The Thermal Design And Thermal-Mechanical Reliability Analysis Of High Power LED Lighting System

As solid state-lighting (SSL) technology advances, high power light emitting diodes (LED) are penetrating into illumination applications due to their low energy consumption, environmental-friendly and long lifetime characteristics. However, the low-light conversion efficiency(10%～20%)causes most of the energy to transform and generate redundant heat which ultimately increases the junction temperature and reduces reliability. Therefore, improving the system’s ability to draw heat away from the LED junction is the critical point to promote the LED light source in general lighting applications. This paper carried out thermal design and thermal-mechanical reliability analysis on high power LED device array packaging system and high power multi-chip LED packaging system, which mainly aims at a series of problems existed in most of high power LED products, such as high junction temperature, low reliability, high cost, and so on.Firstly. Based on analyzing the thermal design and packaging technology of the existing high power LED at home and abroad, the thermal model of LED is abstracted. The experimental analysis on the spreading resistances of the high power LED lighting system and the measuring of the related thermal characteristic parameters are implemented by T3Ster. From the experimental analysis of the spreading resistances, we can obtain that increasing the effective contact area between the LED devices and radiating fins and selecting the optimal mounting position can decrease the spreading resistances and increase the cooling efficiency of heat sink effectively.Secondly, according to the requirements of the thermal and optical performance of the LED packaging, the thermal structure design and material parameters optimization of the high power LED device array packaging system and high power multi-chip LED packaging system are carried out based on the results of the experimental analysis and the thermal characteristic parameters. Then, the thermal resistance and junction temperature of these two high power LED packaging systems are calculated and analyzed by using the thermal resistance network model. The results show that the thermal resistance and the junction temperature of these two systems are(12.48 K/W、74.92℃)and(2.214 K/W、67.066℃), respectively.Thirdly, the distribution of the temperature field and the thermal stress field of the high power LED device array packaging system are simulated by ANSYS. Through the simulation, we can obtain that the thermal resistance and the junction temperature of the high power LED device array packaging system are 12.15K/W and 73.602℃respectively, which are close to the results that are calculated by the thermal resistance network model. During the thermal stress analysis, it can be found that the high thermo-mechanical stress exists in the edge and corner of the chip which lead to delaminate and warp easily. In order to investigate the main parameters leading to excessive stress in the LED chip and optimize the material parameters of the LED device, DOE simulation is performed. The optimal parameters for LED device are acquired: chip substrate (SiC), die-attach (Sn63Pb37), submount (AlN).Finally, the simulation of the distribution of steady state temperature field and the optimization of the cooling structure of the system are carried out for the high power multi-chip LED packaging system. Through the simulation, the thermal resistance ( 2.406K/W) and the junction temperature (70.717℃) of the high power multi-chip LED packaging system are close to the results that are calculated by the thermal resistance network model. In order to investigate the main parameters leading to excessive junction temperature of the LED chip and optimize the structure parameters of the system, DOE simulation is performed. The optimal parameters for system are acquired: when the substrate of LED chip and the material of die-attach are used Si and Sn63Pb37 respectively, the thickness of bonding layer is 0.02mm, the height of the fins is 70mm, the with of the fins is 17.535mm, the number of the fins is 60, and the thickness of the central column is 15mm, the heat dissipation performance and the reliability of the system are the best.