In recent years, under the support of global energy conservation and emission reduction and the support of relevant government policies, LED lighting has developed rapidly. Compared with traditional light sources, it has the advantages of long life, small size, energy saving, high efficiency, fast response, anti-vibration and pollution-free. It is considered to be a green lighting source that can enter the general lighting field. It is an inevitable that LED is widely used in general lighting. the trend of.
As the LED package in the LED industry chain, it plays a key role in the entire industry chain. For the package, the key technology is based on how to extract as much light as possible from the chip in a limited cost range, while reducing the package thermal resistance and improving reliability. In the packaging process, packaging materials and packaging methods are the main factors. With the continuous development of LED high efficiency, power, high reliability and low cost, the requirements for packaging are also getting higher and higher. On the one hand, the LED package must meet enough requirements in terms of both the illumination angle and the uniformity of light color. High light extraction efficiency and luminous flux; on the other hand, the package must meet the heat dissipation requirements of the chip. Therefore, chips, phosphors, substrates, thermal interface materials and other packaging materials and corresponding packaging methods need to be developed to improve the heat dissipation and light extraction efficiency of LEDs.
Packaging material In the packaging process, the performance of the packaging material is the key to determining the long-term reliability of the LED. The reasonable selection and use of high-performance packaging materials can effectively improve the heat dissipation effect of LEDs and greatly extend the service life of LEDs. The packaging materials mainly include chips, phosphors, substrates, and thermal interface materials.
(1) Chip structure With the continuous development of LED device performance and the wide range of applications, especially the development of a single high-power LED, the chip structure is constantly improving. At present, there are four main types of LED chip packaging structures, namely, a formal structure, a flip-chip structure, a vertical structure, and a three-dimensional vertical structure.
At present, the common LED chip adopts the formal structure of the sapphire substrate, and the structure is simple and the manufacturing process is relatively mature. However, due to the poor thermal conductivity of sapphire, the heat generated by the chip is difficult to transfer to the heat sink, which is limited in power LED applications.
Flip-chip packaging is one of the current development directions. Compared with the formal structure, the heat does not have to pass through the sapphire substrate of the chip, but directly to the silicon or ceramic substrate with higher thermal conductivity, and then is radiated through the metal base. In the external environment.
The vertical structure of the blue chip is produced on the basis of the formal mounting. The chip is a reverse-bonded chip of a conventional sapphire substrate to a substrate such as a silicon substrate or a metal having a good thermal conductivity, and then the sapphire substrate is used. Laser peeling. The chip of this structure solves the problem of heat dissipation bottleneck, but the process is complicated, especially the process of substrate conversion is difficult to implement, and the production qualification rate is also low.
Compared with the vertical structure LED chip, the main advantage of the three-dimensional vertical structure LED chip is that it does not need to be gold wire, so that the thickness of the package is thinner, the heat dissipation effect is better, and a larger driving current is more easily introduced.
(2) Phosphors With the increasing demand for LED light quality, phosphors for LEDs of different colors and different systems have been gradually developed. High-efficiency, high color rendering index, long-life phosphor development and coating Research on overlay technology has become the key. At present, the mainstream white light realization form is a blue LED chip combined with a yellow YAG phosphor, but in order to obtain a better illumination effect, a nitride/nitrogen oxide red phosphor, a silicate orange and a green phosphor are also widely used.
The incorporation of multi-color phosphors plays an important role in improving the color rendering index of the light source, broadening the application field of LED light sources, and replacing traditional halogen lamps or metal halide lamps in some occasions where high color reproduction is required. At the same time, people are constantly developing new types of phosphors for LEDs.
The addition of red and green phosphors significantly increases the color rendering index of the source. ZL201210264610.3 [11] discloses a preparation method of a blue-excited continuous spectrum phosphor, which uses zinc oxide, cerium oxide, calcium carbonate and the like to adjust the content of activated ions Ce3 and Eu3, and can be excited in blue light. A continuous spectrum of 470 to 700 nm is emitted. Phosphors of the same matrix will show more advantages in the packaging process.
