1 Introduction
With the rising awareness of global environmental protection, energy saving has become a trend today. The LED industry is one of the most watched industries in recent years. Up to now, LED products have the advantages of energy saving, power saving, high efficiency, fast reaction time, long life cycle, and no mercury, and have environmental benefits. However, usually the input power of LED high-power products is about 20%, which can be converted into light, and the remaining 80% of the energy is converted into heat.
In general, if the thermal energy generated by LED illumination cannot be exported, the junction temperature of the LED will be too high, which will affect the product life cycle, luminous efficiency and stability, and the junction temperature, luminous efficiency and lifetime of the LED. Relationships, which will be further explained below using the relationship diagram.
Figure 1 is a graph of LED junction temperature and luminous efficiency. When the junction temperature rises from 25 °C to 100 °C, the luminous efficiency will decline by 20% to 75%, and the yellow light will decay by 75%. serious. In addition, when the operating temperature of the LED is higher, the production life is lower (as shown in Figure 2). When the operating temperature is raised from 63 °C to 74 °C, the average LED life will be reduced by 3/4. Therefore, to improve the luminous efficiency of LEDs, the heat dissipation management and design of LED systems has become an important issue. Before understanding the heat dissipation problem of LEDs , it is necessary to understand the heat dissipation path and improve the thermal bottleneck.
2, LED heat dissipation
According to different packaging technologies, the heat dissipation method is also different, and the various methods of heat dissipation of LEDs can be roughly illustrated in Figure 3 below:
Description of the heat dissipation route:
1. Dissipating heat from the air 2. Thermal energy is directly derived from the System circuit board. 3. Thermal energy is exported via the gold wire. 4. For eutectic and Flip chip processes, heat is transferred to the system board via the via)
Generally, the LED die (Die) is connected to a substrate (Substrate of LED Die) by gold wire, eutectic or flip chip to form an LED chip, and then the LED chip is fixed to the system. System circuit board. Therefore, the possible heat dissipation path of the LED is to directly dissipate heat from the air (as shown in Figure 3, route 1), or through the LED die substrate to the system board to the atmosphere. The rate at which heat is dissipated from the system board to the atmosphere depends on the design of the entire luminaire or system.
However, most of the thermal bottlenecks of the entire system at this stage occur mainly by transferring heat from the LED die to its substrate to the system board. The possible heat dissipation path of this part: one is to directly dissipate heat to the system board by the die substrate (as shown in Figure 2, route 2). In this heat dissipation path, the heat dissipation capability of the LED die substrate material is equivalent. Important parameters. On the other hand, the heat generated by the LEDs will also pass through the electrode metal wires to the system board. Generally, the heat dissipation is limited by the relatively elongated geometry of the wires themselves. Figure 3 shows the route 3; therefore, there is a recent Eutectic or Flip chip bonding method, which greatly reduces the length of the wire and greatly increases the cross-sectional area of ​​the wire. Thus, by the LED electrode The heat dissipation efficiency of the wire to the system board will be effectively improved (as shown in Figure 4, Route 4).
Through the above explanation of the heat dissipation path, it can be known that the selection of the heat-dissipating substrate material and the packaging method of the LED die occupy an extremely important part in the LED heat dissipation management, and the latter section will be outlined for the LED heat-dissipating substrate.
3, LED heat sink substrate
The LED heat-dissipating substrate mainly uses the heat-dissipating substrate material itself to have better thermal conductivity, and the heat source is derived from the LED die. Therefore, from the description of the LED heat dissipation method, the LED heat dissipation substrate can be subdivided into two major categories: (1) LED die substrate and (2) system circuit board, and the two different heat dissipation substrates respectively carry the LEDs. The heat generated by the die and the LED chip illuminating the LED die is radiated to the system board via the LED die, and then absorbed by the atmosphere to achieve the effect of heat dissipation.
