of the alloy. One-piece aluminium heat sinks can be made by extrusion, casting, skiving or milling.Copper has excellent heat-sink properties in terms of its thermal conductivity, corrosion resistance, biofouling resistance, and antimicrobial resistance. Copper has around twice the thermal conductivity of aluminium, around 400 W/ for pure copper. Its main applications are in industrial facilities, power plants, solar thermal water systems, HVAC systems, gas water heaters, forced air heating and cooling systems, geothermal heating and cooling, and electronic systems.
Copper is three times as dense and more expensive than aluminium, and copper is less ductile than aluminum. One-piece copper heat sinks can be made by skiving or milling. Sheet-metal fins can be soldered onto a rectangular copper body.
Fin efficiency
Fin efficiency is one of the parameters that makes a higher-thermal-conductivity material important. A fin of a heat sink may be considered to be a flat plate with heat flowing in one end and being dissipated into the surrounding fluid as it travels to the other. As heat flows through the fin, the combination of the thermal resistance of the heat sink impeding the flow and the heat lost due to convection, the temperature of the fin and, therefore, the heat transfer to the fluid, will decrease from the base to the end of the fin. Fin efficiency is defined as the actual heat transferred by the fin, divided by the heat transfer were the fin to be isothermal. These equations are applicable for straight fins:where
Fin efficiency is increased by decreasing the fin aspect ratio, or by using a more conductive material.
Spreading resistance
Another parameter that concerns the thermal conductivity of the heat-sink material is spreading resistance. Spreading resistance occurs when thermal energy is transferred from a small area to a larger area in a substance with finite thermal conductivity. In a heat sink, this means that heat does not distribute uniformly through the heat-sink base. The spreading resistance phenomenon is shown by how the heat travels from the heat source location and causes a large temperature gradient between the heat source and the edges of the heat sink. This means that some fins are at a lower temperature than if the heat source were uniform across the base of the heat sink. This nonuniformity increases the heat sink's effective thermal resistance.To decrease the spreading resistance in the base of a heat sink:
- increase the base thickness,
- choose a different material with higher thermal conductivity,
- use a vapor chamber or heat pipe in the heat sink base.
Fin arrangements
Forghan, et al. have published data on tests conducted on pin fin, straight fin, and flared fin heat sinks. They found that for low air approach velocity, typically around 1 m/s, the thermal performance is at least 20% better than straight fin heat sinks. Lasance and Eggink also found that for the bypass configurations that they tested, the flared heat sink performed better than the other heat sinks tested.
Generally, the more surface area a heat sink has, the better its performance. Real-world performance depends on the design and application. The concept of a pin fin heat sink is to pack as much surface area into a given volume as possible, while working in any orientation of fluid flow. Kordyban has compared the performance of a pin-fin and a straight-fin heat sink of similar dimensions. Although the pin-fin has 194 cm2 surface area while the straight-fin has 58 cm2, the temperature difference between the heat-sink base and the ambient air for the pin fin is, but for the straight-fin it was 44 °C, or 6 °C better than the pin fin. Pin fin heat sink performance is significantly better than straight fins when used in their optimal application where the fluid flows axially along the pins rather than only tangentially across the pins.
| Heat-sink fin type | Width | Length | Height | Surface area | Volume | Temperature difference, Tcase − Tair |
| Straight | 2.5 | 2.5 | 3.2 | 58 | 20 | 44 |
| Pin | 3.8 | 3.8 | 1.7 | 194 | 24 | 51 |