Causes of heat build up
The amount of heat generated by an integrated circuit (e.g., a CPU or GPU), the prime cause of heat build up in modern computers, is a function of the efficiency of its design, the technology used in its construction and the frequency and voltage at which it operates.
The dust on the laptop CPU heat sink after three years of use has made the laptop unusable due to frequent thermal shutdowns.
In operation, the temperature levels of a computer's components will rise until the temperature gradient between the computer parts and their surroundings is such that the rate at which heat is lost to the surroundings is equal to the rate at which heat is being produced by the electronic component, and thus the temperature of the component reaches equilibrium.
For reliable operation, the equilibrium temperature must be sufficiently low for the structure of the computer's circuits to survive.
Additionally, the normal operation of cooling methods can be hindered by other causes, such as:
Dust acting as a thermal insulator and impeding airflow, thereby reducing heat sink and fan performance.
Poor airflow including turbulence due to friction against impeding components, or improper orientation of fans, can reduce the amount of air flowing through a case and even create localised whirlpools of hot air in the case.
Poor heat transfer due to a lack or poor application of thermal compounds.
Damage prevention
It is common practice to include thermal sensors in the design of certain computer parts, e.g. CPUs and GPUs, along with internal logic that shuts down the computer if reasonable bounds are exceeded. It is, however, unwise to rely on such preventative measures, as it is not universally implemented, and may not prevent repeated incidents from permanently damaging the integrated circuit.
The design of an integrated circuit may also incorporate features to shut down parts of the circuit when it is idling, or to scale back the clock speed under low workloads or high temperatures, with the goal of reducing both power use and heat generation.
System cooling
Fan from Papst for racks.
Air cooling
Further information: Computer fan
While any method used to move air around or to computer enclosures would count as air cooling, fans are by far the most commonly used implement for accomplishing that task. The term computer fan usually refers to fans attached to computer enclosures, but may also be intended to signify any other computer fan, such as a CPU fan, GPU fan, a chipset fan, PSU fan, HDD fan, or PCI slot fans. Common fan sizes include 40, 60, 80, 92, 120, and 140 mm. Recently, 200mm fans have begun to creep into the performance market, as well as even larger sizes such as 230 and 240mm.
In desktops
Typical airflow through a desktop ATX case.
Desktop computers typically use one or more fans for heat management. Almost all desktop power supplies have at least one fan to exhaust air from the case. Most manufacturers recommend bringing cool, fresh air in at the bottom front of the case, and exhausting warm air from the top rear.
If there is more air being forced into the 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 would be higher than outside. A balanced or neutral airflow is the most efficient[citation needed], although a slightly positive airflow results in less dust build up if dust filters are used. Negative pressure inside the case can create problems such as clogged optical drives due to sucking in air (and dust).
In high density computing
Data centers typically contain many racks of flat 1U servers. Air is drawn in at the front of the rack and exhausted at the rear. Because data centers typically contain such large numbers of computers and other power-consuming devices, they risk overheating of the various components if no additional measures are taken. Thus, extensive HVAC systems are used. Often a raised floor is used so the area under the floor may be used as a large plenum for cooled air and power cabling.
Another way of accommodating large numbers of systems in a small space are blade chassis. In contrast to the horizontal orientation of flat servers, blade chassis are often oriented vertically. This vertical orientation facilitates convection. When the air is heated by the hot components, it tends to flow to the top on its own, creating a natural air flow along the boards. This stack effect can help to achieve the desired air flow and cooling. Some manufacturers expressly take advantage of this effect.
In laptop computing
Most laptops use air cooling in order to keep the CPU and other components within their operating temperature range. Because the air is fan forced through a small port, it can clog the fan and heatsinks with dust or be obstructed by objects placed near the port. This can cause overheating, and can be a cause of component failure in laptops. The severity of this problem varies with laptop design, its use and power dissipation. With recent reductions in CPU power dissipation, this problem can be anticipated to reduce in severity.
