Introduction of heat dissipation technology of high power LED lighting products

Date: 2019-03-14
Viewed: 67

    Due to the continuous progress of Solid State Lighting technology, LED's luminous efficiency has been improved in recent years and can gradually replace the traditional light source. At present, its luminous efficiency has been growing continuously, catching up with incandescent lamps and halogen lamps, as shown in figure 1.

    But some companies have developed more efficiency exceed 100 lm/W LED components, it also makes the LED lighting application is more and more wide, has been used not only in the indoor and outdoor lighting, mobile phone backlight module and the direction of auto lamp, etc., more value in high wattage lamp and street lamp light lighting, such as large size backlight module, and the application of car headlights, etc.Due to the advantages of energy saving, environmental protection and long life, the future trend of LED light source is more and more obvious.   

    In order to make the LED emit brighter light, a higher power input is needed.The WPE value is still limited, typically only about 15 to 25 percent of the input power goes to light and the rest is converted to heat.Due to the small LED chip area (~1mm2), the high power LED unit area heat (heat density) is very high, even more serious than the general IC components, also makes the LED chip Junction Temperature greatly improve, easy to cause overheating.Too high chip interface temperature will reduce the luminance of LED, especially the attenuation of red light.As shown in figure 3, cooling technology has become the bottleneck of current LED technology development.

    Therefore, heat dissipation design faces great challenges. We must attach great importance to heat dissipation design from wafer level, packaging level, PCB level to system module level, and seek for the best heat dissipation method.For LED lighting products, due to the large heat dissipation limitation at the system end, the heat dissipation requirements at other levels are more obvious.For the LED heat transfer problem, the most basic analysis method is to use thermal resistance network analysis.In other words, the main heat dissipation path of LED from chip heat source to ambient temperature is constructed, as shown in figure 4. Then, the characteristics and size of each thermal resistance value are analyzed, so as to calculate the chip temperature under ideal conditions, and countermeasures are taken to reduce the thermal resistance value for each part of the thermal resistance network.It should be noted that figure 4 is the thermal resistance network composed of Chip Level, Package Level, Board Level and System Level.In practical analysis, a more detailed thermal resistance network can be composed based on the system structure, such as the thermal resistance of interface materials such as Die Attach material and Solder, or the thermal resistance of heat dissipation module structure.

    Due to the poor thermal conductivity characteristics of Sapphire substrate of LED wafers, the Rjs value in figure 3 is too high. Therefore, the improvement mode must replace Sapphire with high-thermal conductivity materials such as copper, or remove the substrate from the heat transfer path in the cladding mode to reduce the thermal resistance value.At present, the design of heat dissipation with better performance from wafer to package level, including co-fusion gold substrate and wafer form, makes heat transfer from wafer to package more easily.Increasing wafer size to reduce heating density is also a feasible direction.

    The heat dissipation design of high-power LED is very important, which is related to the luminous quality and service life of LED.Through the thermal resistance network, heat dissipation capacity and requirements can be quickly analyzed and countermeasures can be sought. Due to the high heating density of high-power LED, heat dissipation design must be carried out from Chip Level, Package Level, Board Level and System Level to reduce thermal resistance, so as to obtain the best heat dissipation effect.At present, leading LED chip and packaging manufacturers in the world are committed to the development of products with higher luminous efficiency, through improving the quantum efficiency of light and other ways to improve the photoelectric conversion efficiency, in order to reduce the heating of the chip.In order to accelerate the development and application of LED products, the relevant cooling technologies still need to be developed simultaneously.Due to the continuous improvement of human demand for quality of life, just as the demand for cooling of IC products has always existed, cooling design will still play an important role in the design of various high-power LED products.

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