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LED Cooling Method And Material
Dec 20, 2018

The initial single-chip LEDs were not very powerful, had limited heat generation, and had little thermal problems, so the package was relatively simple. However, with the continuous breakthrough of LED material technology in recent years, the packaging technology of LED has also changed. From the early single-chip gun-type package, it has gradually developed into a flat, large-area multi-chip package module; its operating current is from early 20mA. The low-power LEDs on the left and right have progressed to the current high-power LEDs of about 1/3 to 1A. The input power of a single LED is as high as 1W or more, and even the 3W and 5W package modes are more evolved.

 

Since the heat problem caused by the high-brightness and high-power LED system will be the key to affecting the function of the product, to quickly discharge the heat generated by the LED component to the surrounding environment, it is necessary to start with the thermal management of the package level (L1 & L2). At present, the industry practice is to reduce the thermal impedance of the package module through the heat spreader by soldering the LED chip with solder or thermal paste on a heat spreader.

Many terminal applications, such as mini projectors, automotive and lighting sources, require more than a thousand lumens or tens of thousands of lumens for a given area. Single-chip package modules are clearly inadequate. Going to the multi-chip LED package and directly bonding the chip to the substrate is a future development trend.

 

The heat dissipation problem is the main obstacle in the development of LEDs for lighting objects. The use of ceramic or heat pipes is an effective way to prevent overheating. However, the heat management solution increases the cost of materials. The purpose of high-power LED heat management design is to effectively reduce R junction-to-case is one of the material-based solutions that provide heat dissipation from the chip, providing low thermal resistance but high conductivity, allowing heat to be transferred directly from the chip through die attach or hot metal methods. The outside of the package housing.

 

Of course, the heat dissipation components of LEDs are similar to the heat dissipation of CPUs. They are mainly air-cooled modules composed of heat sinks, heat pipes, fans and thermal interface materials. Of course, water cooling is also one of the thermal countermeasures. In the current hottest large-size LED TV backlight module, the 40-inch and 46-inch LED backlights have input powers of 470W and 550W respectively, 80% of which are converted into heat, and the required heat dissipation is about 360W and 440W or so.

 

So how do you take this heat away? At present, there are water-cooling methods for cooling in the industry, but there are doubts about high unit price and reliability. It is also useful to use heat pipes with heat sinks and fans for cooling.

 

Heat dissipation method

 

In general, the heat sink can be divided into active heat sink and passive heat sink according to the way heat is removed from the heat sink. The so-called passive heat dissipation means that the heat of the heat source LED light source is naturally radiated into the air through the heat sink, and the heat dissipation effect is proportional to the size of the heat sink, but because of the natural heat dissipation, the effect is of course greatly reduced, and is often used in those space spaces. In equipment that is not required, or used to dissipate heat for components that generate less heat. For example, some popular motherboards also adopt passive heat dissipation on the north bridge. Most of them adopt active heat dissipation. Active heat dissipation is forced by heat sinks such as fans. The heat emitted by the heat sink is taken away, which is characterized by high heat dissipation efficiency and small size of the device.

 

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

 

Air-cooled air-cooled heat is the most common form of heat dissipation, and in comparison, it is also a cheaper way. Air cooling is essentially a fan that takes away the heat absorbed by the heat sink. It has the advantages of relatively low price and convenient installation. However, it is highly dependent on the environment, such as rising temperatures and overheating performance.

 

Liquid cooling

 

The liquid cooling heat is forced to circulate away the heat of the radiator by the pump under the driving of the pump. Compared with the air cooling, it has the advantages of quietness, stable temperature stability and low dependence on the environment. The price of liquid cooling is relatively high, and the installation is relatively cumbersome. At the same time, install it as much as possible according to the instructions in the instructions to get the best heat dissipation. For reasons of cost and ease of use, liquid cooling heat dissipation usually uses water as a heat transfer liquid, so liquid cooled heat sinks are often referred to as water cooled heat sinks.

 

  Heat pipe

 

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

 

Semiconductor refrigeration

 

Semiconductor refrigeration uses a special semiconductor refrigeration chip to generate a temperature difference for cooling when energized. As long as the heat at the high temperature end can be effectively dissipated, the low temperature end is continuously cooled. A temperature difference is generated on each of the semiconductor particles, and a cooling sheet is formed by connecting dozens of such particles in series to form a temperature difference on both surfaces of the refrigerant sheet. By using this temperature difference phenomenon, the air cooling/water cooling can be used to cool the high temperature end, and an excellent heat dissipation effect can be obtained. Semiconductor refrigeration has the advantages of low refrigeration temperature and high reliability. The cold surface temperature can reach below 10 °C, but the cost is too high, and it may cause short circuit due to low temperature. Now the process of semiconductor refrigeration film is not mature enough. practical.