Liquid submersion cooling
An uncommon practice is to submerse the computer's components in a thermally conductive liquid. Personal computers that are cooled in this manner do not generally require any fans or pumps, and may be cooled exclusively by passive heat exchange between the computer's parts, the cooling fluid and the ambient air. Extreme density computers such as the Cray-2 may use additional radiators in order to facilitate heat exchange.
The liquid used must have sufficiently low electrical conductivity in order for it not to interfere with the normal operation of the computer's components. If the liquid is somewhat electrically conductive, it may be necessary to insulate certain parts of components susceptible to electromagnetic interference, such as the CPU. For these reasons, it is preferred that the liquid be dielectric.
Liquids commonly used in this manner include various liquids invented and manufactured for this purpose by 3M, such as Fluorinert. Various oils, including but not limited to cooking, motor and silicone oils have all been successfully used for cooling personal computers.
Evaporation can pose a problem, and the liquid may require either to be regularly refilled or sealed inside the computer's enclosure. Liquid may also slowly seep into and damage components, particularly capacitors, causing an initially functional computer to fail after hours or days immersed.
Waste heat reduction
Where full-power, full-featured modern computers are not required, some companies opt to use less powerful computers or computers with fewer features. For example: in an office setting, the IT department may choose a thin client or a diskless workstation thus cutting out the heat-laden components such as hard drives and optical disks. These devices are also often powered with direct current from an external power supply brick which still wastes heat, but not inside the computer itself.
The components used can greatly affect the power consumption and hence waste heat. A VIA EPIA motherboard with CPU typically generates approximately 25 watts of heat whereas a Pentium 4 motherboard typically generates around 140 watts. While the former has considerably less computing power, both types are adequate and responsive for tasks such as word processing and spreadsheets. Choosing a LCD monitor rather than a CRT can also reduce power consumption and excess room heat, as well as the added benefit of increasing available physical desk space.
Conductive and radiative cooling
Some laptop components, such as hard drives and optical drives, are commonly cooled by having them make contact with the computer's frame, increasing the surface area which can radiate and otherwise exchange heat.
Spot cooling
In addition to system cooling, various individual components usually have their own cooling systems in place. Components which are individually cooled include, but are not limited to, the CPU, GPU and the Northbridge chip. Some cooling solutions employ one or more methods of cooling, and may also utilize logic and/or temperature sensors in order to vary the power used in active cooling components.
Passive heat sink cooling
Passive heatsink fitted on a Intel GMA graphics chip
Passive heat sink cooling involves attaching a block of machined or extruded metal to the part that needs cooling. A thermal adhesive may be used, or more commonly for a personal computer CPU, a clamp is used to affix the heat sink right over the chip, with a thermal grease or pad spread between. This block usually has fins and ridges to increase its surface area. The heat conductivity of metal is much better than that of air, and its ability to radiate heat is better than that of the component part it is protecting (usually an integrated circuit or CPU). Until recently, fan cooled aluminium heat sinks were the norm for desktop computers. Today many heat sinks feature copper base-plates or are entirely made of copper, and mount fans of considerable size and power.
Heat sinks tend to get less effective with time due to the build up of dust between their metal fins, which reduces the efficiency with which the heat sink transfers heat to the ambient air. Dust build up can be countered with a gas duster by blowing away the dust along with any other unwanted excess material.
Passive heat sinks are commonly found on older CPUs, parts that do not get very hot (such as the chipset), and low-power computers.
Usually a heatsink is attached to the integrated heat spreader (IHS). It essentially is a large flat plate attached to the CPU (with conduction paste layered between). The plate is used to dissipate or spread the heat locally. Unlike a heatsink, its intent is to redistribute heat and not to remove it. In addition, the IHS offers protection to the fragile CPU.
Passive cooling avoids the generation of fan noise.
Active heat sink cooling
Active heatsink with a 120mm fan located inside the unit and attached fan controller in background
Active heat sink cooling uses the same principle as passive, with the addition of a fan that is directed to blow over or through the heat sink. The moving air increases the rate at which the heat sink can exchange heat with the ambient air. Active heat sinks are the primary method of cooling a modern processor or graphics card.
