Detailed analysis of Heat Dissipation Principle and Technology

As PC computing power is enhanced, power consumption and heat dissipation become unavoidable. Generally, large heat sources in PCs includeCPU,Motherboard(South Bridge, North Bridge and VRM ),Graphics cardAnd other components such as hardware,Optical DriveAnd so on. A considerable amount of energy consumed during their work is converted to heat.

We all know that the operating temperature of an electronic device directly determines its service life and stability. In order to keep the working temperature of each part of the PC within a reasonable range, in addition to ensuring that the temperature of the working environment of the PC is within a reasonable range, it must also be cooled. Especially for the CPU, if the user has a high frequency, it is necessary to ensure the stable operation of the user more effectively heat dissipation.

 Principles and basic methods of Hot Transfer

Although we often call heat as heat energy, heat is not a kind of energy in a strict sense.Energy Transfer Method. From the microscopic point of view, after the molecules in the region are affected by external energy, the molecules in the high-energy regions are transferred to regional molecules with low energy. Therefore, it is generally believed that the transfer of energy is hot in the physics field. Of course, the most important process or form of heat transfer is hot transfer.

Friends who have studied middle school physics know that there are three main hot transfer methods:

Transmission:The transfer of energy is called heat conduction when a substance is in contact with a substance.This is the most common heat transfer method, which is used to transmit energy by directly touching and colliding low-energy particles with high-energy particles. Relatively speaking, the heat conduction method is limited to solids and liquids, because the composition of gas molecules is not very close, and the transfer of energy between them is called heat diffusion.

The basic formula for heat conduction is Q = K × A × △t/△l ". Q indicates the heat, that is, the heat generated or transmitted by Heat Conduction. k indicates the heat conduction coefficient of the material. The heat conduction coefficient is similar to the specific heat, but there are some differences with the specific heat, the heat conduction coefficient is inversely proportional to the specific heat. The higher the thermal conductivity, the lower the specific heat value. For example, the heat conduction coefficient of pure copper is 396.4, and its specific heat is 0.39. In the formula, a represents the area of heat transfer (or the contact area of two objects), and △t represents the temperature difference between the two ends; △l is the distance between the two ends. Therefore, we can find from the formula that the heat transfer is proportional to the heat conduction coefficient and heat transfer area, and is inversely proportional to the distance. The higher the heat transfer coefficient, the larger the heat transfer area, and the shorter the transmission distance, the higher the heat transfer energy, and the easier it is to take away heat.

Convection:Convection refers to the contact between a fluid (gas or liquid) and a solid surface., Resulting in heat transfer of the fluid from the solid surface to the tropical area.

From the practical application, there are two different situations of heat convection: Natural Convection and forced convection. Natural Convection refers to fluid movement. The reason is the temperature difference, the fluid density with high temperature is low, so the quality is light, and the relative movement is upward. On the contrary, a low-temperature fluid has a high density, so it moves downward. This heat transfer is due to the heat transfer of the fluid after it is heated, or when the temperature difference exists; forced Convection is the forced external drive of the fluid (such as the air flow driven by a fan), where the driving force is directed, and where the fluid is directed, so this hot convection is more efficient and directional.

The formula for heat convection is Q = H × A × △t ". In the formula, Q still represents the heat, that is, the heat carried away by the thermal convection; h Represents the thermal convection number, and a represents the effective contact area of the thermal convection; △t represents the temperature difference between the solid surface and the region fluid. Therefore, in thermal convection transmission, the quantity of heat transfer is directly proportional to the thermal convection coefficient, effective contact area, and temperature difference. The higher the thermal convection coefficient, the larger the effective contact area, and the higher the temperature difference, the more heat you can take away.

Radiation:Heat radiation is a heat exchange transfer method that can take place without any media being exposed. That is to say,Heat radiation is actually to achieve the purpose of heat exchange in the form of waves.

Since the heat radiation is transmitted through waves, there will inevitably be wavelengths and frequencies. The transfer efficiency is determined by the heat absorption rate of the desired object without passing through the media. Here there is a heat radiation coefficient with a value between 0 and ~ 1 is the surface property of the object, while the thermal conductivity of the rigid body is the material property of the object. The heat conduction formula of the general heat radiation is Q = E × S × F × △( ta-TB )". In the formula, Q represents the ability of heat radiation exchange, and E is the heat radiation coefficient of the object surface. In reality, when the material is metal and the surface is smooth, the heat radiation coefficient is relatively small, and after the metal surface is processed (such as coloring), the surface heat radiation coefficient is increased. Most of the plastic or non-metallic heat radiation values are relatively high. S is the surface area of the object, and F is the functional relationship between the angle of the Radiation Heat Exchange and the surface, but this function is difficult to explain here. Delta (Ta-TB) is the temperature difference between surface A and surface B. Therefore, there is a proportional relationship between the heat radiation coefficient, the surface area of the object, and the temperature difference.

