High-power LED packaging technology and its development

1. Introduction

Due to the complex structure and process, high-power LED packaging has a direct impact on the performance and life of LEDs. It has been a research hotspot in recent years, especially high-power white LED packaging. The functions of LED packaging mainly include: 1. Mechanical protection to improve reliability; 2. Enhance 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.

The selection of LED packaging methods, materials, structures and processes is mainly determined by factors such as chip structure, optoelectronic/mechanical characteristics, specific applications and costs. After more than 40 years of development, LED packaging has gone through the development stages of bracket type (Lamp LED), SMD type (SMD LED), power type LED (Power LED) and so on. With the increase of chip power, especially the demand for the development of solid-state lighting technology, new and higher requirements are put forward for the optical, thermal, electrical and mechanical structures of LED packaging. In order to effectively reduce the thermal resistance of the package and improve the light output efficiency, a new technical approach must be adopted for package design.

2. Key technologies for high-power LED packaging

High-power LED packaging mainly involves light, heat, electricity, structure and process, as shown in Figure 1. These factors are independent of each other and influence 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 at the same time as 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 costs, and sometimes even impossible.

Specifically, the key technologies of high-power LED packaging include:

1. Low thermal resistance packaging process

For the current 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.

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 absorb the heat generated by the chip and conduct it to the heat sink to achieve heat exchange with the outside world. Common heat dissipation substrate materials include silicon, metals (such as aluminum, copper), ceramics (such as AlN, 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, as shown in Figure 2 (a), and developed the 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 high-power LED array packaging. The high thermal conductivity copper-clad ceramic board developed by Curmilk in Germany is made of a ceramic substrate (AlN or) and a conductive layer (Cu) sintered under high temperature and high pressure without using an adhesive, so it has good thermal conductivity, high strength and strong insulation, as shown in Figure 2 (b). The thermal conductivity of aluminum nitride (AlN) is 160W/mk, and the thermal expansion coefficient is (equivalent to the thermal expansion coefficient of silicon), thereby reducing the thermal stress of the package.

Studies have shown that the package interface has a great influence on thermal resistance. If the interface is not properly handled, it is difficult to obtain a good heat dissipation effect. For example, an interface with good contact at room temperature may have an interface gap at high temperature, and the warping of the substrate may also affect bonding and local heat dissipation. The key to improving LED packaging is to reduce the interface and interface contact thermal resistance and enhance heat dissipation. Therefore, the selection of thermal interface material (TIM) between the chip and the heat dissipation substrate is very important. The commonly used TIMs for LED packaging are conductive adhesives and thermal conductive adhesives. Due to their low thermal conductivity, generally 0.5-2.5W/mK, the interface thermal resistance is very high. The use of low-temperature or eutectic solder, solder paste or conductive adhesive doped with nanoparticles as thermal interface materials can greatly reduce the interface thermal resistance.

2. High light extraction rate packaging structure and process

During the use of LEDs, the losses of photons generated by radiation recombination when they are emitted outward mainly include three aspects: internal structural defects of the chip and absorption of materials; reflection losses of photons at the exit interface due to refractive index differences; and total reflection losses caused by the incident angle being greater than the critical angle of total reflection. Therefore, many rays of light 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 guide 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 obviously superior to epoxy resin because of 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, which affects the LED light efficiency and light intensity distribution.

The function 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 phosphor decreases, the light output decreases, and the radiation wavelength also changes, which causes the color temperature and chromaticity of white light LEDs to change. Higher temperatures will also accelerate the aging of phosphors. The reason is that the phosphor coating is made of epoxy or silicone and phosphors, 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 phosphors at high temperatures. Since the size of commonly used phosphors is above 1um, 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 fact that the size of phosphor particles is much larger than the light scattering limit (30nm), light scattering occurs on the surface of phosphor particles, which reduces the light output efficiency. By adding nano-phosphor to silica gel, the refractive index can be increased to above 1.8, reducing light scattering, improving LED light output efficiency (10%-20%), and effectively improving light color quality.

The traditional phosphor coating method is to mix the phosphor with the potting compound and then apply it on the chip. Since the coating thickness and shape of the phosphor cannot be precisely controlled, the color of the emitted light is inconsistent, with a blue or yellowish color. The conformal coating technology developed by Lumileds can achieve uniform coating of the phosphor and ensure the uniformity of the light color, as shown in Figure 3 (b). However, studies have shown that when the phosphor is directly coated on the chip surface, the light output efficiency is low due to the presence 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 arranges a focusing lens on the chip surface and places a glass sheet containing the phosphor at a certain distance from the chip, which not only improves the reliability of the device, but also greatly improves the light efficiency (60%), as shown in Figure 3 (c).

In general, in order to improve the light extraction efficiency and reliability of LEDs, the encapsulation layer has a tendency to be gradually replaced by high-refractive index transparent glass or microcrystalline glass. By internally doping or externally coating the phosphor on the glass surface, not only the uniformity of the phosphor is improved, but also the encapsulation efficiency is improved. In addition, reducing the number of optical interfaces in the light extraction direction of LEDs is also an effective measure to improve light extraction efficiency.

3. Array packaging and system integration technology

After more than 40 years of development, LED packaging technology and structure have gone through four stages, as shown in Figure 4.