The multi-chip LED integrated package is one of the ways to achieve high-power white LED lighting. The article summarizes the characteristics of integrated packaging, from the aspects of product application, packaging mode, thermal processing and optical design, and analyzes the development trend of integrated packaging, with the wide application of high-power white LED in the field of lighting. The integrated package will also be developed rapidly.
At present, there are two ways to achieve high-power LED lighting: one is to package a single high-power LED chip, and the other is to use a multi-chip integrated package.
For the former, with the development of chip technology, the size is increased, the quality is improved, and high-power LED can be realized by high-current driving, but at the same time, it is limited by the chip size.
The latter has greater flexibility and development potential, can change the number of chips according to different illuminance, and it has a high cost performance, making LED integrated packaging one of the mainstream directions of LED packaging.
Integrated packaged product applications
According to reports, in 2001, UOE Corporation of the United States introduced a multi-chip combination of Norlux series LEDs using hexagonal aluminum plates as substrates;
In 2003, Lanina Ceramics introduced a high-power LED array packaged on the company's proprietary low-temperature sintered ceramic (LTCCM) technology on metal substrates;
Panasonic introduced high-power white LEDs in a combination of 64 chips in 2003;
The 6.4W, 8W, and 12W COB LED series light sources introduced by Everlight use multi-chip integration on the MCPCB substrate to reduce heat transfer distance and lower junction temperature.
Based on the analysis of various technical solutions of LED fluorescent lamps, the COB process is used to directly fix the low-power chips on the aluminum substrate to produce COB LED fluorescent lamps with high efficiency. Since 2009, 45,000 LED fluorescent lamps have been used for 500 Expos. The bus and nearly 4,000 city buses were modified to replace the original fluorescent lamps and received favorable comments from users, serving the Shanghai World Expo and urban traffic.
An LED explosion-proof lamp was developed using a multi-chip integrated package LED light source module, which uses heat pipe cooling technology. The LED explosion-proof lamp has high brightness, long irradiation distance, high reliability, good heat dissipation performance and long service life.
LED integrated package features
Integrated package, also called polycrystalline package, determines the number of LED chips on the substrate base according to the required power. It can be packaged into high-power LED devices with high brightness such as 1W, 2W, 3W, etc. Finally, high refractive index materials are used. The chip is packaged in an optically designed shape.
The unique packaging principle of the integrated package determines its many advantages, such as:
(1) As far as China is concerned, the research and development of high-power chips is in a backward position. The use of integrated packaging is a shortcut for development, and is more in line with China's basic national conditions;
(2) The chip can be designed to be connected in series or in parallel, flexibly adapt to different voltages and currents, facilitate the design of the driver, and improve the light efficiency and reliability of the light source;
(3) The number of chips on a certain area of ​​the substrate can be freely controlled, and can be packaged into a point light source or a surface light source according to the requirements of the customer, and has various forms;
(4) The chip is directly connected to the substrate, which reduces the thermal resistance of the package, and the heat dissipation problem is easy to handle.
However, for integrated packaging, there are also some shortcomings:
(1) Since the multi-chip integrated package is on one substrate, the resulting light source is bulky;
(2) Multiple chips are combined in series and parallel mode, which is less reliable than a single chip, which will cause the overall light source to be affected;
(3) Although the multi-chip package has a strong heat dissipation capability compared to a single chip with the same power, but because of the simultaneous heat dissipation of multiple chips, the degree of heat dissipation is different, which may cause different temperatures between the chips, affecting the life, so the heat dissipation problem Processing is also critical;
(4) The design problem of secondary optics, the multi-chip light-emitting angle is different, and it is necessary to carry out secondary optical design on the basis of one-time optical design to meet the requirements of users.
Mechanical, thermal, and optical research in integrated packaging processes
Due to its outstanding advantages, integrated packaging has become the mainstream of LED packaging. In recent years, it has attracted the attention of many companies and research institutes and carried out a large number of research and applied for related patents. These are extremely Promote the development of integrated packaging technology.
(1) Package structure mode
The current mainstream form of multi-chip integrated package is that multiple chips are directly connected to the substrate in series and parallel, and then the chips are individually packaged or packaged under the same lens.
In the patents applied by Xu Xiangyang, a plurality of chips are directly fixed on an aluminum substrate, and after coating the phosphor, an optical lens is sealed on the outside of each LED chip. The process is simple, the packaging material is simplified, the thermal resistance is reduced, the light efficiency is improved, and the LED lighting product is easily assembled. Compared with the single chip packaging mode of the same power, the COB modular LED packaging technology has many advantages.
Li Jiansheng proposed a COB integrated packaging process based on the layer bonding surface and long heat conduction distance existing in the general integrated package.
That is, the aluminum PCB integrated circuit board is engraved with some reflective cavities which are favorable for the diffusion of the illuminating light of the chip, and a plurality of chips are implanted one by one into the cavity, and a PCB line is drawn around the chip, and the chip electrode leads are soldered thereto, and the conduction circuit is Finally, a barrier is formed around the cavity to form a ring-shaped fence, and silica gel and phosphor are coated therein to form an integrated LED COB assembly at a time.
