Thermal management (electronics)
All electronic devices and circuitry generate excess heat and thus require thermal management to improve reliability and prevent premature failure. The amount of heat output is equal to the power input, if there are no other energy interactions. There are several techniques for cooling including various styles of heat sinks, thermoelectric coolers, forced air systems and fans, heat pipes, and others. In cases of extreme low environmental temperatures, it may actually be necessary to heat the electronic components to achieve satisfactory operation.
Overview
Thermal resistance of devices
This is usually quoted as the thermal resistance from junction to case of the semiconductor device. The units are °C/W. For example, a heatsink rated at 10 °C/W will get 10 °C hotter than the surrounding air when it dissipates 1 Watt of heat. Thus, a heatsink with a low °C/W value is more efficient than a heatsink with a high °C/W value.Given two semiconductor devices in the same package, a lower junction to ambient resistance indicates a more efficient device. However, when comparing two devices with different die-free package thermal resistances, their junction to ambient or junction to case resistance values may not correlate directly to their comparative efficiencies. Different semiconductor packages may have different die orientations, different copper mass surrounding the die, different die attach mechanics, and different molding thickness, all of which could yield significantly different junction to case or junction to ambient resistance values, and could thus obscure overall efficiency numbers.
Thermal time constants
A heatsink's thermal mass can be considered as a capacitor and the thermal resistance as an electrical resistance. Together, these two components form a thermal RC circuit with an associated time constant given by the product of R and C. This quantity can be used to calculate the dynamic heat dissipation capability of a device, in an analogous way to the electrical case.Thermal interface material
A thermal interface material or mastic is used to fill the gaps between thermal transfer surfaces, such as between microprocessors and heatsinks, in order to increase thermal transfer efficiency.It has a higher thermal conductivity value in Z-direction than xy-direction.
Applications
Personal computers
Due to technological developments and public interest, the retail heat sink market rose high. In the early 2000s, CPUs were produced that emitted more and more heat than earlier, escalating requirements for quality cooling systems.Overclocking has always meant greater cooling needs, and the inherently hotter chips meant more concerns for the enthusiast. Efficient heat sinks are vital to overclocked computer systems because the higher a microprocessor's cooling rate, the faster the computer can operate without instability; generally, faster operation leads to higher performance. Many companies now compete to offer the best heat sink for PC overclocking enthusiasts.
Soldering
Temporary heat sinks were sometimes used while soldering circuit boards, preventing excessive heat from damaging sensitive nearby electronics. In the simplest case, this means partially gripping a component using a heavy metal crocodile clip or similar clamp. Modern semiconductor devices, which are designed to be assembled by reflow soldering, can usually tolerate soldering temperatures without damage. On the other hand, electrical components such as magnetic reed switches can malfunction if exposed to higher powered soldering irons, so this practice is still very much in use.Batteries
In the battery used for electric vehicles, Nominal battery performance is usually specified for working temperatures somewhere in the +20 °C to +30 °C range; however, the actual performance can deviate substantially from this if the battery is operated at higher or, in particular, lower temperatures, so some electric cars have heating and cooling for their batteries.Methodologies
Heat sinks
Heat sinks are widely used in electronics and have become essential to modern microelectronics. In common use, it is a metal object brought into contact with an electronic component's hot surface—though in most cases, a thin thermal interface material mediates between the two surfaces. Microprocessors and power handling semiconductors are examples of electronics that need a heat sink to reduce their temperature through increased thermal mass and heat dissipation. Heat sinks have become almost essential to modern integrated circuits like microprocessors, DSPs, GPUs, and more.A heat sink usually consists of a metal structure with one or more flat surfaces to ensure good thermal contact with the components to be cooled, and an array of comb or fin like protrusions to increase the surface contact with the air, and thus the rate of heat dissipation.
A heat sink is sometimes used in conjunction with a fan to increase the rate of airflow over the heat sink. This maintains a larger temperature gradient by replacing warmed air faster than convection would. This is known as a forced air system.