The buildup of dust is greatly increased with active heat sink cooling as the fan is continually taking in the dust present in the surrounding air. As a result, dust removal procedures need to be exercised much more frequently than with passive heat sink methods.
Peltier cooling or thermoelectric cooling
Main article: Thermoelectric cooling
In 1821 T. J. Seebeck discovered that different metals, connected at two different junctions, will develop a micro-voltage if the two junctions are held at different temperatures. This effect is known as the "Seebeck effect"; it is the basic theory behind the TEC (thermoelectric cooling).
In 1834 Jean Peltier discovered the inverse of the Seebeck effect, now known as the "Peltier effect". He found that applying a voltage to a thermocouple creates a temperature differential between two sides. This results in an effective, albeit extremely inefficient heat pump.
Modern TECs use several stacked units each composed of dozens or hundreds of thermocouples laid out next to each other, which allows for a substantial amount of heat transfer. A combination of bismuth and telluride is most commonly used for thermocouples.
Since TECs are active heat pumps, they are capable of cooling PC components below ambient temperatures, which is impossible with common radiator cooled water cooling systems and heatpipe HSFs.
Water cooling
Main article: Water cooling
While originally limited to mainframe computers, water cooling has become a practice largely associated with overclocking in the form of either manufactured kits, or in the form of do-it-yourself setups assembled from individually gathered parts. The past few years has seen water cooling increasing its popularity with pre-assembled, moderate to high performance, desktop computers. Water has the ability to dissipate more heat from the cooled parts than the various types of metals used in heatsinks, making it suitable for overclocking and high performance computer applications. Advantages to water cooling include the fact that a system is not limited to cooling one component, but can be set up to cool the central processing unit, graphics processing unit, and/or other components at the same time with the same system. As opposed to air cooling, water cooling is also influenced less by the ambient temperature. Water cooling's comparatively low noise-level is also favorable to that of active cooling, which can become quite noisy. One disadvantage to water cooling is the potential for a coolant leak. Leaked coolant can damage any electronic components it comes in contact with. Another drawback to water cooling is the complexity of the system; an active heat sink is much simpler to build, install, and maintain than a water cooling solution.
DIY Water cooling setup showing 12v pump, CPU Waterblock and the typical application of a T-Line
DIY Water cooling setup showing 12v MCP655 Vario pump, Swiftech GTZ CPU Waterblock and the non-typical use of a Reservoir.
Heat pipe
Main article: Heat pipe
A graphics card with a heatpipe cooler design.
A heat pipe is a hollow tube containing a heat transfer liquid. As the liquid evaporates, it carries heat to the cool end, where it condenses and then returns to the hot end (under capillary action, or, in earlier implementations, under gravitation). Heat pipes thus have a much higher effective thermal conductivity than solid materials. For use in computers, the heat sink on the CPU is attached to a larger radiator heat sink. Both heat sinks are hollow as is the attachment between them, creating one large heat pipe that transfers heat from the CPU to the radiator, which is then cooled using some conventional method. This method is expensive and usually used when space is tight (as in small form-factor PCs and laptops), or absolute quiet is needed (such as in computers used in audio production studios during live recording). Because of the efficiency of this method of cooling, many desktop CPU's and GPU's, as well as high end chipsets, use heat pipes in addition to active fan-based cooling to remain within safe operating temperatures.
Phase-change cooling
Phase-change cooling is an extremely effective way to cool the processor. A vapor compression phase-change cooler is a unit which usually sits underneath the PC, with a tube leading to the processor. Inside the unit is a compressor of the same type as in a window air conditioner. The compressor compresses a gas (or mixture of gases) which condenses it into a liquid. Then, the liquid is pumped up to the processor, where it passes through an expansion device, this can be from a simple capillary tube to a more elaborate thermal expansion valve. The liquid evaporates (changing phase), absorbing the heat from the processor as it draws extra energy from its environment to accommodate this change (see latent heat). The evaporation can produce temperatures reaching around 15 to -150 degrees Celsius. The gas flows down to the compressor and the cycle begins over again. This way, the processor can be cooled to temperatures ranging from 15 to 150 degrees Celsius, depending on the load, wattage of the processor, the refrigeration system (see refrigeration) and the gas mixture used. This type of system suffers from a number of issues but mainly one must be concerned with dew point and the proper insulation of all sub-ambient surfaces that must be done (the pipes will sweat, dripping water on sensitive electronics).
