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Oct 25 2018

Five Key Technologies of High Power LED Packaging

Introduction 

Article Core

Introduce Technologies of High Power LED Packaging

Purpose

Analyse Appliction of Different Technologies 

Article Name

Five Key Technologies of High Power LED Packaging

Category

Semiconductor Information

Application

High Power LED Packaging

Keywords

High power LED packaging, Fluorescent Powders, Packaging Technology, 

Catalogs  

Sammary
1. Low Thermal Resistance Packaging Process 
2. Packaging Structure and Process with High Luminous Rate 
3.Array Encapsulation and System Integration Technology 
4. Packaging Mass Production Technology 
5. Test and Evaluation of Package Reliability 

 Sammary

       High power LED packaging mainly involves light, heat, electricity, structure and technology. These factors are closely related to each other. Among them, lighting is the purpose of LED packaging, heat dissipation is a key task, energizing, structure and process designing are the means, and performance shows the concrete embodiment of the packaging level. In terms of process compatibility and cost reduction, LED packaging design should be carried out simultaneously with chip design, that is, packaging structure and process should be taken into account in chip design. Otherwise, the chip structure needs to be adjusted after completed due to the needs of the package, thereby prolonging the product development cycle and process cost, and sometimes even causing the product to be infeasible.

Five Key Technologies of High Power LED Packaging

Figure.1 High power LED packaging technology


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

1. Low Thermal Resistance Packaging Process 

        For the existing LED luminescence efficiency level, because about 80% of the input electric energy is converted to heat, and the area of LED chip is small, the heat dissipation of the chip in LED packaging is the key task that must be solved. The process of low thermal resistance packaging 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 material(heat-dissipation substrate and heat sink structure) internal thermal resistance and interface thermal resistance. The function of the heat-dissipation substrate is to absorb the heat generated by the chip and transmit it to the heat sink to realize the heat exchange with the outside. Common heat-dissipation substrate materials include silicon, metals (such as aluminum, copper), ceramics (such as AlN and SiC) and composites, etc.

        For example, in the third generation LED, Nichia Company uses CuW as the substrate and inverted the 1mm chip on it, which reduced the thermal resistance of the package, and improved the luminous power and efficiency; As shown in figure 2 (a), Lamina Ceramics Company has developed the low temperature co-fired ceramic metal substrate and 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 substrate presents a solution for high power LED array packaging as it has advantages of simple structure, high thermal conductivity, small thermal interface and improved heat dissipation performance; 

        The high thermal conductivity copper-clad ceramic plate, developed by Curmilk Company in Germany, is made of ceramic substrate (AlN) and conductive layer (Cu) that sintered under high temperature and high pressure without the use of binder. So it has good thermal conductivity, high strength and strong insulation. As shown in figure 2 (b), the thermal conductivity of AlN is 160 W / mk, and the thermal expansion coefficient is equal to that of silicon, thus reducing the Package thermal stress.

Five Key Technologies of High Power LED Packaging

Figure 2

        The results show that the package interface attaches a great impact on thermal resistance, and if the interface is not handled correctly, it is difficult to dissipate heat. For example, the interfacial gap may exist at high temperature when the interface is in good contact at room temperature, and the warping of the substrate may also affect bonding and local heat dissipation. Reducing interfaces and interface-contact thermal resistance, and enhancing heat-dissipation are the key to improve LED packaging. Therefore, the choice of thermal interface material (TIM) between chip and heat dissipation substrate is very important. Conductive adhesive and thermal conductive adhesive are the common TIM in LED packaging. Low thermal conductivity of 0.5-2.5 W / mK results in high interface thermal resistance. However, using conductive adhesive that has nano-particles inside as a thermal interface material can greatly reduce the interface thermal resistance.


  2. Packaging Structure and Process with High Luminous Rate 

        When applied LED, the loss of photons, which produced by radiation recombination, in outward emission mainly includes three aspects: defective chip internal structure and low materials utilization; the reflection loss of photons at the exit interface due to refractive index difference; and the total reflection loss caused by the incident angle larger than the critical angle of total reflection.

         As a result, a lot of light cannot be sent out of the chip. Since the adhesive layer wraps the chip, it can effectively reduces the loss of photons at the interface by coating a transparent adhesive layer(pouring sealant), which has relatively high refractive index on the chip surface, thereby improving extraction efficiency. In addition, pouring sealant plays the role of mechanical protecting the chip, stress releasing, and being a photoconductive structure.

        Therefore, the adhesive layer is required to have high light transmittance, high refractive index, good thermal stability, good fluidity, and to be easy spraying. In order to improve the reliability of the LED package, the pouring sealant is also required to have low hygroscopicity, low stress, and aging resistance.

