A Study On Thermal Performance Of Packaging Of High-power LEDs

Light emitting diode(LED),being widely used in mobile flashlight,large and medium sizes displays(NB,LCD-TV) and illumination system,has become one of the most promising high technologies in 21st century with a series of distinctive advantages such as lower-power consumption,pure light,solid-state,environmental protection.Many countries such as the United States,the European Union,Japan and China have established national strategies and invested to accelerate LED industry.The eletrophotic conversion efficiency ranges from 20%to 30%,thus the rest energy converts into thermopower which can't be released through radiation.With this low heat removal efficiency,the raise of temperature in the chip will result in non-uniform distribution of thermal stress,decline of the luminous efficiency of the chip and the maser efficiency of the phosphor,and ultimately lead to device failure. So the packaging of high-power LEDs,especially the packaging of white high-power LEDs has been the obstacles to the industrilization.To solve the problems of the current packaging of high-power LEDs,we put forward a new structure of packaging of high-power LEDs.Through research both theoretically and experimentally as well as processing on the structure and materials of packaging for high-power LEDs and the simulation and optimal design of 5W single-chip white LED packaging and 5W multi-chip white LEDs packaging,we designed and prepared a thermal packaging module for 5W white LEDs with the thermal resistance of 8.05K/W and the luminous efficiency of 1601m/W.The major works and innovation in this paper are as follows:(1) To solve the common problems about the present packaging of high-power LEDs with worse thermal performance,complicate process and high cost,we presented an integrated packaging of high-power LEDs and a packaging of fluorescent lamp composed of LEDs on the basis of both theory and experiment,and applied for two relevant patents.By preparing the insulating layer and the electrode layer directly on the aluminum plate,the amount and thickness of internal heat sinks about the packaging of high-power LEDs is reduced.Thus,these methods,which could improve the thermal performance of packaging for high-power LEDs with easy processing,lost cost and environmental protection according to RollS,could lead to a effective solution to the heat-release problem for packaging of high-power LEDs.(2) Through theoretical analysis and comparison about performance among the materials used for the cooling plate,the insulator layer and the electrode layer,we chose the aluminum for cooling plate,Al₂O₃ for insulator layer and Cr/Ni-Cu/Ag or ITO layer for the electrode layer.(3) With the simulation and optimal design of thermodynamics for packaging modules of 5W single-chip white LED and 5W multi-chip white LEDs,we found that the thermal resistance of the first packaging is 5.8K/W and This packaging,of which the highest temperature of the chip is 73.197℃when the ambient temperature is 50℃,could well satisfy heat-release needs for the high-power LED packaging.And the second packaging,of which the thermal resistance is 3.986K/W and the highest temperature of the chip is 65.994℃when the ambient temperature is 50℃,could well satisfy heat-release needs for the high-power LED packaging.The thermal resistance of the second one is 2.563K/W with aluminum plate 1.1 mm thick and the convection coefficient of 10 W/(m²·K) after the optimal of structure parameters. The packaging both for single-chip and multi-chips could meet the heat-release requirement for high-power LEDs.(4)We prepared the aluminum dioxide layer with the thickness of 30.2urn and the dielectric probability of 32.5V/um by the method of hard acid anodizing.Through theoretical analysis and test,we analyzed the explanation for non-insulation about the insulator layer after coating and pointed out some improvements.(5) After the research about the preparation and materials of the electrode layer, we produced each layer of it as Cr/Ni-Cu/Ag.The results shown that the sample of which the resistivity achieved 3×10^-6Ω·cm,average bonding force up to 4.22 MPa with little surface defaults and good density and fine welding probability could meet the acquirement of the electrode layer.(6) We presented a new method with the introduction of online ultraviolet radiation during the preparation of ITO thin-film as electrode layer by RF magnetron sputtering and applied for a relevant patent.These results shown that the resistivity, the surface topography and the growth orientation of samples produced with UV radiation are much better than samples produced without UV radiation.The sample produced with UV radiation of which the smallest square resistance is 5Ω/□,the resistivity achieved 2.5×10^-4Ω·cm,average bonding force up to 5.3 MPa with little surface defaults and good density and the peak of[222]and good welding probability could meet the acquirement of the electrode layer.(7)Based on the study above,we prepared a packaging of 5W single-chip white LED with thermal resistance of 8.05K/W.This result is highly consistent with the predictions of the theoretical model.This packaging,of which the highest temperature of the chip is 82.2℃when the ambient temperature is 50℃,could well satisfy heat-release needs for the high-power LED packaging.