LED (light-emitting diode) has become the international emerging strategic industry competition hot spots. In the LED industry chain, the upstream includes substrate materials, epitaxial, chip design and manufacturing, midstream covers packaging technology, equipment and testing technology, downstream LED display, lighting and lighting applications. At present, the main use of blue LED + yellow phosphor technology to achieve high-power white LED, that is, through the GaN-based blue LED part of the blue light to stimulate YAG (yttrium aluminum garnet) yellow phosphor emitting yellow, and the other part of the blue light emitted through the phosphor, The yellow phosphor emits yellow light and blends the blue light to get white light. Blu-ray LED chip issued by the blue light through the coating around the yellow phosphor, the phosphor is part of the blue light after the issue of yellow light, blue light spectrum and yellow light spectrum overlap after the formation of white light.
High-power LED packaging as an important link in the industrial chain, is to promote the semiconductor lighting and display to the practical core manufacturing technology. Only through the development of low thermal resistance, high efficiency and high reliability of LED packaging and manufacturing technology, the LED chip for good mechanical and electrical protection, reduce the mechanical, electrical, thermal, wet and other external factors on the chip performance, Chip stable and reliable work in order to provide efficient and sustained high-performance lighting and display, LED-specific energy-saving advantages, and promote the entire semiconductor lighting and display industry chain benign development. In view of the foreign companies concerned for the interests of the market considerations, the relevant core technologies and equipment are taken blockade measures, and thus the development of independent high-power LED packaging technology, especially white LED packaging equipment is imminent. This paper will briefly introduce the research and application of high-power LED packaging field, analyze and summarize the key technical problems in the process of high-power LED packaging, in order to attract the attention of domestic counterparts, in order to achieve high-power LED key technology and equipment autonomy The
Packaging technology plays an important role in LED performance. 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. With the increase of power, especially the development of solid-state lighting technology needs, 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 to carry out packaging design. From the process compatibility and reduce the cost of production point of view, LED package design should be carried out at the same time with the chip design, that is, the chip design should take into account the packaging structure and process. Current power LED package structure of the main trends are: small size, device thermal resistance minimization, flat patch, resistant to the highest junction temperature, single light flux maximization; goal is to improve the luminous flux, light efficiency, reduce light Failure, failure rate, increased consistency and reliability. Specifically, the key technologies of high-power LED packaging include: thermal dispersion technology, optical design technology, structural design technology, phosphor coating technology, eutectic welding technology.
1, Cooling technology
The general node temperature can not exceed 120 ℃, even LumiLEDs, Nichia, CREE, etc. introduced the latest device, the maximum node temperature still can not exceed 1500 ℃. So the thermal radiation effect of LED devices can be negligible, heat conduction and convection is the main way of LED heat dissipation. In the thermal design from the heat conduction aspects, because the heat from the LED package module first conduction to the radiator. So the bonding material, the substrate is the key to LED cooling technology.
Bonding materials mainly include thermal plastic, conductive silver paste and alloy solder three main ways. The conductive paste is a kind of composite material formed by adding silver powder into the epoxy resin, and the adhesive is hardened by adding some high thermal conductivity filler, such as SiC, A1N, A12O3, SiO2, etc., to improve its thermal conductivity. The temperature is generally lower than 200 ℃, has good thermal conductivity, bonding performance and reliable, but the silver absorption of light is relatively large, resulting in decreased light efficiency.
The substrate mainly includes ceramic substrate, ceramic substrate and composite substrate three main ways. Ceramic substrates are mainly LTCC substrates and AIN substrates. LTCC substrate has easy to shape, simple process, low cost and easy to make a variety of shapes and many other advantages; Al and Cu are LED packaging substrate excellent material, due to the conductivity of metal materials, in order to make its surface insulation, often through Anodized to form a thin insulating layer on its surface. Metal-based composite materials are mainly Cu-based composite materials, Al-based composite materials. Occhionero et al. Explored the use of AlSiC in flip chip, optoelectronic devices, power devices and high-power LED thermal substrates, and the addition of pyrolytic graphite to AlSiC also meets the requirements for higher heat dissipation. The future of the composite substrate is mainly five kinds: monolithic circuit carbonaceous materials, metal matrix composites, polymer matrix composites, carbon composites and advanced metal alloys.
In addition, the package interface on the thermal resistance is also great, the key to improve the LED package is to reduce the interface and interface contact thermal resistance, enhance heat dissipation. Therefore, the choice of thermal interface material between the chip and the heat dissipation substrate is very important. 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.
2, Optical design technology
The optical design of the LED package includes both optical design and external optical design.
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The key to the optical design is the choice and application of potting. In the choice of potting, requiring its high transmittance, high refractive index, good thermal stability, good fluidity, easy to spray. In order to improve the reliability of LED packaging, but also requires potting with low moisture absorption, low stress, temperature and environmental protection and other characteristics. Commonly used potting agents include epoxy and silicone. Among them, the silica gel has a 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 (Young's modulus Low), low moisture absorption (less than 0.2%) and other characteristics, significantly better than epoxy resin, in high-power LED packaging has been widely used. But the silica gel performance by the ambient temperature greater impact, thus affecting the LED light efficiency and light intensity distribution, so the silica gel preparation process to be improved.
