Development Trend of High Power LED Heat Dissipation Substrate

Due to the advancement of LED technology, LED applications are becoming more and more diversified, from early power indicators to LED lighting products with advantages such as power saving, long life and high visibility. However, since only 15 to 20% of the input power of high- power LEDs is converted into light, and the remaining 80 to 85% is converted into heat, if this heat is not discharged to the outside world in time, the temperature of the LED grain interface will be too high, affecting the luminous efficiency and luminous life.

Heat dissipation is the key to LED development . With the continuous evolution of LED materials and packaging technology, the brightness of LED products has been continuously improved, and the application of LEDs has become more and more extensive, providing a stable and growing market for the LED industry. Using LED as the backlight source of displays has become a hot topic recently. It has gradually developed from the backlight source of small-sized displays to the backlight source of medium and large-sized LCD TVs, and it is likely to gradually replace the CCFL backlight source. The main reason is that LED has more advantages than traditional cold cathode tubes (CCFL) in color, brightness, life, power consumption and environmental protection, thus attracting industry players to actively invest. In the early days, the power of single-chip LEDs was not high, the heat generation was limited, and the heat problem was not big, so its packaging method was relatively simple. However, in recent years, with the continuous breakthroughs in LED material technology, LED packaging technology has also changed, from the early single-chip cannonball-shaped packaging to a flat, large-area multi-chip packaging module; its operating current has evolved from the early low-power LED of about 20mA to the current high-power LED of about 1/3 to 1A, and the input power of a single LED is as high as 1W or even 3W or 5W. The packaging method has evolved Since the heat problem derived from the high-brightness and high-power LED system will be the key to affecting the product function, in order 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 packaging level (L1&L2); the current practice is to connect the LED die to a heat spreader with solder or thermal paste, and reduce the thermal impedance of the packaging module through the heat spreader. This is also the most common LED packaging module on the market. The main sources are internationally renowned LED manufacturers such as Lumileds, OSRAM, Cree and Ni ic ha. In actual applications, these LED modules can be assembled in a row to form a linear light source, or in an array or circular arrangement, and then joined to a heat dissipation substrate as a surface light source. However, for many terminal application products, such as mini projectors , automotive and lighting sources, the required lumen volume in a specific area needs to exceed thousands or tens of thousands of lumens. Single- crystal packaging modules alone are obviously not enough to cope with it. Moving towards multi-chip LED packaging and direct chip bonding to the substrate is the future development trend. In the actual application of LED products, whether it is used for display backlight, indicator light or general lighting, multiple LEDs are usually 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 output power of LEDs becomes higher and higher, the substrate must also play the role of heat dissipation to transfer the heat generated by the LED chip . Therefore, in the selection of materials, both structural strength and heat dissipation requirements must be taken into account. Material costs need to be considered Traditional LEDs have low power and are not a serious heat dissipation problem. It is sufficient to use a copper foil printed circuit board used in general electronics. However, as high-power LEDs become more and more popular, this board is no longer sufficient to meet the heat dissipation needs. Therefore, the printed circuit board needs to be attached to a metal plate, the so-called MetalCore PCB , to improve its heat transfer path. Another method is to directly make an insulating layer or dielectric layer on the surface of the aluminum substrate, and then make a circuit layer on the surface of the dielectric layer. In this way, the LED module can directly connect the wires to the circuit layer. At the same time, in order to avoid increasing thermal impedance due to the poor thermal conductivity of the dielectric layer, perforation is sometimes used to allow the heat spreader at the bottom of the LED module to directly contact the metal substrate, which is the so-called direct chip bonding. In addition to metal substrates, in order to respond to the development of high-power LED packaging and chip direct bonding substrates, the selection of substrate materials must not only consider heat dissipation, but also consider the matching of the thermal expansion coefficient with the chip to avoid thermal deformation and reliability problems caused by thermal stress. Therefore, 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 a thermal expansion coefficient (between 4~8ppm/K) that matches the LED chip. The only drawback is that they are more expensive than general metal substrates. Focus on fanless heat dissipation In fact, the heat dissipation components of LEDs are similar to CPU heat dissipation. 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 heat countermeasures. For the most popular large-size LEDTV backlight modules , the input power of 40-inch and 46-inch LED backlight sources is 470W and 550W respectively. Assuming that 80% of them are converted into heat, the required heat dissipation is about 360W and 440W. How to take away this heat? There are water cooling methods, but there are concerns about high unit price and reliability; there are also heat pipes with heat sinks and fans for cooling (such as SONY46”LEDTV), but there are problems such as fan power consumption and noise. Therefore, how to design fanless heat dissipation is an important key to determine who will win in the future.