Uncover the secrets of heat dissipation methods an

2022-08-11
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With the continuous evolution of LED materials and packaging technology, the brightness of LED products continues to improve, and the application of LED is becoming more and more widely. Taking LED as the backlight of display is a hot topic recently, mainly because different kinds of LED backlight technology have more advantages than traditional cold cathode tube (CCFL) in color, brightness, life, power consumption and environmental protection demands, Therefore, it attracts the industry to actively invest

the power of the original single-chip LED is not high, the calorific value is limited, and the heat problem is not big, so its packaging method is relatively simple. However, in recent years, with the continuous breakthrough of LED material technology, the packaging technology of LED has also changed. From the early single-chip shell packaging, it has gradually developed into a flat, large-area multi chip packaging module; Its working current has evolved from low-power LED of about 20mA in the early stage to high-power LED of about 1/3 to 1a at present. The input power of a single LED is as high as 1W or more, and even the packaging mode of 3W and 5W is more evolved

since the thermal problems derived from the high brightness and high power LED system will be the key to the quality of product functions, to quickly discharge the heat of LED components to the surrounding environment, we must first start with the thermal management of the packaging level (L1 & L2). At present, the industry's practice is to connect the LED chip with solder or heat transfer paste on a soaking sheet, and reduce the thermal impedance of the packaging module through the soaking sheet. This is also the most common LED packaging module on the market at present, mainly from internationally renowned LED manufacturers such as Lumileds, OSRAM to remove the gas in the tubing, Cree and Nicha

many terminal application products, such as mini projectors, automotive and lighting light sources, need more than thousands of lumens or tens of thousands of lumens in a specific area. It is obviously not enough to rely on single-chip packaging modules alone. Moving towards multi-chip LED packaging, and the chip is directly adhered to the substrate, and has released the first physical entrepreneurship platform in China - Jinmin we work, which is the future development trend

the problem of heat dissipation is the main obstacle in the development of LED as a lighting object. The use of ceramics or heat dissipation tubes is an effective method to prevent overheating, but the heat dissipation management solution increases the cost of materials. The purpose of high-power LED heat dissipation management design is to effectively reduce the thermal resistance between chip heat dissipation and the final product. R junction to case is one of the solutions using materials, which provides low thermal resistance but high conductivity, Heat is transferred directly from the chip to the outside of the package housing by chip attachment or hot metal methods

of course, the heat dissipation components of LED are similar to that of CPU. They are mainly air-cooled modules composed of fins, heat pipes, fans and thermal interface materials. Of course, water cooling is also one of the thermal countermeasures. In terms of the most popular large-size LED TV backlight module at present, the input power of 40 inch and 46 inch LED backlight is 470w and 550W respectively, and 80% of them turn into heat, the required heat dissipation is about 360W and 440W

so how to take away these calories? At present, the industry uses water cooling for cooling, but there are doubts about high unit price and reliability; Heat pipes are also used to cooperate with heat sinks and fans for cooling. For example, the 46 inch LED backlight LCD TV of Sony, a Japanese manufacturer, still has problems such as fan power consumption and noise. Therefore, how to design a fan free cooling method may be an important key to determine who can win in the future

next, we will introduce several heat dissipation methods and materials

heat dissipation mode

generally speaking, according to the way of taking heat away from the radiator, the radiator can be divided into active heat dissipation and passive heat dissipation. The so-called passive heat dissipation means that the heat of the heat source LED light source is naturally dissipated into the air through the heat sink, and its heat dissipation effect is directly proportional to the size of the heat sink. However, because the heat is naturally dissipated, the effect is of course greatly reduced. It is often used in equipment that does not require space, or for heat dissipation of components with low heat generation. For example, some popular motherboards also adopt passive heat dissipation on the north bridge, Most of them adopt active heat dissipation, which is to forcibly take away the heat from the heat sink through the fan and other heat dissipation equipment. Its characteristics are high heat dissipation efficiency and small size of the equipment

active cooling can be divided into air cooling cooling, liquid cooling, heat pipe cooling, semiconductor cooling, chemical cooling and so on

air cooling is the most common way of heat dissipation, which is also relatively cheap. In essence, air cooling is to use a fan to take away the heat absorbed by the radiator. It has the advantages of relatively low price and convenient installation. However, it is highly dependent on the environment. For example, when the temperature rises and overclocking, its heat dissipation performance will be greatly affected

liquid cooling is the forced circulation of liquid driven by the pump to take away the heat of the radiator. Compared with air cooling, it has the advantages of quietness, stable cooling, less dependence on the environment and so on. The price of liquid cooling is relatively high, and the installation is relatively troublesome. At the same time, try to install according to the instructions in order to obtain the best heat dissipation effect. For the consideration of cost and ease of use, water is usually used as the heat conducting liquid for liquid cooling, so liquid cooling radiator is often called water-cooled radiator