Alternately a new breed of cooling system is being developed inserting a pump into the thermo siphon loop. This adds another degree of flexibility for the design engineer as the heat can now be effectively transported away from the heat source and either reclaimed or dissipated to ambient. Junction temperature can be tuned by adjusting the system pressure; higher pressure equals higher fluid saturation temperatures. This allows for smaller condensers, smaller fans and/or the effective dissipation of heat in a high ambient environment. These systems are in essence the next generation liquid cooling paradigm as they are approximately 10 times more efficient than single phase water. Since the system uses a dielectric as the heat transport media, leaks do not cause a catastrophic failure of the electric system.
This type of cooling is seen as a more extreme way to cool components, since the units are relatively expensive compared to the average desktop. They also generate a significant amount of noise, since they are essentially refrigerators, however the compressor choice and air cooling system is the main determinant of this, allowing for flexibility for noise reduction based on the parts chosen.
Liquid nitrogen
Liquid nitrogen may be used to cool an overclocked PC.
As liquid nitrogen evaporates at -196 C, far below the freezing point of water, it is valuable as an extreme coolant for short overclocking sessions.
In a typical installation of liquid nitrogen cooling, a copper or aluminum pipe is mounted on top of the processor or graphics card. After being heavily insulated against condensation, the liquid nitrogen is poured into the pipe, resulting in temperatures well below -100C.
By welding an open pipe onto a heat sink, and insulating the pipe, it is possible to cool the processor either with liquid nitrogen, which has a temperature below 196C, or dry ice. However, after the nitrogen evaporates, it has to be refilled. In the realm of personal computers, this method of cooling is seldom used in contexts other than overclocking trial-runs and record-setting attempts, as the CPU will usually expire within a relatively short period of time due to temperature stress caused by changes in internal temperature.
Liquid helium
Liquid helium, colder than liquid nitrogen, has also been used for cooling. Liquid helium evaporates at -269 C, and temperatures ranging from -230 to -240 C have been measured from the heatsink.
Soft cooling
Softcooling is the practice of utilizing software to take advantage of CPU power saving technologies to minimize energy use. This is done using halt instructions to turn off or put in standby state CPU subparts that aren't being used or by underclocking the CPU.
Undervolting
Undervolting is a practice of running the CPU or any other component with voltages below the device specifications. An undervolted component draws less power and thus produces less heat. The ability to do this varies by manufacturer, product line, and even different production runs of the same exact product (as well as that of other components in the system), but modern processors are typically shipped with voltages higher than strictly necessary. This provides a buffer zone so that the processor will have a higher chance of performing correctly under sub-optimal conditions, such as a lower quality mainboard (motherboard). However, too low a voltage will not allow the processor to function correctly, producing errors, system freezes or crashes, or the inability to turn the system on. (Undervolting too far does not typically lead to hardware damage, though in worst-case scenarios, program or system files can be corrupted)
This technique is generally employed by those seeking low-noise systems, as less cooling is needed because of the reduction of heat production.
Integrated chip cooling techniques
Conventional cooling techniques all attach their ooling component to the outside of the computer chip, or via IHS and/or heat sinks. This ttaching technique will always exhibit some thermal resistance, reducing its effectiveness. The heat can be more efficiently and quickly be removed by directly cooling the local hot spots. At these locations, power dissipation of over 300W/cm2 (typical CPU are less than 100W/cm2, although future systems are expected to exceed 1000W/cm2 ) can occur. This form of local cooling is essential to developing high power density chips. This ideology has led to the investigation of integrating cooling elements into the computer chip. Currently there are two techniques: micro-channel heat sinks, and jet impingement cooling.