 

Chemical refrigeration

 

The so-called chemical refrigeration is the use of some ultra-low temperature chemicals, which use them to absorb a large amount of heat during melting to lower the temperature. It is more common to use dry ice and liquid nitrogen in this regard. For example, using dry ice can lower the temperature to below minus 20 °C, and some more "perverted" players use liquid nitrogen to lower the CPU temperature below minus 100 °C (in theory), of course, because of the high price and short duration, this Methods are more common in the laboratory or extreme overclocking enthusiasts.

 

Material selection

 

Heat transfer coefficient (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 type aluminum alloy 201

 

6061 type aluminum alloy 155

 

In general, ordinary air-cooled radiators naturally choose metal as the material of the radiator. For the materials selected, it is desirable to have both high specific heat and high thermal conductivity. From the above, silver and copper are the best thermal materials, followed by gold and aluminum. However, gold and silver are too expensive, so the current heat sink is mainly made of aluminum and copper. In comparison, both copper and aluminum alloy have their own advantages and disadvantages: copper has good thermal conductivity, but the price is relatively expensive, the processing is difficult, the weight is too large, and the heat capacity of the copper heat sink is small, and it is easy to oxidize. . On the other hand, pure aluminum is too soft to be used directly. The aluminum alloy used can provide sufficient hardness. The advantage of aluminum alloy is that it is cheap and light, but the thermal conductivity is much worse than copper. Therefore, in the history of the development of the radiator, the following materials have appeared:

 

Pure aluminum radiator

 

Pure aluminum radiators are the most common radiators in the early days. The manufacturing process is simple and the cost is low. So far, pure aluminum radiators still occupy a considerable part of the market. In order to increase the heat dissipation area of the fins, the most common processing method for pure aluminum radiators is aluminum extrusion technology, and the main indicator for evaluating a pure aluminum radiator is 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 the thickness of the base) divided by Fin. The larger the Pin-Fin ratio means the larger the effective heat dissipation area of the heat sink, which means that the aluminum extrusion technology is more advanced.

 

Pure copper radiator

 

Copper has a heat transfer coefficient of 1.69 times that of aluminum, so pure copper heat sinks can remove heat from the heat source more quickly, all other things being equal. However, the texture of copper is a problem. Many of the "pure copper radiators" are not really 100% copper. In the copper list, copper containing more than 99% is called acid-free copper, and the next grade of copper is copper containing less than 85% copper. The copper content of most pure copper heat sinks on the market today is somewhere in between. Some inferior pure copper radiators do not even contain 85% of copper. Although the cost is very low, their heat transfer capacity is greatly reduced, which affects heat dissipation. In addition, copper also has obvious shortcomings, high cost, difficult processing, and too large heat sinks hinder 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, etc.) is not as good as aluminum; copper has a much higher melting point than aluminum, which is not conducive to extrusion and other problems.

 

Copper-aluminum bonding technology

 

After considering the shortcomings of the two materials, copper and aluminum, some high-end heat sinks in the market often use a combination of copper and aluminum. These heat sinks usually use a copper metal base, while the heat sink fins use aluminum alloy. In addition to the copper base, there are also methods such as using a copper post for the heat sink, which is the same principle. With a high thermal conductivity, the copper bottom can quickly absorb the heat released by the CPU; the aluminum fins can be made into a shape that is most favorable for heat dissipation by means of complicated processes, and provide a large heat storage space and quick release. A balance point found in all aspects.

 

To improve the LED luminous efficiency and service life, solving the heat dissipation problem of LED products is one of the most important issues at this stage. The development of the LED industry is also focused on the development of high-power, high-brightness and small-size LED products. The heat-dissipating substrate with high heat dissipation and precise size has also become a trend in the future development of LED heat-dissipating substrates. At present, the aluminum oxide substrate is replaced by an aluminum nitride substrate, or the die/substrate bonding method of replacing the gold wire by a eutectic or flip chip process is used to improve the luminous efficiency of the LED. Under this development trend, the accuracy of the alignment of the heat-dissipating substrate itself is extremely strict, and it is required to have high heat dissipation, small size, and good adhesion of metal lines. Therefore, a thin film ceramic heat-dissipating substrate is produced by using yellow light micro-shadow. It will become one of the important catalysts to promote LED's continuous high power.