LED (light-emitting diode) has become a hot spot in the international emerging strategic industries. In the LED industry chain, the upstream includes substrate materials, epitaxy, chip design and manufacturing. The middle reaches include packaging technology, equipment and testing technology, and downstream applications include LED display, lighting and lighting. The main use of blue LED + yellow phosphor technology to achieve high-power white LED, that is part of the GaN-based blue LED blue excitation YAG (yttrium aluminum garnet) yellow phosphor emitting yellow light, the other part of the blue light emitted through the phosphor by The yellow phosphor emitted by the yellow phosphor is mixed with the transmitted blue light to give white light. The blue light emitted by the blue LED chip is transmitted through the yellow phosphor coated around it. The phosphor is partially excited by the blue light to emit yellow light, and the blue light and yellow light are overlapped to form white light.
High-power LED packaging as an important link in the industry chain, is to promote semiconductor lighting and display to practical core manufacturing technology. Only through the development of low thermal resistance, high luminous efficiency and high reliability LED packaging and manufacturing technology, the LED chip for good mechanical and electrical protection, reduce mechanical, electrical, thermal, humidity and other external factors on the chip performance, protect the LED Chip stable and reliable work in order to provide efficient and sustained high-performance lighting and display effects, to achieve LED-specific advantages of energy-saving longevity and promote the sound development of the semiconductor lighting and display industry chain. Due to the consideration of market interests by foreign related companies, the relevant core technologies and equipment are taken to block measures. Therefore, the development of independent high-power LED packaging technology, especially white LED packaging equipment, is imminent. This article will briefly introduce the status of research and application of high-power LED packaging field, analysis and summarizes the key technical issues in the process of high-power LED packaging with a view to arouse the attention of domestic counterparts, in order to achieve the autonomy of high-power LED key technologies and equipment efforts .
Packaging process technology plays a crucial role in LED performance. LED packaging methods, materials, structure and process of choice mainly by the chip structure, optoelectronic / mechanical properties, the specific application and cost and other factors. With the increase of power, especially the demand for the development of solid-state lighting technology, new and higher requirements are put forward on the optical, thermal, electrical and mechanical structures of the LED package. In order to effectively reduce the package thermal resistance, improve the light efficiency, we must adopt a new technical ideas for package design. From process compatibility and reduce production costs point of view, LED package design should be carried out simultaneously with the chip design, the chip design should take into account the package structure and process. At present, the main development trends of power LED package structure are as follows: miniaturization of dimensions, minimization of thermal resistance of devices, planarization of patches, maximization of withstand junction temperature and maximization of single lamp flux; the goal is to improve luminous flux, luminous efficiency and reduce light Failure, failure rate, improved consistency and reliability. Specifically, the key technologies of high-power LED package include: thermal dispersion technology, optical design technology, structural design technology, phosphor coating technology, eutectic solder technology.
1, cooling technology
General LED node temperature can not exceed 120 ℃, even the latest devices such as Lumileds, Nichia, CREE and other devices, the maximum node temperature can not exceed 1500 ℃. Therefore, the heat radiation effects of LED devices can be negligible, the heat conduction and convection is the main method of LED cooling. Thermal design considerations in the first thermal design, because the heat is first conducted from the LED package module to the radiator. Therefore, the bonding material, the substrate LED cooling technology is the key link.
Bonding materials include thermal plastic, conductive silver paste and alloy solder in three main ways. Thermal plastic is added within the matrix of some high thermal conductivity of the filler, such as SiC, A1N, A12O3, SiO2, etc., thereby enhancing its thermal conductivity; conductive silver paste is added to the silver in the epoxy resin to form a composite material, The temperature is generally lower than 200 ℃, has good thermal conductivity, bonding performance and other advantages, but the silver absorption of light is relatively large, resulting in decreased luminous efficiency.
Substrates mainly include ceramic substrates, ceramic substrates and composite substrates in three main ways. Ceramic substrates are mainly LTCC substrate and AIN substrate. LTCC substrate is easy to shape, process is simple, low cost and easy to make a variety of shapes and many other advantages; Al and Cu are LED package substrate excellent material, due to the conductivity of metal materials, in order to make the surface insulation, often through Anodized to form a thin insulating layer on the surface. Metal-based composites are mainly Cu-based composites, Al-based composites. Occhionero, who explored the AlSiC in flip chip, optoelectronic devices, power devices and high-power LED cooling substrate on the application of pyrolytic graphite in AlSiC can also meet the cooling requirements of higher operating conditions. The future of the composite substrate there are five main types: monolithic circuit carbonaceous materials, metal-based composites, polymer-based composites, carbon composites and advanced metal alloys.