3.1 system board
The system board is mainly used as an LED cooling system, and finally the thermal energy is led to the fins, the outer casing or the atmosphere. In recent years, the production technology of printed circuit boards (PCBs) has been very mature. The system boards of early LED products are mostly PCB-based. However, with the increasing demand for high-power LEDs, the heat dissipation capability of PCB materials is limited, making it unusable. In order to improve the heat dissipation problem of high-power LEDs, high-power products have recently developed a high thermal conductivity aluminum substrate (MCPCB), which has achieved the purpose of heat dissipation of high-power products by utilizing the characteristics of better heat dissipation characteristics of metal materials. However, with the continuous development of LED brightness and performance requirements, although the system board can effectively dissipate the heat generated by the LED chip to the atmosphere, the heat generated by the LED die cannot be effectively transferred from the die to the system circuit. Board, in other words, when the LED power is more efficient, the thermal bottleneck of the entire LED will appear on the LED die heat sink substrate. The next article will discuss the LED die substrate in more depth.
3.2 LED die substrate
The LED die substrate is mainly used as a medium for deriving thermal energy between the LED die and the system board, and is combined with the LED die by a wire bonding, eutectic or flip chip process. Based on the consideration of heat dissipation, the current LED die substrates on the market are mainly ceramic substrates, and the circuit preparation methods can be roughly divided into three types: thick film ceramic substrate, low temperature co-fired multilayer ceramic, and thin film ceramic substrate. Conventional high-power LED components are mostly made of a thick film or a low-temperature co-fired ceramic substrate as a die-dissipating substrate, and the LED die is bonded to the ceramic substrate by a gold wire. As mentioned in the introduction, this gold wire connection limits the effectiveness of heat dissipation along the electrode contacts. Therefore, in recent years, major domestic and foreign manufacturers have all worked hard to solve this problem. There are two solutions. One is to find a substrate material with a high heat dissipation coefficient instead of alumina, and includes a germanium substrate, a tantalum carbide substrate, an anodized aluminum substrate or an aluminum nitride substrate, wherein the tantalum and the tantalum carbide substrate are made of a material semiconductor. The characteristics make it encounter the more severe test at the present stage, and the anodized aluminum substrate is easily turned on due to the insufficient strength of the anodized oxide layer, which makes it limited in practical application, and thus, at this stage The more mature and generally accepted is the use of aluminum nitride as the heat sink substrate; however, the current aluminum nitride substrate is not suitable for the traditional thick film process (the material must be subjected to atmospheric heat treatment at 850 ° C after silver paste printing). The material reliability problem arises. Therefore, the aluminum nitride substrate line needs to be prepared by a thin film process. The aluminum nitride substrate prepared by the thin film process greatly accelerates the heat from the LED die to the system board through the substrate material, thereby greatly reducing the burden of heat from the LED die to the system board through the metal wire, thereby achieving high heat dissipation. Effect.
Another solution for heat dissipation is to bond the LED die to its substrate in a eutectic or flip chip manner, thus greatly increasing the heat dissipation efficiency through the electrode wires to the system board. However, this process requires extremely high precision of the wiring of the substrate and the flatness of the surface of the substrate. This makes the accuracy of the thick film and the low-temperature co-fired ceramic substrate incapable of being affected by the problem of the process screen and the shrinkage ratio. At this stage, a thin film ceramic substrate is introduced to solve this problem. The thin film ceramic substrate is prepared by a yellow light lithography method, and the thickness of the line is increased by electroplating or electroless plating, so that the product has high line precision and high flatness. The eutectic/cladding process combined with the thin film ceramic heat sink substrate is bound to greatly increase the LED's luminous power and product life.