In micro-channel heat sinks, channels are fabricated into the silicon chip (CPU), and coolant is pumped through them. The channels are designed with very large surface area which results in large heat transfers. Heat dissipation of 3000W/cm2 has been reported with this technique . In comparison to the Sun power density of around 7400W/cm2. The heat dissipation can be further increased if two-phase flow cooling is applied. Unfortunately the system requires large pressure drops, due to the small channels, and the heat flux is lower with dielectric coolants used in electronic cooling. Another local chip cooling technique is jet impingement cooling. In this technique, a coolant is flown through a small orifice to form a jet. The jet is directed toward the surface of the CPU chip, and can effectively remove large heat fluxes. Heat dissipation of over 1000W/cm2has been reported . The system can be operated at lower pressure in comparison to the micro-channel method. The heat transfer can be further increased using two-phase flow cooling and by intergrading return flow channels (hybrid between micro-channel heat sinks and jet impingement cooling).
Cooling and overclocking
Extra cooling is usually required by those who run parts of their computer (such as the CPU and GPU) at higher voltages and frequencies than manufacturer specifications call for, called overclocking. Increasing performance by this modification of settings results in a greater amount of heat generated and thus increasing the risk of damage to components and/or premature failure.
The installation of higher performance, non-stock cooling may also be considered modding. Many overclockers simply buy more efficient, and often, more expensive fan and heat sink combinations, while others resort to more exotic ways of computer cooling, such as liquid cooling, Peltier effect heatpumps, heat pipe or phase change cooling.
There are also some related practices that have a positive impact in reducing system temperatures:
Heat sink lapping
Heat sink lapping is the smoothing and polishing of the contact (bottom) part of a heat sink to increase its heat transfer efficiency. The desired result is a contact area which has a more even surface, as a less even contact surface creates a larger amount of insulating air between the heat sink and the computer part it is attached to. Polishing the surface using a combination of fine sandpaper and abrasive polishing liquids can produce a mirror-like shine, an indicator of a very smooth metal surface. However, it should be noted that even a curved surface can become extremely reflective, yet not particularly flat, as is the case with curved mirrors; thus heat sink quality is based on overall flatness, more than optical properties. Lapping a high quality heat sink can damage it, because, although the heat sink may become shiny, it is likely that more material will be removed from the edges, making the heat sink less effective overall.
If attempted, a piece of float glass should be used, as it self-levels as it cools and offers the most economical solution to producing a perfectly flat surface.
Use of exotic thermal conductive compounds
Some overclockers use special thermal compounds whose manufacturers claim to have a much higher efficiency than stock thermal pads. Heat sinks clean of any grease or other thermal transfer compounds have a very thin layer of these products applied, and then are placed normally over the CPU. Many of these compounds have a high proportion of silver as their main ingredient due to its high thermal conductivity. The resulting difference in the temperature of the CPU is measurable (several degrees celsius), so the heat transfer does appear to be superior to stock compounds. Some people experience negligible gains and have called to question the advantages of these exotic compounds, calling the style of application more important than the compound itself. Also note that there may be a 'setting' or 'curing' period and negligible gains may improve over time as the compound reaches its optimum thermal conductivity.
Use of rounded cables
Most older PCs use flat ribbon cables to connect storage drives (IDE or SCSI). These large flat cables greatly impede airflow by causing drag and turbulence. Overclockers and modders often replace these with rounded cables, with the conductive wires bunched together tightly to reduce surface area. Theoretically, the parallel strands of conductors in a ribbon cable serve to reduce crosstalk (signal carrying conductors inducing signals in nearby conductors), but there is no empirical evidence of rounding cables reducing performance. This may be because the length of the cable is short enough so that the effect of crosstalk is negligible. Problems usually arise when the cable is not electromagnetically protected and the length is considerable, a more frequent occurrence with older network cables.
These computer cables can then be cable tied to the chassis or other cables to further increase airflow.
