I. Introduction High-power LED package has been a research hotspot in recent years due to its complicated structure and process, and directly affects the performance and life of LED. In particular, high-power white LED package is a hot topic in research. The functions of the LED package mainly include: 1. mechanical protection to improve reliability; 2. enhanced heat dissipation to reduce chip junction temperature and improve LED performance; 3. optical control, improve light extraction efficiency, optimize beam distribution; 4. power supply management, Includes AC/DC transitions, as well as power control.
The choice of LED packaging methods, materials, structures, and processes is primarily determined by factors such as chip structure, optoelectronic/mechanical characteristics, specific applications, and cost. After more than 40 years of development, LED packaging has experienced the development stages of stent (LampLED), SMD (SMDLED), and power LED (PowerLED). With the increase of chip power, especially the development of solid-state lighting technology, new and higher requirements are put forward for the optical, thermal, electrical and mechanical structures of LED packages. In order to effectively reduce the thermal resistance of the package and improve the light extraction efficiency, a new technical idea must be adopted for the package design.
Second, high-power LED packaging key technology High-power LED package mainly involves light, heat, electricity, structure and technology, as shown in Figure 1. These factors are 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 the means, and performance is the concrete embodiment of the packaging level. In terms of process compatibility and lower production costs, the LED package design should be carried out simultaneously with the chip design, that is, the package design and process should be considered in the chip design. Otherwise, after the chip is manufactured, the chip structure may be adjusted due to the needs of the package, thereby prolonging the product development cycle and process cost, sometimes even impossible.
Figure 1 High-power white LED packaging technology
(1) Low thermal resistance packaging process
For the existing LED light effect level, since about 80% of the input power is converted into heat, and the LED chip area is small, the chip heat dissipation is a key problem that the LED package must solve. It mainly includes chip layout, packaging material selection (substrate material, thermal interface material) and process, heat sink design and so on.
LED package thermal resistance mainly includes internal thermal resistance and interface thermal resistance of materials (heat dissipation substrate and heat sink structure). 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. Commonly used heat sink materials include silicon, metals (such as aluminum, copper), ceramics (such as Al2O3, AlN, SiC) and composite materials. For example, Nichia's third-generation LED uses CuW as the substrate, and the 1mm chip is flip-chip mounted 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. The technique first prepares a high-power LED chip suitable for eutectic soldering and a corresponding ceramic substrate, and then directly solders the LED chip to the substrate. Since the eutectic solder layer, the electrostatic protection circuit, the driving circuit and the control compensation circuit are integrated on the substrate, the structure is simple, and the thermal conductivity of the material is high, the thermal interface is small, and the heat dissipation performance is greatly improved, and the high-power LED array package is provided. Proposed a solution. The high thermal conductivity copper-clad ceramic plate developed by Curmilk Company of Germany is made of ceramic substrate (AlN or Al2O3) and conductive layer (Cu) sintered under high temperature and high pressure. No adhesive is used, so the thermal conductivity is good, the strength is high, and the insulation is good. Strong, as shown in Figure 2 (b). Among them, aluminum nitride (AlN) has a thermal conductivity of 160 W/mk and a thermal expansion coefficient of 4.0 ???10-6/°C (corresponding to a thermal expansion coefficient of silicon of 3.2 ???10-6/°C), thereby reducing the package. Thermal Stress.
Figure 2 (b) Schematic cross-section of a copper-clad ceramic substrate
Studies have shown that the package interface has a great influence on the thermal resistance. If the interface cannot be processed correctly, it is difficult to obtain a good heat dissipation effect. For example, a well-contacted interface at room temperature may have interfacial gaps at high temperatures, and warpage of the substrate may also affect bonding and local heat dissipation. The key to improving LED packaging is to reduce interface and interface contact thermal resistance and enhance heat dissipation. Therefore, the choice of thermal interface material (TIM) between the chip and the heat sink substrate is important. The TIM commonly used in LED packaging is conductive adhesive and thermal conductive adhesive. Due to the low thermal conductivity, it is generally 0.5-2.5 W/mK, resulting in high interface thermal resistance. The use of low temperature or eutectic solder, solder paste or conductive paste with nano-particles as the thermal interface material can greatly reduce the interface thermal resistance.
