High-power LED packaging technology

High-power LED packaging technology

Summary

In this paper, the design and research progress of high-power white LED package are reviewed in detail from the aspects of optics, thermal, electrical and reliability, and the key technologies of high-power LED packaging are reviewed. It is proposed that the package design of the LED should be carried out simultaneously with the chip design, and the performance of light, heat, electricity and structure should be considered uniformly. In the packaging process, although the choice of materials (heat sink, phosphor, potting) is very important, the thermal and optical interfaces should be minimized in the package structure to reduce the thermal resistance of the package and improve the light extraction efficiency. Finally, the design and packaging requirements of LED luminaires are described.

Keywords: solid state lighting, high power LED, white LED package

I. Introduction

High-power LED package has been a research hotspot in recent years due to its complicated structure and process, and directly affects the performance and life of LED. Especially, high-power white LED package is a hot spot in research. The functions of the LED package mainly include: 1. mechanical protection to improve reliability; 2. enhanced heat dissipation to reduce chip 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 chip 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 chip power, especially the development of solid-state lighting technology, new and higher requirements are 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 carried out simultaneously with the chip design, that is, the package design and process should be considered in the chip design. Otherwise, after the chip is manufactured, the chip structure may be adjusted due to the needs of the package, thereby prolonging the product development cycle and process cost, sometimes even impossible.

Figure 1 High-power white LED packaging technology

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 chip heat dissipation is a key problem that the LED package must solve. It mainly includes chip layout, packaging material selection (substrate material, thermal interface material) and process, heat sink design and so on.

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 dissipation substrate is to absorb the heat generated by the chip and conduct it to the heat sink to achieve heat exchange with the outside world. Commonly used heat sink materials include silicon, metals (such as aluminum, copper), ceramics (such as Al2O3, AlN, SiC) and composite materials. For example, Nichia's third-generation LED uses CuW as the substrate, and the 1mm chip is flip-chip mounted 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 (a), and developed the corresponding LED packaging technology. The technique first prepares a high-power LED chip suitable for eutectic soldering and a corresponding ceramic substrate, and then directly solders the LED chip 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 package is provided. Proposed a solution. The high thermal conductivity copper-clad ceramic plate developed by Curmilk Company of Germany is made of ceramic substrate (AlN or Al2O3) and conductive layer (Cu) sintered under 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 2 (b). Among them, aluminum nitride (AlN) has a thermal conductivity of 160 W/mk and a coefficient of thermal expansion of 4.0×10 −6 /° C. (corresponding to a thermal expansion coefficient of silicon of 3.2×10 −6 /° C.), thereby reducing thermal stress of the package.

Studies have shown that the package interface has a great influence on the thermal resistance. If the interface cannot be processed correctly, it is difficult to obtain a good heat dissipation effect. For example, a well-contacted interface at room temperature may have interfacial gaps at high temperatures, and warpage of the substrate may also affect bonding and local heat dissipation. The key to improving LED packaging is to reduce interface and interface contact thermal resistance and enhance heat dissipation. Therefore, the choice of thermal interface material (TIM) between the chip and the heat sink substrate is important. The TIM commonly used in LED packaging is conductive adhesive and thermal conductive adhesive. Due to the low thermal conductivity, it is generally 0.5-2.5 W/mK, resulting in high interface thermal resistance. The use of low temperature or eutectic solder, solder paste or conductive paste with nano-particles as the thermal interface material can greatly reduce the interface thermal resistance.