        The commonly used sealants include epoxy resin and silica gel. Silica gel is better than epoxy resin due to its advantages of high light transmittance, large refractive index, good thermal stability, low stress and low hygroscopicity. And it is widely used in high power LED packaging, but its cost is high. In addition, it is shown that increasing the refractive index of silica gel can effectively reduce the photon loss caused by the refractive index and improve the external quantum efficiency. However, the properties of silica gel are greatly affected by ambient temperature. With the increase of temperature, thermal stress inside the silica gel increases, which results in the decrease of the refractive index, affecting the luminous efficiency and light intensity distribution of the LED.

        The function of fluorescent powder lies in the combination of light and color to form white light. Its characteristics include particle size, shape,luminous efficiency, conversion efficiency, thermal and chemical stability, etc. Luminous efficiency and conversion efficiency are the key. It is shown that with the increase of temperature, the quantum efficiency of fluorescent powders decreases, the emission decreases, and the radiation wavelength changes, which results in the change of colour temperature and chromaticity of white LED, thereby accelerating the aging of fluorescent powders. The reason is that the fluorescent powder coating is from fluorescent powder mixing with epoxy or silica gel, and has poor heat-dissipation property. When exposed to violet or ultraviolet radiation, temperature quenching and aging are liable to occur, resulting in reduced luminous efficiency.

        In addition, the thermal stability problem of potting and fluorescent powders exsists at high temperature. The size of common fluorescent powders is above 1um, the refractive index is no less than 1.85. The refractive index mismatch as that of silica gel is about 1.5, and the size of the fluorescent powder particle is much larger than that of the light scattering limit (30nm), so fluorescent powder has light scattering on its surface, which reduces the output efficiency. By adding nano-fluorescent powders into silica gel, the refractive index can be increased to more than 1.8, the light scattering can be reduced, the light efficiency of LED can be improved (10- 20%) and the quality of light and color can be improved.

        The traditional fluorescent powder coating method is to mix the fluorescent powder with the coating glue and then apply it on the chip. Because the coating thickness and shape of fluorescent powders can not be controlled accurately, the emitted color of the fluorescent powders is inconsistent, and there may be blue or yellow light. The conformal coating technology developed by Lumileds Company can realize the uniform coating of fluorescent powders and ensure the uniformity of light and color, as shown in figure 3 (b). However, when the fluorescent powder is directly coated on the chip surface, the light efficiency is low due to the existence of light scattering. In view of this, the Rensselaer Institute of the United States proposed a Scattered Photon Extraction method(SPE). By placing a focusing lens on the surface of the chip and placing the glass containing fluorescent powder in a certain position from the chip, it not only improved the reliability of the device but greatly improved the light efficiency to 60%, as shown in Fig. 3 (c).

Five Key Technologies of High Power LED Packaging

Figure 3

        In general, to improve the luminous efficiency and reliability of LED, the encapsulated adhesive is gradually replaced by high refractive index transparent glass or glass-ceramics. Doping phosphors or coating it on the glass surface can not only improves the uniformity of phosphors, but also improves the packaging efficiency. In addition, reducing the number of optical interfaces of LED is also an effective measure to improve the light efficiency.


3.Array Encapsulation 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.

Five Key Technologies of High Power LED Packaging

Figure 4 Development of packaging technology and structure

        ① Lamp LED Packaging 

        Lamp LED packaging is a commonly encapsulation structure of 3-5mm. It is generally used in LED packaging with low current (20-30mA) and low power (less than 0.1W). Mainly used for instrument display or indication, large-scale integration can also be used as a display screen. It has disadvantages of large thermal resistance (generally higher than 100K/W) and short life.

        ② SMT-LED Packaging 

        Surface mount technology (SMT) is a kind of packaging technology which can directly weld the encapsulated device to the designated position of the PCB surface. Specifically, using a specific tool or device to point the chip pin at the pads precoated with adhesive and paste, and then directly attach it to the surface of the PCB that has not been drilled, after wave soldering or reflow welding, a reliable mechanical and electrical connection between the device and the circuit is established. SMT technology, the most popular packaging technology in electronic industry, has the advantages of high reliability, high frequency characteristic, easy automation and so on.

