LED lighting applications, in order to achieve the brightness requirements close to the original alternative light source, usually adopt a design method of winning by "quantity", that is, designing a large number of LED light-emitting components per unit area, or designing to improve the luminous efficiency of a single component. However, the temperature of the surface or the point component is not required to be processed, and the heat dissipation method can be carried out by active or passive design, especially the design of active heat dissipation is more complicated, how to achieve optimal heat dissipation design. Avoiding the light decay of LED components affects the life, which is an important key to the development of high-brightness LED lighting.
LED (LightEmittingDiode) solid-state lighting is a future industry that is considered to have great potential in recent years, because consumers and industry are looking forward to using LED solid-state lighting to solve a lot of energy waste in inefficient light source lighting problems, precisely because LED has volume Small, luminous efficiency, power saving and other advantages, therefore, most people also have high hopes for the future development of LED solid-state lighting. For LED solid-state lighting and traditional light source technology, so there are many advantages in environmental protection, energy saving, first observe the common daily lighting source, nothing more than incandescent and fluorescent lamps, incandescent lamps are basically in luminous efficiency That is to say, it tends to be inferior. Even if it has the advantages of low cost and established habits, it has become an environmentally friendly and inefficient lighting product under the social atmosphere of environmental protection. In terms of fluorescent lamps, although photovoltaic technology using high-frequency gas discharge to achieve power-saving benefits, in fact, the fluorescent lamp control process can not avoid mercury harmful to the environment, and is not the best light source choice in environmental protection appeal.
Back to the development of LED solid-state light source, early LEDs are mostly used for indicating light sources, such as medium/low brightness and low power driving light source applications such as signal lights and indicator lights. Therefore, there is no heat dissipation consideration, and on the other hand, the indicator light source. It is only used to identify the current status of the device and the status of the switch. It is not for lighting purposes, because the driving power is not high, and there is no obvious heat problem to be solved. However, the problem is that the purpose of using high-brightness LEDs is mostly to develop for alternative environmentally friendly light sources. This design method will have many effects.
When the LED solid-state light source is thinking in the direction of daily lighting application, there will be a problem of insufficient brightness. It is necessary to try to increase the power and increase the luminous efficiency on the LED component, or use a larger number of high-brightness LEDs for modular design. The light source has the requirement of "high brightness" for lighting applications.
High-performance, environmentally-friendly lighting efficiency is a key to thermal design
The core design of the LED component is to use a voltage applied to a piece of LED die to produce a light-emitting result. Similar to a general silicon wafer, the LED chip will also have a light decay phenomenon due to long-term use, and most designs are designed to enhance the component. The brightness of the light is increased by increasing the bias of the crystal, that is, increasing the power of the electric energy applied to the LED, so that the wafer can excite higher brightness. Thus, strengthening the power of the LED will also accelerate the problem of light decay and lifetime of the crystal. The appearance of even the high temperature of the component itself due to the enhancement of brightness can also shorten the life of the product.
When the brightness of a single LED die increases, the wattage of a single LED will increase from 0.1W to 1, 3, or even 5W. However, most LED light source modules will be analyzed and analyzed, and package modules will also appear. The thermal impedance is increased by increasing the luminous efficiency, and generally increases from 250K/W to 350K/W.
The inspection test results will show that the LED will also have a decrease in "power" and "lifetime", which will cause the LED light source component, which may have a lifetime of 20,000 hours, to be reduced due to heat dissipation. Only 1,000 hours of life is left. Especially when the component is operating at a temperature of 50 degrees Celsius, it can maintain an optimum lifetime of 20,000 hours, but when the LED component is operated at 70 degrees Celsius, the average lifespan is reduced to 10,000 hours. Operating in a 100 degree Celsius environment, life expectancy is only 5,000 hours.
