LED packaging technology and application of phosphor in packaging
1. LED packaging technology
According to different application needs, LED chips can be made into devices with different structures and appearances through various packaging methods, and LED products with various color temperature, color rendering index, variety and specifications are produced. LEDs are available in pin-type and surface-mount packages depending on whether the package has pins. Typical low-power LED packages include: in-line DIP LEDs, surface mount SMD LEDs, Piranha LEDs and PCB integrated packages. Power LED is the core of semiconductor lighting in the future, and its packaging is a hot spot of research. Here are a few main package formats:
(1) Lead-type package The lead frame is used as a pin for various package types. Round-headed LEDs are a common form of package. This package is commonly used as an encapsulant for epoxy or silicone. About 90% of the heat is transferred from the lead frame to the printed circuit board (PCB) and then to the surrounding air. The diameter of the epoxy resin is 7mm, 5mm, 4mm, 3mm and 2mm. The illuminating angle (2θ1/2) can range from 18 to 120°.
(2) Surface mount package It is an important package form that appears after the lead package. It usually uses a plastic leaded chip carrier (PLCC) with the LED chip placed in the top recess and the bottom with a metal chip pin. LED surface mount package, which solves the problems of brightness, viewing angle, flatness, consistency and reliability, is an important development direction of LED packaging technology.
(3) Power type LED package Power type LED is divided into two types: ordinary power LED (less than 1W) and watt level power LED (1W and above). Among them, the tile-level power LED is the core of future lighting. The single-chip watt-level power LED was first introduced by Lumileds in 1998. The package structure is characterized by thermoelectric separation, and the Flip Chip is directly soldered to the heat sink with a silicon carrier. New structures and materials such as reflective cups, optical lenses and flexible transparent adhesives are used.
2. Phosphor
At present, white LEDs are mainly realized by three types: 1) using a combination of red, green and blue LEDs, that is, multi-chip white LEDs; 2) using blue LED chips and yellow phosphors, which are complemented by blue and yellow colors. White light, or use blue LED chip with red and green phosphors, blue light emitted by the chip, red light and green light emitted by the phosphor to obtain white light; 3) excitation of trichromatic fluorescence by near ultraviolet light emitted by the ultraviolet LED chip The powder gets white light. The white LEDs obtained in the latter two methods require phosphors, called phosphor co nverted Light Emitting Diodes (PC-LEDs), which are compared with multi-chip white LEDs in control circuits, production costs, and heat dissipation. It has advantages in other aspects and is dominant in the current LED product market.
Phosphor has become one of the key materials in semiconductor lighting technology. Its characteristics directly determine the brightness, color rendering index, color temperature and lumen efficiency of phosphor-converted LEDs. The current yellow phosphors mainly have yttrium activated yttrium aluminum garnet (Y3Al5O12: Ce3+, YAG: Ce) and yttrium activated alkaline earth metal silicate; red phosphors mainly include: Ca1-xSrxS: Eu2+, YVO4: Bi3+, Eu3+ and M2Si5N8 :Eu2+ (M=Ca, Sr, Ba), etc.; green phosphors mainly include: SrGa2S4: Eu2+, M2SiO4: Eu2+ (M=Ca, Sr, Ba) and MSi2N2O2: Eu2+ (M=Ca, Sr, Ba); The blue phosphors mainly include: BaMg2Al16O27: Eu2+, Sr5(PO4)Cl: Eu2+, Ba5SiO4Cl6: Eu2+, and LiSrPO4: Eu2+.
3. Application of phosphor in packaging
In addition to the package structure before packaging, you need to choose the chip and phosphor. For high color temperature cold white LEDs, InGaN chips are usually used together with YAG:Ce yellow phosphors. Warm white LEDs with low color temperature need to add red phosphors or UV chips to match three primary phosphors. There is a matching problem between the LED chip and the phosphor, and the efficiency of the LED chip and the phosphor can be maximized only when the emission peak of the LED chip and the excitation peak of the phosphor are maximally overlapped.
