I. Introduction
High-power LED package due to the complexity of the structure and process, and directly affect the use of LED performance and life, has been a hot topic in recent years, especially high-power white LED package is hotspot in the hot spots. LED packaging functions include:
1. Mechanical protection to improve reliability;
2. Strengthen the heat to reduce the chip junction temperature, improve LED performance;
3. Optical control, improve the efficiency of light, optimize the beam distribution;
4. Power supply management, Including AC / DC conversion, and power control.
LED packaging methods, materials, structure and process selection mainly by the chip structure, photoelectric / mechanical properties, specific application and cost factors such as the decision. After 40 years of development, LED package has experienced a bracket (Lamp LED), SMD (SMD LED), power LED (Power LED) and other stages of development. With the increase of chip power, especially the development of solid-state lighting technology, LED packaging optical, thermal, electrical and mechanical structure put forward new and higher requirements. In order to effectively reduce the package thermal resistance, improve the efficiency of light, you must use a new technical ideas for packaging design.
Second, the key technology of high power LED packaging
High-power LED package mainly involves light, heat, electricity, structure and technology, as shown in Figure 1. These factors are independent of each other and interact with each other. Among them, the purpose of LED packaging alone, the heat is the key, electricity, structure and technology is a means, and performance is the specific embodiment of the level of packaging. From the process of compatibility and reduce production costs, LED packaging design and chip design should be carried out at the same time, that is, the chip design should take into account the packaging structure and process. Otherwise, after the completion of the chip manufacturing, may be due to the need for packaging chip structure to adjust, thus extending the product development cycle and process costs, and sometimes even impossible
In particular, the key technologies for high-power LED packaging include:
(A) low thermal resistance packaging process
For the existing LED light level, because the input power of about 80% into heat, and LED chip area is small, so the chip cooling LED packaging must solve the key issues. Including the chip layout, packaging materials selection (substrate materials, thermal interface materials) and technology, heat sink design.
LED package thermal resistance mainly includes material (heat sink and heat sink structure) internal thermal resistance and interface thermal resistance. The role of the heat dissipation substrate is to absorb the heat generated by the chip, and conduction to the heat sink, to achieve heat exchange with the outside world. Commonly used heat dissipation substrate materials include silicon, metals (such as aluminum, copper), ceramics (such as Al2O3, AlN, SiC) and composite materials. Such as Nichia's third-generation LED using CuW substrate, 1mm chip flip on the CuW substrate, reducing the package thermal resistance, improve the luminous power and efficiency; Lamina Ceramics company has developed a low temperature co-fired ceramic metal substrate , As shown in Figure 2 (a), and developed the corresponding LED packaging technology. The technology is first prepared for eutectic welding of high-power LED chip and the corresponding ceramic substrate, and then the LED chip and the substrate directly welded together. Because the substrate has integrated eutectic layer, electrostatic protection circuit, driving circuit and control compensation circuit, not only the structure is simple, but also because of the high thermal conductivity of the material, less thermal interface, greatly improving the thermal performance, high power LED array package Proposed a solution. Germany's Curmilk company developed a high thermal conductivity of copper-clad ceramic plate, ceramic substrate (AlN or Al2O3) and conductive layer (Cu) at high temperature and pressure sintering, without the use of adhesives, so good thermal conductivity, high strength, insulation Strong as shown in Figure 2 (b). Wherein the thermal conductivity of aluminum nitride (AlN) is 160 W / mk and the coefficient of thermal expansion is 4.0 x 10-6 / C (equivalent to the thermal expansion coefficient of silicon 3.2 x 10-6 / C), thereby reducing the thermal stress of the package.
Research shows that the package interface on the thermal resistance is also great, if you can not properly handle the interface, it is difficult to get a good cooling effect. For example, a good interface at room temperature may have an interfacial gap at high temperatures, and warping of the substrate may also affect bonding and local heat dissipation. The key to improving LED packaging is to reduce the interface and interface contact thermal resistance, enhance heat dissipation. Therefore, the thermal interface material (TIM) between the chip and the heat sink substrate is very important. LED package commonly used TIM conductive adhesive and thermal plastic, because the thermal conductivity is low, generally 0.5-2.5W / mK, resulting in high thermal resistance at the interface. The use of low temperature or eutectic solder, solder paste or within the nano-particles of conductive adhesive as a thermal interface material, can greatly reduce the interface thermal resistance.
