What is high-power LED packaging technology?

High-power LED packaging has been a research hotspot in recent years due to its complex structure and process, and directly affects the performance and life of LED. In particular, high-power white light LED packaging is a hotspot among research hotspots.The functions of LED packaging mainly include:1. Mechanical protection to improve reliability;2. Strengthen heat dissipation to reduce chip junction temperature and improve LED performance;3. Optical control to improve light output efficiency and optimize beam distribution;4. Power supply management, including AC/DC conversion and power supply control.

What is high-power LED packaging technology? How do SMD packaging and COB packaging promote，The selection of LED packaging methods, materials, structures and processes is mainly determined by factors such as chip structure, optoelectronic/mechanical properties, specific applications and costs. After more than 40 years of development, LED packaging has gone through the development stages of bracket type (Lamp LED), patch type (SMD LED), bare core LED (COB LED) and so on.

Key technologies of high-power LED packaging，High-power LED packaging mainly involves light, heat, electricity, structure and process. These factors are both independent of each other and affect each other. Among them, light is the purpose of LED packaging, heat is the key, electricity, structure and process are means, and performance is the specific embodiment of the packaging level. In terms of process compatibility and reducing production costs, LED packaging design should be carried out simultaneously with chip design, that is, the packaging structure and process should be considered when designing the chip. Otherwise, after the chip is manufactured, the chip structure may be adjusted due to the need for packaging, thereby extending the product development cycle and process cost, and sometimes even impossible.

Low thermal resistance packaging process，For the existing LED light efficiency level, since about 80% of the input electrical energy is converted into heat and the LED chip area is small, chip heat dissipation is a key issue that must be solved in LED packaging. It mainly includes chip layout, packaging material selection (substrate material, thermal interface material) and process, heat sink design, etc.What is high-power LED packaging technology? How does SMD packaging and COB packaging promote

The thermal resistance of LED packaging mainly includes the internal thermal resistance of the material (heat dissipation substrate and heat sink structure) and the interface thermal resistance. The function of the heat dissipation substrate is to attract the heat generated by the chip and conduct it to the heat sink to achieve heat exchange with the outside world. Commonly used heat dissipation substrate materials include silicon, metal (such as aluminum, copper), ceramic (such as Al2O3, AIN, Sic) and composite materials. For example, Nichia's third-generation LED uses CuW as a substrate, and flips the 1mm chip on the CuW substrate, which reduces the thermal resistance of the package and improves the luminous power and efficiency; Lamina Ceramics has developed a low-temperature co-fired ceramic metal substrate and developed corresponding LED packaging technology. This technology first prepares a high-power LED chip and a corresponding ceramic substrate suitable for eutectic welding, and then directly welds the LED chip to the substrate. Since the substrate integrates the eutectic welding layer, electrostatic protection circuit, drive circuit and control compensation circuit, it is not only simple in structure, but also greatly improves the heat dissipation performance due to the high thermal conductivity of the material and the small thermal interface, which provides a solution for the high-power LED array packaging. The high thermal conductivity copper-clad ceramic board developed by Curmilk in Germany is sintered by a ceramic substrate (AIN and Al2O3) and a conductive layer (Cu) under high temperature and high pressure without using an adhesive, so it has good thermal conductivity, high strength and strong insulation. Among them, the thermal conductivity of aluminum nitride (AIN) is 160W/mk, and the thermal expansion coefficient is 4.0×10-6/℃ (equivalent to the thermal expansion coefficient of silicon 3.2×10-6/℃), which reduces the thermal stress of the package.Studies have shown that the package interface also has a great influence on thermal resistance. If the interface is not properly handled, it is difficult to obtain a good heat dissipation effect。What is high-power LED packaging technology? How do SMD packaging and COB packaging promote，High light extraction efficiency packaging structure and process.

During the use of LEDs, the loss of photons generated by radiation recombination when emitted outward mainly includes three aspects: internal structural defects of the chip and absorption of materials; reflection loss of photons at the exit interface due to the refractive index difference; and total reflection loss caused by the incident angle being greater than the critical angle of total reflection. Therefore, many light rays cannot be emitted from the chip to the outside. By coating a layer of transparent glue (potting glue) with a relatively high refractive index on the surface of the chip, since the glue layer is between the chip and the air, the loss of photons at the interface is effectively reduced, and the light extraction efficiency is improved. In addition, the role of potting glue also includes mechanical protection of the chip, stress release, and as a light-conducting structure. Therefore, it is required to have high light transmittance, high refractive index, good thermal stability, good fluidity, and easy spraying. In order to improve the reliability of LED packaging, the potting glue is also required to have low hygroscopicity, low stress, and aging resistance. Currently commonly used potting glues include epoxy resin and silicone. Silicone is significantly better than epoxy resin due to its high light transmittance, high refractive index, good thermal stability, low stress, and low hygroscopicity. It is widely used in high-power LED packaging, but the cost is relatively high. Studies have shown that increasing the refractive index of silicone can effectively reduce the photon loss caused by the physical barrier of the refractive index and improve the external quantum efficiency, but the performance of silicone is greatly affected by the ambient temperature. As the temperature rises, the thermal stress inside the silicone increases, resulting in a decrease in the refractive index of the silicone, thereby affecting the LED light efficiency and light intensity distribution.