Semiconductor nanocrystalline phosphors are also a hot trend in recent years, as they are expected to change the current dependence of LEDs on rare earth materials and break through foreign patent barriers. At the same time, the semiconductor nanocrystalline phosphor has the characteristics of small size, adjustable wavelength, wide luminescence spectrum and small self-absorption, and has a potential market in white LED applications.
(3) Heat-dissipating substrate With the development of LED technology, the power is getting higher and higher, the heat flux density of the LED chip is larger, and the requirements for thermal resistance and expansion coefficient of the package substrate material are getting higher and higher. The heat-dissipating substrate has developed rapidly and has many varieties. At present, it mainly consists of a metal core printed circuit board, a metal matrix composite material, and a ceramic matrix composite material.
The metal core printed circuit board (MCPCB) is to attach the original printed circuit board (PCB) to another metal (aluminum, copper) with better heat conduction effect to enhance the heat dissipation effect. Inside the printed circuit board. This technology can effectively solve the heat dissipation problem caused by the high power device in the trend of compact structure. The thermal conductivity of MCPCB can reach 1~2.2W/(mK).
Since the dielectric layer of the MCPCB does not have a very good thermal conductivity (0.3 W/(mK)), it becomes a thermal bottleneck with the heat sink. Metal-based heat sinks have high thermal conductivity and provide good heat dissipation for the device. The polymer insulation layer and the copper foil circuit and the epoxy resin are directly bonded to the aluminum and copper plates, and then the LEDs are disposed on the insulating substrate, and the thermal conductivity of the insulating substrate is relatively high, reaching 1.12 W/(mK). ).
Ceramic substrate packaging substrate stability is probably the most promising research direction. Compared with the metal material packaging substrate, it eliminates the complicated manufacturing process of the insulating layer. Multilayer Ceramic Metal Package (MLCMP) technology offers significant improvements in heat treatment compared to traditional packaging methods. The new AlN ceramic material, which has high thermal conductivity, low dielectric constant and low dielectric loss, is considered to be an ideal material for a new generation of semiconductor packages. The ceramic copper clad laminate (DBC) [12] is also a ceramic substrate with excellent thermal conductivity. The ultra-thin composite substrate has excellent electrical insulation properties and high thermal conductivity, and its thermal conductivity can reach 24-28 W/(mK). .
For LED packaging applications, in addition to the basic high thermal conductivity and layout circuit functions, the heat dissipation substrate also requires a certain insulation, heat resistance, and matching expansion coefficient. Transparent ceramic material technology not only has high heat dissipation efficiency, heat resistance, expansion coefficient matching, etc., but also hopes to make breakthroughs in the optical performance of packaged devices, enabling full-space LED package.
(4) Thermal interface materials At present, research on heat dissipation pays more attention to the materials and structures of chips, substrates, and heat sinks, but often ignores the influence of thermal interface materials. The thermal interface material is used as a filler between the two materials and acts as a bridge during heat transfer. The LED luminaire is a combination of multi-layer structures. To quickly derive the heat generated by the chip, minimize the thermal resistance between the materials, and improve the thermal conductivity, the thermal conductivity of the thermal interface material is crucial. There are currently four types of thermal interface materials for LED packages: thermally conductive adhesives, conductive silver paste, solder paste, and tin-gold alloy eutectic soldering.
The thermal paste is made by adding some high thermal conductivity fillers such as SiC, AlN, Al2O3, SiO2, etc. inside the substrate to improve its thermal conductivity. The advantages of the thermal conductive adhesive are low cost, insulating property and simple process, but the thermal conductivity is generally poor, and the thermal conductivity is about 0.7 W/(m?K).
The conductive silver paste is added with silver powder in the epoxy resin. The curing temperature is generally lower than 200 ° C, the heat transfer coefficient is about 20 W / (m? K), has good thermal conductivity, and the bonding strength is also good, but the silver paste is good. The absorption of light is relatively large, resulting in a decrease in light efficiency. For low-power LED chips, the heat generation is small, and the conductive silver paste can be used as the bonding layer to completely satisfy the heat dissipation and reliability problems [13]. The thermal conductivity of the conductive solder paste is about 50W/(m?K), which is generally used for metal. Inter-weld, excellent electrical conductivity.
