Difference between revisions of "Thermal management of electronic devices and systems"
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Latest revision as of 13:49, 10 December 2011
Heat generated by electronic devices and circuitry must be dissipated to improve reliability and prevent premature failure.[1] Techniques for heat dissipation can include heatsinks and fans for air cooling, and other forms of computer cooling such as liquid cooling.[2]
In cases of extreme low environmental temperatures, it may actually be necessary to heat the electronic components to achieve satisfactory operation.[3]
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.[4]
Thermal time constants
A heatsink's thermal mass can be considered as a capacitor (storing heat instead of charge) and the thermal resistance as an electrical resistance (giving a measure of how fast stored heat can be dissipated). 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.[5] A specific type of thermal interface material is put between the heat sink and the heat source to increase thermal throughput, such as a microprocessor chip or other power handling semiconductor to stabilise its temperature through increased thermal mass and heat dissipation (primarily by conduction and convection and to a lesser extent by radiation).[citation needed]
Thermal interface material
A Thermal Interface Material or Mastic (aka TIM) is used to fill the gaps between thermal transfer surfaces, such as between microprocessors and heatsinks, in order to increase thermal transfer efficiency. These gaps are normally filled with air which is a very poor conductor.[citation needed]
Applications
Personal Computers
Due to recent technological developments and public interest, the retail heat sink market has reached an all time 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. Prominent aftermarket heat sink manufacturers include: Aero Cool, Foxconn, Thermalright, Thermaltake, Swiftech, and Zalman.[citation needed]
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.[6]
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[7]
Methodologies
Heat sinks
Heat sinks are widely used in electronics, and have become almost essential to modern central processing units. 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 (primarily by conduction and convection and to a lesser extent by radiation). 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.
A heat sink (or heatsink) is an environment or object that absorbs and dissipates heat from another object using thermal contact (either direct or radiant). Heat sinks are used in a wide range of applications wherever efficient heat dissipation is required; major examples include refrigeration, heat engines and cooling electronic devices.[citation needed]
Principle
Heat sinks function by efficiently transferring thermal energy ("heat") 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 (for example coolants in refrigeration) and thermal interface material (in cooling electronic devices) 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.[citation needed]
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.[citation needed]
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 (401 W/(m·K) at 300 K) is significantly more expensive than aluminum (237 W/(m·K) at 300 K) 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 (the base) 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.[citation needed]
Performance
Heat sink performance (including free convection, forced convection, liquid cooled, and any combination thereof) 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 (usually by adding extended surfaces, such as fins or foam metal) and by increasing the overall area heat transfer coefficient (usually by increase fluid velocity, such as adding fans, pumps, etc.).
Online heat sink calculators from companies such as Novel Concepts, Inc., can accurately estimate forced convection heat sink performance. For more complex heat sink geometries, and/or heat sinks with multiple materials, and/or heat sinks with multiple fluids, computation fluid dynamics (CFD) analysis is recommended (see graphics on this page).[citation needed]
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.[citation needed]
Forced air cooling
If there is more air being forced into a system than being pumped out (due to an imbalance in the number of fans), 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.[citation needed]
Heat pipes
A heat pipe is a heat transfer mechanism that can 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. The pipe contains a relatively small quantity of a "working fluid" or coolant (such as water, ethanol or mercury) with the remainder of the pipe being filled with the vapour phase of the working fluid, all other gases being excluded. The advantage of heat pipes is their great efficiency in transferring heat. They are actually more "conductive" than a copper bar of equivalent cross-section.[citation needed]
Peltier cooling plates
Peltier cooling plates (11px /ˈpɛlti.eɪ/) take advantage of what is known as the Peltier effect to create a heat flux between the junction of two different types of materials. This effect is commonly used for cooling electronic components and small instruments.
There are no moving parts and such a device is maintenance free. Due to the relatively low efficiency, thermoelectric cooling is generally only used in environments where the solid state nature outweighs the poor efficiency. Thermoelectric junctions are generally only around 10% as efficient as the ideal refrigerator (Carnot cycle), compared with 40% achieved by conventional compression cycle systems.[citation needed]
Synthetic Jet Air Cooling
A Synthetic Jet is produced by a continual flow of vortices that are formed by alternating brief ejection and suction of air across an opening such that the net mass flux is zero. A unique feature of these jets is that they are formed entirely from the working fluid of the flow system in which they are deployed can produce a net momentum to the flow of a system without net mass injection to the system.
Synthetic jet air movers have no moving parts and are thus maintenance free. Due to the high heat transfer coefficients, high reliability but lower overall flow rates, Synthetic jet air movers are usually used at the chip level and not at the system level for cooling. However depending on the size and complexity of the systems they can be used for both at times.[citation needed]
Electrostatic Fluid Acceleration
An electrostatic fluid accelerator (EFA) is a device which pumps a fluid such as air without any moving parts. Instead of using rotating blades, as in a conventional fan, an EFA uses an electric field to propel electrically charged air molecules. Because air molecules are normally neutrally charged, the EFA has to create some charged molecules, or ions, first. Thus there are three basic steps in the fluid acceleration process: ionize air molecules, use those ions to push many more neutral molecules in a desired direction, and then recapture and neutralize the ions to eliminate any net charge.
The basic principle has been understood for some time but only in recent years have seen developments in the design and manufacture of EFA devices that may allow them to find practical and economical applications, such as in micro-cooling of electronics components.
Recent developments
More recently, synthetic diamond cooling sinks are being researched to provide better cooling. Also, some heat sinks are constructed of multiple materials with desirable characteristics, such as phase change materials, which can store a great deal of energy due to their heat of fusion.[citation needed]
See also
References
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zh:散热片- ↑ The Basics of Package/Device Cooling
- ↑ Your Cooling Options - From Air To Water To Refrigeration: Cooling Goes High Tech
- ↑ OSHA Technical Manual
- ↑ The Effect of Forced Air Cooling on Heat Sink Thermal Ratings
- ↑ Materials for High-Density Electronic Packaging and Interconnection (1990)
- ↑ Reed Switches
- ↑ http://www.mpoweruk.com/thermal.htm
