The Influences Of Package Substrate And Chip Interconnect Layer For Optical And Thermal Properties Of High-power Light-emitting Diodes

LED has the advantages of smaller size, high brightness, energy conservation, environmental protection, long life, high efficiency, etc. Therefore they are widely used in traffic lights, the rear lights of car, the liquid crystal display, night view lighting and general lighting, etc. White LED will replace incandescent and fluorescent lighting as a new generation of lighting source. But the LED electro-optic conversion efficiency is low; the extra energy is converted into heat at pn junction. If heat can’t urgent be exported, it will result in higher junction temperature gradually. Higher junction temperature will affect the performance of LED devices. The heat dissipation problems of LED devices become the main bottleneck for the development of LED industry, so we need a good thermal design to reduce the thermal resistance and increase the performance of LED devices. Interconnect materials and the voids ratio of interconnect layer have important influence on HP-LED heat dissipation, and the substrate of HP-LED packaging has certain influence on the thermal spreading resistance. So the interconnect materials, the voids ratio of interconnect layer and package substrate are important for the thermal design of HP-LED.Firstly, thermal analysis was carried out by combing T3 ster thermal resistance tester with ANSYS simulation. The thermal resistance and junction temperature of HP-LED with interconnect materials(Au80Sn20, Solder paste and silver paste) are compared firstly, and then the thermal conductivity of interconnect materials, the thickness of interconnect layer and the effective contact area ratio between the interconnect layer with the chip and substrate are simulated respectively. The experiment results show that thermal resistance of interconnect layer(thermal resistance caused by interconnect materials and voids in interconnect layer) is about 1/3 of the total thermal resistance of HP-LED devices, and it is one of the main factors which affect the junction temperature of HP-LED; Gold-tin eutectic interconnect has the best interconnect quality, so the thermal resistance of interconnect layer is the smallest; Gold-tin eutectic interconnection has the minimum thickness of interconnect layer, the smaller the thickness, the better for HP-LED heat dissipation. The simulated results show that the properties of HP-LED heat dissipation is no longer significantly when the thermal conductivity of interconnect materials more than 20 W/m K; The effective contact area ratio of interconnect layer with chip and the substrate reduced from 100% to 10%, the LED junction temperature rise up 8.7%. The thermal conductivity of interconnect materials, the thickness of interconnect layer and the effective contact area ratio of the interconnect layer with chip and substrate all can influence on the junction temperature of HP-LED. So these three parameters should be made an overall consideration for HP-LED thermal design.Secondly, study on the influence of voids ratio for optical, thermal and electrical properties of high-power LED by a series of experiments and analysis of simulation data. The experiment results show that the voids ratio of interconnect layer reduced from 62.45% to 16.53%, the thermal resistance of interconnect layer decreased by 82.7% when the eutectic force increased from 0 N to 2 N; Luminous flux increases with the decrease of thermal resistance of interconnect layer, Luminous flux increases by 6.87%. The simulated results show that the voids play an important role on thermal stress and thermal strain of chip, thermal stress and thermal strain increases by 49.87% and 50% respectively; The voids in interconnect layer plays an important role in the distribution of electric filed intensity and maximum current density of LED chip, maximum current density and electric filed intensity increases by 161.06% and 37.15% respectively.Finally, study on the influence factors of thermal spreading resistance of HP-LED module. So, thermal analysis was conducted by combining T3 ster, finite element simulation with calculation of thermal spreading resistance of HP-LED devices. For single chip LED, the results show: ① Thermal spreading resistance plays an important role on total thermal resistance of HP-LED devices, accounts for about 60.49%; ② The contact area between the heating source and substrate is the main factor which influence on thermal spreading resistance; It is beneficial to decrease thermal spreading resistance when increases the contact area between the heat source and substrate; ③ The thermal spreading resistance shows a declined and then increased trend with the increase of substrate thickness; ④ The center distance between chip and substrate plays a distinct impact on thermal spreading resistance and junction temperature. The results of finite element simulation are in good agreement with calculation of thermal spreading resistance. For the LED multichip packaging model, the finite element software simulates temperature field distribution of multichip LED. To establish the thermal resistance model tree of the substrate to the air, and puts forward a method to directly solve the thermal spreading resistance of more heat source, the effects of different heat source location on the temperature field distribution were simulated. The research results show: ① Compared direct method with indirect method, the results verify the accuracy of the direct method; ② Uses MATLAB to solve the temperature field results of heat source, the accuracy of temperature field distribution by finite element simulation were verified; ③ Study on the influence of substrate sizes on thermal spreading resistance and the total thermal resistance, the results show that the thermal spreading resistance increases with the increase of substrate area, while the total thermal resistance is gradually decline. Therefore it needs to comprehensively consider the thermal resistance of conductor, the thermal resistance of air convection and thermal spreading resistance when design the substrate size in order to achieve the best value in the total thermal resistance.