heat sinks. Left for TO-3 package, right for TO-220 package, middle for two TO-220.
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 (°C/W). 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 (heat sink to ambient air) 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 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. The most common heat sink materials are
aluminium alloys. Aluminium alloy
1050 has one of the higher thermal conductivity values at 229 W/(m·K) and heat capacity of 922 J/(kg·K), but is mechanically soft. Aluminium alloys 6060 (low-stress),
6061, and
6063 are commonly used, with thermal conductivity values of 166 and 201 W/(m·K) 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 (see also
Copper in heat exchangers). Copper has around twice the thermal conductivity of aluminium, around 400 W/(m·K) 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
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. 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 A pin fin heat sink is a heat sink that has pins that extend from its base. The pins can be cylindrical, elliptical, or square. A second type of heat sink fin arrangement is the straight fin. A variation on the straight fin heat sink is a cross-cut heat sink. A third type of heat sink is the flared fin heat sink, where the fins are not parallel to one another. Flaring the fins decreases flow resistance and makes more air go through the heat-sink fin channel; otherwise, more air would bypass the fins. Slanting them keeps the overall dimensions the same, but offers longer fins. Examples of the three types are shown in the image on the right. 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.
Conductive thick plate between the heat source and the heat sink Placing a conductive thick plate as a heat-transfer interface between a heat source and a cold flowing fluid (or any other heat sink) may improve the cooling performance. In such arrangement, the heat source is cooled under the thick plate instead of being cooled in direct contact with the cooling fluid. It is shown that the thick plate can significantly improve the heat transfer between the heat source and the cooling fluid by conducting the heat current in an optimal manner. The two most attractive advantages of this method are that no additional pumping power and no extra heat-transfer surface area, that is quite different from fins (extended surfaces).
Surface color with a black heat sink The
heat transfer from the heat sink occurs by convection of the surrounding air, conduction through the air, and
radiation. Heat transfer by radiation is a function of both the heat-sink temperature and the temperature of the surroundings that the heat sink is optically coupled with. When both of these temperatures are on the order of 0 °C to 100 °C, the contribution of radiation compared to convection is generally small, and this factor is often neglected. In this case, finned heat sinks operating in either natural-convection or forced-flow will not be affected significantly by surface
emissivity. In situations where convection is low, such as a flat non-finned panel with low airflow,
radiative cooling can be a significant factor. Here the surface properties may be an important design factor. Matte-black surfaces radiate much more efficiently than shiny bare metal. A shiny metal surface has
low emissivity. The emissivity of a material is tremendously frequency-dependent and is related to absorptivity (of which shiny metal surfaces have very little). For most materials, the emissivity in the
visible spectrum is similar to the emissivity in the infrared spectrum; however, there are exceptions notably, certain metal oxides that are used as "
selective surfaces". In
vacuum or
outer space, there is no convective heat transfer, thus in these environments, radiation is the only factor governing heat flow between the heat sink and the environment. For a satellite in space, a surface facing the
Sun will absorb a lot of radiant heat, because the
Sun's surface temperature is nearly 6000 K, whereas the same surface facing deep space will radiate a lot of heat, since deep space has an
effective temperature of only several Kelvin. ==Engineering applications==