External optical design refers to the convergence of light beam, shaping, to form a uniform distribution of light intensity of the light field. Mainly including reflective condenser cup design (primary optical) and plastic lens design (secondary optics), the array module, also includes the chip array distribution. The lens is commonly used in the shape of convex lens, concave lens, spherical, Fresnel lens, modular lens, lens and high power LED assembly method can be used in airtight and semi-airtight package. In recent years, with the deepening of research, taking into account the integration requirements after packaging, for the beam shaping lens using a microlens array, microlens array in the optical path can play a two-dimensional parallel convergence, shaping, collimation and so on, It has the advantages of high arrangement accuracy, easy manufacture and easy coupling with other planar devices. The research shows that the use of diffractive microlens array instead of ordinary lens or Fresnel microlens can greatly improve the beam quality and improve 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 lead-type, power-based packaging, SMD (SMD), on-board chip directly loaded (COB) four stages.
4, Phosphor coating technology
Light conversion structure, that is, phosphor coating structure, mainly for LED white lighting technology, the purpose is to LED chip issued by the shorter wavelength of light into complementary (color complementary white light) long wavelength of light.
At present, there are three ways to produce white light with phosphor: blue LED with yellow phosphor; blue LED with red, green phosphor; UV-LED with red, green and blue phosphor. One of the commercial white LED is mostly blue LED with yellow phosphor single-chip type, blue LED with red, green phosphor white way only in Osram, Lumileds and other companies reported on the patent, but still not commercialized Products appear, and UV-LED with three-color phosphor is still in the development of the way. The advantages and disadvantages of different phosphors to produce white LED are shown in the following table.
Existing coating methods, as shown below, have their advantages and disadvantages. Currently widely used in the phosphor coating method is to mix the phosphor and potting, and then directly on the chip. Since it is difficult to precisely control the thickness and shape of the coating of the phosphor, the color of the outgoing light does not coincide with the occurrence of partial blue or yellowing. GE's research by Arik et al. Shows that the phosphor is directly covered on the chip, resulting in a rise in phosphor temperature, which in turn reduces the quantum efficiency of the phosphor and seriously affects the conversion efficiency of the package.
And based on the coating process of the conformal coating technology can achieve uniform coating of phosphor, thus ensuring the uniformity of light color. But this technology is difficult, and a large part of the blue light emitted by the LED directly by the phosphor layer reflected back to the chip, which is directly absorbed by the chip, seriously affecting the light efficiency. Yamada, Narendran, etc. found that the backscattering characteristics of the phosphor will make 50% ~ 60% of the positive incident light backwards scattering.
There is also a coating method in which the phosphor layer is far away from the LED chip (for example, the phosphor layer is located on the reflective cup or the astigmatic cup outside the LED chip), the amount of light absorbed by the phosphor layer reflected back to the chip can be drastically reduced , Thereby improving the light efficiency. In addition, since the phosphor layer is not in direct contact with the chip, the heat generated by the chip is not transferred to the phosphor layer, thereby extending the service life of the phosphor layer. Researchers at Schlbert Institute of Technology, Schubert et al. Found that the use of away from the phosphor coating process can reduce the risk of heat dissipation from the chip, the LED luminous efficiency can be increased by 7% to 16%. Sun Yat-sen Wang Gang, who also carried out a related study, the results show that the use of away from the phosphor coating can reduce the phosphor coating temperature of about 16.8 ℃, significantly improve the efficiency of phosphor conversion. However, far from the coating method has its shortcomings, because of its use of more phosphor, phosphor plate manufacturing and installation process is also relatively complex and other cost considerations, the current can not be widely promoted and industrial applications.
In addition, You et al. Proposed the use of multi-layer phosphor structure on the basis of the optimization of phosphor coating. The red phosphor layer was separated from the yellow phosphor layer and the yellow phosphor was placed on the red phosphor. The experimental results showed , Such a phosphor coating structure can reduce the mutual absorption between the phosphor coating, packaging lumen efficiency can be improved by 18%.
5, Eutectic welding technology
Eutectic welding technology is one of the most important core technologies in high power LED flip chip packaging process. Eutectic welding technology in the LED packaging process is the core of the heat problem and the advantages of solid crystal problems, and will be the future direction of the future development of LED packaging. Eutectic alloy has a lower melting point than the pure component, the melting process is simple; eutectic alloy has better fluidity than pure metal, which can prevent the formation of dendrites that impede the flow of liquid in the solidification, thus improving the casting performance; eutectic alloy But also has a constant temperature transition characteristics (no solidification temperature range), can reduce casting defects, such as segregation and shrinkage; cured eutectic alloy toughness (close to the toughness of metal), should not break; eutectic solidification can be a variety of forms Of the microstructure, especially the regular arrangement of lamellar or rod eutectic structure, can be excellent performance in situ composite materials. It is precisely because eutectic has so many advantages, so the use of eutectic process to produce the LED package will have to reduce the impedance and enhance the advantages of heat conduction efficiency.