With the rising awareness of global environmental protection, energy saving and power saving have become the current trend. LED industry is one of the most watched industries in recent years. Since its development, LED products have become energy-efficient, energy-saving, high-efficiency, fast response time, long life cycle, and no mercury, and they have environmental benefits. However, usually about 20% of input power of LED high-power products can be converted into light, and 80% of the remaining power is converted into heat energy.
In general, if the heat generated by the LED when the light is emitted cannot be derived, the junction temperature of the LED will be too high, thereby affecting the product life cycle, luminous efficiency, and stability, and the LED junction temperature, luminous efficiency and lifespan Relationships, the following will use the relationship diagram for further explanation.
1, LED heat dissipation methods based on different packaging technologies, the heat dissipation method is also different, and LED various heat dissipation methods can roughly indicate:
Thermal path description:
(1). Heat from the air (2). Thermal energy is directly derived from the System circuit board (3). Deriving heat energy via a gold wire (4). In the case of eutectic and Flip chip processes, thermal energy is drawn through the vias to the system board. In general, the LED die (Die) is bonded to the substrate by gold wire, eutectic or flip chip (Substrate of LEDDie) An LED chip is formed, and then the LED chip is fixed on a system circuit board. Therefore, the LED may be cooled by direct heat dissipation from the air or through the LED die substrate to the system board and then to the atmosphere. The rate of heat dissipation from the system board to the atmosphere depends on the design of the entire light fixture or system.
However, the bottleneck of heat dissipation in the entire system at the present stage is mostly the conduction of heat from the LED die to its substrate to the system circuit board. The possible heat-dissipation path in this part: one is to directly dissipate heat from the die substrate to the system circuit board. In this heat-dissipation path, the heat dissipation capability of the LED die substrate material is an important parameter. On the other hand, the heat generated by the LED also passes through the electrode metal wires to the system circuit board. Generally speaking, under the condition that the gold wire is used for electrode bonding, the heat dissipation is limited by the relatively slender geometry of the metal wire; therefore Recently, Eutectic or Flipchip bonding methods have been adopted. This design significantly reduces the length of the wire and significantly increases the cross-sectional area of ​​the wire. As a result, the efficiency of heat dissipation from the LED electrode wire to the system board will be effective. Promote.
Through the explanation of the above heat dissipation methods, it can be known that the selection of the heat-dissipating substrate material and the packaging method of the LED die occupies a very important part in the LED heat dissipation management, and the outline of the LED heat-dissipating substrate will be outlined in the following paragraph.
2. LED heat-dissipating substrate The LED heat-dissipating substrate mainly uses its heat-dissipating substrate material itself to have better thermal conductivity, and the heat source is led out from the LED die. Therefore, from the description of LED heat dissipation methods, LED heat-dissipating substrates can be subdivided into two categories, namely, LED die substrates and system circuit boards. These two different heat-dissipating substrates carry LED die and LED chips respectively. The heat energy generated when the LED die emits light passes through the LED die to heat the substrate to the system circuit board, and then is absorbed by the atmospheric environment to achieve the effect of heat dissipation.
2.1 System Board System Board is mainly used as the LED heat dissipation system, and finally the heat energy is conducted to the heat sink fins, the shell or the atmosphere. In recent years, the production technology of printed circuit boards (PCBs) has become very sophisticated. The system circuit boards of early LED products are mainly PCBs. However, as the demand for high-power LEDs increases, the heat dissipation capacity of PCB materials is limited, making it impossible to apply them. For its high-power products, in order to improve the heat dissipation of high-power LEDs, a high thermal conductivity coefficient aluminum substrate (MCPCB) has recently been developed. The use of metal materials with better heat dissipation characteristics has achieved the purpose of heat dissipation for high-power products. 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 can not be efficiently conducted from the die to the system circuit. The board, in other words, when the LED power is increased more efficiently, the heat sink bottleneck of the entire LED will appear on the LED die heat sink substrate.
