Research status and development trend of LED packaging

 In recent years, with the advocacy of global energy conservation and emission reduction and the support of relevant policies of various governments, 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 speed, earthquake resistance, and no pollution. It is considered to be a "green lighting source" that can enter the field of general lighting . The large-scale application of LED in general lighting is an inevitable trend.

　　As the link between the upper and lower parts of the LED industry chain , LED packaging plays a key role in the entire industry chain. For packaging, the key technology ultimately lies in how to extract as much light as possible from the chip within a limited cost range, while reducing the thermal resistance of the package and improving reliability. In the packaging process, packaging materials and packaging methods are the main influencing factors. With the continuous development of LED high light efficiency, power, high reliability and low cost, the requirements for packaging are getting higher and higher. On the one hand, LED packaging must meet the requirements of sufficiently high light extraction efficiency and luminous flux while taking into account the light emitting angle, light color uniformity and other aspects; on the other hand, the packaging must meet the heat dissipation requirements of the chip. Therefore, chips, phosphors, substrates, thermal interface materials and other packaging materials and corresponding packaging methods are in urgent need of development and innovation to improve the heat dissipation capacity and light extraction efficiency of LEDs.

　　Packaging materials

　　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 the LED and greatly extend the service life of the LED. 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 continuous expansion of application scope, especially the development of single high-power LED, the chip structure is also constantly improving. At present, there are four main types of LED chip packaging structures, namely: upright structure, flip-chip structure, vertical structure and three-dimensional vertical structure.

　　At present, ordinary LED chips use a sapphire substrate with a straight-mounted structure, which is simple and has a relatively mature manufacturing process. However, due to the poor thermal conductivity of sapphire, the heat generated by the chip is difficult to transfer to the heat sink, which limits its application in power LEDs.

　　Flip-chip packaging is one of the current development directions. Compared with the upright structure, the heat does not have to pass through the sapphire substrate of the chip, but is directly transferred to the silicon or ceramic substrate with higher thermal conductivity, and then dissipated to the external environment through the metal base.

　　The vertical blue chip is produced on the basis of the normal assembly. This chip is made by bonding the traditional sapphire substrate chip upside down on a silicon substrate or metal substrate with good thermal conductivity, and then laser peeling the sapphire substrate. This chip structure solves the heat dissipation bottleneck problem, but the process is complicated, especially the substrate conversion process is difficult to implement, and the production qualification rate is also low.

　　Compared with vertical structure LED chips, the main advantage of three-dimensional vertical structure LED chips is that they do not require gold wire, making their packaging thinner, heat dissipation better, and easier to introduce larger driving currents.

　　(2) Phosphor

　　As people's requirements for LED light quality become higher and higher, LED phosphors of different colors and systems are gradually developed. The development of high-light-efficiency, high-color rendering index, and long-life phosphors and the research of coating technology have become the key. The current mainstream white light realization form is blue LED chips combined with yellow YAG phosphors, but in order to obtain better lighting effects, nitride/nitride oxide red phosphors, silicate orange and green phosphors have also been widely used.

　　The addition of multi-color phosphors plays an important role in improving the color rendering index of light sources, broadening the application field of LED light sources and replacing traditional halogen lamps or metal halide lamps in some occasions that require high color reproduction. At the same time, people are also constantly developing new types of phosphors for LEDs.

　　The addition of red and green phosphors significantly improves the color rendering index of the light source. ZL201210264610.3[11] discloses a method for preparing a continuous spectrum phosphor excited by blue light. The phosphor uses zinc oxide, lanthanum oxide, calcium carbonate and other raw materials, and adjusts the content of activated ions Ce3+ and Eu3+ to obtain a continuous spectrum of 470 to 700 nm under blue light excitation. Phosphors with the same matrix will show more advantages during the packaging process.

　　Semiconductor nanocrystalline phosphors are also a hot research topic 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, semiconductor nanocrystalline phosphors have the characteristics of small size, adjustable wavelength, wide luminescence spectrum, and low self-absorption, and have a potential market in white light LED applications.

　　(3) Heat dissipation substrate

　　With the development of LED technology , the power is getting higher and higher, the heat flux density of LED chips is getting higher, and the requirements for the thermal resistance and expansion coefficient of packaging substrate materials are getting higher and higher. The heat dissipation substrate is developing rapidly and there are many varieties. At present, it mainly consists of metal core printed circuit boards, metal matrix composite materials, and ceramic matrix composite materials.

　　　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, and this piece of metal is located inside the printed circuit board. This technology can effectively solve the heat dissipation problem caused by the trend of compact structure of high-power devices. The thermal conductivity of MCPCB can reach 1~2.2 W/(m·K).