heat pipe is a kind of heat transfer element, which makes full use of the principle of heat conduction and the rapid heat transfer property of refrigeration medium. The voice of seeking new wood substitutes for forest resources is growing. It transfers heat through the evaporation and condensation of liquid in the fully enclosed vacuum pipe. It has a series of advantages, such as high thermal conductivity, good isothermal, arbitrary change of heat transfer area on both sides of cold and hot, long-distance heat transfer, temperature control, etc, And the heat exchanger composed of heat pipes has the advantages of high heat transfer efficiency, compact structure, small fluid resistance loss and so on. Its thermal conductivity has far exceeded that of any known metal

semiconductor refrigeration

semiconductor refrigeration is to use a special semiconductor refrigeration chip to produce temperature difference when powered on. As long as the heat at the high temperature end can be effectively dissipated, the low temperature end will be continuously cooled. There is a temperature difference on each semiconductor particle. A refrigeration sheet is composed of dozens of such particles in series, thus forming a temperature difference on the two surfaces of the refrigeration sheet. Using this kind of temperature difference to cool the high-temperature end with air cooling/water cooling can get excellent heat dissipation effect. Semiconductor refrigeration has the advantages of low refrigeration temperature and high reliability. The cold surface temperature can reach below minus 10 ℃, but the cost is too high, and it may cause short circuit due to low temperature. Moreover, the process of semiconductor refrigeration chip is not mature and practical enough

chemical refrigeration

the so-called chemical refrigeration is to use some ultra-low temperature chemicals to absorb a lot of heat when melting to reduce the temperature. Dry ice and liquid nitrogen are commonly used in this aspect. For example, using dry ice can reduce the temperature to below minus 20 ℃, and some more "abnormal" players use liquid nitrogen to reduce the CPU temperature to below minus 100 ℃ (in theory). Of course, due to the high price and short duration, this method is often used in laboratories or extreme overclocking enthusiasts

material selection

thermal conductivity (unit: w/MK)

Silver 429

copper 401

Gold 317

Aluminum 237

lead 34.8

1070 aluminum alloy 226

1050 aluminum alloy 209

6063 aluminum alloy 201

6061 aluminum alloy 155

generally speaking, ordinary air-cooled radiators naturally choose metals.As the material of radiator. For the selected materials, it is hoped that they have high specific heat and high thermal conductivity at the same time. It can be seen from the above that silver and copper are the best heat conducting materials, followed by gold and aluminum. But gold and silver are too expensive, so at present, heat sinks are mainly made of aluminum and copper. In comparison, copper and aluminum alloy have their own advantages and disadvantages at the same time: copper has good thermal conductivity, but the price is expensive, the processing is difficult, the weight is too large, and the heat capacity of copper radiator is small, and it is easy to oxidize. On the other hand, pure aluminum is too soft to be used directly. Only aluminum alloy can provide enough hardness. The advantages of aluminum alloy are low price and light weight, but its thermal conductivity is much worse than that of copper. Therefore, in the history of radiator development, there are also products made of the following materials:

pure aluminum radiator

pure aluminum radiator is the most common radiator in the early stage. Its manufacturing process is simple and its cost is low. So far, pure aluminum radiator still occupies a considerable part of the market. In order to increase the heat dissipation area of its fins, the most commonly used processing method for pure aluminum radiators is aluminum extrusion technology, and the main indicators for evaluating a pure aluminum radiator are the thickness of the radiator base and the pin fin ratio. Pin refers to the height of the fins of the heat sink, and fin refers to the distance between two adjacent fins. Pin fin ratio is the height of pin (excluding base thickness) divided by fin. The larger the pin fin ratio, the larger the effective heat dissipation area of the radiator, and the more advanced the aluminum extrusion technology. Pure copper radiator

the thermal conductivity of copper is 1.69 times that of aluminum, so under the same conditions, pure copper radiator can take heat away from the heat source faster. However, the texture of copper is a problem. Many "pure copper radiators" are not really 100% copper. In the list of copper, copper with a copper content of more than 99% is called acid free copper, and the next grade of copper is Dan copper with a copper content of less than 85%. At present, the copper content of most pure copper radiators on the market is between the two. The copper content of some inferior pure copper radiators is even less than 85%. Although the cost is very low, its heat conduction capacity is greatly reduced, affecting the heat dissipation. In addition, copper also has obvious disadvantages, such as high cost, difficult processing, and too much radiator quality, which hinders the application of all copper fins. The hardness of red copper is not as good as that of aluminum alloy al6063, and the performance of some machining (such as grooving) is not as good as that of aluminum; The melting point of copper is much higher than that of aluminum, which is not conducive to extrusion and other problems

copper aluminum combination technology

after considering the respective shortcomings of copper and aluminum, at present, some high-end radiators in the market often use copper aluminum combination manufacturing technology. These radiators usually use copper metal base, while the heat dissipation fins use aluminum alloy. Of course, in addition to the copper bottom, there are also methods such as using copper columns for heat sinks, which is the same principle. With high thermal conductivity, the copper bottom can quickly absorb the heat released by the CPU; Aluminum fins can be made into the shape that is most conducive to heat dissipation with the help of complex technological means, and provide large heat storage space and rapid release, which has found an equilibrium point in all aspects

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