2.2 LED die substrate The LED die substrate is mainly used as the medium for the thermal energy between the LED die and the system circuit board. It is combined with the LED die by the process of wire bonding, eutectic or flip chip. Based on heat dissipation considerations, the current market for LED die substrates is mainly based on ceramic substrates. The circuit preparation methods can be roughly divided into three types: thick-film ceramic substrates, low-temperature co-fired multilayer ceramics, and thin-film ceramic substrates. Traditional high-power LED devices mostly use a thick film or a low-temperature co-fired ceramic substrate as a crystal heat-dissipating substrate, and then combine the LED crystal grains with a ceramic substrate by using gold wires.
As mentioned in the introduction, this gold wire connection limits the heat dissipation along the electrode contact. Therefore, in recent years, major manufacturers at home and abroad have all worked hard to solve this problem. There are two ways to solve this problem. One is to find a substrate material with a high heat dissipation coefficient to replace aluminum oxide, which includes a tantalum substrate, a tantalum carbide substrate, an anodized aluminum substrate, or an aluminum nitride substrate. Among them, a material semiconductor for tantalum and a tantalum carbide substrate. The characteristics have caused it to face severe challenges at the current stage, and the anodized aluminum substrate is susceptible to cracking due to insufficient strength of the anodized oxide layer, which limits its practical application. Therefore, at this stage, Aluminum nitride is used as a heat-dissipating substrate for more mature and commonly accepted substrates; however, it is currently limited to aluminum nitride substrates that are not suitable for conventional thick-film processes (the material must be subjected to atmospheric heat treatment at 850°C after printing in silver paste, so that 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 performance of heat from the LED die through the substrate material to the system circuit board, thereby greatly reducing the heat burden caused by the LED die via the metal line to the system circuit board, thereby achieving high heat dissipation. Effect.
Another heat dissipation solution is to connect the LED die with its substrate in a eutectic or flip-chip manner, which greatly increases the heat dissipation efficiency through the electrode lead to the system circuit board. However, this process requires extremely high precision for the wiring of the substrate and the flatness of the circuit surface of the substrate. This makes the accuracy of the thick film and the low-temperature co-fired ceramic substrate unsatisfactory due to the problem of the screen mesh and the problem of sintering shrinkage. At this stage, more and more film ceramic substrate is introduced to solve this problem. The thin film ceramic substrate is prepared by a yellow lithography method, and the thickness of the circuit is increased by electroplating or electroless plating, so that the product has high line accuracy and high flatness characteristics. The eutectic/flip chip process, supplemented by thin-film ceramic heat-dissipating substrates, is bound to significantly increase the LED's luminous power and product lifetime.
In recent years, due to the development of aluminum substrates, the problem of heat dissipation in the system circuit boards has gradually improved, and even gradually, flexible flexible circuit boards have been developed. On the other hand, the LED die substrate also gradually strives to reduce its thermal resistance.
3. Introduction of LED ceramic heat-dissipating substrates How to reduce the thermal resistance of LED crystal ceramic heat-dissipation substrates is one of the most important topics for improving LED luminous efficiency. If it is based on its circuit manufacturing method, it can be divided into thick-film ceramic substrates and low-temperature co-fired multilayers. Three kinds of ceramics and thin-film ceramic substrates are described as follows:
3.1 Thick-film ceramic substrates Thick-film ceramic substrates are produced using screen printing technology. The material is printed on the substrate by a doctor blade and dried, sintered, and lasered. The major domestic manufacturers of thick-film ceramic substrates are: Teng Tang, Jiu Hao and other companies. In general, screens made by screen printing methods are prone to rough lines and inaccurate alignment due to the problem of screens. Therefore, the accuracy of thick-film ceramic substrates for future LED products with smaller and smaller size requirements, finer-grained high-power LED products, or LED products that require accurate eutectic or flip-chip manufacturing processes. Has been gradually insufficient.
3.2 Low-temperature co-fired multilayer ceramic low-temperature co-fired multilayer ceramic technology uses ceramics as a substrate material, and the lines are printed on the substrate by means of screen printing, then the multilayer ceramic substrate is integrated, and finally the sintered ceramics are formed through low-temperature sintering. The major domestic manufacturers include Junde Electronics and Yuxin. The metal circuit layer of the low-temperature co-fired multi-layer ceramic substrate is also manufactured by using a screen printing process, and the alignment error may also be caused by the tension network problem. In addition, after the multi-layer ceramics are laminated and sintered, the shrinkage ratio may also be considered. . Therefore, if the low-temperature co-fired multilayer ceramics are used in eutectic/flip chip LEDs that require precise alignment of the circuits, they will be even more severe.