Sikkim alloy eutectic soldering uses the eutectic point of the metal to weld the two metals together and is suitable as a bonding material for high-power LED chips. Kim et al [14] compared the heat dissipation properties of thermally conductive conductive silver paste, Sn-Ag-Cu solder and Au-Sn eutectic solder as thermal interface materials, and found that for the bonding of SiC substrate and Si substrate, Au/ The thermal resistance of Sn eutectic solder is significantly lower than that of silver paste and Sn-Ag-Cu solder.
At present, domestic thermal interface materials lag far behind foreign standards. With the increasing integration of LED packages and the increase of heat flux density, new thermal interface materials with higher thermal conductivity are needed to improve the heat transfer capability between LED package devices, such as The composite thermal interface material technology with low thermal resistance is prepared by using graphene, carbon nanotubes and nano-silver wires as fillers, and modifying the binder with inorganic functional groups. For LED packaging applications, in addition to low thermal resistance, the ideal thermal interface material should have a matching expansion coefficient and elastic modulus, as well as better mechanical properties, high heat distortion temperature, and lower cost.
Package structure In the rapid development of LED chip technology, the package form of LED products has also evolved from a single chip package to a multi-chip package. Its package structure also ranges from Lamp package to SMD package to COB package and RP package technology.
The lead-type package (Lamp) adopts a lead frame as a pin for various package appearances, and is the first LED package structure successfully developed on the market, with a large variety and high technical maturity. Surface mount package (SMD) is a relatively advanced process because it reduces the space occupied by the product, reduces the weight, and allows the working current to be large. It is a relatively advanced process, from Lamp package to SMD package. The development trend of the electronics industry. However, in applications, there are problems such as heat dissipation, uniformity of illumination, and decrease in luminous efficiency.
The CoB (ChiponBoard) package structure is developed on the basis of multi-chip package technology. The CoB package directly mounts the exposed chip on the circuit board, bonds with the circuit board through the bonding wires, and then performs passivation of the chip. And the advantages of protection [15].CoB are: soft light, simple circuit design, high cost efficiency, saving system space, etc. [16], but there are technical problems of chip integration brightness, color temperature adjustment and system integration.
The remote fluorescent packaging technology (RP) is a method in which a plurality of blue LEDs are placed separately from the phosphors, and the blue light emitted by the LEDs is uniformly incident on the phosphor layer after being mixed by a reflector, a diffuser, etc., and finally emits a uniform white light. LED light source form. Compared with other package structures, the performance of RP package technology is more special: first, the phosphor powder is far away from the LED chip, and the phosphor is not easily affected by the heat of the PN junction, especially some silicate phosphors are susceptible to high temperatures. The effect of high humidity, after away from the heat source, can reduce the probability of hot quenching of the phosphor and prolong the life of the light source. Secondly, the structure of the phosphor away from the chip design facilitates the removal of light and improves the luminous efficiency of the light source. Moreover, the light color space emitted by the structure is evenly distributed and the color consistency is high. In recent years, UV-excited remote packaging technology has attracted people's attention. Compared with traditional UV light sources, it has unique advantages, including low power consumption, fast luminescence response, high reliability, high radiation efficiency, long life and no pollution to the environment. It has many advantages, such as compact structure, and has become one of the new research hotspots of major companies and research institutions in the world.
Development Trends In recent years, many research institutes and enterprises at home and abroad have continued to research LED packaging technology. Excellent packaging materials and high-efficiency packaging technologies have been proposed. High-reliability LED lighting new products have emerged, such as LED filaments and soft substrates. Packaging technology, etc. (Figure 4) also has certain performance requirements.
Under the continuous development of new materials, superconducting and superconducting materials have been successively launched, which provides a solid foundation for the further development of LED packaging technology, such as graphene. The Chinese Academy of Sciences semiconductor invented a flip-chip light-emitting diode using graphene as a heat-conducting layer. By using the superior electrical conductivity of graphene, part of the heat can be transferred to the substrate via the graphene heat-conducting layer, which increases the heat conduction path of the device and improves Dispersed
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