AnyRadiatorThe above three hot transfer methods will also be used at the same time, but the focus is different. Taking CPU heat dissipation as an example, the heat is constantly emitted by the CPU, transmitted to the heat sink through the heat sink base that is closely in contact with its core, and then reaches the heat sink, in other ways, such as fan blowing, the heat is sent away. The entire heat dissipation process includes four links: CPU, heat source, heat sink, heat conduction body, and fan, a medium for increasing heat conduction and directing to heat conduction; the fourth is air, which is the final flow of heat exchange.

Heat Dissipation Mode

 

GenerallyRadiatorTake away the heat, you canRadiatorActive heat dissipation and passive heat dissipation. Passive Heat Dissipation refers to heat sources suchCPUThe heat is naturally emitted into the air, and the heat dissipation effect is proportional to the size of the Heat Sink. However, because the heat is naturally emitted, the effect is of course compromised, it is often used in devices that have no requirements for space, or for heat dissipation of components with little heat, such as some popularMotherboardPassive Heat Dissipation is also adopted on the North Bridge. For PCs used by individuals, the vast majority adopt active heat dissipation methods. Active heat dissipation is to forcibly remove the heat from the heat sink through fans and other heat dissipation devices, which is characterized by high heat dissipation efficiency, the device size is small.

Heat Dissipation Mode

Active Heat Dissipation can be divided into air cooling, liquid cooling, heat pipe cooling, semiconductor cooling, and chemical cooling.

Air Cooling

Air-cooled heat dissipation is the most common heat dissipation method, which is also a cheaper method. In essence, air-cooled heat dissipation is to use a fan to take away the heat absorbed by the radiator. WithPriceRelatively low, easy to install and other advantages. However, the environment is highly dependent. For example, the heat dissipation performance is greatly affected when the temperature increases and the frequency is too high.

Liquid Cooling

Liquid Cooling is forced to take away the heat of the radiator by the circulation of the liquid driven by the pump. Compared with air cooling, liquid cooling has the advantages of quiet, stable cooling, and low environmental dependence. The prices of Liquid Cooling are relatively high, and installation is also relatively troublesome. At the same time, try to install according to instructions in order to obtain the best heat dissipation effect.

For cost and ease of use, water is usually used as the thermal conductivity for Liquid Cooling. Therefore, liquid cooling radiators are often calledWater-cooled radiator.

Heat Pipe

Heat Pipe is a heat transfer element. It fully utilizes the principle of heat conduction and the fast thermal transfer property of the cooling medium, and transmits heat by the evaporation and condensation of the liquid in the fully enclosed vacuum tube, it has a series of advantages, such as high thermal conductivity, good isothermal, the heat transfer area on both sides of the hot and cold sides can be arbitrarily changed, long distance heat transfer, controllable temperature, etc, the heat exchanger composed of heat pipes has the advantages of high heat transfer efficiency, compact structure, and low fluid loss. Its Thermal Conductivity far exceeds the thermal conductivity of any known metal.

Semiconductor Refrigeration

Semiconductor cooling is the use of a special semiconductor cooling tablet to generate a temperature difference during power-on cooling, as long as the heat of the high temperature end can be effectively emitted, the low temperature end will be continuously cooled. Each semiconductor particle has a temperature difference. A refrigerating plate is made up of dozens of such particles in series, thus forming a temperature difference between the two surfaces of the refrigerating plate. This temperature difference can be used in combination with air-cooled/water-cooled cooling on the high-temperature end to achieve excellent heat dissipation.

Semiconductor Refrigeration has the advantages of low cooling temperature and high reliability. The cold surface temperature can reach below-10 ℃, but the cost is too high, and the CPU condensation may be short-circuited due to the low temperature, moreover, the technology of the semiconductor refrigeration unit is not mature and not practical enough.

Chemical Refrigeration

The so-called chemical cooling refers to the use of some super-low temperature chemicals that absorb a large amount of heat during the melting to reduce the temperature. Dry ice and liquid nitrogen are common in this regard. For example, the use of dry ice can reduce the temperature to below-20 ℃, and some more "abnormal" Players use liquid nitrogen to reduce the CPU temperature to below-100 ℃ (theoretically ), of course, because the price and duration are too short, this method is mostly used in laboratories or extreme frequency enthusiasts.

 

Improve Heat Conduction capability of Heat Sink

No matter which heat dissipation method is adopted, we must first solve how to efficiently extract heat from the heat source suchCPUFast transfer to the heat dissipation body. For example, for air cooling, it needs to transfer the heat generated by the CPU to the heat sink, then, the high-speed rotation of the fan will take away most of the heat by convection (including forced convection and Natural Convection); the same is true for liquid cooling. In this process, the radiation method directly emits a very small amount of heat, which determines the role of the first step, to improve the efficiency of heat conduction, the heat is taken away from the heat source.

To improve the heat conduction efficiency, according to the formula q = K × A × △t/△l, the heat conduction capability is proportional to the heat conduction coefficient, contact area and temperature difference of the heat sink, it is inversely proportional to the combination distance. We will discuss this one by one.