In addition, the impact of the package interface on the thermal resistance is also great, LED package to improve the key is to reduce interface and interface contact resistance, and enhance heat dissipation. Therefore, the choice of thermal interface material between the chip and the heat sink substrate is important. The use of low temperature or eutectic solder, solder paste or conductive paste with nano-particles as a thermal interface material can greatly reduce the thermal interface resistance.
2, optical design technology
The optical design of the LED package includes the inner optical design and the outer optical design.
The key to the optical design is the choice and application of potting. In the choice of potting, the requirements of its high transmittance, high refractive index, thermal stability, good liquidity, easy to spray. In order to improve the reliability of LED package, but also requires a low moisture absorption potting, low stress, temperature and environmental protection and other characteristics. Currently used potting adhesives include epoxy resin and silicone. Among them, the silica gel with high light transmittance (visible light transmittance greater than 99%), high refractive index (1.4 ~ 1.5), good thermal stability (can withstand 200 ℃ high temperature), low stress Low), low hygroscopicity (less than 0.2%) and other characteristics, significantly better than the epoxy resin, widely used in high-power LED package. But the performance of silica gel is greatly affected by the ambient temperature, thus affecting the LED luminous efficiency and light intensity distribution, so the preparation of silica gel to be improved.
Outer optical design refers to the convergence of the outgoing beam, shaping, in order to form a light field uniform distribution of light. Including the reflective condenser cup design (primary optical) and plastic lens design (secondary optics), the array module, also includes the distribution of the chip array. Lens shape commonly used convex lens, concave lens, spherical mirror, Fresnel lens, modular lens, lens and high-power LED assembly methods can be used hermetic packaging and semi-airtight package. In recent years, with the deepening of research, taking into account the integration requirements after encapsulation, a microlens array is adopted for the lens used for beam shaping. The microlens array can perform two-dimensional parallel convergence, shaping and collimation in the optical path, The invention has the advantages of high arrangement precision, convenient and reliable fabrication and easy coupling with other planar devices. Studies have shown that the use of a diffractive microlens array instead of an ordinary lens or a Fresnel microlens can greatly improve the beam quality and increase the intensity of outgoing light, LED is the most promising new technology for beam shaping.
3, LED package structure
LED packaging technology and structure has a pin, power package, SMD (SMD), chip-on-board (COB) four stages.
(1) Pin LED package
LED pin package lead frame for a variety of package appearance of the pin, is the first to successfully launch the market structure of the package, a large number of varieties, high technology maturity, package structure and reflective layer is still continuous improvement. Common 3 ~ 5mm package structure, generally used for current smaller (20 ~ 30mA), low power (less than 0.1W) LED package. Mainly used for instrumentation or instructions, large-scale integration can also be used as a display. The disadvantage is the larger package thermal resistance (generally higher than 100K / W), shorter life expectancy.
(2) Power LED package
LED chip and package to the direction of high-power development, in large current than Φ5mmLED produce 10 to 20 times the luminous flux must be effective heat dissipation and non-degradable packaging materials to solve the problem of light failure, the case and the package is also the key Technology, LED packages that can withstand several watts of power have emerged. 5W series of white, green, blue and green power LED from the beginning of 2003, white LED light output up to 1871m, 44.31 lm / W luminous efficiency green failure, developed to withstand 10W power LED, large area Tube; the size of 2.5mm X2.5mm, 5A current work, the light output of 2001 lm, as a solid light source has great room for development.
(3) surface mount (SMD) type (SMD) LED package
As early as 2002, SMD LED (SMDLED) gradually accepted by the market and gain a certain market share from pin-type package to SMD in line with the development trend of the entire electronics industry, many manufacturers introduced such products.
SMDLED is currently the LED market, the highest occupancy package structure, this LED package structure using injection molding process of metal lead frame wrapped in PPA plastic and the formation of a specific shape of the reflector cup, the metal lead frame extends from the reflector bottom to the device side , By outward flat or bent inward to form the device pins. The improved SMDLED structure is accompanied by the emergence of white LED lighting technology. In order to increase the power usage of a single LED device to increase the brightness of the device, engineers are looking for ways to reduce the thermal resistance of the SMDLED and introduce the concept of a heat sink. This improved structure reduces the height of the original SMDLED structure, the metal lead frame is placed directly on the bottom of the LED device, a reflective cup is formed around the metal frame by injecting plastic, the chip is placed over the metal frame, the metal frame is soldered directly Circuit board, the formation of vertical cooling channels. Due to the development of material technology, SMD packaging technology has overcome the early problems of heat dissipation, service life and the like, and can be used for packaging high-power white LED chips of 1 to 3W.
