| The heat dissipation technology of LED is becoming more and more important with the persistent requirement for high power and high brightness due to the increasing application in general lighting. Increase of junction temperature may lead to decrease of quantum efficiency and excitation efficiency of phosphors, red shift of LED peak wavelength and deteriorate of light uniformity for white LEDs. Elevate junction temperature may generate reliability issues or even catastrophic failure. Therefore, advanced packaging technology with optimized thermal design is important precondition to fulfill the application of LED in general lighting.In order to improve the thermal performance of high power LED device and module, two aspects were investigated in this paper. Firstly, thermal design and reliability issues in high power LED were investigated. Secondly, design optimization of RGB LED modules combining theoretical calculation and experiments was carried out.Firstly, based on the chip on board (COB) technology, a ceramic substrate with high thermal conductivity coefficient, low cost is designed and fabricated. The substrate is AIN-based muti-layer low temperature co-fired ceramic (LTCC) substrate. Experiments were carried out to compare the effects of LED module using different substrates. The thermal resistance of the LED module with A1N based substrate (17.16℃/W) is 54.9%and 40.2%lower than that with Al (38.1℃/W) and A12O3 (28.7℃/W) based substrate, respectively. When the electric current becomes higher than 800 mA, the total luminous flux increase ratio of modules with Al or Al2O3 based substrate are lower than that of the one with AIN-based substrate. By simulation, the highest junction temperature for the module with AlN based substrate is 18.6%and 12.9%lower than the one with Al and the Al2O3 based substrate respectively. All the simulated and experimental results show that the AlN based substrate is an effective solution to improve the heat dissipation of the HP-LED heat dissipation.Secondly, performance and reliability issues of HP-LED utilizing different die attaching materials were investigated. It was found that eutectic die attaching technology is an effective method to improve the thermal resistance of HP-LED and the reliability of HP-LED is closely related to the die attach materials and the film on the substrate. There are mainly two points were concluded:a) the thermal resistance of HP-LED attached by gold-tin and solder paste is 14.3%and 17.3% lower than the samples attached by solder paste before aging test; b) deposited film on the substrate greatly affects the reliability of the LED. For the HP-LED with the Ag film, the luminous flux durability of the HP-LED adhesive by 80Au20Sn, solder paste and silver epoxy are 75.4%,78.9%and 93.6%after aging 2580 hours aged at the condition of 450 mA & 55℃temperature. For the HP-LED with the Au film substrate, the luminous flux durability is 97%,96%and 96%respectively for the HP-LED after 1100 hours aging at 85℃/85RH. At the same time, the junction temperature of HP-LED attached by solder paste and silver paste increased 6.6%and 9.6%respectively. But it remains almost the same (0.4%) for the HP-LED attached by gold-tin. The above results indicate that the substrate with Au film is suited for gold-tin eutectic process.Thirdly, the effects of void ratio and the eutectic pressure on the thermal and optical performance of LEDs are studied, where the LED module utilized ceramic substrate with deposited copper. The investigations were carried out by theoretical calculation and experimental measurements. The results show that:a) both packaging thermal resistance and spreading thermal resistance increase with the void ratio. The spreading thermal resistance proportion is about 80%of the total thermal resistance for the HP-LED according to the theoretical calculation. The results indicate that spreading thermal resistance can not be ignored in the HP-LED packaging design, b) The eutectic pressure can efficiently improve the thermal performance. The thermal resistance of the die attach layer increased by 65%(0.07 ℃/W) and the spreading thermal resistance increased by 25%(0.45℃/W). The calculated total thermal resistance is 7.8℃/W which can be regarded as agree with the measured thermal resistance (8.1℃/W). In order to reduce the voids in the adhesive layer, pressure is added on the surface of LED chip during the eutectic process. The junction temperature was decreased from 75℃to 70℃when the eutectic pressure changed from 0 g to 15 g. But the thermal performance can not be improved when further increasing the eutectic pressure. Crack can be found in some of the HP-LED surface when the eutectic pressure increased up to 45 g. It can be concluded that proper pressure can reduce the voids and thickness of attaching layer during the eutectic process. Scanning Electron Microscope (SEM) tested results show that intermetallic compound (Au,Ni)3Sn2 are formed between the interconnection layer and the LED bottom surface and the substrate top surface.Fourth, a multi-chip of RGB LED module is designed and packaged based on the numerical calculation. Comprehensive investigations are developed based on the RGB module. The results show both the thermal and optical performance can be affected by the distribution of the RGB LED The fabricated module can emit white light with color temperature from 2300 K to 7000 K by the self designed intelligent control program. The measured results of the RGB module are in accordance to the calculated or the simulated results.In summary, the designation and fabrication of A1N based LTCC ceramic substrate with high conductivity and low cost, low thermal resistance eutectic die attach processes, the effects of spreading thermal resistance to the total thermal resistance with the variations of the void ratio, low void ratio alloy eutectic processes and flexible color temperature white LED designation and multi-chip LED module packaging are investigated. The results show that the advanced packaging technologies have important effect to the reliability of the HP-LED.
|
PDF Full Text Request