In recent years, under the support of global energy conservation and emission reduction and the support of relevant government policies, LED lighting has developed rapidly. Compared with traditional light sources, it has the advantages of long life, small size, energy saving, high efficiency, fast response, shock resistance and pollution-free. It is considered to be a “green lighting source†that can enter the general lighting field. An inevitable trend.
As the LED package in the LED industry chain, it plays a key role in the entire industry chain. For the package, the key technology is based on how to extract as much light as possible from the chip in a limited cost range, while reducing the package thermal resistance and improving reliability. In the packaging process, packaging materials and packaging methods are the main factors. With the continuous development of LED high efficiency, power, high reliability and low cost, the requirements for packaging are also getting higher and higher. On the one hand, the LED package must meet enough requirements in terms of both the illumination angle and the uniformity of light color. High light extraction efficiency and luminous flux; on the other hand, the package must meet the heat dissipation requirements of the chip. Therefore, chips, phosphors, substrates, thermal interface materials and other packaging materials and corresponding packaging methods need to be developed to improve the heat dissipation and light extraction efficiency of LEDs.
Packaging material
In the packaging process, the performance of the packaging material is the key to determining the long-term reliability of the LED. The reasonable selection and use of high-performance packaging materials can effectively improve the heat dissipation effect of LEDs and greatly extend the service life of LEDs. The packaging materials mainly include chips, phosphors, substrates, and thermal interface materials.
(1) Chip structure
With the continuous development of LED device performance and the wide range of applications, especially the development of a single high-power LED, the chip structure is constantly improving. At present, there are four main types of LED chip packaging structures, namely, a formal structure, a flip-chip structure, a vertical structure, and a three-dimensional vertical structure.
At present, the common LED chip adopts the formal structure of the sapphire substrate, and the structure is simple and the manufacturing process is relatively mature. However, due to the poor thermal conductivity of sapphire, the heat generated by the chip is difficult to transfer to the heat sink, which is limited in power LED applications.
Flip-chip packaging is one of the current development directions. Compared with the formal structure, the heat does not have to pass through the sapphire substrate of the chip, but directly to the silicon or ceramic substrate with higher thermal conductivity, and then is radiated through the metal base. In the external environment.
The vertical structure of the blue chip is produced on the basis of the formal mounting. The chip is a reverse-bonded chip of a conventional sapphire substrate to a substrate such as a silicon substrate or a metal having a good thermal conductivity, and then the sapphire substrate is used. Laser peeling. The chip of this structure solves the problem of heat dissipation bottleneck, but the process is complicated, especially the process of substrate conversion is difficult to implement, and the production qualification rate is also low.
Compared with the vertical structure LED chip, the main advantage of the three-dimensional vertical structure LED chip is that it does not need to be gold wire, so that the thickness of the package is thinner, the heat dissipation effect is better, and a larger driving current is more easily introduced.
(2) Phosphor
As people's requirements for LED light quality are getting higher and higher, phosphors for LEDs of different colors and different systems are gradually developed. Research on high light efficiency, high color rendering index, long-life phosphor development and coating technology has become The essential. At present, the mainstream white light realization form is a blue LED chip combined with a yellow YAG phosphor, but in order to obtain a better illumination effect, a nitride/nitrogen oxide red phosphor, a silicate orange and a green phosphor are also widely used.
The incorporation of multi-color phosphors plays an important role in improving the color rendering index of the light source, broadening the application field of LED light sources, and replacing traditional halogen lamps or metal halide lamps in some occasions where high color reproduction is required. At the same time, people are constantly developing new types of phosphors for LEDs.
The addition of red and green phosphors significantly increases the color rendering index of the source. ZL201210264610.3 [11] discloses a preparation method of blue-excited continuous spectrum phosphor, which uses zinc oxide, cerium oxide, calcium carbonate and other raw materials to adjust the content of activated ions Ce3+ and Eu3+, and can be excited in blue light. A continuous spectrum of 470 to 700 nm is emitted. Phosphors of the same matrix will show more advantages in the packaging process.
Semiconductor nanocrystalline phosphors are also a hot trend in recent years, as they are expected to change the current dependence of LEDs on rare earth materials and break through foreign patent barriers. At the same time, the semiconductor nanocrystalline phosphor has the characteristics of small size, adjustable wavelength, wide luminescence spectrum and small self-absorption, and has a potential market in white LED applications.
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