3.3 Thin-Film Ceramic Substrates In order to improve the problem of thick-film process screens and shrinkage ratio after multi-laminated pressure sintering, thin-film ceramic substrates have recently been developed as heat-dissipating substrates for LED dies. The thin film heat sink substrate is fabricated using sputtering, electro/electrochemical deposition, and a yellow photolithography process, and includes:
(1) Low temperature process (below 300°C), avoiding the possibility of high temperature material damage or size variation;
(2) Using a yellow photolithography process to make the circuit on the substrate more accurate;
(3) The metal circuit is not easy to fall off ... and so on, so the thin film ceramic substrate is suitable for high power, small size, high brightness LEDs, as well as eutectic/flip chip packaging process with high alignment accuracy. At present, the company mainly produces electronic film and ceramic substrate production capacity with companies such as Qisibai Electronics and Tongxin Electric.
4. Development trend of LED products of international manufacturers The current trend of development of LED lamps can be seen from the LED power and size of LED packaging manufacturers recently announced that high-power, small-size products are the focus of the current development of LED industry. Both ceramic heat sinks are used as a way to dissipate heat from their LED die. Therefore, the ceramic heat-dissipating substrate has become a very important part in the structure of high-power, small-size LED products. The following Table II is a simple combination of the status quo of domestic and foreign LED product development and product categories.
5. Conclusions To improve LED luminous efficiency and service life, solving the heat dissipation problem of LED products is one of the most important topics 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. Therefore, providing a heat-dissipating substrate with high heat dissipation and precise dimensions has also become a trend for the development of LED heat-dissipating substrates in the future. At this stage, aluminum nitride substrates are used instead of alumina substrates, or eutectic or flip-chip processes are used to replace the gold/grain-bonded die/substrate bonding method to improve the luminous efficiency of LEDs. In this development trend, the accuracy of the alignment of the heat-dissipating substrate itself is extremely stringent, and the characteristics such as high heat dissipation, small size, and good adhesion of metal lines are required. Therefore, a thin-film ceramic heat-dissipating substrate is fabricated using a yellow light lithography. It will become one of the important catalysts to promote the continuous improvement of LED power.
In general, if the heat generated by the LED when the light is emitted cannot be derived, the junction temperature of the LED will be too high, thereby affecting the product life cycle, luminous efficiency, and stability, and the LED junction temperature, luminous efficiency and lifespan Relationships, the following will use the relationship diagram for further explanation.
1, LED heat dissipation methods based on different packaging technologies, the heat dissipation method is also different, and LED various heat dissipation methods can roughly indicate:
Thermal path description:
(1). Heat from the air (2). Thermal energy is directly derived from the System circuit board (3). Deriving heat energy via a gold wire (4). In the case of eutectic and Flip chip processes, thermal energy is drawn through the vias to the system board. In general, the LED die (Die) is bonded to the substrate by gold wire, eutectic or flip chip (Substrate of LEDDie) An LED chip is formed, and then the LED chip is fixed on a system circuit board. Therefore, the LED may be cooled by direct heat dissipation from the air or through the LED die substrate to the system board and then to the atmosphere. The rate of heat dissipation from the system board to the atmosphere depends on the design of the entire light fixture or system.
However, the bottleneck of heat dissipation in the entire system at the present stage is mostly the conduction of heat from the LED die to its substrate to the system circuit board. The possible heat-dissipation path in this part: one is to directly dissipate heat from the die substrate to the system circuit board. In this heat-dissipation path, the heat dissipation capability of the LED die substrate material is an important parameter. On the other hand, the heat generated by the LED also passes through the electrode metal wires to the system circuit board. Generally speaking, under the condition that the gold wire is used for electrode bonding, the heat dissipation is limited by the relatively slender geometry of the metal wire; therefore Recently, Eutectic or Flipchip bonding methods have been adopted. This design significantly reduces the length of the wire and significantly increases the cross-sectional area of ​​the wire. As a result, the efficiency of heat dissipation from the LED electrode wire to the system board will be effective. Promote.