　　Since the dielectric layer of MCPCB does not have a very good thermal conductivity (0.3W/(m·K)), it becomes a bottleneck for heat dissipation with the heat sink. Metal-based heat sinks have high thermal conductivity and can provide good heat dissipation for devices. The polymer insulation layer and copper foil circuit are directly bonded to the aluminum and copper plates with epoxy resin, and then the LED is configured on the insulating substrate. The thermal conductivity of this insulating substrate is relatively high, reaching 1.12 W/(m·K).

　　Ceramic packaging substrates have good stability and may be the most promising research direction. Compared with metal packaging substrates, they do not require the complex manufacturing process of the insulating layer. Multilayer ceramic metal packaging (MLCMP) technology has greatly improved thermal treatment compared with traditional packaging methods. The new AlN ceramic material has the characteristics of high thermal conductivity, low dielectric constant and dielectric loss, and is considered to be an ideal material for the new generation of semiconductor packaging. Ceramic copper clad laminate (DBC)[12] is also a ceramic substrate with excellent thermal conductivity. The ultra-thin composite substrate has excellent electrical insulation performance and high thermal conductivity. Its thermal conductivity can reach 24~28W/(m·K).

　　For LED packaging applications, the heat dissipation substrate, in addition to the basic functions of high thermal conductivity and circuit layout, is also required to have certain insulation, heat resistance, and matching expansion coefficient. Transparent ceramic material technology not only has high heat dissipation efficiency, heat resistance, and expansion coefficient matching, but also has the potential to make breakthroughs in the optical performance of packaging devices and realize full-space luminous LED packaging.

　　(4) Thermal interface materials

　　At present, people pay more attention to the materials and structures of chips, substrates, and heat sinks in the research of heat dissipation, but often ignore the influence of thermal interface materials. Thermal interface materials are fillers used between two materials, and they act as a bridge in the process of heat transfer. LED lamps are a combination of multi-layer structures. If you want to quickly export the heat generated by the chip, minimize the thermal resistance between materials, and improve thermal conductivity, the thermal conductivity of thermal interface materials is crucial. There are currently four types of thermal interface materials used for LED packaging: thermal conductive adhesives, conductive silver glue, solder paste, and tin-gold alloy eutectic welding.

　　Thermal conductive adhesive is to add some high thermal conductivity fillers such as SiC, AlN, Al2O3, SiO2, etc. inside the matrix to improve its thermal conductivity. The advantages of thermal conductive adhesive are low price, insulation performance, and simple process, but the thermal conductivity is generally poor, and the thermal conductivity coefficient is about 0.7W/(m?K).

　　Conductive silver paste is made by adding silver powder to epoxy resin. Its curing temperature is generally lower than 200°C and its thermal conductivity is about 20W/(m·K). It has good thermal conductivity and good bonding strength. However, silver paste absorbs a lot of light, which leads to a decrease in light efficiency. For low-power LED chips that generate less heat, conductive silver paste as an adhesive layer can fully meet the heat dissipation and reliability issues [13]. The thermal conductivity of conductive solder paste is about 50W/(m·K). It is generally used for welding between metals and has excellent conductivity.

　　Tin-gold alloy eutectic welding uses the eutectic point of metals to weld two metals together, and is suitable as a bonding material for high-power LED chips. Kim et al. [14] compared the heat dissipation performance of thermally conductive and electrically conductive silver glue, Sn-Ag-Cu solder, and Au-Sn eutectic solder as thermal interface materials, and found that for the bonding of SiC substrates to Si substrates, the packaging thermal resistance of Au/Sn eutectic solder was significantly lower than that of silver glue and Sn-Ag-Cu solder.

　　At present, domestic thermal interface materials lag far behind the foreign level. With the improvement of LED packaging integration and the increase of heat flux density, new thermal interface materials with higher thermal conductivity are needed to improve the heat transfer capacity between LED packaging devices, such as using graphene, carbon nanotubes, and nano silver wires as fillers for compounding, and using inorganic functional groups to modify the base material to prepare new low thermal resistance composite thermal interface material technology. For LED packaging applications, in addition to low thermal resistance, ideal thermal interface materials should also have matching expansion coefficients and elastic moduli, as well as good mechanical properties, high heat deformation temperature, and low cost.

　　Package structure

　　With the rapid development of LED chip technology, the packaging form of LED products has also developed from single-chip packaging to multi-chip packaging. Its packaging structure has also evolved from Lamp packaging to SMD packaging to COB packaging and RP packaging technology.