(4) COB-LED package
COB package can be packaged in multiple chips directly on the metal-based printed circuit board MCPCB, direct cooling through the substrate, not only can reduce the manufacturing process of the stent and its cost, but also has the thermal resistance to reduce thermal advantages. The PCB can be a low-cost FR-4 material (fiberglass-reinforced epoxy) or a highly thermally conductive metal-based or ceramic matrix composite such as an aluminum or copper-clad ceramic substrate. The wire bonding can be used under high temperature thermo-ultrasonic bonding (gold ball welding) and ultrasonic bonding at room temperature (aluminum chopper welding). COB technology is mainly used for high power multi-chip LED array package, compared with the SMD, not only greatly improve the package power density, but also reduces the package thermal resistance (typically 6-12W / m · K).
From the cost and application point of view, COB will become the mainstream of the future of lighting design. COB LED module package installed in the backplane multiple LED chips, the use of multiple chips not only can improve the brightness, but also help to achieve a reasonable LED chip configuration, reducing the input current of a single LED chip to ensure high efficiency. And such a surface light source can greatly expand the package's heat dissipation area, so that heat is more easily transmitted to the shell. The traditional LED lighting practices are: LED light source discrete devices - MCPCB light source module - LED lamps, mainly based on the core components of the light source is not applicable to the practice, not only time-consuming and costly. In fact, if the "COB light source module - LED lamps" route, not only can save time and labor, but also can save the cost of device packaging.
In short, whether it is single-device package or modular COB package, from low-power to high-power, LED package structure is designed around how to reduce device thermal resistance, improve the light effect and improve reliability.
4, phosphor coating technology
Light conversion structure, namely phosphor coating structure, mainly for LED white lighting technology, the purpose is to LED light emitted by the shorter wavelength of light into complementary (complementary to form white light) of the wavelength of light.
Currently there are three ways to use white light to produce white light: blue LED with yellow phosphor; blue LED with red, green phosphor; UV-LED with red, green and blue phosphor. Among them, the white LED commercialized mostly belongs to the single chip type of blue LED combined with the yellow phosphor. The blue LED produced with the red and green phosphors is only reported in the patents of OSRAM, Lumileds and other companies but has not yet been commercialized Products appear, and UV-LED with trichromatic phosphor is currently under development. The advantages and disadvantages of different phosphors to produce white LEDs are shown in the following table.
The existing coating methods, as shown below, have their own advantages and disadvantages. Currently widely used in phosphor coating is the phosphor and potting compound, and then directly coated on the chip. Due to the difficulty of precisely controlling the thickness and shape of the phosphor coating, the color of the emitted light is inconsistent and the blue light or the yellowish light appears. The research by GE company Arik et al shows that directly covering the chip with the phosphor can cause the temperature of the phosphor to increase, thereby reducing the quantum efficiency of the phosphor and seriously affecting the conversion efficiency of the package.
The conformal coating technology based on the spray process can achieve uniform phosphor coating, thus ensuring the uniformity of light color. However, this technique is very difficult, and a large part of the blue light emitted by the LED is directly reflected by the phosphor layer back to the chip, which is directly absorbed by the chip, which seriously affects the light output efficiency. Yamada, Narendran, etc. found that the backscattering properties of the phosphors cause 50% to 60% of backscattered forward incident light.
In addition, a coating method is to make the phosphor layer far away from the LED chip (for example, the phosphor layer is located on the reflector cup or the astigmatism cup outside the LED chip), the amount of light absorbed by the phosphor layer and reflected back to the chip can be greatly reduced , Thus improving the light efficiency. In addition, since the phosphor layer has no direct contact with the chip, heat generated by the chip is not transmitted to the phosphor layer, thereby extending the service life of the phosphor layer. A study by Schubert et al. At the Leinster Institute found that the use of a phosphor coating process to reduce the chances of back-emitting light being absorbed by the chip increases the LED's luminous efficiency by 7% to 16%. Zhongshan University, who also launched a study Wang Gang et al. The results show that the use of far away from the phosphor coating can reduce the phosphor coating temperature of about 16.8 ℃, significantly improve the conversion efficiency of the phosphor. But far from the coating method also has its drawbacks, because out of its use of more phosphor, phosphor version