Abstract: With the continuous advancement of manufacturing technology, the increasing use of LEDs in the field of illumination is an inevitable trend. This paper reviews the LED packaging technology for lighting, mainly expounds the key technologies such as heat dissipation, white light, testing, screening, and electrostatic protection, and puts forward suggestions for accelerating the development of LED technology for lighting.
Keywords: light-emitting diode; illumination; luminous flux; package
CLC number: TN312 + 18 Document code: A Article ID: 1002 - 6339 (2005) 05 - 0430 - 02
Encapsulation Technical Keys of illumination LED
ZHANGDong - chun ,SUN Qiu - yan ,ZHENGJi - yu
(Light Industry College of Hebei Polytechnic University, Tangshan 063020, China)
Abstract : It is a inevitable trend that LED is more and more used in the illumination field as the progress of manganese technology. Encapsulation technology of illumination LED is summarized in the paper , including key technology , such as emanating heat , white light , test , filter and Static prevention , etc. The advice thattechnology of illumination LED should bedevelopedisputforwardinthepaper.
Keywords :LED ;illumination ;luminousflux ;encapsulation
Light-emitting diodes (LEDs), developed for the market in the 1960s and gradually becoming market-oriented, are constantly improving and developing their packaging technologies. From the earliest development of glass tube packages to stent-type epoxy and surface mount packages, low-power LEDs are used in a wide range of applications. Since the 1990s, due to breakthroughs in LED epitaxy and chip technology, quaternary Al2GaInP and GaN-based LEDs have been introduced, achieving full-color LED illumination, greatly improved luminance, and combining various colors and white light. 〔1〕. The input power of the device is greatly improved. At present, single-chip 1W high-power LED has been industrialized and brought to market, which makes the application of ultra-high-brightness LEDs continue to expand, and the market for special lighting is gradually moving toward the general lighting market. Due to the continuous improvement of the input power of LED chips, higher requirements are placed on its packaging technology. In order to develop LEDs, it is important to increase the luminous efficiency and luminous flux to the level of existing lighting sources. The epitaxial materials used in high-power LEDs use MOCVD epitaxial growth technology and multiple quantum well structures. Although the external quantum efficiency needs to be further improved, the biggest obstacle to obtaining high luminous flux is the low light extraction efficiency of the chip. The existing high-power LED design adopts a new flip-chip structure to improve the light extraction efficiency of the chip, improve the thermal characteristics of the chip, and increase the photoelectric conversion efficiency of the device by increasing the chip area and increasing the operating current. Higher luminous flux. In addition to the chip, the packaging technology of the device is also important. The key packaging technology processes are:
1 Heat Dissipation Technology The traditional indicator type LED package structure generally uses conductive or non-conductive glue to mount the chip in a small-sized reflector cup or on a carrier table. The gold wire is used to complete the internal and external connection of the device and then encapsulated with epoxy resin. It has a thermal resistance of up to 250~300 °C/W. If the new high-power chip adopts the traditional LED package form, the junction temperature will rise rapidly and the epoxy carbonization will turn yellow due to poor heat dissipation. accelerated light fades until failure, because of the rapid and even heat stress generated by the expansion caused by an open circuit failure. Therefore, for high-power LED chips with large operating current, a new package structure with low thermal resistance, good heat dissipation and low stress is the key technology. The material is bonded by a material with low resistivity and high thermal conductivity; a copper or aluminum heat sink is added to the lower part of the chip, and a semi-encapsulated structure is used to accelerate heat dissipation; and even a secondary heat sink is designed to reduce the thermal resistance of the device. Inside the device, a flexible silicone rubber filled with high transparency is used in the temperature range of the silicone rubber (generally -40 to 200 °C). The colloid does not open due to sudden changes in temperature, and the yellowing phenomenon does not occur. . Part materials should also take into account their thermal and thermal properties to achieve good overall thermal properties.