　　The pin-type package (Lamp) uses a lead frame as the pins of various package shapes. It is the first LED package structure successfully developed and put on the market. There are many varieties and the technology is highly mature. The surface mount package (SMD) has become a relatively advanced process because it reduces the space occupied by the product, reduces the weight, and allows a large working current to pass, which is particularly suitable for automated mounting production. The conversion from Lamp package to SMD package is in line with the development trend of the entire electronics industry. However, there are problems such as heat dissipation, luminous uniformity and reduced luminous efficiency in application.

　　The CoB (Chip on Board) packaging structure is developed based on the multi-chip packaging technology. The CoB packaging is to directly mount the exposed chip on the circuit board, bond it to the circuit board through bonding wires, and then passivate and protect the chip[15]. The advantages of CoB are: soft light, simple circuit design, high cost-effectiveness, saving system space, etc.[16], but there are technical problems in chip integration brightness, color temperature adjustment and system integration.

　　Remote fluorescent packaging technology (RP) is a form of LED light source that places multiple blue LEDs and phosphors separately. The blue light emitted by the LED is evenly incident on the phosphor layer after being mixed by reflectors, diffusers, etc., and finally emits uniform white light. Compared with other packaging structures, the performance of RP packaging technology is more outstanding: first, the phosphor is far away from the LED chip, and the phosphor is not easily affected by the heating of the PN junction, especially some silicate phosphors, which are easily affected by high temperature and high humidity. After being away from the heat source, the probability of thermal quenching of the phosphor can be reduced, and the life of the light source can be extended. Secondly, the structure designed with the phosphor away from the chip is conducive to the extraction of light and improves the luminous efficiency of the light source. Furthermore, the light color space distribution of this structure is uniform and the color consistency is high. In recent years, ultraviolet-excited remote packaging technology has attracted great attention. Compared with traditional ultraviolet light sources, it has unique advantages, including low power consumption, fast luminous response, high reliability, high radiation efficiency, long life, no pollution to the environment, compact structure and many other advantages, becoming one of the new research hotspots of major companies and research institutions in the world.

　　Development Trend

　　In recent years, many scientific research institutions and enterprises at home and abroad have continued to conduct research on LED packaging technology. Excellent packaging materials and efficient packaging processes have been proposed one after another, and new high-reliability LED lighting products have appeared one after another, such as: LED filaments, soft substrate packaging technology, etc. (as shown in Figure 4) and have certain performance requirements.

　　With the continuous research and development of new materials, superconducting and superthermal conducting materials have been introduced, providing a solid foundation for the further development of LED packaging technology, such as graphene. The Chinese Academy of Sciences Semiconductor invented a flip-chip structure light-emitting diode with graphene as the thermal conductive layer. By utilizing the excellent electrical conductivity of graphene, part of the heat can be transferred to the substrate through the graphene thermal conductive layer, increasing the thermal conduction channel of the device and improving the heat dissipation effect [17].

　　At present, LED chips are driven by low-voltage DC, which requires voltage reduction and rectification in the power driver, causing energy loss and reliability problems. People have proposed using high-voltage LED chips and AC LED chips to improve the situation. In September 2008, Taiwan Industrial Technology Research Institute won the US R&D 100 Awards for its On Chip Alternating Current LED Lighting Technology. AC LED (Alternating Current LED) has excellent performance such as low energy consumption, high efficiency, and easy use, and it has also subverted the application of traditional LEDs.

　　Three-dimensional packaging technology is a brand-new concept for LED packaging. It puts forward more innovative requirements for design ideas and concepts, material properties and packaging technology itself. Since its emergence, three-dimensional printing technology has made great progress, making LED three-dimensional packaging technology a possibility, but there are many problems that need to be overcome, such as composite preparation of materials, thermal stress balance control between materials, production efficiency, etc. Therefore, it can be said that LED packaging technology based on three-dimensional printing technology is still a relatively distant concept.

　　In the long run, LED packaging technology needs to accelerate the exploration of packaging materials, packaging structures and multi-functional systematic integration for three-dimensional packaging, improve the miniaturization of LED packaging according to the packaging concept of integrated circuits, adopt the packaging method of bondless alloy wire, realize the heat dissipation capability of high-efficiency light source module devices, solve the contradiction between light, heat and electricity in LED applications, and finally realize intelligent and systematic LED packaging technology to meet the increasingly complex LED application requirements.

　　Conclusion

　　With the continuous development of LED power, high efficiency, low cost and high reliability, the requirements for packaging technology will become more and more stringent, especially packaging materials and packaging processes. Packaging technology is relatively complex and requires comprehensive consideration of optical, thermal, electrical, structural and other factors. At the same time, low thermal resistance, stable packaging materials and novel and excellent packaging structures are still the key to LED packaging technology. With the perfect combination of new packaging materials and new packaging structures, comfortable, beautiful and intelligent LED lighting products will continue to emerge.