In recent years, due to the development of aluminum substrates, the heat dissipation problem of system boards has been gradually improved, and even gradually developed into flexible flexible boards. On the other hand, LED die substrates are gradually working towards reducing their thermal resistance. Table 1 below is the common system board and LED die substrate types and major suppliers in China:
4, LED ceramic heat sink substrate introduction
How to reduce the thermal resistance of LED die ceramic heat sink substrate is one of the most important topics to improve LED luminous efficiency. If it is based on its circuit manufacturing method, it can be divided into thick film ceramic substrate, low temperature co-fired multilayer ceramic, and thin film ceramic substrate. Species, respectively, are as follows:
4.1 Thick film ceramic substrate
The thick-film ceramic substrate is produced by screen printing technology. The material is printed on the substrate by a doctor blade and dried, sintered, and lasered. The main manufacturers of thick film ceramic substrates in China are He Shentang and Jiuhao. And other companies. Generally speaking, the line produced by the screen printing method is prone to rough lines and inaccurate alignment due to the problem of the screen version. Therefore, for future high-power LED products with smaller and smaller size requirements, finer-line circuits, or LED products that require accurate eutectic or flip-chip processing, the accuracy of thick-film ceramic substrates Has gradually become inadequate.
4.2 Low temperature co-fired multilayer ceramics
Low-temperature co-fired multi-layer ceramic technology, using ceramic as the substrate material, printing the circuit on the substrate by screen printing, integrating the multilayer ceramic substrate, and finally sintering through low temperature, and its main domestic manufacturer has Jude Electronics, Yan Xin and other companies. The metal circuit layer of the low-temperature co-fired multi-layer ceramic substrate is also made by the screen printing process, and the alignment error may also be caused by the problem of the web. In addition, after the multi-layer ceramics are laminated and sintered, the shrinkage ratio is also considered. . Therefore, if the low-temperature co-fired multi-layer ceramics are used in eutectic/flip-chip LED products requiring precise line alignment, it will be more stringent.
4.3 Thin film ceramic substrate
In order to improve the problem of thick film process webpage and shrinkage ratio after multi-layer pressure sintering, a thin film ceramic substrate has recently been developed as a heat sink substrate for LED dies. The thin film heat sink substrate is fabricated by sputtering, electro/electrochemical deposition, and yellow light lithography. It has:
(1) Low temperature process (below 300 °C), avoiding the possibility of high temperature material damage or dimensional variation;
(2) Using the yellow light lithography process to make the circuit on the substrate more precise;
(3) The metal circuit is not easy to fall off, etc. Therefore, the thin film ceramic substrate is suitable for high power, small size, high brightness LED, and eutectic/ricon crystal packaging process requiring high alignment accuracy. At present, the company mainly has the production capacity of professional thin film ceramic substrates, such as Yusibo Electronics and Tongxin Electric.
5. Development trend of LED products of international manufacturers
At present, the trend of LED product development can be observed from the recent LED product power and size published by LED packaging manufacturers. High-power, small-size products are the development focus of the current LED industry, and both use ceramic heat-dissipating substrates as their The way LED crystals dissipate heat. Therefore, ceramic heat-dissipating substrates have become a very important part in the structure of high-power, small-size LED products. Table 2 below is a simple summary of the development of major LED products at home and abroad and product categories.
6 Conclusion
To improve the LED luminous efficiency and service life, solving the heat dissipation problem of LED products is one of the most important issues at this stage. The development of the LED industry is also focused on the development of high-power, high-brightness and small-size LED products. The heat-dissipating substrate with high heat dissipation and precise size has also become a trend in the future development of LED heat-dissipating substrates. At present, the aluminum oxide substrate is replaced by an aluminum nitride substrate, or the eutectic or flip chip process is used instead of the gold/wire substrate/substrate bonding method to improve the LED luminous efficiency. Under this development trend, the accuracy of the alignment of the heat-dissipating substrate itself is extremely strict, and it is required to have high heat dissipation, small size, and good adhesion of metal lines. Therefore, a thin film ceramic heat-dissipating substrate is produced by using yellow light micro-shadow. It will become one of the important catalysts to promote LED's continuous high power.
Author: Bo Electronics Co., Ltd. R & D Associate at the Ai River Division I clean product marketing applications engineer at the tour Huiru
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