I. Preface
Due to the complexity of structure and technology, high-power LED packaging has a direct impact on the performance and life of LED. It has been a research hotspot in recent years, especially the high-power white LED packaging.
The functions of LED package mainly include:
1. Mechanical protection to improve reliability;
2. Strengthen heat dissipation to reduce chip junction temperature and improve led performance;
3. Optical control to improve the light efficiency and optimize the beam distribution;
4. Power supply management, including AC / DC conversion, power control, etc.
The choice of LED packaging method, material, structure and process is mainly determined by chip structure, photoelectric / mechanical characteristics, specific application and cost. After more than 40 years of development, LED packaging has experienced the development stages of lamp LED, SMD LED and power LED.
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 structure of LED package. In order to effectively reduce the thermal resistance and improve the optical efficiency, we must adopt a new technology to design the package.
2、 Key technology of high power LED package
High power LED package mainly involves light, heat, electricity, structure and technology, as shown in Figure 1. 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 it is impossible to be specific. 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 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.
（4） Packaging mass production technology
Wafer bonding technology refers to that the chip structure and circuit are made and packaged on the wafer, and then cut to form a single chip after the package is completed; die bonding corresponding to it refers to that after the chip structure and circuit are completed on the chip, the chip is cut to form a die, and then the single chip is cut to form a die Packaging (similar to the current LED packaging process), as shown in Figure 6. Obviously, the efficiency and quality of wafer bonding package are higher. Because packaging cost accounts for a large proportion of the manufacturing cost of LED devices, changing the existing LED packaging form (from chip bonding to chip bonding) will greatly reduce the packaging cost. In addition, wafer bonding packaging can also improve the cleanliness of LED device production, prevent the damage of chip cutting and chip splitting process to the device structure before bonding, improve the packaging yield and reliability, so it is an effective means to reduce the packaging cost.
（5） Packaging reliability test and evaluation
The failure modes of LED devices mainly include electrical failure (such as short circuit or open circuit), optical failure (such as yellowing of potting adhesive caused by high temperature, deterioration of optical performance, etc.) and mechanical failure (such as lead fracture, desoldering, etc.), which are related to packaging structure and process. The service life of LED is defined by mean time to failure (MTTF). For lighting purposes, it generally refers to the service time when the attenuation of LED output luminous flux is 70% of the initial value (50% of the initial value for display purposes). Because of the long life of LED, the method of accelerated environmental test is usually used for reliability test and evaluation. The test contents mainly include 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, solubility resistance, mechanical shock, etc. However, the accelerated environmental test is only one aspect of the problem, and the research of LED life prediction mechanism and method is still a problem to be studied.
