In the semiconductor lighting industry chain, the package plays a role of connecting the upper and lower. In addition to the basic electrical signal connection, it can also protect the chip from environmental influences, improve the light efficiency and enhance the heat dissipation capability, and even determine the LED performance (affecting the light shape). And light color) is good or bad. This report focuses on the high-efficiency, high-reliability, and intelligent requirements for LED packaging. It covers the package design, packaging materials, packaging process, and integrated design of light, heat, electricity, and machinery.

optical design

Optical performance is the most direct indicator of the quality of LED packaging. Good optical design requires high lumen efficiency, high light controllability, high spatial color consistency, etc. Therefore, LEDs must not only save energy, but also adapt to the comfort requirements of the human eye and improve the lighting quality. Therefore, computer optical simulation, designing a unique lens, has become an important part of the package. In addition, the optical design also involves the coating process of the phosphor. From the early dispensing process to the conformal coating process, the uniformity and uniformity of the LED color are better, which is in line with the development of the packaging process.

Electrical Design

Good electrical design should ensure the safety and reliability of the LED. Due to the easy-to-control characteristics of LEDs, future LED applications should be combined with control technology, so combining with control technology at the package level is one of the options for the future.

Thermal design

LED heat dissipation design is an extremely important part of the packaging process, which directly determines the efficiency and life of the LED. At present, the high-power LED light efficiency is greatly improved, but more than 50% of the electrical energy is converted into heat energy, and the heat dissipation is poor, resulting in an increase in the junction temperature of the chip, thereby reducing the internal quantum efficiency of the chip, affecting the life span, and directly causing failure. In addition, after the packaged LED, the heat source is not only from the chip, but also from the phosphor particles, silica gel, frame self-absorption, and the like. Therefore, it is necessary to comprehensively consider the heat source and perform a good package heat dissipation design to effectively dissipate the heat. The main method to enhance the heat dissipation of the LED package is to make the interface of the heat dissipation path larger and the path shorter. The heat dissipation path often experiences the heat dissipation interface, and the interface connection is often the bottleneck of heat dissipation, so it is necessary to focus on the design.

Reliability design

The reliability design of the package involves material selection, process selection, reasonable operation, and compact structure. Good packaging needs to reduce deformation, voids, delamination, cracks, etc.

To meet the above-mentioned LED package design requirements, it is necessary to improve the packaging materials, including heat sink substrates, phosphors, thermal interface materials, silicone materials, lead materials, etc. For this purpose, the packaging materials are elaborated in this section.

In terms of substrate materials, the early package was adapted to low-power LEDs, using metal brackets, ranging from in-line LED brackets, piranha LED brackets, and patch LED brackets to high-power LED brackets. However, since the thermal expansion coefficient of the metal material and the thermal expansion coefficient of the LED chip are largely mismatched, considering the LED duty cycle and the junction temperature change, it is easy to cause micro cracks and increase the interface thermal resistance, so the metal substrate is not the best choice. The ceramic substrate has a thermal expansion coefficient close to that of the sapphire chip, and some ceramics have good thermal conductivity, such as AlN ceramics, which are close to the thermal conductivity of the metal, and thus can be used as an ideal package substrate for high-power LEDs. Foreign high-end packaging products have adopted ceramic packaged LEDs, which greatly reduces the thermal resistance, and in order to achieve miniaturization, it is possible to directly produce a silicone lens on a ceramic substrate by using the Molding process. Typical products include REBEL from LumiLEDs, OSLON from Osram, etc. These LEDs are suitable for use in high temperature enclosed environments such as high-end automotive LED lights. In addition, other substrate materials, such as silicon and MCPCB, have certain disadvantages and are limited in use. In addition, as a high-power integrated package COB metal substrate, the surface is covered with silver or copper, and the chip is directly connected thereto, which has good heat dissipation, but also has the problem of thermal mismatch. From the perspective of comprehensive performance, in the future, the mainstream of package substrates is metal brackets, while high-end products are mainly ceramic substrates.

