LED thermal design cooling methods and material Secret

With the continuous evolution of LED materials and packaging technology, LED products continue to improve brightness, LED applications are more and more widely, with LED as the display backlight, is a hot topic recently, mainly different types of LED backlight technology The color, brightness, lifespan, power consumption, and environmental protection appeal are all advantages over traditional cold cathode fluorescent lamps (CCFLs), thus attracting active input from the industry.

The initial monolithic LED has low power, limited heat, and little heat, so its packaging is relatively simple. However, with the continuous breakthrough of LED material technology in recent years, LED packaging technology has also changed along with it, from the early one-chip artillery shell type packaging gradually developed into a flat, large-area multi-chip packaging module; its operating current from early 20mA About the low-power LED, progressed to the current high-power LED of about 1/3 to 1A, single LED input power up to 1W or more, even to 3W, 5W package more evolution.

Since the heat problem derived from a high-brightness high-power LED system will be the key to affecting the function of a product, to rapidly dissipate heat from the LED component to the surrounding environment, the thermal management of the package level (L1 & L2) must first be carried out. At present, the industry practice is to connect the LED chip with a solder or thermal paste on a soaking plate to reduce the thermal impedance of the package module through the soaking plate. This is the most common LED package module on the market. The main source is Lumileds, OSRAM, Cree and Nicha LED international famous manufacturers.

Many end-user applications, such as mini projectors, automotive and lighting sources, require more than a thousand lumens or tens of thousands of lumens in a given area. Obviously, the single-chip package is not enough to cope with. , Towards a multi-chip LED package, and chip directly attached to the substrate is the future trend.

Heat dissipation is a major obstacle in the development of LED lighting objects. The use of ceramics or heat pipes is an effective way to prevent overheating, but thermal management solutions increase the cost of materials. The purpose of high power LED thermal management design is to effectively reduce Heat dissipation from the chip to the final product, R junction-to-case is one of the materials-based solutions that offers low thermal resistance but high conductivity, allowing heat to be transferred directly from the chip via die attach or hot metal methods Enclose the outside of the shell.

Of course, the heat dissipation components of the LED are similar to the heat dissipation of the CPU, and are mainly composed of an air-cooling module composed of a heat sink, a heat pipe, a fan, and a thermal interface material. Of course, water cooling is also one of the heat countermeasures. According to the current most popular large-size LED TV backlight module, the input power of the 40-inch and 46-inch LED backlight sources is 470 W and 550 W, respectively. When 80% of them are converted into heat, the required heat dissipation is about About 360W and 440W.

So how do you take these heat away? At present, there are water-cooled cooling methods in the industry, but there are doubts such as high unit price and reliability; also use heat pipes with cooling fins and fans to cool, for example, SONY's 46 LED backlight LCD TVs, but fan power consumption and noise, etc. The problem still exists. Therefore, how to design a fanless cooling method may be an important key to decide who will win in the future.

Here are just a few ways to introduce heat dissipation and heat dissipation.

Cooling method

In general, the heat sink can be divided into active heat dissipation and passive heat dissipation in accordance with the manner in which heat is taken away from the heat sink. The so-called passive heat dissipation refers to the fact that the heat of the heat source LED light source is naturally emitted into the air through the heat sink. The effect of heat dissipation is proportional to the size of the heat sink, but since it is natural to dissipate heat, the effect is of course greatly compromised, and often used in those places. There is no required equipment, or is used to dissipate heat for parts that do not generate much heat. For example, some popular motherboards adopt passive heat dissipation on the North Bridge. Most of them adopt active heat dissipation, and active heat dissipation is forced by fans and other heat dissipation equipment. The heat generated by the heat sink is taken away, which is characterized by high heat dissipation efficiency and a small device size.

Active cooling, subdivided from the cooling method, can be divided into air cooling, liquid cooling, heat pipe cooling, semiconductor refrigeration, chemical refrigeration and so on.

Air-cooled, air-cooled heat dissipation is the most common method of heat dissipation. In comparison, it is a relatively inexpensive method. Air-cooled heat dissipation is essentially the use of fans to remove the heat absorbed by the heat sink. With relatively low prices, easy installation and other advantages. However, it is highly dependent on the environment. For example, when the temperature rises and the overclocking frequency is exceeded, the heat dissipation performance will be greatly affected.

Liquid cooling

Liquid-cooled heat dissipation is the heat that is forced by the pump to take away the heat from the radiator under the drive of the pump. Compared with the air-cooling, it has the advantages of quietness, stable temperature drop, and low dependence on the environment. The price of liquid cooling is relatively high, and the installation is relatively troublesome. At the same time, install as much as possible according to the instruction manual to get the best cooling effect. For reasons of cost and ease of use, liquid cooling usually uses water as a heat-conducting liquid, so liquid-cooled radiators are often also referred to as water-cooled radiators.

