The LED package connects 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 casing to realize the connection between the chip and the external circuit, but also fixes and seals the chip to protect the chip circuit from the erosion of water, air and the like to cause electrical performance degradation. In addition, the package can also improve the light-emitting efficiency of the LED chip and provide convenience for installation and transportation of applications in the downstream industry. Therefore, packaging technology plays an important role in the performance and reliability of LEDs. The following is an introduction to LED packaging technology, phosphors and their applications 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 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 (2Î¸1/2) can range from 18 to 120Â°.
(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.
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 converted Light Emitting Diodes (PC-LEDs), which are compared with multi-chip white LEDs in control circuits, production costs, heat dissipation, etc. It has advantages 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+.