2 High-power LED white light technology Commonly used to achieve white light, there are three methods [2]: the blue chip is coated with YAG phosphor, and the blue light-excited phosphor of the chip emits yellow-green light with a typical value of 500-560 nm, yellow-green light. Synthetic white light with blue light. The method is relatively simple to prepare, high in efficiency and practical. The disadvantage is that the consistency of the cloth is poor, the phosphor is easy to precipitate, the uniformity of the light surface is poor, the color tone is not uniform, the color temperature is high, and the color rendering is not ideal. RGB three primary colors multiple chips or multiple devices illuminate white light; or use blue + yellow-green two-chip complementary color to produce white light. As long as the heat is dissipated, the white light produced by the method is more stable than the former method, but the driving is complicated, and the different light decay speeds of different color chips are also considered. The RGB phosphor is coated on the ultraviolet light chip, and the phosphor is excited by the violet light to generate a three-color light mixed color to form white light. However, the current ultraviolet light chip and RGB phosphor are less efficient, and the epoxy resin is easily decomposed and aged under ultraviolet light. Accumulated a certain amount of experience and experience, we believe that the use of W-class power LED products for lighting to achieve industrialization must also solve the following technical problems: 1 powder coating Volume control: LED chip + phosphor process used in the coating method is usually fluorescent The powder is mixed with the glue and applied to the chip with a dispenser. During the operation process, because the viscosity of the carrier glue is the dynamic parameter, the specific gravity of the phosphor is larger than that of the carrier gel, and the precision of the dispenser and the accuracy of the dispenser, the control of the uniformity of the coating amount of the phosphor is difficult, resulting in white light. Uneven color. 2 chip photoelectric parameter matching: the characteristics of the semiconductor process, it is determined that the same material may have optical parameters (such as wavelength, light intensity) and electrical (such as forward voltage) parameter differences between the same wafer chip. This is especially true for RGB tricolor chips, which have a large impact on white chromaticity parameters. This is one of the key technologies that must be solved in industrialization. 3 According to the application requirements, the color chromaticity parameter control: for different purposes, the requirements for color coordinates, color temperature, color rendering, optical power (or light intensity) and spatial distribution of light of white LEDs are different. It relates to a product with many factors control structure, process methods, the material of the parameters. In industrial production, it is important to control the above factors to obtain products that meet the application requirements and have good consistency.
3 Testing Technology and Standards With the development of W-class power chip manufacturing technology and white LED technology, LED products are gradually entering the (special) lighting market. The traditional LED product parameter testing standards and test methods used for display or indication can no longer meet the lighting requirements. The needs of the application. Semiconductor equipment manufacturers at home and abroad have also launched their own test instruments. There are certain differences in the test principles, conditions and standards used by different instruments, which increases the difficulty and complexity of the test application and product performance comparison work. China Optical Optoelectronics Industry Association Optoelectronics Sub-Committee Industry Association released the "LED Test Method (Trial)" in 2003 according to the needs of LED product development. This test method has increased the LED color parameter. However, LEDs need to expand into the lighting industry. Establishing LED lighting product standards is an important means of industrial standardization.
4 Screening technology and reliability guarantee Due to the limitation of the appearance of the luminaire, the assembly space of the LED for lighting is sealed and limited. The sealed and limited space is not conducive to the heat dissipation of the LED, which means that the lighting LED environment is inferior to the traditional display and indication. LED products. In addition, the lighting LEDs operate under high current drive, which puts higher reliability requirements on them. In industrial production, it is necessary to formulate appropriate thermal aging, temperature cycle impact, load aging process screening tests for different product uses, and to eliminate early failure products to ensure product reliability.
5 Electrostatic protection technology The blue chip of sapphire substrate has positive and negative electrodes on the chip with small spacing. For InGaN/AlGaN/GaN double heterojunction, InGaN active layer is only tens of nm, and it is resistant to static electricity. Small, extremely easy to be broken down by static electricity, causing the device to fail. Therefore, in industrial production, the prevention of static electricity is appropriate, directly affecting the product yield and economic benefits. There are several kinds of anti-static technology: 1 from the human body, workbench, ground, space and product transmission, stacking, etc., anti-static clothing, gloves, bracelets, shoes, pads, boxes, ion fans, testing equipment Wait. 2 Electrostatic protection circuit is designed on the chip. A protective device is mounted on the 3LED. China's LED packaging products are mainly ordinary low-power LEDs, and also have certain high-power LED packaging technology and level. However, due to a variety of reasons, China's high-power LED packaging technology level is still quite different from the international level [3].
In order to speed up the development of LED packaging technology, we suggest that: 1 The country should focus on supporting the pre-extension of LEDs, the key research units (universities) and enterprises with strong chips, focusing on the advantages, focusing on key technical difficulties, and developing and producing independent property rights as soon as possible. The high-power LED chips such as 1 W, 3 W, 5 W and 10 W can only ensure the smooth development of high-power LEDs in China. 2 The country should focus on supporting powerful high-power LED packaging companies, research and development of LED packaging products with independent property rights, and to achieve scale production level and participate in international market competition. 3 Pay attention to the research and development and industrialization of basic materials such as phosphors and epoxy. 4 According to market requirements, develop various high-power LED products that adapt to the market, first aim at the market of special lighting applications, and gradually move toward the general lighting source market.
References [1] Semiconductor lighting industry chain is beginning to appear in Shenzhen [J]. Lighting, 2004 (2).
[2] High-brightness LEDs illuminate the future of mankind [J]. Urban Lighting Business, 2004 (1, 2).
[3] Understanding Lighting LED [J]. China Lighting Appliances, 2004 (2) . [1] [J]
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