Results and Discussions With the increase in sintering temperature from 180 ℃ to 240 ℃, the resistivity of the sintered samples decreases gradually, and the shear strength increases. High temperatures afford a large sintering driving force, which can accelerate the sintering and diffusion of nanoparticles. Therefore, nano-silver sintered at a high temperature has the characteristics of compactness and low resistivity. Nano-silver has better electrical conductivity and mechanical properties in the case of a higher sintering temperature (Fig. 2). The bonding layer of the high-power LED chip has a compact structure without holes and cracks. Moreover, the interfaces between the nano-silver layer and the upper and lower substrates exhibit good metallurgical bonding, which indicates that the LED sintered with nano-silver paste has excellent packaging quality (Fig. 3). The surface temperature of LED samples packaged with different bonding materials increases slowly with the increasing driving current. Compared with SAC305 solder, nano-silver has lower interface thermal resistance after sintering, which can improve the heat dissipation performance of LED devices and achieve lower operating temperature (Fig. 4). Due to the high thermal conductivity of nano-silver, the heat of the LED chip is dissipated faster (Fig. 5). The low interface thermal resistance of nano-silver is contributed to reducing the chip temperature and thereby lowering the junction temperature, which helps improve the reliability of LED devices (Fig. 6).

Light emitting diodes (LEDs) are semiconductor light emitting devices based on the electroluminescence principle of p-n junction. Due to the advantages of high lighting efficiency, long life, environmental protection, energy saving, and compact structure, they have been widely used in the field of lighting and backlight display, such as road lighting, indoor lighting, automobile headlights, TV backlight, and mobile phone flash. With the increasing demand for lighting and display, LED technology is developing toward high power and high density. Therefore, luminescence performance has become an important indicator. Research shows that the luminescence performance of LEDs can be improved by employing the embedded ceramic circuit board technology and doping metals and other chemical compounds. However, the influence of LED heat dissipation on luminescence performance cannot be ignored. As the electro-optic conversion efficiency of LED is less than 60%, part of the input electric energy is converted into heat, and more heat is generated by the LED chip with the increase in the input power. For avoiding thermal damage to LED chips under high temperatures, it is significant to enhance the heat dissipation performance of LEDs, which can thereby improve the luminescence performance of high-power LEDs. This paper uses nano-silver paste for high-power LED packaging and systematically investigates the thermal resistance and luminescence performance of a nano-silver sintered interface in a high-power LED. Further, the paper analyzes the resistivity, bonding strength, and micromorphology of the nano-silver bonding layer at different sintering temperatures and compares the thermal resistance, junction temperature, and optical properties of LED devices sintered with nano-silver paste and Sn-Ag-Cu (SAC) solder.

The nano-silver paste in this study is the silver paste after pressureless sintering. The chip used is a vertically packaged high-power blue LED chip with a direct plated copper (DPC) ceramic substrate, and the packaging substrate is a hexagonal copper substrate. Firstly, the chip and packaging substrate were cleaned by ultrasonic wave with acetone and anhydrous ethanol solution to remove the oil stain and impurities on the metal layer surface. Then, the nano-silver paste was coated on the hexagonal copper substrate by screen printing. The metal welding layer on the back of the DPC ceramic substrate containing chips was aligned to the copper substrate line layer and placed horizontally. After that, the LED samples were put into an oven for sintering to obtain the packaged high-power LED devices. Finally, the thickness of nano-silver films sintered at different temperatures was measured by a step profiler, and the sheet resistance of the sintered silver films was determined by a four-probe tester. The shear strengths of bonded joints were tested by a multifunctional shear force tester. The crystal structure of nano-silver paste after sintering was detected by X-ray diffraction. A scanning electron microscope (SEM) was used to observe the cross-sectional microstructure and fracture surface of the bonding joints. The cross-sectional structure of the high-power LED was observed under an optical microscope. A thermal imaging system was used to record the surface operating temperature of the high-power LED. The junction temperature change and structural thermal resistance of the LED devices with different bonding materials were tested by a thermal resistance tester. In addition, a photoelectric analysis system was adopted to test the emission spectrum and light output power of LED samples.

In this paper, high-power LED devices are fabricated by nano-silver sintering technology, and the interface thermal resistance and luminescence performance of the bonding layer are emphatically investigated. The experimental results illustrate that with the increase in the sintering temperature, the resistivity of nano-silver decreases, and the bonding strength of joints increases. The interface of the nano-silver sintered LED bonding layer is compact and crack-free, forming good metallurgical bonding. The LED sample sintered with nano-silver paste has a lower working temperature and lower total thermal resistance than that sintered with SAC305 solder, and the interface thermal resistance of the LED sample sintered with SAC305 solder is 8.9% higher than that of the nano-silver sintered one. These findings indicate that nano-silver has higher thermal conductivity and better heat dissipation performance. In addition, the luminous efficiency of the aged LED sample sintered with nano-silver paste is invariably higher than that of the sample sintered with SAC305 solder at different input currents.