Heat pipe

The heat pipe belongs to a kind of heat transfer element, which fully utilizes the principle of heat conduction and the rapid heat transfer property of the refrigerant, transfers heat through the evaporation and condensation of the liquid in the totally enclosed vacuum tube, and has extremely high thermal conductivity and good isothermal. The heat transfer area on both sides of the sex, cold and hot can be arbitrarily changed, long-distance heat transfer, temperature control and other advantages, and the heat exchanger composed of heat pipes has high heat transfer efficiency, compact structure, small fluid loss, etc. advantage. Its thermal conductivity has far exceeded the thermal conductivity of any known metal.

Semiconductor refrigeration

Semiconductor refrigeration is the use of a special semiconductor cooling sheet to generate a temperature difference when the power is switched on for cooling. As long as the heat of the high temperature side can be effectively dissipated, the low temperature side is continuously cooled. A temperature difference is generated on each semiconductor particle, and one cooling sheet is formed by connecting several tens of such particles in series, thereby forming a temperature difference on both surfaces of the cooling sheet. Using this phenomenon of temperature difference, in combination with air cooling/water cooling, the high temperature end can be cooled to obtain excellent heat dissipation effect. Semiconductor refrigeration has the advantages of low cooling temperature and high reliability. The cold surface temperature can reach below minus 10°C, but the cost is too high, and the short circuit may be caused by too low temperature. Now, the technology of semiconductor cooling sheet is not mature enough. practical.

Chemical refrigeration

The so-called chemical cooling, is the use of some ultra-low temperature chemicals, use them to absorb a lot of heat to reduce the temperature when melting. It is common to use dry ice and liquid nitrogen in this regard. For example, dry ice can be used to lower the temperature below -20°C, and some more "metamorphic" players use liquid nitrogen to reduce the CPU temperature below -100°C (in theory). Of course, this is due to the high price and short duration. Methods are more common in labs or extreme overclockers.

Material selection

In general, ordinary air-cooled radiators naturally choose metal as the material of the radiator. For the selected materials, it is desirable to have both high specific heat and high thermal conductivity. From the above, it can be seen that silver and copper are the best thermal materials, followed by gold and aluminum. However, gold and silver are too expensive. Therefore, the current heat sink is mainly made of aluminum and copper. In comparison, both copper and aluminum alloys have their own advantages and disadvantages: Copper has good thermal conductivity, but its price is relatively expensive, it is difficult to process, and its weight is too large. The copper heat sink has a small heat capacity and is easily oxidized. . On the other hand, pure aluminum is too soft to be used directly. All aluminum alloys are used to provide sufficient hardness. The advantages of aluminum alloys are low cost and light weight, but the thermal conductivity is much lower than that of copper. Therefore, in the history of the development of the radiator, there are also the following materials:

Aluminum radiator

Pure aluminum radiator is the most common early radiator, 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 common processing method for pure aluminum heat sinks is aluminum extrusion technology, and the main indicator for evaluating a pure aluminum heat sink is the thickness of the heat sink base and the Pin-Fin ratio. Pin refers to the height of the fin of the heat sink, and Fin refers to the distance between two adjacent fins. The Pin-Fin ratio is calculated by dividing the height of Pin (excluding the base thickness) by Fin. The larger the Pin-Fin ratio, the greater the effective heat dissipation area of ​​the heat sink and the more advanced the aluminum extrusion technology. Copper radiator

The thermal conductivity coefficient of copper is 1.69 times that of aluminum, so under the same conditions, the pure copper radiator can quickly remove heat from the heat source. However, the texture of copper is a problem, many advertised as "pure copper radiators" are not really 100% copper. In the copper list, the copper content of more than 99% is called acid-free copper, and the next copper is copper copper content of 85% or less. Most copper radiators on the market today have copper content in between. The copper content of some poor quality pure copper radiators is even less than 85%. Although the cost is very low, its thermal conductivity is greatly reduced, which affects the heat dissipation. In addition, copper also has obvious shortcomings, high cost, difficult processing, and too much heat sink quality hinders the application of all-copper heat sinks. The hardness of red copper is not as good as that of aluminum alloy AL6063. Some mechanical processing (such as grooving) is not as good as aluminum. The melting point of copper is much higher than that of aluminum, which is not conducive to Extrusion.

Copper-aluminum bonding technology

After taking into account the disadvantages of both copper and aluminum materials, some high-end heat sinks in the market currently use a copper-aluminum alloy manufacturing process. These heat sinks usually use a copper metal base, while the heat sink fins use aluminum alloy. Of course, In addition to the copper base, there are also methods such as the use of copper posts for heat sinks, which is the same principle. With a high thermal conductivity, the bottom surface of the copper can quickly absorb the heat released by the CPU; the aluminum fins can be made into the most heat-dissipating shape by a complicated process, and provide a large space for heat storage and rapid release. An equilibrium point has been found in all aspects.