Study On Thermal Management And Failure Analysis Of Solid State Lighting Packaging

With the improvement of luminous efficiency, the application of high-power light-emitting diode (LED) in general lighting area becomes wider than the past. For normal use and device lifetime of high-power LED, the thermal management is a key issue. In this dissertation, the thermal management of high-power LED is investigated by numerical simulation and experimental methods. Meanwhile, the reliability of the LED is also studied by experimental and theoretical analysis.Thermal resistance is the key parameter for thermal management. The correct thermal management makes the package thermal resistance of the LED the smallest by choosing the suitable packaging materials and packaging structure. Die-attach material is the most important one of all the LED packaging materials, its thermal physical properties and geometric dimensions have a great effect on the total thermal resistance of the package. Because AuSn-eutectic alloy (80Au20Sn) has high thermal conductivity and high melting point, it can be used as die-attach material to reduce the thermal resistance between the LED die and the heat sink. In this dissertation, the AuSn-eutectic alloy thin foil and preformed solder are prepared by casting toughening method and its chemical compositions, properties and microstructures are measured with investigation of its application in the high-power LED package. The LED packaging experiments are conducted by adopting different chips and lead frames, which use 80Au20Sn and Sn3.5Ag0.5Cu as die-attach materials, respectively. The thermal models of different LED structures are established and their thermal resistances are calculated. The results show that the thermal resistance of the LED using Sn3.5Ag0.5Cu as the die-attach material is larger than the thermal resistance of the LED using 80Au20Sn as the die-attach material under the same package structure. The test results show that the junction temperature of the LED using Sn3.5Ag0.5Cu as die-attach material is higher than the junction temperature of the LED using 80Au20Sn as die-attach material under the same package structure and working condition. Theoretical calculation and experimental test results fit well.Because thermal management for LED luminous performance and device lifetime is essential, mechanical designers must consider the thermal problem in the initial stage of developing the new LED products. The evaluation and optimal design for thermal management solutions of high-power LED from package level to system level can be achieved by thermal simulation, which plays an important role in cost saving and development cycle of a new product curtailing. In this dissertation, thermal simulation for high-power LED and multi-chip module from package level to system level are carried out by using the finite volume method and the finite element method, respectively.Thermal path and the corresponding thermal resistance of the high-power LED is analyzed and calculated. The heat flow analysis and heat dissipation optimal design of the high-power LED are realized by using commercial computational fluid dynamics (CFD) software. Theoretical calculation results show that, to the high-power LED, the total thermal resistance from the junction to the ambient is 28.67℃/W. When the dissipating power is 1W and the ambient temperature is 25℃, the junction temperature is 53.67℃while the simulation results show that junction temperature is 54.85℃under the same working condition. It is a good agreement with the theoretical calculation result. The results show that when the area of heat dissipation increases to a certain value, the heat dissipation effect remains a constant. Thus, considering to lower product costs, the area of heat sink should have a threshold range. The air-fluid flow up unimpeded, when the heat sink is installed with vertical fins to the left, of which the heat dissipation has the best effect and the junction temperature is lowest.In this dissertation, three-dimensional steady-state finite element numerical thermal simulation is performed on high-power LED multi-chip module with heat sink which is composed of heat pipe and fins. An optimal packaging configuration is obtained by simulating different materials and geometrical parameters of the heat sink, sub-mount, die attachment, as well as different chip array layouts and chip spacing parameters. The numerical simulation results show that the optimized thermal conductivity of the heat pipe is 5000W/(m·K). Circular is a better chip layout in this paper than square chip layout. Silicon is the best sub-mount material for InGaN dies and 0.27mm is the optimized thickness.80Au20Sn alloy is the best material of die attachment and 0.02mm is the optimized thickness. Taking packaging density and heat coupling into account,1.5mm is the optimized spacing between chips. Numerical simulation results of heat transfer also show that an ideal junction temperature of the chips can be obtained through appropriate selection of packaging material and structure under general working condition. The delamination may occur in the LED package when GaN-based high flux LED work for a long time at the room temperature or aging because of high temperature and high humidity. The delamination may occur between the LED chip and the encapsulation, which generates a thin chip-air-silicone interface within the package. The darkening may occur on the interface between LED chip and epoxy when high-power LED work for a long time at the room temperature or aging because of high temperature and high humidity. The darkening may occur on the surface of the LED, which makes the chip surface covered with a layer of dark material. These problems do not cause a catastrophic failure, but they can cause a permanent reduction in light output, leading to device failure eventually.In this dissertation, the mechanism of delamination and darkening failure due to thermal overstress is detected and analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersive Analysis by X-ray (EDAX). The results show that, as input current of the light-emitting diode increases, junction temperature of the LED also increases, leading to thermal overstress which makes the thermal stress between LED chip and epoxy mismatch, as well as the stress will be induced by the moisture expansion of epoxy, which leads to the delamination in the interface between the die and epoxy. The epoxy material, in contact with the chip surface, will be aged under high temperature and the internal structure of epoxy after degradation turns out a significant change, which form carbon and carbonization deposited on the surface of the die. That is the main reason of the darkening on the surface.