Through the explanation of the above heat dissipation methods, it can be known that the selection of the heat-dissipating substrate material and the packaging method of the LED die occupies a very important part in the LED heat dissipation management, and the outline of the LED heat-dissipating substrate will be outlined in the following paragraph.
2. LED heat-dissipating substrate The LED heat-dissipating substrate mainly uses its heat-dissipating substrate material itself to have better thermal conductivity, and the heat source is led out from the LED die. Therefore, from the description of LED heat dissipation methods, LED heat-dissipating substrates can be subdivided into two categories, namely, LED die substrates and system circuit boards. These two different heat-dissipating substrates carry LED die and LED chips respectively. The heat energy generated when the LED die emits light passes through the LED die to heat the substrate to the system circuit board, and then is absorbed by the atmospheric environment to achieve the effect of heat dissipation.
2.1 System Board System Board is mainly used as the LED heat dissipation system, and finally the heat energy is conducted to the heat sink fins, the shell or the atmosphere. In recent years, the production technology of printed circuit boards (PCBs) has become very sophisticated. The system circuit boards of early LED products are mainly PCBs. However, as the demand for high-power LEDs increases, the heat dissipation capacity of PCB materials is limited, making it impossible to apply them. For its high-power products, in order to improve the heat dissipation of high-power LEDs, a high thermal conductivity coefficient aluminum substrate (MCPCB) has recently been developed. The use of metal materials with better heat dissipation characteristics has achieved the purpose of heat dissipation for high-power products. 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 can not be efficiently conducted from the die to the system circuit. The board, in other words, when the LED power is increased more efficiently, the heat sink bottleneck of the entire LED will appear on the LED die heat sink substrate.
2.2 LED die substrate The LED die substrate is mainly used as the medium for the thermal energy between the LED die and the system circuit board. It is combined with the LED die by the process of wire bonding, eutectic or flip chip. Based on heat dissipation considerations, the current market for LED die substrates is mainly based on ceramic substrates. The circuit preparation methods can be roughly divided into three types: thick-film ceramic substrates, low-temperature co-fired multilayer ceramics, and thin-film ceramic substrates. Traditional high-power LED devices mostly use a thick film or a low-temperature co-fired ceramic substrate as a crystal heat-dissipating substrate, and then combine the LED crystal grains with a ceramic substrate by using gold wires.
As mentioned in the introduction, this gold wire connection limits the heat dissipation along the electrode contact. Therefore, in recent years, major manufacturers at home and abroad have all worked hard to solve this problem. There are two ways to solve this problem. One is to find a substrate material with a high heat dissipation coefficient to replace aluminum oxide, which includes a tantalum substrate, a tantalum carbide substrate, an anodized aluminum substrate, or an aluminum nitride substrate. Among them, a material semiconductor for tantalum and a tantalum carbide substrate. The characteristics have caused it to face severe challenges at the current stage, and the anodized aluminum substrate is susceptible to cracking due to insufficient strength of the anodized oxide layer, which limits its practical application. Therefore, at this stage, Aluminum nitride is used as a heat-dissipating substrate for more mature and commonly accepted substrates; however, it is currently limited to aluminum nitride substrates that are not suitable for conventional thick-film processes (the material must be subjected to atmospheric heat treatment at 850°C after printing in silver paste, so that 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 performance of heat from the LED die through the substrate material to the system circuit board, thereby greatly reducing the heat burden caused by the LED die via the metal line to the system circuit board, thereby achieving high heat dissipation. Effect.
Another heat dissipation solution is to connect the LED die with its substrate in a eutectic or flip-chip manner, which greatly increases the heat dissipation efficiency through the electrode lead to the system circuit board. However, this process requires extremely high precision for the wiring of the substrate and the flatness of the circuit surface of the substrate. This makes the accuracy of the thick film and the low-temperature co-fired ceramic substrate unsatisfactory due to the problem of the screen mesh and the problem of sintering shrinkage. At this stage, more and more film ceramic substrate is introduced to solve this problem. The thin film ceramic substrate is prepared by a yellow lithography method, and the thickness of the circuit is increased by electroplating or electroless plating, so that the product has high line accuracy and high flatness characteristics. The eutectic/flip chip process, supplemented by thin-film ceramic heat-dissipating substrates, is bound to significantly increase the LED's luminous power and product lifetime.
