Due to advances in LED technology, LED applications are increasingly diversified, from early power indicator lights to LED lighting products with advantages of power saving, long life and high visibility. However, since only 15 to 20% of the input power of the high-power LED is converted into light, the remaining 80 to 85% is converted into heat. If the heat is not properly discharged to the outside world, the interface temperature of the LED die will be too high and the illumination will be affected. Efficiency and luminescence lifetime.
LED development is the key to heat dissipation
With the continuous evolution of LED materials and packaging technology, the brightness of LED products is continuously improved, the application of LEDs is more and more extensive, and a stable market map for the LED industry is provided. LED as the backlight of the display is a hot topic recently. From the small-size display backlight to the medium-sized LCDTV backlight, it has gradually replaced the CCFL backlight. Mainly LEDs have more advantages in color, brightness, life, power consumption and environmental protection than traditional cold cathode tubes (CCFLs), thus attracting active investment.
Early single-chip LEDs were not very powerful, had limited heat generation, and had little heat problems, so their packaging 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 working current is about 20mA early. The low-power LEDs have progressed to the current high-power LEDs of 1/3 to 1A. The input power of a single LED is as high as 1W or more, even to 3W and 5W.
Encapsulation is more evolutionary
Since the thermal 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); The method is to reduce the thermal impedance of the package module by using the solder or thermal paste on a heat spreader, and the heat shield of the package module is reduced by the heat spreader. This is also the most common LED package module on the market, mainly from Lumileds, OSRAM, Cree and Nicha and other LED internationally renowned manufacturers.
In practical applications, these LED modules can be assembled in a whole row of line sources, or arranged in an array or a circular array, and then bonded to a heat dissipation substrate as a surface light source. However, for many terminal applications, such as mini projectors, automotive and lighting sources, the required lumens in a given area must exceed thousands of lumens or tens of thousands of lumens. Single-die package modules are clearly insufficient. In order to cope with, the trend toward multi-chip LED packaging, and the direct adhesion of the chip to the substrate is a future development trend.
In LED actual product applications, whether for display backlights, indicator lights or general illumination, multiple LEDs are typically assembled on a circuit substrate as needed. On the one hand, the circuit substrate plays the role of carrying the LED module structure. On the other hand, as the LED output power is higher and higher, the substrate must also play the role of heat dissipation to transfer the heat generated by the LED chip, in terms of material selection, Consider both structural strength and heat dissipation requirements.
Need to worry about material costs
The traditional LED power is not large, the heat dissipation problem is not serious, as long as the general electronic copper foil printed circuit board is enough to cope with, but with the high-power LED becoming more and more popular, this board is not enough to meet the heat demand, so it will be The printed circuit board is attached to a metal plate, the so-called MetalCore PCB, to improve its heat transfer path.
In addition, there is also a method of directly forming an insulating layer or a dielectric layer on the surface of the aluminum substrate, and then forming a circuit layer on the surface of the dielectric layer, so that the LED module can directly bond the wires to the circuit layer. At the same time, in order to avoid increasing the thermal impedance due to the poor thermal conductivity of the dielectric layer, a perforation method is sometimes adopted, so that the heat spreader at the bottom end of the LED module directly contacts the metal substrate, that is, the so-called chip is directly adhered. In addition to the metal substrate, in order to cope with the development of the high-power LED package and the direct adhesion of the substrate, in addition to considering the heat dissipation, the substrate material must also be considered to match the thermal expansion coefficient of the chip to avoid thermal deformation caused by thermal stress. Since reliability is a problem, ceramic substrates and metal composite substrates are also being developed at home and abroad. These newly developed substrate materials not only have good heat dissipation, but also have thermal expansion coefficients (between 4 and 8 ppm/K) matching with LED chips. The only drawback is that the price is higher than that of ordinary metal substrates.
Focus on fanless cooling
In fact, 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 LEDTV backlight module, the 40-inch and 46-inch LED backlights have input powers of 470W and 550W respectively, and 80% of them are converted into heat. The required heat dissipation is about 360W. 440W or so. How to take away this heat, there is water cooling to cool, but there are doubts about high unit price and reliability; also use heat pipes with heat sinks and fans for cooling (such as SONY46" LEDTV), but the fan consumes power and noise. Therefore, how to design a fanless cooling method is an important key to determine who will win in the future.
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