RadiatorMaterial

Note: In this section, we discussRadiatorThe conduction capability is related to the base of the radiator in the general sense, rather than the whole radiator. This is especially confusing when it comes to air-cooled cooling, because the base and the fin are mostly integrated for Air-cooled cooling, but the functions and technical implementation of the two are completely different: the base of the heat sink is in contact with the CPU, and its function is to absorb heat and transmit it to the fin with a high heat capacity conductor, while the fin is the end of the conduction process, the heat is finally lost to the air through a large heat dissipation area and heat exchange with the air. These two parts are mutually independent. Of course, how to properly combine the two is the skill of the manufacturer.

CPU die is usually less than 2 cm², but the power consumption reaches dozens or hundreds of Watts. If the heat cannot be transmitted in time, once the heat accumulates in the die, it will lead to serious consequences.

For the radiator, the most important thing is that its base can absorb as much heat as possible from the CPU in a short time, that is, the instantaneous heat absorption capability. Only metals with high heat conductivity can be competent. For metal thermal conductive materials, the specific heat and thermal conductivity are two important parameters.

The heat conduction coefficient is defined as the number of W energy that can be transferred per unit length and Per K. The unit is W/mK. "W" indicates the unit of thermal power, "M" indicates the unit of length, and "K" indicates the absolute temperature unit. The greater the value, the better the thermal conductivity. The heat conduction coefficient tables of several common metals are as follows:

 

Thermal Conductivity (unit: W/mK)
Silver	429	Copper	401
Gold	317	Aluminum	237
Iron	80	Lead	34.8
1070 Aluminum Alloy	226	1050 Aluminum Alloy	209
6063 Aluminum Alloy	201	6061 aluminum alloy	155 Silver and copper are the best thermal materials, followed by gold and aluminum. However, gold and silver are too expensive. At present, the heat sink is mainly made of aluminum and copper. However, due to the high copper density and complicated process,PriceMore expensive, so the current common fan is made of light aluminum, of course, for the air-cooled radiator, when considering the material, in addition to the heat conduction coefficient, you must also consider the heat capacity of the radiator, combining these two parameters, the superiority of aluminum is shown. However, this article only discusses the heat conduction, which will be discussed in detail in the next section. Radiator Bottom Surface Processing Technology To improveRadiatorBase Heat Conduction ability, choose material with high heat conduction coefficient is one hand, but on the other hand also to solve the hot source suchCPUAndRadiatorThe closeness of the base. According to the Law of heat conduction, the conductivity is proportional to the contact area and inversely proportional to the contact distance under the premise of fixed materials. The larger the contact area, the faster the heat is distributed, but the die of the CPU is fixed, so the distance is more important. Although theoretically, the heat sink base can be closely connected with the CPU, objectively speaking, no matter how smooth the two contact surfaces are, there is still a gap between them, that is, there is air, the heat conduction performance of the air is very poor, which requires an excellent design, a strong grasp of the buckle to closely buckle the heat sink on the CPU, In addition, you need to replace the air with something with better thermal conductivity and deformation to fill these gaps, such as thermal grease or heat dissipation tape. The ideal condition is that the buckle closely binds the heat sink to the CPU, and the contact between the heat sink and the CPU is completely parallel to keep the maximum contact area, some tiny gaps between them are completely filled with silicon grease to minimize the contact thermal resistance. However, it must be clear that no matter what kind of Thermal Insulation silicon grease or heat dissipation tape, its role can only be auxiliary, compared with the copper heat dissipation base material, its thermal resistance is much greater. To maximize the heat conduction capability of the radiator base, we must first ensure smooth and smooth the radiator base so as to reduce the gap between the radiator and the CPU contact surface. Radiator Bottom Surface Processing Technology Common bottom-layer processing techniques include: Drawing Process (grinding) The drawing process is also the most widely used bottom surface processing technology. When drawing, a tool with a certain degree of roughness and hardness is used. Common tools such as sandpaper and files are used to carry out one-way, repeated or rotating friction on the surface of the object, with the help of tool rough surface friction when the shear effect to remove the surface of the protruding thing; of course, polishing the protruding thing will also cause scratches on the original smooth surface. Therefore, the process from coarse to fine should be adopted to gradually reduce the roughness of the processing surface.   Disk milling (cutting) The disc milling process refers to cutting the radiator surface by means of high-speed rotating tool after the bottom surface of the radiator is fixed, and the tool is always rotated in the same plane, so the bottom surface of the cut is very smooth. Similar to the drawing process, the finer the tool used in the disc milling process, the higher the smoothness of the bottom surface. The manufacturing cost of the disk milling process is relatively high, but it only requires two or three processes for drawing, which is time-saving and has an ideal effect.   NC machine tools The main process used by the numerical control machine tool to flat the bottom of the heat sink is still milling. However, unlike traditional disc milling, the NC milling tool can precisely control the relative distance between the tool and the heat sink through a single chip microcomputer. After the tool is in contact with the bottom surface of the heat sink, the horizontal movement of the two sides can be used to cut the unprocessed part left by the tool gap in the traditional disk milling to achieve the complete plane effect, do not allow any subsequent processing to obtain the General mirror effect, and the flatness can be less than 0.001mm. Other processes In addition to the above, there are other processing techniques for the radiator base, such as polishing. However, the polishing process is more out of the aesthetic considerations of the radiator, the flatness of the bottom of the radiator is not greatly improved, and the processing cost is high.