In this paper, optical, thermal, electrical, reliability, etc., are reviewed in detail the high-power white LED packaging deSign and research progress, and key high-power LED packaging technology are reviewed. LED package design should be made with the Chip design at the same time, and the need for light, heat, electricity, consider a unified structure and other properties. In the packaging process, although the material (thermal substrate, phosphor, potting materials) choice is important, but the package structure should be minimized thermal and optical interface, thereby reducing the package thermal resistance, improve the light efficiency. Finally, the paper design of LED lamps and packaging requirements are described.
Key words: solid-state white LED lighting, high-power LED package
I. Introduction
High-power LED package structure and process as complex and directly affect the use of LED performance and life, has been a research focus in recent years, especially in high-power white light LED package is research hotspot in the hot spots. LED package features include: 1. Mechanical protection to improve reliability; 2. To enhance heat dissipation, to reduce the chip junction temperature and improve the LED performance; 3. Optical control and improve the light efficiency and optimize the beam distribution; 4. Power Management including AC / DC shift and power control.
LED packaging methods, materials, structure and process of selection mainly by the chip architecture, optical / mechanical properties, the specific application and cost factors. After 40 years of development, LED package has gone through the frame type (Lamp LED), SMD (SMD LED), power type LED (Power LED) and other stages of development. As the power increases, in particular, the demand for the development of solid-state lighting technology, LED packaging of the optical, thermal, electrical and mechanical structure and put forward new and higher requirements. In order to effectively reduce the package thermal resistance, improve the efficiency of light, new technology must be used for package design ideas.
Second, the key to high-power LED packaging technology
High-power LED package mainly related to light, heat, electricity, structure and process, etc., shown in Figure 1. These factors are mutually independent of each other, but also influence each other. Among them, the purpose of just LED packaging, heat is the key, power, structure and process is the means, the level of performance is a concrete manifestation of package. From the process in terms of compatibility and reduce production cost, LED chip design package design should be the same time, the chip design should take into account the package structure and process. Otherwise, the other chip manufacturing is completed, probably because of chip package structure need to be adjusted, resulting in longer product development cycle and process costs, and sometimes even impossible.
Figure 1 high-power white LED packaging technology
Specifically, the key technology of high-power LED package includes:
(A) low thermal resistance packaging technology
LED light effects for the existing level, since about 80% of the input energy turned into heat, and the LED chip size, therefore, is the LED chip cooling package must address the key issues. Including chip layout, packaging material selection (substrate material, thermal interface materials) and process, heat sink design.
Thermal resistance LED package including materials (substrate and heat sink thermal structure) internal resistance and interfacial thermal resistance. The role of the substrate is absorbing heat generated by the chip's heat and transfer to the heat sink to achieve the heat exchange with the outside world. Commonly used substrate materials including silicon thermal, metal (such as aluminum, copper), ceramic (eg Al2O3, AlN, SiC) and composite materials. Companies such as Nichia LED with the third generation to do CuW substrate, flip chip will be in the CuW 1mm substrate, reducing the package thermal resistance, enhanced light-emitting power and efficiency; Lamina Ceramics companies developed LTCC metal substrate, Figure 2 (a), and the development of the corresponding LED packaging technology. First, the technology for eutectic solder was prepared high-power LED chips and the corresponding ceramic substrate, the LED chip and the substrate and then directly welded together. As the substrate integrated eutectic solder layer, electrostatic protection circuit, drive circuit and control compensation circuit is not only simple in structure, and because of high thermal conductivity materials, thermal interface less, greatly improving the thermal performance for high-power LED array package proposed solutions. Germany Curmilk company developed high thermal conductivity copper ceramic plate, the ceramic substrate (AlN or Al2O3) and conductive layer (Cu) sintered at high temperature, do not use binders, so good thermal conductivity, high strength, insulation strong, as shown in Figure 2 (b) below. In which aluminum nitride (AlN) the thermal conductivity is 160W/mk, thermal expansion coefficient of 4.0 × 10-6 / ℃ (coefficient of thermal expansion of silicon 3.2 × 10-6 / ℃ equivalent), which reduces the package thermal stress.
