High power LED packaging mainly involves light, heat, electricity, structure and technology. These factors are independent and influence each other. Among them, light is the purpose of LED packaging, heat is the key, electricity, structure and technology are the means, and performance is the embodiment of packaging level. In terms of process compatibility and production cost reduction, LED packaging design should be carried out at the same time as chip design, that is to say, packaging structure and process should be considered in chip design. Otherwise, after the chip manufacturing is completed, the chip structure may be adjusted due to the need of packaging, thus prolonging the product development cycle and process cost, sometimes even impossible.
Specifically, the key technologies of high-power LED packaging include:
1、 Low thermal resistance packaging technology
For the existing LED light efficiency level, because about 80% of the input power is converted into heat, and the LED chip area is small, so chip heat dissipation is the key problem that must be solved in LED packaging. It mainly includes chip layout, packaging material selection (substrate material, thermal interface material), technology, heat sink design, etc.
The thermal resistance of LED package 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 realize the heat exchange with the outside world. Commonly used heat dissipation substrate materials include silicon, metal (such as aluminum, copper), ceramics (such as AlN, SiC) and composite materials. For example, Nichia's third generation led uses CuW as the substrate, and the 1mm chip is inverted on CuW substrate, which reduces the thermal resistance of packaging and improves the luminous power and efficiency; lamina ceramics has developed the low-temperature CO fired ceramic metal substrate, as shown in Figure 2 (a), and developed the corresponding led sealing technology. Firstly, high-power LED chips and corresponding ceramic substrates suitable for eutectic welding are prepared, and then led chips and substrates are directly welded together. Due to the integration of eutectic welding layer, electrostatic protection circuit, driving circuit and control compensation circuit on the substrate, not only the structure is simple, but also due to the high thermal conductivity of the material, less thermal interface, greatly improving the heat dissipation performance, a solution for high-power LED array packaging is proposed. The high thermal conductivity copper-clad ceramic plate developed by curmilak company in Germany is made of ceramic substrate (AlN or) and conductive layer (Cu) sintered under high temperature and pressure, without using binder, 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 coefficient of thermal expansion is (equivalent to the coefficient of thermal expansion of silicon), which reduces the thermal stress of packaging.
The results show that the packaging interface has a great influence on the thermal resistance. If the interface can not be treated correctly, it is difficult to get a good heat dissipation effect. For example, the interface with good contact at room temperature may have interface gap at high temperature, and warpage of substrate may also affect bonding and local heat dissipation. The key to improve the LED package is to reduce the thermal resistance of interface and interface contact and enhance the heat dissipation. Therefore, it is very important to choose the thermal interface material (TIM) between the chip and the heat dissipation substrate. The commonly used Tim for LED packaging is conductive adhesive and thermal conductive adhesive. Due to the low thermal conductivity, generally 0,5-2,5w / MK, the interface thermal resistance is very high. Using low temperature or eutectic solder, solder paste or conductive adhesive mixed with nano particles as thermal interface material can greatly reduce the thermal resistance of interface.
2、 Packaging structure and technology of high optical efficiency
In the process of using LED, the loss of photons produced by radiation recombination is mainly composed of three aspects: internal structure defects of chip and absorption of materials; reflection loss caused by refractive index difference of photons at the exit interface; and total reflection loss caused by the incident angle greater than the critical angle of total reflection. As a result, a lot of light cannot be emitted from the chip to the outside. By coating a transparent adhesive layer (potting adhesive) with relatively high refractive index on the chip surface, as the adhesive layer is between the chip and the air, the loss of photons at the interface is effectively reduced, and the optical efficiency is improved. In addition, the function of the potting glue also includes mechanical protection of the chip, stress release, and as an optical waveguide structure. Therefore, it is required to have high transmittance, high refractive index, good thermal stability, good fluidity and easy to spray. In order to improve the reliability of LED packaging, it is also required that the potting adhesive has the characteristics of low moisture absorption, low stress and aging resistance. At present, the commonly used potting adhesives include epoxy resin and silica gel. Due to its high transmittance, high refractive index, good thermal stability, small stress and low moisture absorption, silica gel is obviously superior to epoxy resin. It is widely used in high-power LED packaging, but its cost is high. The results show that increasing the refractive index of silica gel can effectively reduce the photon loss caused by the refractive index physical barrier and improve the external quantum efficiency, but the performance of silica gel is greatly affected by the ambient temperature. With the increase of temperature, the thermal stress inside the silica gel increases, which leads to the decrease of refractive index of silica gel, thus affecting the light efficiency and light intensity distribution of LED.
The function of phosphor is to combine light and color to form white light. Its characteristics mainly include particle size, shape, luminous efficiency, conversion efficiency, stability (thermal and chemical), among which luminous efficiency and conversion efficiency are the key. The results show that with the increase of temperature, the quantum efficiency of phosphor decreases, the output light decreases, and the radiation wavelength also changes, resulting in the change of color temperature and chroma of white LED, and the aging of phosphor will be accelerated at a higher temperature. The reason is that the fluorescent powder coating is composed of epoxy or silica gel and fluorescent powder, which has poor heat dissipation performance. When it is radiated by violet light or ultraviolet light, it is easy to occur temperature quenching and aging, which reduces the luminous efficiency. In addition, there are some problems in the thermal stability of potting glue and phosphor at high temperature. Because the size of commonly used phosphors is more than 1um, the refractive index is greater than or equal to 1,85, while the refractive index of silica gel is generally about 1,5. Due to the mismatch of refractive index between the two and the fact that the size of phosphor particles is far larger than the light scattering limit (30 nm), light scattering exists on the surface of phosphor particles, which reduces the light output efficiency. By adding nano phosphor 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 (10% - 20%) can be improved, and the light color quality can be effectively improved.