Cold plate
Placing a conductive thick metal plate, referred to as a cold plate, as a heat transfer interface between a heat source and a cold flowing fluid 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 way of 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.Principle
Heat sinks function by efficiently transferring thermal energy from an object at high temperature to a second object at a lower temperature with a much greater heat capacity. This rapid transfer of thermal energy quickly brings the first object into thermal equilibrium with the second, lowering the temperature of the first object, fulfilling the heat sink's role as a cooling device. Efficient function of a heat sink relies on rapid transfer of thermal energy from the first object to the heat sink, and the heat sink to the second object.The most common design of a heat sink is a metal device with many fins. The high thermal conductivity of the metal combined with its large surface area result in the rapid transfer of thermal energy to the surrounding, cooler, air. This cools the heat sink and whatever it is in direct thermal contact with. Use of fluids and thermal interface material ensures good transfer of thermal energy to the heat sink. Similarly, a fan may improve the transfer of thermal energy from the heat sink to the air.
Construction and materials
A heat sink usually consists of a base with one or more flat surfaces and an array of comb or fin-like protrusions to increase the heat sink's surface area contacting the air, and thus increasing the heat dissipation rate. While a heat sink is a static object, a fan often aids a heat sink by providing increased airflow over the heat sink—thus maintaining a larger temperature gradient by replacing the warmed air more quickly than passive convection achieves alone—this is known as a forced-air system.Ideally, heat sinks are made from a good thermal conductor such as silver, gold, copper, or aluminum alloy. Copper and aluminum are among the most-frequently used materials for this purpose within electronic devices. Copper is significantly more expensive than aluminum but is also roughly twice as efficient as a thermal conductor. Aluminum has the significant advantage that it can be easily formed by extrusion, thus making complex cross-sections possible. Aluminum is also much lighter than copper, offering less mechanical stress on delicate electronic components. Some heat sinks made from aluminum have a copper core as a trade off. The heat sink's contact surface must be flat and smooth to ensure the best thermal contact with the object needing cooling. Frequently a thermally conductive grease is used to ensure optimal thermal contact; such compounds often contain colloidal silver. Further, a clamping mechanism, screws, or thermal adhesive hold the heat sink tightly onto the component, but specifically without pressure that would crush the component.
Performance
Heat sink performance is a function of material, geometry, and overall surface heat transfer coefficient. Generally, forced convection heat sink thermal performance is improved by increasing the thermal conductivity of the heat sink materials, increasing the surface area and by increasing the overall area heat transfer coefficient.Online heat sink calculators from companies such as Novel Concepts, Inc. and at www.heatsinkcalculator.com can accurately estimate forced and natural convection heat sink performance. For more complex heat sink geometries, or heat sinks with multiple materials or multiple fluids, computation fluid dynamics analysis is recommended.
Convective air cooling
This term describes device cooling by the convection currents of the warm air being allowed to escape the confines of the component to be replaced by cooler air. Since warm air normally rises, this method usually requires venting at the top or sides of the casing to be effective.Forced air cooling
If there is more air being forced into a system than being pumped out, this is referred to as a 'positive' airflow, as the pressure inside the unit is higher than outside.A balanced or neutral airflow is the most efficient, although a slightly positive airflow can result in less dust build up if filtered properly
Heat pipes
A heat pipe is a heat transfer device that uses evaporation and condensation of a two-phase "working fluid" or coolant to transport large quantities of heat with a very small difference in temperature between the hot and cold interfaces. A typical heat pipe consists of sealed hollow tube made of a thermoconductive metal such as copper or aluminium, and a wick to return the working fluid from the evaporator to the condenser. The pipe contains both saturated liquid and vapor of a working fluid, all other gases being excluded. The most common heat pipe for electronics thermal management has a copper envelope and wick, with water as the working fluid. Copper/methanol is used if the heat pipe needs to operate below the freezing point of water, and aluminum/ammonia heat pipes are used for electronics cooling in space.The advantage of heat pipes is their great efficiency in transferring heat. The thermal conductivity of heat pipes can be as high as 100,000 W/m K, in contrast to copper, which has a thermal conductivity of around 400 W/m K.