The phosphor material directly determines the color of the light after the chip is packaged. The current mainstream products still use the blue chip yellow phosphor to obtain white light, but the packaged LED color is not good. The erbium-activated silicate phosphor can emit green light, yellow light, orange red light and red light under the activation of blue light. The encapsulated LED is rich in color, good in color rendering and high in efficiency, and can replace yellow phosphor. However, the silicate is resistant to water and heat, and the surface of the phosphor needs to be treated. Therefore, the use of nanotechnology to prepare the encapsulated silicate phosphor material will be an important means to improve the application of silicate phosphors in the future.

In recent years, oxynitride phosphor materials have been developed, mainly for the preparation of high-performance red phosphors. The white LED obtained by using the oxynitride and the YAG phosphor not only has a high color rendering index but also has high color stability. However, the oxynitride phosphor is expensive, and more is used as a light-filling material for the yellow phosphor, thereby improving the problem of insufficient light emission of the red portion of the LED and improving color rendering. On the whole, in the next few years, the phosphors are mainly yellow phosphors, but the LEDs emphasize color rendering more, and some of the efficiency must be sacrificed to add red phosphors. In addition, silicate phosphors have been partially applied and have great potential for application.

In addition, as mentioned above, the thermal interface is often the bottleneck determining the thermal resistance of the LED. Therefore, the thermal interface material is very important. At present, the thermal interface materials commonly used in LED packaging are thermal conductive adhesive, thermal conductive silver adhesive, metal solder paste and the like. Thermal paste is limited to low power LED package applications; conductive silver paste is slightly better than thermal paste, mainly used in small and medium power LED packages; solder paste solid crystal and LED chip form chemical bond connection, good heat dissipation effect, improve the reliability of LED devices, It is the main thermal interface material for high-power LEDs in the future. In addition, in recent years, some new TIM materials and technologies have begun to be applied to high-power LED packages, such as the use of nanotechnology to achieve thermal interface materials. Future thermal interface materials will continue to evolve to reduce thermal resistance.

In addition, in order to improve the light extraction efficiency, potting materials and lens materials will continue to develop, requiring high refractive index, high temperature resistance, impact resistance, UV resistance and appropriate hardness, suitable for molding, easy processing, and optical design. . In terms of lead materials, gold wire is still the mainstream of the future.

The development of various packaging materials mentioned above has made great progress in the packaging form and process of LEDs. The early LED package was mainly used to realize the chip protection function and electrical connection, so the heat dissipation and optical design were less considered, and the package form was also a simple lead package. With the increase of power, it is necessary to consider the heat dissipation performance, so it has been developed into the current chip package, further considering the heat dissipation to the typical high-power "Luxeon LED" bracket package; until recently, the Molding process was used to make COB on the ceramic substrate. Technology encapsulates high power LEDs. From the development of the above package forms, the heat dissipation requirements are getting higher and higher, the power is getting larger and larger, and the package form is gradually simplified. This is the development trend of single-particle package.

With the rapid development of high-power LED lighting, more and more applications require a single LED package module to generate more luminous flux. Therefore, multi-chip packaging is one of the future development directions. The multi-chip LED package has a higher concentration of heat flux and heat. It mainly uses a metal substrate as a thermal lining. It is packaged with COB technology, with a short heat dissipation path and a compact module. In addition, the high-performance multi-chip integrated COB package utilizes a ceramic substrate. In order to further reduce costs, research and development of low-cost, high-volume, high-quality LED packaging technology has become a difficult and hot issue to be solved. Promising wafer-level packaging and roll-to-roll LED packaging are also emerging. The packaging form will gradually move toward the upstream industrial chain, and the packaging cost will be lower and lower. Even with the integration of LED packages, it has been proposed to integrate the driver circuit into the LED package to implement a system on package.

In the future, computer simulation technology will be further developed. The optical field, thermal field and reliability of the package can be simulated by computer, which is also the trend of future development.

Edit: Cedar

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