Thermal impedance status of LED module design
In addition to the critical component LEDs are susceptible to temperature, most of the light source design has also been developed using modular concepts. Even in order to replace the traditional light source, the light-emitting components and electronic circuits can only be integrated in a very small space because the LEDs are DC-DC drive components. Most of the lamps are connected to AC power supply. In order to simplify the application of LED light source, the current mainstream practice is to directly integrate the power rectifier, transformer module and LED lighting components, but the problem is because The available circuit space is relatively small. When the convection space in the device is relatively small, it is naturally unable to obtain a better heat dissipation effect, and the heat dissipation process of the module can only be performed through the active countermeasures for forced heat dissipation. .
If the heat flow model is observed by the thermal impedance module, the temperature of the LED die is predicted, the junction point means the pn junction of the semiconductor, and the thermal impedance R is defined as the temperature difference and the corresponding power dissipation ratio, and the thermal impedance. The formation factors are quite a lot, but through the inspection method of the heat flow model, it can be more clearly confirmed that the heat dissipation treatment is because of which key problems reduce the efficiency, and the heat can be dissipated from components, assembly methods, substrate materials, structures. project Improvement. The heat flow model of a typical LED solid-state light source can be viewed from several key points.
For example, LED light-emitting components can be disassembled into LED dies, die and pin wires, packaged plastics, and then extended to the LED light source module, that is, there will be LED components, bonded metal pins, MetalCorePCB ( The MCPCB) circuit board, and finally the aluminum extruded heat sink for heat dissipation, and the heat flow model can observe several heat flow impedances in series, such as bonding points, metal pieces of the carrier die, circuit board and environment, etc. The thermal circuit of the series impedance tries to find the problem node with low heat dissipation efficiency.
Further observation from the model reveals that the heat dissipation process from the junction of the die to the entire external environment is actually a combination of several heat dissipation methods, for example, the material properties of the die and the carrier metal sheet, The optical resin contact that encapsulates the LED die material is in contact with the thermal resistance characteristics of the circuit board material, the surface contact of the LED component, or the aluminum extruded heat sink fin adhesive for heat dissipation, and even the combination of the cooling device and the air, etc. The heat dissipation process of heat flow. How the operating temperature of the LED solid-state light source is effectively dissipated will affect the lighting efficiency, energy utilization efficiency, and device life of the entire light source application. The way to improve the heat dissipation can be from the wafer level technology, the technology of packaging the LED die, and the circuit board. Level of technology to improve.
In terms of heat dissipation treatment at the wafer level, due to the traditional wafer fabrication method, sapphire is mostly used as the substrate, and the thermal conductivity of the sapphire substrate is close to 20W/mK. In fact, it is difficult to quickly dissipate the heat generated by LED epitaxy. In the case of the heat-dissipation enhancement treatment for the LED chip level, especially for the high-power, high-brightness LED element, in order to use the Flip-Chip form, the flip-chip is effectively utilized to conduct the epitaxial heat.
There is also a way to make LED elements by adopting a "vertical" electrode. Since the LED elements are provided with metal electrodes on both the upper and lower ends, this can be more beneficial in terms of heat dissipation. For example, if a GaN substrate is used as a material, since the GaN substrate is a conductive material, the electrodes can be directly connected under the substrate to obtain the benefit of rapidly dissipating the epitaxial temperature, but this method is also expensive because of the high material cost. The cost of the conventional sapphire substrate is much more expensive, which increases the manufacturing cost of the component.
As for the heat dissipation enhancement of the package level, there are quite a few common practices. Here are a few common practices. In general, the LED manufacturing process uses optical grade epoxy to wrap the entire LED, thereby making the LED component perform better in terms of mechanical strength, and even protect the relevant circuitry within the component, but the ring Oxygen resin can increase the strength of the component, but at the same time limit the temperature operating range of the component. Because the optical grade epoxy resin is used at high temperatures, the optical properties of the epoxy resin will deteriorate due to high temperature or strong light. Even the material itself can cause deterioration.
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