LED packaging is to connect the outer leads to the electrodes of the LED chip to facilitate connection with other devices. It not only connects the electrodes on the chip to the package shell with wires to realize the connection between the chip and the external circuit, but also fixes and seals the chip to protect the chip circuit from being corroded by water, air and other substances and causing electrical performance degradation.
Packaging can also improve the light extraction efficiency of LED chips, and provide convenience for the application, installation and transportation of downstream industries. Therefore, packaging technology plays an important role in the performance and reliability of LEDs. The following is an introduction to LED packaging technology, phosphor and its application in LED 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 temperatures, color rendering indexes, varieties and specifications can be produced. According to whether the package has pins, LED can be divided into two types: pin package and surface mount package. The packaging forms of conventional low-power LEDs mainly include: in-line DIP LED, surface mount SMD LED, Piranha LED and PCB integrated packaging. Power LED is the core of future semiconductor lighting, and its packaging is a hot research topic. Several main packaging forms are described below:
- Lead-type packages use lead frames as pins for various package shapes. Round pin LED is a commonly used packaging form. Epoxy resin or silicone resin is commonly used as the encapsulation material in this package, and about 90% of the heat of the chip is transferred from the lead frame to the printed circuit board (PCB), and then dissipated into the surrounding air. The diameter of epoxy resin has specifications such as 7mm, 5mm, 4mm, 3mm and 2mm. The range of light emitting angle (2θ1/2) can reach 18~120°.
- Surface mount package It is an important package form following the pin package. It usually uses a plastic leaded chip carrier (Plastic Leaded Chip Carrier, PLCC), the LED chip is placed in the top groove, and the bottom is sealed with metal chip pins. LED adopts surface mount packaging, which better solves the problems of brightness, viewing angle, flatness, consistency and reliability, and is an important development direction of LED packaging technology at present.
- Power LED packaging Power LEDs are divided into two types: ordinary power LEDs (less than 1W) and watt-level power LEDs (1W and above). Among them, watt-level power LED is the core of future lighting. The single-chip watt-level power LED was first launched by Lumileds in 1998 as the LUXEON LED. The packaging structure is characterized by the use of thermoelectric separation, and the flip chip (Flip Chip) is directly soldered to the heat sink with a silicon carrier, and New structures and new materials such as reflective cups, optical lenses and flexible transparent glue are adopted.
At present, white LEDs are mainly realized in three types: 1) using red, green, and blue LEDs to combine light, that is, multi-chip white LEDs; 2) using blue LED chips and yellow phosphors, which are obtained by complementing blue and yellow colors White light, or use a blue LED chip with red and green phosphors, and mix the blue light emitted by the chip, the red light emitted by the phosphor powder, and the 6-Leaf High Power LED High Bay Light to obtain white light; The powder gets white light. The white light LED obtained by the latter two methods needs to use phosphor powder, which is called phosphor converted LED (phosphor converted Light Emitting Diode, pc-LED). In terms of advantages, it occupies a dominant position in the current LED product market.
Phosphor powder has become one of the key materials in semiconductor lighting technology, and its characteristics directly determine the brightness, color rendering index, color temperature and lumen efficiency of phosphor-converted LEDs. The current yellow phosphors mainly include cerium-activated yttrium aluminum garnet (Y3Al5O12:Ce3+, YAG:Ce) and europium-activated alkaline earth metal silicate; the 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), etc.; Blue phosphors mainly include: BaMg2Al16O27:Eu2+, Sr5(PO4)Cl:Eu2+, Ba5SiO4Cl6:Eu2+ and LiSrPO4:Eu2+, etc.
#3 The Application Of Phosphor Powder In Encapsulation
Before packaging, in addition to determining the packaging structure, it is also necessary to select the chip and phosphor. For cool white LEDs with high color temperature, InGaN chips are usually used with YAG:Ce yellow phosphors. To obtain warm white LEDs with low color temperatures, red phosphors must be added on this basis or ultraviolet chips with tricolor phosphors. There is a matching problem between the LED chip and the phosphor. Only when the emission peak of the LED chip overlaps with the excitation peak of the phosphor to the greatest extent can the efficiency of the LED chip and the phosphor be maximized.
There are still many blue LED chips actually used, and the emission wavelength is generally between 450nm and 470nm. Therefore, we need to develop a series of YAG:Ce phosphors with different color coordinates for LED chips of each emission wavelength, which are 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 led light parts and kits, 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, it is necessary to add red phosphor (such as Sr2Si5N8:Eu2+) and use it with YAG:Ce. Therefore, it is crucial to develop efficient and stable red phosphors for warm white LEDs with high color rendering and low color temperature.