(2) High light extraction rate package structure and process
In the process of using LED, the loss of photon generated by radiation recombination mainly includes three aspects: internal structural defects of the chip and absorption of materials; reflection loss of photons at the exit interface due to refractive index difference; The total reflection loss caused by the incident angle being greater than the critical angle of total reflection. Therefore, a lot of light cannot be emitted from the chip to the outside. By coating a surface of the chip with a relatively high refractive index transparent layer (potting glue), 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.
Figure 3 High-power white LED package structure
In addition, the potting function 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 the LED package, the potting compound is also required to have low hygroscopicity, low stress, and aging resistance. Currently used potting compounds include epoxy resin and silica gel. Because of its high light transmittance, large refractive index, good thermal stability, low stress and low hygroscopicity, silica gel is superior to epoxy resin and is widely used in high-power LED packaging, but the cost is high. Studies have shown that increasing the refractive index of silica gel can effectively reduce the photon loss caused by the physical barrier of refractive index and improve the external quantum efficiency, but the performance of silica gel is greatly affected by the ambient temperature. As the temperature increases, the thermal stress inside the silica gel increases, causing the refractive index of the silica gel to decrease, thereby affecting the LED light efficiency and light intensity distribution.
The role of the phosphor is to combine light and color to form white light. Its characteristics mainly include particle size, shape, luminous efficiency, conversion efficiency, stability (heat and chemistry), etc., among which luminous efficiency and conversion efficiency are key. Studies have shown that as the temperature rises, the quantum efficiency of the phosphor decreases, the light emission decreases, and the radiation wavelength also changes, which causes the color temperature and chromaticity of the white LED to change, and the higher temperature accelerates the aging of the phosphor. The reason is that the phosphor coating is prepared by epoxy or silica gel and phosphor, and the heat dissipation performance is poor. When subjected to ultraviolet light or ultraviolet light, temperature quenching and aging are liable to occur, and the luminous efficiency is lowered. In addition, the thermal stability of potting and phosphors at high temperatures is also problematic. Since the common phosphor size is above 1 um, the refractive index is greater than or equal to 1.85, and the refractive index of the silica gel is generally about 1.5. Due to the mismatch in refractive index between the two, and the phosphor particle size is much larger than the light scattering limit (30 nm), light scattering exists on the surface of the phosphor particles, which reduces the light extraction efficiency. By incorporating nano-phosphor in silica gel, the refractive index can be increased to above 1.8, the light scattering can be reduced, the LED light-emitting efficiency can be improved (10%-20%), and the light color quality can be effectively improved.
The traditional method of phosphor coating is to mix the phosphor with the potting compound and then apply it to the chip. Due to the inability to accurately control the thickness and shape of the phosphor coating, the color of the emitted light is inconsistent, and blue or yellowish light appears. The Conformal coating technology developed by Lumileds can achieve uniform coating of phosphors and ensure the uniformity of light color, as shown in Figure 3(b). However, studies have shown that when the phosphor is directly coated on the surface of the chip, the light extraction efficiency is low due to the presence of light scattering. In view of this, the Rensselaer Institute of the United States proposed a Scattered Photon Extraction (SPE) method by placing a focusing lens on the surface of the chip and placing the phosphor-containing glass sheet at a certain position from the chip. Improved device reliability and greatly improved light efficiency (60%), as shown in Figure 3(c).
Figure 3 High-power white LED package structure
In general, in order to improve the light-emitting efficiency and reliability of LEDs, the encapsulant layer has a tendency to be gradually replaced by high-refractive-index transparent glass or glass-ceramics, and the phosphor is not only improved by being doped or externally coated on the glass surface. The uniformity of the phosphor and the increase in packaging efficiency. In addition, reducing the number of optical interfaces in the direction in which the LEDs are emitted is also an effective measure to improve the light extraction efficiency.
Figure 4 LED packaging technology and structure development
(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.
1, lead (Lamp) LED package
The lead package is a commonly used Æ 3-5mm package structure. Generally used for LED packages with low current (20-30mA) and low power (less than 0.1W). It is mainly used for instrument display or indication, and can also be used as a display screen for large-scale integration. The disadvantage is that the package has a large thermal resistance (generally higher than 100K/W) and has a short life.
2, surface mount (sMD) type (SMT-LED) package
Surface Mount Technology (SMT) is a packaging technology that directly attaches and solders packaged devices to specified locations on the surface of the PCB. Specifically, the chip is pinned to a pad pattern previously coated with an adhesive and solder paste using a specific tool or device, and then directly attached to the surface of the PCB on which the hole is not drilled, and subjected to wave soldering. Or reflow soldering to establish a reliable mechanical and electrical connection between the device and the circuit. SMT technology has the advantages of high reliability, high frequency characteristics, and easy automation. It is the most popular packaging technology and process in the electronics industry.
