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A brief analysis of the current situation and development of ceramic substrates in the field of LED packaging
Introduction: Ceramic substrate materials are widely used in power electronics, electronic packaging, hybrid microelectronics and multi-chip modules due to their excellent thermal conductivity and air tightness. This article briefly introduces the current status and future development of LED packaging ceramic substrates.
Keywords: LED ceramic substrate LED industry
· Ceramic substrate materials are widely used in power electronics, electronic packaging, hybrid microelectronics and multi-chip modules due to their excellent thermal conductivity and air tightness. This paper briefly introduces the current status and future development of ceramic substrates.
1. Comparison of plastic and ceramic materials
Plastics, especially epoxy resins, still occupy a dominant position in the entire electronic market due to their relatively good economy. Not suitable, even adding a large amount of organic bromide to the epoxy will not help.
Compared with plastic materials, ceramic materials also play an important role in the electronics industry. They have high electrical resistance, outstanding high-frequency characteristics, and have the advantages of high thermal conductivity, good chemical stability, thermal stability and high melting point. These properties are very required in the design and manufacture of electronic circuits, so ceramics are widely used as substrate materials for different thick films, thin films or circuits. They can also be used as insulators, as thermal paths in circuits with demanding thermal performance, and as a to manufacture various electronic components.
2. Comparison of various ceramic materials
So far, alumina substrate is the most commonly used substrate material in the electronics industry, because of its high strength and chemical stability compared to most other oxide ceramics in terms of mechanical, thermal and electrical properties, and rich sources of raw materials, it is suitable for various Manufactured in a variety of techniques as well as in different shapes.
It has a higher thermal conductivity than metal aluminum, and is used in occasions requiring high thermal conductivity, but the temperature decreases rapidly after the temperature exceeds 300 °C.
Most importantly, it limits its own development due to its toxicity.
AlN has two very important properties worth noting: one is high thermal conductivity, and the other is the expansion coefficient that matches Si. The disadvantage is that even a very thin oxide layer on the surface will have an impact on thermal conductivity, and only strict control of materials and processes can produce AlN substrates with better consistency. At present, the large-scale AlN production technology is still immature in China. Compared with Al2O3, the price of AlN is relatively high, which is also a bottleneck restricting its development. Based on the above reasons, it can be known that alumina ceramics are still widely used in the fields of microelectronics, power electronics, hybrid microelectronics, power modules and other fields due to their superior comprehensive performance.
3. Manufacturing of ceramic substrates
It is very difficult to manufacture high-purity ceramic substrates. Most ceramics have high melting points and hardness, which limits the possibility of ceramic machining. Therefore, ceramic substrates are often doped with glass with a lower melting point for fluxing or adhesion. connected, making the final product easy to machine. The preparation process of Al2O3, BeO, and AlN substrates is very similar. The base material is ground into a powder with a diameter of about a few microns, mixed with different glass fluxes and binders (including powdered MgO, CaO), and added to the mixture. Some organic binders and different plasticizers are then ball milled to prevent agglomeration to make the composition uniform, green ceramic sheets are formed, and finally sintered at high temperature. At present, there are mainly the following methods for ceramic molding:
Roller rolling: The slurry is sprayed onto a flat surface, partially dried to form a putty-like flake, and then the flake is sent to a pair of large parallel rollers for rolling to obtain a green ceramic sheet of uniform thickness.
●Casting The slurry is coated on a moving belt by a sharp blade to form a thin sheet. This is a low pressure process compared to other processes.
●Powder pressing The powder is sintered in a hard mold cavity and under a large pressure (about 138MPa), although the uneven pressure may cause excessive warpage, the sintered parts produced by this process are very dense and have small tolerances.
●Isostatic powder compaction This process uses molds surrounded by water or glycerin and uses pressures up to 69MPa. This more uniform pressure produces less warpage of parts.
●Extrusion The slurry is extruded through a die. The viscosity of the slurry used in this process is low, and it is difficult to obtain small tolerances, but this process is very economical and can obtain thinner parts than other methods.
4. Comparison of substrate types and their characteristics
At present, there are four common types of ceramic heat dissipation substrates: HTCC, LTCC, DBC, and DPC. Among them, HTCC belongs to a relatively early development technology, but the selection of electrode materials is limited due to the high sintering temperature, and the production cost is relatively expensive. These factors have prompted the development of LTCC. Although LTCC reduces the co-firing temperature to about 850°C, the disadvantage is that dimensional accuracy and product strength are not easily controlled. While DBC and DPC are domestically developed and mature technologies for energy production in recent years, DBC uses high-temperature heating to combine Al2O3 and Cu plates. The technical bottleneck is that it is difficult to solve the problem of micro-porosity between Al2O3 and Cu plates. However, the DPC technology uses direct copper plating technology to deposit Cu on the Al2O3 substrate. The process combines materials and thin film technology, and its products It is the most commonly used ceramic heat dissipation substrate in recent years. However, its material control and process technology integration capabilities are relatively high, which makes the technical threshold for entering the DPC industry and stable production relatively high.