In recent years, due to the development of aluminum substrates, the problem of heat dissipation in the system circuit boards has gradually improved, and even gradually, flexible flexible circuit boards have been developed. On the other hand, the LED die substrate also gradually strives to reduce its thermal resistance.
3. Introduction of LED ceramic heat-dissipating substrates How to reduce the thermal resistance of LED crystal ceramic heat-dissipation substrates is one of the most important topics for improving LED luminous efficiency. If it is based on its circuit manufacturing method, it can be divided into thick-film ceramic substrates and low-temperature co-fired multilayers. Three kinds of ceramics and thin-film ceramic substrates are described as follows:
3.1 Thick-film ceramic substrates Thick-film ceramic substrates are produced using screen printing technology. The material is printed on the substrate by a doctor blade and dried, sintered, and lasered. The major domestic manufacturers of thick-film ceramic substrates are: Teng Tang, Jiu Hao and other companies. In general, screens made by screen printing methods are prone to rough lines and inaccurate alignment due to the problem of screens. Therefore, the accuracy of thick-film ceramic substrates for future LED products with smaller and smaller size requirements, finer-grained high-power LED products, or LED products that require accurate eutectic or flip-chip manufacturing processes. Has been gradually insufficient.
3.2 Low-temperature co-fired multilayer ceramic low-temperature co-fired multilayer ceramic technology uses ceramics as a substrate material, and the lines are printed on the substrate by means of screen printing, then the multilayer ceramic substrate is integrated, and finally the sintered ceramics are formed through low-temperature sintering. The major domestic manufacturers include Junde Electronics and Yuxin. The metal circuit layer of the low-temperature co-fired multi-layer ceramic substrate is also manufactured by using a screen printing process, and the alignment error may also be caused by the tension network problem. In addition, after the multi-layer ceramics are laminated and sintered, the shrinkage ratio may also be considered. . Therefore, if the low-temperature co-fired multilayer ceramics are used in eutectic/flip chip LEDs that require precise alignment of the circuits, they will be even more severe.
3.3 Thin-Film Ceramic Substrates In order to improve the problem of thick-film process screens and shrinkage ratio after multi-laminated pressure sintering, thin-film ceramic substrates have recently been developed as heat-dissipating substrates for LED dies. The thin film heat sink substrate is fabricated using sputtering, electro/electrochemical deposition, and a yellow photolithography process, and includes:
(1) Low temperature process (below 300°C), avoiding the possibility of high temperature material damage or size variation;
(2) Using a yellow photolithography process to make the circuit on the substrate more accurate;
(3) The metal circuit is not easy to fall off ... and so on, so the thin film ceramic substrate is suitable for high power, small size, high brightness LEDs, as well as eutectic/flip chip packaging process with high alignment accuracy. At present, the company mainly produces electronic film and ceramic substrate production capacity with companies such as Qisibai Electronics and Tongxin Electric.
4. Development trend of LED products of international manufacturers The current trend of development of LED lamps can be seen from the LED power and size of LED packaging manufacturers recently announced that high-power, small-size products are the focus of the current development of LED industry. Both ceramic heat sinks are used as a way to dissipate heat from their LED die. Therefore, the ceramic heat-dissipating substrate has become a very important part in the structure of high-power, small-size LED products. The following Table II is a simple combination of the status quo of domestic and foreign LED product development and product categories.
5. Conclusions To improve LED luminous efficiency and service life, solving the heat dissipation problem of LED products is one of the most important topics 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. Therefore, providing a heat-dissipating substrate with high heat dissipation and precise dimensions has also become a trend for the development of LED heat-dissipating substrates in the future. At this stage, aluminum nitride substrates are used instead of alumina substrates, or eutectic or flip-chip processes are used to replace the gold/grain-bonded die/substrate bonding method to improve the luminous efficiency of LEDs. In this development trend, the accuracy of the alignment of the heat-dissipating substrate itself is extremely stringent, and the characteristics such as high heat dissipation, small size, and good adhesion of metal lines are required. Therefore, a thin-film ceramic heat-dissipating substrate is fabricated using a yellow light lithography. It will become one of the important catalysts to promote the continuous improvement of LED power.
NINGBO SANCO ELECTRONICS CO., LTD. , https://www.sancobuzzer.com