2010 LED heat sink substrate trend

January 11, 2023

Topic: Technology
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, no mercury, and 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 life cycle, luminous efficiency and stability of the LED, and the junction temperature, luminous efficiency and lifetime of the LED. The following will be further explained using the diagram.

Figure 1 is the relationship between LED junction temperature and luminous efficiency. When the junction temperature is from 25 °C to 100 °C, the luminous efficiency will decline by 20% to 75%, and then the yellow light will decay 75. % is the most 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 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.

Figure 1 The relationship between junction temperature and luminous efficiency? (Source: PNNL)

Figure 2 LED temperature and life off? (Source: Lighting Research Center)

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:

Figure 3 is a schematic diagram of the LED heat dissipation path.
(Description of heat dissipation: 1. Dissipate heat from the air
2. Thermal energy is directly exported by the System circuit board. 3. Heat energy is exported via gold wire
4. For eutectic and Flip chip processes, thermal energy will be derived via vias to the system board)

Generally, an LED chip (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 circuit of 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 chip 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 chip 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 through the chip substrate (as shown in Figure 2, route 2). In this heat dissipation path, the heat dissipation capability of the LED chip substrate material is quite important. parameter. 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 chip are very important 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 chip. Therefore, from the description of the LED heat dissipation method, the LED heat dissipation substrate can be subdivided into two major categories: (1) LED chip substrate and (2) system circuit board. The two different heat dissipation substrates respectively carry the LED chip. The thermal energy generated when the LED chip emits the LED chip is radiated to the system circuit board via the LED chip, and then absorbed by the atmospheric environment to achieve the effect of heat dissipation.

3.1 System Board System The circuit board is mainly used as the LED heat dissipation system, and finally the thermal energy is led to the heat dissipation sheet, the outer casing or the material in 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, the tube system board can effectively dissipate the heat generated by the LED chip to the atmosphere, but the heat generated by the LED chip cannot be effectively transmitted from the chip to the system board. In other words, when the LED power is more efficient, the thermal bottleneck of the entire LED will appear on the LED chip heat sink substrate. The next article will discuss the LED chip substrate in more depth.

3.2 LED chip substrate
The LED chip substrate is mainly used as a medium for deriving thermal energy between the LED chip and the system board, and is combined with the LED chip by a wire bonding, eutectic or flip chip process. Based on the consideration of heat dissipation, the current LED chip substrate is mainly based on ceramic substrate, and the circuit preparation method can be roughly divided into three types: thick film ceramic substrate, low temperature co-fired multilayer ceramic, and thin film ceramic substrate. For high-power LED components, a thick film or a low-temperature co-fired ceramic substrate is used as a chip heat-dissipating substrate, and the LED chip and the ceramic substrate are combined 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 high heat dissipation substrate material to replace the aluminum oxide, including a silicon substrate, a silicon carbide substrate, an anodized aluminum substrate or an aluminum nitride substrate, wherein the silicon and silicon carbide substrate materials The characteristics of the semiconductor make it encounter more severe tests at this 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 applications. The stage is more mature and the higher acceptance is aluminum nitride as the heat sink substrate; however, the current aluminum nitride substrate is not suitable for the traditional thick film process (the material is subjected to atmospheric heat treatment at 850 ° C after silver paste printing, This causes material reliability problems. Therefore, the aluminum nitride substrate circuit 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 chip to the system board through the substrate material, thereby greatly reducing the burden of heat from the LED chip to the system board through the metal line, thereby achieving high heat dissipation effect. .

Another heat dissipation solution is to bond the LED chip to its substrate in a eutectic or flip chip manner, thereby greatly increasing the heat dissipation efficiency through the electrode lead to the system board. However, this process requires extremely high accuracy of the substrate and flatness of the surface of the substrate. This makes the accuracy of the thick film and low temperature co-fired ceramic substrate inferior to 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, the LED chip substrate is gradually working towards reducing its thermal resistance. Table 1 below is the common system board and LED chip substrate types and major suppliers in China:

Table 1 LED heat sink substrate types and their main suppliers

4, LED ceramic heat sink substrate introduction

How to reduce the thermal resistance of the LED chip ceramic heat-dissipating substrate is one of the most important topics for improving the LED luminous efficiency. If it is based on its circuit manufacturing method, it can be divided into a thick-film ceramic substrate, a low-temperature co-fired multilayer ceramic, and a 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 manufacturer of thick film ceramic substrates in China is Wo. Shentang, 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 multi-layer ceramic low-temperature co-fired multi-layer ceramic technology, using ceramic as the substrate material, the circuit is printed on the substrate by screen printing, then the multi-layer ceramic substrate is integrated, and finally sintered through low temperature, and The main domestic manufacturers are Dede Electronics, Xinxin 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. problem. 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 the thick-film process, and the shrinkage ratio after multi-layer pressure sintering, a thin-film ceramic substrate has recently been developed as a heat-dissipating substrate for an LED chip. The thin film heat-dissipating substrate is made by sputtering, electric/electrochemical deposition, and yellow lithography. It has: (1) low temperature process (below 300 °C), avoiding the possibility of high temperature material damage or dimensional variation; (2) The yellow-light lithography process is used 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-sized, high-brightness LEDs, and requires high alignment accuracy. Crystal/clad crystal packaging process. At present, the company mainly has the production capacity of professional thin film ceramic substrates, such as Sibo 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 LED chip cooling method. 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.

Table 2 Recent developments and product categories of major LED products at home and abroad

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. Providing a heat-dissipating substrate with high heat dissipation and precision dimensions 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 chip/substrate bonding method of replacing the gold wire by a eutectic or flip chip process is adopted to achieve 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 LEDs to high power.

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Author:

Mr. Wayne Tang

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