        ③ COB LED Packaging 

        Chip On Board (COB) is a packaging technology that directly attach the LED chip to the PCB through glue or solder, and the electrical interconnection between chip and PCB is realized by wire bonding. The PCB can not only be low-cost FR-4 material (Glass fiber reinforced epoxy resin), but metal matrix or ceramic matrix composites with high thermal conductivity (such as aluminum substrates or copper clad ceramic substrates, etc.). Wire bonding can adopt hot ultrasonic bonding (gold wire ball welding) at high temperature and ultrasonic bonding at room temperature. COB technology is mainly used in LED packaging of high power multi-chip array. Compared with SMT, COB technology can not only greatly improves the power density of the package, but also reduces the thermal resistance of the package (generally 6-12W/mK).

        ④ SiP LED Packaging 

        Based on the System on Chip (SOC), System in Package (SiP) is a new packaging and integration method to meet the requirements of portable development and miniaturization of the whole system. For SiP-LED, it is not only to assemble multiple luminous chips in a single package, but to integrate various types of devices (such as power supply, control circuits, optical microstructures, sensors, etc.) into a more complex and complete system. Compared with other packaging structures, SiP has better process compatibility (SiP can use existing packaging material and process). Including easy to block test, SiP has the advantages of high integration, low cost, more new functions, short development cycle and so on. According to the type of technology, SiP can be divided into four types: chip stacking, module, MCM and three dimensional (3D) encapsulation.

        At present, in order to replace incandescent and high pressure mercury lamps, the high brightness LED devices must improve the total or the available luminous flux. The increase of luminous flux can be achieved by increasing integration, increasing current density or using large size chips. But all of these will increase the power density of LED, such as poor heat-dissipation, which will increase the junction temperature of LED chip and thereby directly affect the performance of LED devices (such as decreased luminous efficiency, red shift of outgoing light, lower lifetime, etc.). 

        Multi-chip array packaging is one of the most feasible methods to obtain high luminous flux at present, but the density of LED array packaging is limited by price, available space, electrical connection, especially heat-dissipation and so on. Due to the high density integration of the luminous chip, the temperature on the heat-dissipation substrate is very high, so it is necessary to adopt effective heat sink structure and proper packaging technology. The commonly used heat sink structure is divided into passively and actively heat dissipation. The passively heat dissipation usually uses the fin with high rib coefficient and dissipates the heat into the environment through the natural convection between the fin and the air. The scheme is simple in structure and high in reliability, but due to the low heat transfer coefficient of natural convection, it is only suitable for packaging with low power density and low integration. For high-power LED packaging, actively heat dissipation must be used, such as fin + fan, heat pipe, liquid forced convection, microchannel cooling, phase change cooling, etc.


4. Packaging Mass Production Technology 

        Wafer bonding technology refers to making chip structures and packaging circuits on wafer, and then cutting wafer to form a single chip. Die bonding refers to after the chip structure and circuit completed on wafer, cutting the wafer to form the die, and then package single die (similar to the current LED packaging process), as shown in figure 5.

        Obviously, the wafer bonding is more efficient and of higher quality. Because the cost of packaging accounts for a large proportion of the manufacturing cost of LED devices, changing the existing LED packaging form (from die bonding to wafer bonding) will greatly reduce the packaging and manufacturing cost. In addition, wafer bonding can also prevent the damage of the device structure caused by scribing and slicing before bonding, improve the cleanliness of LED device production, the package yield and reliability. Therefore, wafer bonding is an effective mean to reduce packaging cost.

Five Key Technologies of High Power LED Packaging

Figure 5

        In addition, for high power LED packaging, it is necessary to adopt few packaging steps as far as possible in the process of chip design and package design. It can simplify the package structure and minimize the number of thermal and optical interfaces so as to reduce the thermal resistance of the package, and improve the efficiency of light production.


5. Test and Evaluation of Package Reliability 

        The failure modes of LED devices mainly include electrical failure (such as short circuit or open circuit), optical failure (such as high temperature yellowing of sealant, deterioration of optical properties, etc.) and mechanical failure (such as lead breakage, desoldering, etc.), and all these factors are related to the packaging structure and process. The service life of LED is defined by the mean time to failure (MTTF), and it generally refers to the LED output flux attenuation to the initial 70% (for display purposes generally defined as 50% of the initial value) of the service time. Due to the long life of LED, the method of accelerated environmental test is usually used to test and estimate reliability. Test content mainly includes high temperature storage (100 ℃, 1000h), low temperature storage (-55 ℃, 1000h), high temperature and high humidity (85 ℃ / 85 ~ 1000h), high and low temperature cycle (85 ℃ ~ -55 ℃), thermal shock, corrosion resistance, resistance to solubility, mechanical impact and so on. However, accelerated environmental testing is only one aspect of the problem, and the research on the prediction mechanism and method of LED life is still a difficult problem to be studied.




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