The role of phosphor is to compound light and color to form white light. Its characteristics mainly include particle size, shape, luminous efficiency, conversion efficiency, stability (thermal and chemical), etc. Among them, luminous efficiency and conversion efficiency are the key. Studies have shown that as the temperature rises, the quantum efficiency of the phosphor decreases, the light output decreases, and the radiation wavelength also changes, causing changes in the color temperature and chromaticity of the white light LED. Higher temperatures will also accelerate the aging of the phosphor. The reason is that the phosphor coating is made of epoxy or silicone and phosphor, and has poor heat dissipation performance. When irradiated by purple or ultraviolet light, it is prone to temperature quenching and aging, which reduces the luminous efficiency. In addition, there are also problems with the thermal stability of potting glue and phosphor at high temperatures. Since the size of commonly used phosphors is above 1μm, the refractive index is greater than or equal to 1.85, while the refractive index of silicone is generally around 1.5. Due to the mismatch of the refractive index between the two, and the size of the phosphor particles is much larger than the light scattering limit (30nm), there is light scattering on the surface of the phosphor particles, which reduces the light output efficiency. By adding nano-fluorescent powder to silica gel, the refractive index can be increased to above 1.8, light scattering can be reduced, the light output efficiency of LED can be increased (10%-20%), and the light color quality can be effectively improved.

The traditional method of applying fluorescent powder is to mix the fluorescent powder with the potting glue and then apply it on the chip. Since the coating thickness and shape of the fluorescent powder cannot be precisely controlled, the color of the emitted light is inconsistent, and the light is blue or yellow. The conformal coating technology developed by Lumileds can achieve uniform coating of fluorescent powder and ensure the uniformity of light color, as shown in Figure 4b. However, studies have shown that when the fluorescent powder is directly coated on the surface of the chip, the light output efficiency is low due to the existence of light scattering. In view of this, the Rensselaer Institute in the United States proposed a photon scattering extraction process (Scattered Photon Extraction method, SPE), which not only improves the reliability of the device, but also greatly improves the light efficiency (60%) by arranging a focusing lens on the surface of the chip and placing the glass sheet containing fluorescent powder at a certain distance from the chip.

What is high-power LED packaging technology? How do SMD packaging and COB packaging promote it?In recent years, LED chips have gradually become the darling of the display industry. With the advantages of high brightness, self-luminescence and full color, they have continuously entered various display venues, and SMD packaging and COB packaging are the main factors driving this development.Surface mount three-in-one (SMD) LED emerged in 2002 and gradually occupied the market share of LED display devices, turning from pin-type packaging to SMD. Surface mount packaging is to bond a single or multiple LED chips to a metal bracket with a plastic "cup-shaped" outer frame (the outer pins of the bracket are connected to the P and N poles of the LED chip respectively), and then fill the plastic outer frame with liquid packaging glue, then bake it at high temperature, and finally cut and separate it into a single surface mount package device. Because surface mount technology (SMT) can be used, the degree of automation is relatively high. Compared with pin-type packaging technology, SMD LED has good performance in brightness, consistency, reliability, viewing angle, appearance, etc.

Technical advantages of SMD packaging:1. Mature technology, complete related production equipment and process, and sound supply system.2. Wide application, mature display control compatibility and high stability.3. High one-time pass rate of lamp beads, stable quality and high quality.

COB packaging adopts surface-like light emission, so the viewing angle is wider, which makes Voury Zhuohua COB packaged LED display have a vertical and horizontal bidirectional 170° ultra-wide viewing angle. Any viewing angle is the central viewing angle, and any angle ensures the consistency of color and brightness. It has a better optical diffuse color effect, a larger display coverage area, and ensures that there is no dead angle or color cast at any angle, and the image can always be displayed perfectly.

In addition, COB packaging technology encapsulates pixels on the PCB board, realizes the comprehensive sealing of PCB circuit boards, crystal particles, solder feet and leads, and has a smooth surface without exposed components, achieving IP65 full protection capability. The surface of the pixels is smooth and hard, with anti-knock, anti-collision, anti-shock, anti-pressure, waterproof, moisture-proof, dust-proof, anti-oil, anti-oxidation, anti-static and other properties. It is highly stable and easy to maintain. Daily cleaning and maintenance can directly wipe off surface stains with a wet cloth.