3, chip-on-board (COB) LED package
COB is the abbreviation of Chip On Board. It is a kind of adhesive that directly pastes the LED chip onto the PCB through adhesive or solder, and then electrically interconnects the chip and the PCB through wire bonding. Packaging technology. The PCB board can be a low cost FR-4 material (glass fiber reinforced epoxy resin) or a high thermal conductivity metal based or ceramic based composite material (such as an aluminum substrate or a copper clad ceramic substrate). The wire bonding can be performed by thermosonic bonding (gold wire ball bonding) at a high temperature and ultrasonic bonding (aluminum boring tool welding) at a normal temperature. COB technology is mainly used for LED packaging of high-power multi-chip arrays. Compared with SMT, it not only greatly increases the package power density, but also reduces the package thermal resistance (generally 6-12W/mK).
4, system package (SiP) LED package
SiP (System in Package) is a new type of package integration method developed on the basis of system on chip (SOC) in order to meet the requirements of portable development and system miniaturization of the whole machine in recent years. For SiP-LEDs, not only can multiple light-emitting chips be assembled in one package, but also various types of devices (such as power supplies, control circuits, optical microstructures, sensors, etc.) can be integrated into one. For a complex, complete system. Compared with other package structures, SiP has good process compatibility (using existing electronic packaging materials and processes), high integration, low cost, more new functions, easy block testing, short development cycle, etc. . Depending on the type of technology, SiP can be divided into four types: chip-stacked, modular, MCM and three-dimensional (3D) packages.
At present, high-brightness LED devices have to replace incandescent lamps and high-pressure mercury lamps, and it is necessary to increase the total luminous flux, or the luminous flux that can be utilized. The increase in luminous flux can be achieved by increasing integration, increasing current density, and using large-size chips. These will increase the power density of the LED, such as poor heat dissipation, which will lead to an increase in the junction temperature of the LED chip, which directly affects the performance of the LED device (such as reduced luminous efficiency, red shift of emitted light, reduced lifetime, etc.). Multi-chip array packages are currently the most viable solution for achieving high luminous flux, but the density of LED array packages is limited by price, available space, electrical connections, and especially heat dissipation. Due to the high density integration of the light-emitting chip, the temperature on the heat-dissipating substrate is high, and an effective heat sink structure and a suitable packaging process must be employed. Commonly used heat sink structures are divided into passive and active heat dissipation. Passive heat dissipation generally uses fins with a high ribbing coefficient to dissipate heat into the environment through natural convection between the fins and the air. The scheme has simple structure and high reliability, but because of the low natural convection heat transfer coefficient, it is only suitable for the case where the power density is low and the integration degree is not high. For high-power LED packages, active heat dissipation must be used, such as fins + fans, heat pipes, liquid forced convection, microchannel refrigeration, phase change refrigeration, and so on.
In terms of system integration, Taiwan Xinqiang Optoelectronics Co., Ltd. adopted System Packaging Technology (SiP) and developed a 72W, 80W high-brightness white LED light source through a fin + heat pipe with high-efficiency heat dissipation module, as shown in Figure 5 (a ). Due to the low thermal resistance of the package (4.38 ° C / W), when the ambient temperature is 25 ° C, the LED junction temperature is controlled below 60 ° C, thus ensuring the life of the LED and good luminescence performance. Huazhong University of Science and Technology uses COB packaging and micro-injection active cooling technology to package 220W and 1500W ultra-high power LED white light sources, as shown in Figure 5(b).
LED lighting module
(4) Packaging large production technology
Wafer bonding technology refers to the fabrication and packaging of the chip structure and circuit on the wafer (wafer). After the package is completed, the chip is cut to form a single chip; the corresponding chip bonding (Die bonding) means that the chip structure and circuit are completed on the wafer, that is, cutting to form a chip, and then packaging a single chip (similar to the current LED packaging process), as shown in FIG. 6. It is clear that the efficiency and quality of wafer bonded packages is higher. Since packaging costs account for a large percentage of LED device manufacturing costs, changing the existing LED package form (from chip bonding to wafer bonding) will greatly reduce package manufacturing costs. In addition, the wafer bonding package can improve the cleanliness of the LED device production, prevent the dicing and fragmentation process before bonding, damage the device structure, improve the package yield and reliability, and thus is an effective method for reducing the packaging cost. means.