The traditional coating method of phosphor is to mix phosphor and potting glue, and then dot on the chip. Because the coating thickness and shape of phosphor can not be precisely controlled, the color of the emitted light is not consistent, resulting in blue light or yellow light. The conformal coating technology developed by Lumileds company can realize the uniform coating of fluorescent powder and ensure the uniformity of light and color, as shown in Figure 3 (b). However, the results show that when the phosphor is directly coated on the surface of the chip, the light efficiency is low due to the existence of light scattering. In view of this, Rensselaer Institute of the United States proposed a scattered photon extraction method (SPE). By arranging a focusing lens on the chip surface and placing the glass plate containing fluorescent powder at a certain position away from the chip, not only the reliability of the device is improved, but also the optical efficiency is greatly improved (60%), as shown in Figure 3 (c).
In general, in order to improve the light efficiency and reliability of LED, the encapsulation adhesive layer is gradually replaced by high refractive index transparent glass or microcrystalline glass. By adding or coating the fluorescent powder on the glass surface, not only the uniformity of the fluorescent powder is improved, but also the encapsulation efficiency is improved. In addition, reducing the number of optical interfaces in the light direction of LED is also an effective measure to improve the light efficiency.
3、 Array packaging and system integration technology
After more than 40 years of development, LED packaging technology and structure has experienced four stages, as shown in Figure 4.
1. Lamp LED package
Pin packaging is a commonly used 3-5mm packaging structure. It is generally used for LED packaging 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 display screen in large-scale integration. Its disadvantage is that the package thermal resistance is large (generally higher than 100k / W), and the service life is short.
2. Smt-led package
Surface mount technology (SMT) is a kind of packaging technology that can directly paste and weld the packaged devices to the designated position on the PCB surface. Specifically, it is to use specific tools or equipment to align the chip pin with the pad pattern coated with adhesive and solder paste in advance, and then directly mount it on the PCB surface without drilling the mounting hole. After wave soldering or reflow soldering, reliable mechanical and electrical connection between the device and the circuit can be established. SMT technology has the advantages of high reliability, high frequency characteristics, easy to realize automation and so on. It is the most popular packaging technology and process in the electronic industry.
3. On board chip direct mount (COB) led package
Cob is the abbreviation of chip on board. It is a kind of packaging technology that directly pastes LED chip onto PCB by adhesive or solder, and then realizes the electrical interconnection between chip and PCB by wire bonding. PCB can be FR-4 material with low cost (glass fiber reinforced epoxy resin), or metal base or ceramic base composite material with high thermal conductivity (such as aluminum base plate or copper-clad ceramic base plate). The lead wire bonding can be made by using the hot ultrasonic bonding (gold wire ball welding) at high temperature and the ultrasonic bonding (aluminum cleaver welding) at normal temperature. Cob technology is mainly used in LED packaging of high-power multi chip array. Compared with SMT, it not only greatly improves the packaging power density, but also reduces the packaging thermal resistance (generally 6-12w / m, K).
4. System package (SIP) led package
SIP (system in package) is a new type of packaging and integration method developed on the basis of system on chip (SOC) in recent years to meet the requirements of portable development and system miniaturization of the whole machine. For SIP led, not only multiple light-emitting chips can be assembled in one package, but also various types of devices (such as power supply, control circuit, optical microstructure, sensor, etc.) can be integrated together to build a more complex and complete system. Compared with other packaging structures, SIP has the advantages of good process compatibility (existing electronic packaging materials and processes can be used), high integration, low cost, more new functions, easy block testing, short development cycle, etc. According to different technology types, SIP can be divided into four types: chip stack type, die set type, MCM type and three-dimensional (3D) packaging type.
At present, in order to replace incandescent lamp and high pressure mercury lamp, the total luminous flux, or usable luminous flux, must be increased. The increase of optical flux can be realized by improving integration, increasing current density and using large-scale chips. All of these will increase the power density of LED, such as poor heat dissipation, which will lead to the increase of junction temperature of LED chip, thus directly affect the performance of LED devices (such as the reduction of luminous efficiency, red shift of outgoing light, and the reduction of life span). At present, multi chip array packaging is the most feasible solution to obtain high light flux, but the density of LED array packaging is limited by the price, available space, electrical connection, especially heat dissipation and other issues. Due to the high density integration of light-emitting chips and the high temperature on the heat dissipation substrate, effective heat sink structure and appropriate packaging technology must be adopted. The commonly used heat sink structure is divided into passive and active heat sink. Generally, fins with high rib coefficient are used for passive heat dissipation, which pass between fins and air