I. Introduction
Since the high power LED package structure and process complex, and directly affect the performance and life of LED, has been studied in recent years, in particular, high-power white LED package is the focus of research focus. The functions of LED package mainly include: 1. Mechanical protection to improve reliability; 2. Strengthen heat dissipation to reduce wafer junction temperature and improve LED performance; 3. Optical control, improve light extraction efficiency, optimize beam distribution; 4. Power supply management, Includes AC/DC transitions, as well as power control.
The choice of LED packaging methods, materials, structures, and processes is primarily determined by factors such as wafer structure, optoelectronic/mechanical characteristics, specific applications, and cost. After more than 40 years of development, LED packaging has experienced the development stages of stent (Lamp LED), SMD (SMD LED), and power LED (Power LED). With the increase of wafer power, especially the development of solid-state lighting technology, new and higher requirements have been put forward for the optical, thermal, electrical and mechanical structures of LED packages. In order to effectively reduce the thermal resistance of the package and improve the light extraction efficiency, a new technical idea must be adopted for the package design.
Second, the key technology of high-power LED packaging
High-power LED packages mainly involve light, heat, electricity, structure and process, as shown in Figure 1. These factors are independent of each other and affect each other. Among them, light is the purpose of LED packaging, heat is the key, electricity, structure and process are the means, and performance is the concrete embodiment of the packaging level. In terms of process compatibility and lower production costs, the LED package design should be performed simultaneously with the wafer design, ie the package design and process should be considered in the design of the wafer. Otherwise, after the wafer is manufactured, the wafer structure may be adjusted due to the needs of the package, thereby prolonging the product development cycle and process cost, sometimes even impossible.
Specifically, key technologies for high-power LED packaging include:
(1) Low thermal resistance packaging process
For the existing LED light effect level, since about 80% of the input power is converted into heat, and the LED chip area is small, the heat dissipation of the chip is a key problem that the LED package must solve. It mainly includes wafer layout, packaging material selection (substrate material, thermal interface material) and process, heat sink design and so on.
Figure 1 high power LED packaging technology
Figure 2 Low temperature co-fired ceramic metal substrate
LED package thermal resistance mainly includes internal thermal resistance and interface thermal resistance of materials (heat dissipation substrate and heat sink structure). The function of the heat dissipating substrate is to attract the heat generated by the wafer and conduct it to the heat sink to achieve heat exchange with the outside world. Commonly used heat sink materials include tantalum, metals (such as aluminum, copper), ceramics (such as Al2O3, AIN, SiC) and composite materials. For example, Nichia's third-generation LED uses CuW as the substrate, and the 1mm wafer is flipped on the CuW substrate, which reduces the thermal resistance of the package and improves the luminous power and efficiency. Lamina Ceramics has developed a low-temperature co-fired ceramic metal substrate. , as shown in Figure 2, and developed the corresponding LED packaging technology. The technique first prepares a high power LED wafer suitable for eutectic soldering and a corresponding ceramic substrate, and then directly solders the LED wafer to the substrate. Since the eutectic solder layer, the electrostatic protection circuit, the driving circuit and the control compensation circuit are integrated on the substrate, the structure is simple, and the thermal conductivity of the material is high, the thermal interface is small, and the heat dissipation performance is greatly improved, and the high-power LED array is packaged. Proposed a solution. The high thermal conductivity copper-clad ceramic plate developed by Curmilk Company of Germany is made of ceramic substrate (AIN and Al2O3) and conductive layer (Cu) sintered at high temperature and high pressure. No adhesive is used, so the thermal conductivity is good, the strength is high, and the insulation is good. Strong, as shown in Figure 3. Among them, aluminum nitride (AIN) has a thermal conductivity of 160 W/mk and a thermal expansion coefficient of 4.0×10 −6 /° C. (corresponding to a thermal expansion coefficient of 3.2×10 −6 /° C.), thereby reducing the thermal stress of the package.
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