Figure 2 (a) LTCC metal substrate
Figure 2 (b) Schematic cross-section copper ceramic substrates
The results show that the interface on the thermal resistance of package is also a great, if not correctly handle the interface, would be difficult to get a good cooling effect. For example, room temperature, good interfacial contact with high temperatures may exist in the interface gap, the substrate warpage may also affect the local bonding and heat dissipation. The key to improve the LED package is to reduce the interface and the interface contact resistance and enhance heat dissipation. Therefore, the chip and the cooling thermal interface between the substrate material (TIM) selection is very important. TIM LED package commonly used for the conductive plastic and thermal plastic, the low thermal conductivity, generally 0.5-2.5W/mK, resulting in high interfacial thermal resistance. The low temperature or eutectic solder, solder paste or conductive nanoparticles doped within the gel as a thermal interface material, can greatly reduce the interface thermal resistance.
(B) high rate of take-ray structure and technology package
LED used in the process, the radiative recombination of photons generated in the loss of firing out, mainly in three aspects: the internal structure of the chip defect as well as the absorption; photons in the outgoing interface, the refractive index difference caused by the reflection losses; as a result of angle of incidence greater than the total reflection critical angle of total reflection caused by the loss. Therefore, a lot of chips out of the light can not reach all parts from the outside. Chip surface coated by a layer of relatively high refractive index of the transparent layer (potting), because the air layer between the chip and is, thus effectively reducing the loss of photons in the interface to improve the light extraction efficiency. In addition, the role of potting chip also includes a mechanical protection, stress release, and as a light guide structure. Therefore, the requirements of its high transmittance, high refractive index, thermal stability, good fluidity, easy to spray. To improve the reliability of LED package, also called potting materials with low moisture absorption, low stress, anti-aging properties. Currently used include epoxy and silicone encapsulants. Due to high transparency silica gel, a large refractive index, thermal stability, stress is small, low moisture absorption, better than epoxy resin, high-power LED package is widely used, but the cost is higher. Studies show that increasing the refractive index can be effective in reducing the refractive index of silica physical barriers caused by photon losses and improve the external quantum efficiency, but silicone performance influenced by ambient temperature. As the temperature increases, the silica gel to increase the internal thermal stress, resulting in lower refractive index of silica gel, thus affecting the LED light efficiency and light intensity distribution.
The role of phosphor is light color composite, the formation of white light. Its features include size, shape, luminous efficiency, conversion efficiency, stability (thermal and chemical), etc., in which light-emitting efficiency and conversion efficiency is the key. Studies show that as temperature rises, lower quantum efficiency phosphor, the light decrease radiation wavelength will also change, causing the white LED color temperature, color changes, the higher temperature will accelerate the aging of phosphors. Because the phosphor coating by epoxy or silicone and phosphor from the deployment, lower heat dissipation, when subject to violet or UV radiation, temperature, quenching and aging-prone, so luminous efficiency. In addition, potting materials and high temperature thermal stability of the phosphor is also problematic. Size of the phosphor used in more than 1um, refractive index greater than or equal to 1.85, while the silica refractive index is generally 1.5. Since the refractive index mismatch between the two, and the phosphor light scattering particle size is much larger than the limit (30nm), which in the light scattering surface of the phosphor particles, reducing the light efficiency. Through the incorporation of nano-silica gel phosphor, the refractive index increased to 1.8 or more can reduce the light scattering and improve the optical efficiency LED (10% -20%), and can effectively improve the quality of light and color.
The traditional way is to phosphor phosphor coating and potting mix, then coated on the chip. The phosphor coating can not be precisely controlled thickness and shape, leading to inconsistent color of light emitted appears blue or yellow light side. The Lumileds has developed a conformal coating (Conformal coating) technology can achieve the uniformity of the phosphor coating to protect the uniformity of light color, as shown in Figure 3 (b). But studies show that when the phosphor coating on the chip surface directly, because the presence of light scattering, the light efficiency is low. In view of this, the United States Institute Rensselaer proposed a scattering photon extraction (Scattered Photon Extraction method, SPE), through the layout of the chip surface, a focusing lens, and glass with phosphor from the chip placed in a certain position, not only improved device reliability, but also improves the optical efficiency (60%), as shown in Figure 3 (c).
Figure 3 High power white light LED package structure
Overall, of the LED light to improve the efficiency and reliability, packaging, high refractive index layer has gradually been transparent glass or ceramic glass replacement trend, by the phosphor within the doped or coated on the glass surface, not only improve uniformity of phosphor, and improve the packaging efficiency. In addition, the direction of reducing the optical LED light out of the number of interfaces, but also improve the light extraction efficiency and effective measures.