This design directly connects the chip to the heat sink, which reduces the structural thermal resistance, and the heat dissipation effect is much better than the ordinary package structure, which improves the light output rate of the LED.
Li Binggan et al. used COB technology and array interconnection to prepare white LED light source modules. They applied a phosphor layer on the light-emitting plate to improve the uniformity of light emission and the stability of the phosphor.
At the same time, the array interconnection method is combined with the current derating, which reduces the defects of one chip damage on the working state of other chips in the traditional series and parallel connection methods, and improves the system reliability, and the package structure is simplified. The purpose of the process.
In general, the structural patterns and principles of the integrated package described in different patents are similar. The difference lies mainly in the selected welding method, the coating material of the inner wall of the reflective cavity and the selected substrate, changing the thinking mode of the integrated package to enable integration. The package is used in a wide range of applications in white LED packages.
(2) Heat treatment
Although integrated packaging technology is one of the main directions of packaging, the heat dissipation problem has always been the bottleneck of integrated packaging technology. We know that the LED high-power products usually have a photoelectric conversion efficiency of 20%, and the remaining 80% of the energy is converted into heat. Handling the heat dissipation problem will make the quality of the LED light source a step up.
The heat treatment ideas for integrated packages are currently focused on:
Select a substrate with a high thermal conductivity;
Shorten the distance of heat transfer;
Optimize solid crystal technology and other aspects.
From the aspects of the number of chips and the integration density of the chip, Antze Pure found that the junction temperature of the multi-chip white LED of the integrated package increases with the increase of the number of integrated chips, and the luminous efficiency decreases with the increase of the number of integrated chips. Therefore, the number of chips and the integration density are also important factors in the application of integrated packaging technology.
In the publication No. CN 102042500 A, an improvement scheme is proposed for the heat dissipation performance of the light source module, that is, a columnar heat conduction device is added as a heat dissipation area at the center of the substrate, so that when the light source module is illuminated, the heat generated by each light emitting chip can be further improved. Fast divergence from the substrate.
Among the choices of heat-dissipating substrate materials, ceramic substrates, which have the advantages of good heat dissipation, high temperature resistance and moisture resistance, are becoming the material of choice for high-power LED heat-dissipating substrates. Cheng Zhiguo and others used ceramic substrates (aluminum oxide and aluminum nitride, thickness 0.5 to 1.0 mm) as heat-dissipating substrates, and applied for invention patents.
In the patent, ceramic substrate metallization technology and eutectic soldering technology are used for LED integrated packaging, and the thermal conductivity is greatly improved. The integrated package can make the power of the light source reach 200W.
Luqiao Yin developed a ceramic substrate with a surface layer of LTCC and a bottom layer of AlNx. The integrated package test found that the PN junction temperature was only 70. 8 ° C after long-term lighting. The temperature of the aluminum heat sink connected to the ceramic substrate was observed by ANSYS. 39. At 3 ° C, when the drive current reaches 500 mA, it is only 41.0 °C.
(3) Optical design
High-power LED lighting components typically require two optical designs before becoming a lighting product.
The purpose of an optical design is to remove as much light as possible from the LED chip.
The purpose of the secondary optics design is to allow the light emitted by the entire luminaire system to meet the design requirements.
Due to the presence of multiple chips in the integrated package, the requirements for secondary optical system design are higher!
In order to realize the rectangular spot distribution required for road illumination, Liu Hong et al. established the slope equation of the lens busbar by the law of refraction according to the characteristics of the light source and the spot distribution of the road surface. According to the equation, the LED street lamp lens for rectangular spot distribution is designed. The optimization method was used to simulate and compare the designed lens optical system with Light Tools software, and an optical lens with rectangular spot distribution was obtained.
The simulation results show that the lens optical system has an illumination area of ​​40m & TImes; a 10m rectangular spot with a uniformity of 0.31 under the illumination condition of 10m. This method provides a simple and effective design approach for LED street light lenses with spot size requirements.
Song Chunfa et al. designed a high-power LED lens and its luminaire for multi-chip integrated packaging.
The lens includes a light incident surface and a light exiting surface, and further includes a circular reflecting surface, the light emitting surface is intersected with the reflecting surface, and the light incident surface is a quadric surface, and the curved surface coefficient is: K=-1. 2~-1.5, R = 35 ~ 41mm, the light-emitting surface is a plane, the reflective surface is a quadric surface, the surface coefficient is: K = -0.2. 24 ~ -0.26, R = 23 ~ 29mm.
In this design, the light in the central area of ​​the LED is emitted through the light exiting surface, and the light at the edge of the LED is emitted through the annular reflecting surface, thereby avoiding loss of the LED light energy due to the limited field of view of the lens, thereby maximally collecting the light emitted by the LED and improving the light. The luminous efficiency of the luminaire.
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