Wafer bonding and chip bonding
In addition, for high-power LED packages, package-less packaging must be used as much as possible in the chip design and package design process, while simplifying the package structure and minimizing the number of thermal and optical interfaces to reduce Encapsulate thermal resistance to improve light extraction efficiency.
(5) Package reliability testing and evaluation
The failure modes of LED devices mainly include electrical failure (such as short circuit or open circuit), light failure (such as high temperature caused by potting yellowing, optical performance degradation, etc.) and mechanical failure (such as lead breakage, desoldering, etc.). Both are related to the package structure and process. The lifetime of an LED is defined by the mean time to failure (MTTF). For lighting purposes, it is generally indicated that the output luminous flux of the LED is attenuated to an initial 70% (typically defined as 50% of the initial value for display usage). Due to the long life of LEDs, accelerated environmental testing is often used for reliability testing and evaluation. The test content mainly includes high temperature storage (100 ° C, 1000 h), low temperature storage (-55 ° C, 1000 h), high temperature and high humidity (85 ° C / 85%, 1000 h), high and low temperature cycle (85 ° C ~ -55 ° C), thermal shock , corrosion resistance, solubility resistance, mechanical shock, etc. However, accelerating environmental testing is only one aspect of the problem. The research on the prediction mechanism and method of LED life is still a difficult problem to be studied.
Third, solid-state lighting requirements for high-power LED packaging
Compared with traditional lighting fixtures, LED fixtures do not require the use of filters or filters to produce colored light, which is not only efficient, pure in color, but also allows for dynamic or gradual color changes. Maintain a high color rendering index while changing the color temperature to meet different application needs. However, new requirements have been put forward for its packaging, which are embodied in:
(a) modular
A good lumen output stack can be achieved by interconnecting multiple LED lights (or modules) to meet the requirements of high brightness illumination. Through modular technology, multiple point light sources or LED modules can be combined in a random shape to meet the lighting requirements of different fields.
(2) Maximizing system efficiency
In order to improve the light-emitting efficiency of LED lamps, in addition to the need for suitable LED power supplies, efficient heat dissipation structures and processes must be employed, as well as optimized internal/external optical design to improve overall system efficiency.
(3) Low cost
In order to go to the market, LED lamps must have a competitive advantage in cost (mainly refers to the initial installation cost), and the package accounts for a large part of the production cost of the entire LED lamp. Therefore, the new package structure and technology are used to improve the light efficiency/cost. Than, is the key to commercialization of LED lamps.
(four) easy to replace and maintain
Due to the long life of the LED light source and low maintenance cost, high requirements are placed on the package reliability of the LED lamp. LED luminaire design is required to be easily modified to meet the future requirements of more efficient LED chip packaging, and the interchangeability of LED chips is required to be good, so that the luminaire manufacturers can choose which chip to use.
The LED luminaire light source can be composed of a plurality of distributed point light sources. Due to the small size of the chip, the packaged luminaire is light in weight, compact in structure, and can meet various shapes and different integration requirements. The only downside is that there are no ready-made design standards, but at the same time provide ample imagination for the design. In addition, the primary goal of LED lighting control is to supply power. Since the mains power supply is high voltage alternating current (220V, AC), and LEDs require constant current or current limiting power supplies, conversion circuits or embedded control circuits (ASICs) must be used to implement advanced calibration and closed loop feedback control systems. In addition, through the use of digital lighting control technology, the use and control of solid-state light sources mainly rely on intelligent control and management software to achieve a new connection between users, information and light sources, and can give full play to the imagination of designers and consumers. force.
Fourth, the conclusion
LED packaging is a research topic involving multidisciplinary (such as optics, thermals, mechanics, electricity, mechanics, materials, semiconductors, etc.). In some ways, LED packaging is not only a manufacturing technology, but also a basic science. Good packaging requires understanding and application of physical properties such as thermal, optical, materials and process mechanics. The LED package design should be carried out simultaneously with the chip design, and the performance of light, heat, electricity, structure and other properties must be considered. In the packaging process, although the choice of materials (heat sink, phosphor, potting glue) is very important, the package structure (such as thermal interface, optical interface) has a great influence on LED light efficiency and reliability, high power white LED package New materials, new processes, and new ideas must be adopted. For LED luminaires, it is necessary to integrate the light source, heat dissipation, power supply and lamps.
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