(D) packaging large production technology
Wafer bonding (Wafer bonding) technology refers to the chip structure and circuit production, packaging are in the wafer (Wafer) on the package is completed and then cut to form a single chip (Chip); with the corresponding chip bonding (Die bonding) refers to the chip structure and circuit after the completion of the chip, that is, cutting the formation of the chip (Die), and then a single chip package (similar to the current LED packaging process), as shown in Figure 6. It is clear that the wafer bonding package is more efficient and of higher quality. Since packaging costs account for a large proportion of LED device manufacturing costs, changing the existing LED package form (from chip bonding to wafer bonding) will significantly reduce package manufacturing costs. In addition, the wafer bonding package can also improve the cleanliness of LED device production, to prevent the bonding before the scribing, fragmentation process on the device structure damage, improve packaging yield and reliability, and thus is a cost reduction package effective means.
In addition, for high-power LED packages, package-less packaging must be used in the chip design and packaging design as much as possible, while simplifying the package structure, minimizing the number of thermal and optical interfaces to reduce Package thermal resistance, improve the efficiency of light.
(5) package reliability testing and evaluation
The failure mode of LED devices mainly includes electrical failure (such as short circuit or open circuit), light failure (such as high temperature caused by potting paste yellowing, optical performance degradation, etc.) and mechanical failure (such as lead fracture, desoldering, etc.), and these factors Are related to the package structure and process. The life of the LED is defined by the MTTF. For lighting applications, the output luminous flux of the LED is generally reduced to the initial 70% (50% of the initial value for the display). As the LED long life, usually take the accelerated environmental test method for reliability testing and evaluation. The main contents of the test include high temperature storage (100 ℃, 1000h), low temperature storage (-55 ℃, 1000h), high temperature and high humidity (85 ℃ / 85%, 1000h), high and low temperature cycle (85 ℃ ~ -55 ℃) , Corrosion resistance, resistance to solubility, mechanical shock and so on. However, accelerating environmental testing is only one aspect of the problem, and the research on the prediction mechanism and method of LED life is still a difficult problem to be studied.
Third, solid-state lighting on high-power LED package requirements
Compared with the traditional lighting, LED lamps do not need to use filters or filters to produce colored light, not only high efficiency, light color pure, and can achieve dynamic or gradient color changes. While changing the color temperature while maintaining a high color rendering index to meet the needs of different applications. But its packaging also put forward new requirements, embodied in:
(A) modular
Through the interconnection of multiple LED lights (or modules) can achieve a good lumen output stack, to meet the requirements of high brightness lighting. Through the modular technology, can be a number of point light source or LED module in accordance with the arbitrary shape of the combination to meet the lighting requirements of different areas.
(B) system efficiency maximization
In order to improve the efficiency of LED lamps, in addition to the need for appropriate LED power, but also must use efficient cooling structure and technology, and optimize the internal / external optical design to improve the efficiency of the entire system.
(C) low cost
LED lamps to the market, must have a competitive advantage in cost (mainly refers to the initial installation costs), and packaging in the entire LED lighting production costs accounted for a large part, therefore, the use of new packaging structure and technology to improve light efficiency / cost Than to achieve the key to the commercialization of LED lamps.
(D) easy to replace and maintain
As the LED light source long life, low maintenance costs, so the reliability of LED lighting package put forward higher requirements. Requiring LED lighting design easy to improve to meet the future more efficient LED chip packaging requirements, and require LED chip interchangeability is better, so that the lighting manufacturers choose their own chips.
LED light source can be composed of a number of distributed point light source, the chip size is small, so that the packaging of the light weight, compact structure, and to meet the needs of various shapes and different integration. The only downside is that there is no ready-made design standard, but at the same time gives the design a full imagination. In addition, the primary goal of LED lighting control is power supply. Since the general utility power supply is high voltage alternating current (220V, AC), and LED needs constant current or current limiting power supply, it must use conversion circuit or embedded control circuit (ASICs) to achieve advanced calibration and closed-loop feedback control system. In addition, through the use of digital lighting control technology, the use and control of solid-state light source mainly rely on intelligent control and management software to achieve, so that users, information and light source to establish a new relationship between, and can give full play to designers and consumers force.
Fourth, concluding remarks
LED encapsulation is a research topic involving a multidisciplinary (such as optics, thermal, mechanical, electrical, mechanical, materials, semiconductors, etc.). From a certain point of view, LED packaging is not only a manufacturing technology (Technology), but also a basic science (Science), a good package requires heat, optics, materials and process mechanics and other physical understanding and application. LED package design should be carried out at the same time with the chip design, and the need for light, heat, electricity, structure and other performance considerations. In the packaging process, although the material (thermal substrate, phosphor, potting) selection is very important, but the package structure (such as thermal interface, optical interface) on the LED light efficiency and reliability is also great, high-power white LED package Must use new materials, new technology, new ideas. For LED lamps, it is necessary to light, heat, power and lighting and other integrated considerations.