This is less of a problem with new computers that use Serial ATA which has a much narrower cable.
Airflow optimization
The colder the cooling medium (the air), the more effective the cooling. Cooling air temperature can be reduced by these guidelines:
Supply cool air to the hot components as directly as possible. Examples are air snorkels and tunnels that feed outside air directly and exclusively to the CPU or GPU cooler. For example, the BTX case design prescribes a CPU air tunnel.
Expel warm air as directly as possible. Examples are: Conventional PC (ATX) power supplies blow the warm air out the back of the case. Many dual-slot graphics card designs blow the warm air through the cover of the adjacent slot. There are also some aftermarket coolers that do this. Some CPU cooling designs blow the warm air directly towards the back of the case, where it can be ejected by a case fan.
Air that has already been used to spot-cool a component should not be reused to spot-cool a different component (this follows from the previous items). The ATX case design can be said to violate this rule, since the power supply gets its "cool" air from the inside of the case, where it has been warmed up already. The BTX case design also violates this rule, since it uses the CPU cooler's exhaust to cool the chipset and often the graphics card.
Prefer cool intake air, avoid inhaling exhaust air (outside air above or near the exhausts). For example, a CPU cooling air duct at the back of a tower case would inhale warm air from a graphics card exhaust. Moving all exhausts to one side of the case, conventionally the back, helps to keep the intake air cool.
Hiding cables behind motherboard tray or simply apply ziptie and tucking cables away to provide unhindered airflow.
Fewer fans strategically placed will improve the airflow internally within the PC and thus lower the overall internal case temperature in relation to ambient conditions. The use of larger fans also improves efficiency and lowers the amount of waste heat along with the amount of noise generated by the fans while in operation.
There is little agreement on the effectiveness of different fan placement configurations, and little in the way of systematic testing has been done. For a rectangular PC (ATX) case, a fan in the front with a fan in the rear and one in the top has been found to be a suitable configuration. However, AMD's (somewhat outdated) system cooling guidelines notes that "A front cooling fan does not seem to be essential. In fact, in some extreme situations, testing showed these fans to be recirculating hot air rather than introducing cool air." It may be that fans in the side panels could have a similar detrimental effect -- possibly through disrupting the normal air flow through the case. However, this is unconfirmed and probably varies with the configuration.
See also
Thermal management of electronic devices and systems
Full immersion cooling
References
^ Verari Systems uses vertical air flow for cooling
^ The tower case Silverstone Raven RV01 has been designed to make use of the stack effect
^ Tom's Hardware - "Strip Out The Fans", 9 January 2006, presented as 11 web pages.
^ oilcooledcomputer.com
^ AMD Phenom II Overclocked to 6.5GHz - New World Record for 3DMark
^ I. Mudawar, ssessment of High-Heat-Flux Thermal Management Schemes, IEEE Trans. -Components and Packaging Tech., Vol. 24, pp. 122-141, 2001.
^ M.B. Bowers and I. Mudawar, igh Flux Boiling inLow Flow Rate, Low Pressure Drop Mini-Channel and Micro-Channel Heat Sinks, Int. J. Heat Mass Transfer, Vol. 37, pp. 321-332, 1994.
^ M.K. Sung and I. Mudawar, ingle-phase and two-phase hybrid cooling schemes for high-heat-flux thermal management of defense electronics, Thermal and Thermomechanical Phenomena in Electronic Systems, 2008. ITHERM 2008,Issue 28-31, pp.121 131, 2008.
^ AMD Athlon System Cooling Guidelines -- Although somewhat out of date, it appears to be backed up by some amount of systematic testing -- which is lacking in many other guides.
External links
Wikibooks has a book on the topic of
How To Assemble A Desktop PC/Silencing#Liquid nitrogen cooling
Online Heat Sink Performance Calculators
CPU Cooler Rules of Thumb
Submersion Cooling Patent Application
Categories: Computer hardware cooling | Central processing unitHidden categories: All articles with unsourced statements | Articles with unsourced statements from April 2007