Figure 1 shows the fluorescence spectra of InGaN chips and YAG:Ce phosphors, where the shaded portion on the left is the emission spectrum of the InGaN chip, the light gray shade on the left is the excitation spectrum of YAG:Ce, and the emission spectrum on the right is the excitation at 460 nm. . It can be seen from the figure that the emission spectrum of the InGaN chip and the excitation spectrum of YAG:Ce are very well matched, so that YAG:Ce is under the most effective excitation condition, so that the luminous efficiency of YAG:Ce is the highest. When the excitation main peak of YAG:Ce shifts the emission peak of the InGaN chip to the left or right, the degree of overlap between the two is greatly reduced, resulting in a significant decrease in the luminous efficacy of the LED after packaging.
Fig.1 InGaN chip and YAG:Ce fluorescence spectrumFig.2 LED color coordinates of different YAG:Ce addition amount
Figure 2 is the LED color coordinates of different YAG:Ce phosphor additions, where 1 is the color coordinate of the InGaN blue chip, 7 is the color coordinate of the YAG:Ce phosphor, and 2 to 6 is the YAG:Ce fluorescence. The thin layer of powder is placed on the glass and the color coordinates measured by the LED chip are excited. Two points are added with a layer of YAG:Ce phosphor, three points are added with two layers of YAG:Ce phosphor, and so on. As can be seen from the figure, appropriately adjusting the thickness of the YAG:Ce phosphor can make the color coordinates of the white LED move on the line connecting the chip color coordinate and the fluorescent pink coordinate. In addition, there is a triangle from Fig. 2, and the coordinates of the three vertices are the color coordinates of the red, green and blue phosphors specified by the National Television Standards Committee (NTSC). In Fig. 2, we can also see that there is a black arc. This is the color coordinate curve of the blackbody at different temperatures calculated according to the blackbody radiation formula. It is called the blackbody trajectory, which is an important basis for measuring the color temperature of white LEDs.
Figure 3 Using a phosphor to modulate the color temperature of a white LED
Figure 3 shows the color temperature of a white LED with a phosphor. The left side of Figure 3 shows the color line of the InGaN chip and a series of different YAG:Ce color coordinates and the correlated color temperature line of 4500K~10000K. A partial enlarged view of the white light region. As can be seen from FIG. 3, when the color coordinate of YAG:Ce is close to the green light region, the intersection of the color coordinate line of the InGaN chip and YAG:Ce and the respective color temperature lines is gradually increased as the color temperature decreases. Big. This indicates that the green-green YAG:Ce is not suitable for packaging white LEDs with low color temperature, because if the white LED with low color temperature in the package will make the color coordinates of the white LED deviate larger above the black body track, the color rendering is poor. This will exceed the allowable error limits set by the International Electrotechnical Commission (IEC). Similarly, when the color coordinate of YAG:Ce is close to the orange light region, it is not suitable for packaging white LEDs with high color temperature, so the color coordinates of the encapsulated white LEDs will also deviate greatly below the black body track. Therefore, it is necessary to select the phosphor of the appropriate color coordinate according to the color temperature of the white LED of the package, and adjust the amount of the phosphor to make the color coordinate of the white LED after packaging as close as possible to the black body trajectory, so that it conforms to the regulations of the International Electrotechnical Commission. standard.
Only the case where the YAG:Ce matches the 450 nm blue LED chip is given above. There are still many blue LED chips actually used, and the emission wavelength is generally between 450 nm and 470 nm. Therefore, we need to develop a series of YAG:Ce phosphors with different color coordinates for each LED chip with emission wavelength, which is used to package a series of white LEDs with different color temperatures. For low color temperature white LEDs (below 3300K), YAG:Ce cannot meet the requirements due to the lack of red light components, and needs to be improved. For example, by co-doping YAG with Ce and Pr, the color rendering index (Ra) of the packaged white LED can reach about 83. To obtain a white LED with a color rendering index Ra greater than 90, a red phosphor (such as Sr2Si5N8:Eu2+) is required in combination with YAG:Ce. Therefore, for a warm white LED with high color rendering and low color temperature, it is important to develop an efficient and stable red phosphor.
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