4.1 LTCC (Low-Temperature Co-fired Ceramic)
LTCC is also known as low temperature co-fired multi-layer ceramic substrate. In this technology, inorganic alumina powder and about 30%~50% glass material plus organic binder are firstly mixed to form a muddy slurry. Use a scraper to scrape the slurry into flakes, and then go through a drying process to form the flake slurry into thin green embryos, and then drill through holes according to the design of each layer, as the signal transmission of each layer, the internal circuit of LTCC Then, screen printing technology is used to fill holes and print circuits on the green embryo, respectively, and the inner and outer electrodes can be made of silver, copper, gold and other metals respectively. Sintering and molding in a sintering furnace can be completed. Detailed manufacturing process LTCC production flow chart 4.1
Figure 4.1 LTCC production flow chart
4.2 HTCC (High-Temperature Co-fired Ceramic)
HTCC is also known as high temperature co-fired multi-layer ceramics. The manufacturing process is very similar to LTCC. The main difference is that the ceramic powder of HTCC is not added with glass material. Therefore, HTCC must be dried and hardened at a high temperature of 1300~1600 °C. The green embryo is then drilled with via holes, and the holes and printed circuits are filled with screen printing technology. Due to the high co-firing temperature, the choice of metal conductor materials is limited. The main material is high melting point but conductive Metals such as tungsten, molybdenum, manganese...
4.3 DBC (Direct Bonded Copper)
The direct copper coating technology is to directly bond copper on the ceramic by using the oxygen-containing eutectic liquid of copper. The basic principle is to introduce an appropriate amount of oxygen between the copper and the ceramic before or during the bonding process. In the range of ℃, copper and oxygen form a Cu-O eutectic liquid. DBC technology uses this eutectic liquid to chemically react with the ceramic substrate to generate CuAlO2 or CuAl2O4 phase, and on the other hand, infiltrate the copper foil to realize the combination of the ceramic substrate and the copper plate. The manufacturing flow chart of the direct copper clad plate on the ceramic substrate is shown in Figure 4.2 below.
(a) Al2O3 ceramic substrate copper cladding process (b) AlN ceramic substrate copper cladding process
Figure 4.2 Schematic diagram of direct copper-clad ceramic substrate process
Direct copper-clad ceramic substrates are widely used because they have the advantages of excellent electrical conductivity and thermal conductivity of copper and high mechanical strength and low dielectric loss of ceramics. Over the past few decades, copper-clad substrates have made great contributions to power electronic packaging, mainly due to the following performance characteristics of direct-coated substrates:
● Good thermal performance;
● Capacitance performance;
● High insulation performance;
● Si matching thermal expansion coefficient;
● Excellent electrical performance and strong current carrying capacity.
· The initial research of direct copper-clad ceramic substrate was developed to solve the problem of high current and heat dissipation, and later it was applied to the metallization of AlN ceramics. In addition to the above characteristics, it also has the following characteristics that make it widely used in high-power devices:
● Strong mechanical stress, stable shape; high strength, high thermal conductivity, high insulation; strong bonding force, anti-corrosion;
● Excellent thermal cycle performance, the number of cycles can reach 50,000 times, and the reliability is high;
● Same as PCB board (or IMS substrate), it can etch various patterns of structure; no pollution and no pollution;
● The operating temperature is -55℃～850℃; the thermal expansion coefficient is close to that of silicon, which simplifies the production process of power modules.
Due to the characteristics of direct copper-clad ceramic substrates, it has the irreplaceable characteristics of PCB substrates. The thermal expansion coefficient of DBC is close to that of silicon chip, which can save the transition layer Mo chip, save labor, material and cost. Since the direct copper-clad ceramic substrate does not add any brazing components, it reduces the solder layer, reduces thermal resistance, and reduces holes. Improve the yield, and the line width of 0.3mm thick copper foil is only 10% of that of ordinary printed circuit boards under the same current carrying capacity; its excellent thermal conductivity makes the package of the chip very compact, thereby greatly improving the power density and improving the system. and device reliability.
In order to improve the thermal conductivity of the substrate, it is generally necessary to reduce the thickness of the substrate. Ultra-thin (0.25mm) DBC board can replace BeO, and the thickness of direct copper bonding can reach 0.65mm. The current and the temperature rise are not obvious. The 100A current continuously passes through the 1mm wide and 0.3mm thick copper body, and the temperature rise is about 17℃; the 100A current continuously passes through the 2mm wide and 0.3mm thick copper body, and the temperature rise is only about 5℃. Compared with brazing and Mo-Mn method, DBC has very low thermal resistance characteristics, take the thermal resistance of 10×10mm DBC board as an example:
The thermal resistance of the 0.63mm thick ceramic substrate DBC is 0.31K/W, the 0.38mm thick ceramic substrate DBC's thermal resistance is 0.19K/W, and the 0.25mm thick ceramic substrate DBC's thermal resistance is 0.14K/W.
Alumina ceramics have the highest resistance and high insulation withstand voltage, which ensures personal safety and equipment protection capabilities; in addition, DBC substrates can realize new packaging and assembly methods, making products highly integrated and small in size.
4.3.1 Development Trend of Direct Copper Clad Ceramic Substrate
In high-power and high-density packaging, the heat generated by electronic components and chips during operation is mainly dissipated to the environment through the ceramic substrate, so the ceramic substrate plays an important role in the heat dissipation process. The thermal conductivity of Al2O3 ceramics is relatively low, and it is necessary to force heat dissipation in the operation of high-power, high-density packaged devices to meet the requirements. BeO ceramics have the best thermal conductivity, but they are basically eliminated due to environmental problems. The bonding of SiC ceramics after metallization is unstable, and when used as an insulating substrate, it will cause changes in thermal conductivity and dielectric constant. AlN ceramics have high thermal conductivity and are suitable for high-power semiconductor substrates, which can be achieved by natural cooling during the heat dissipation process. At the same time, they also have good mechanical strength and excellent electrical properties. Although the current domestic manufacturing technology needs to be improved and the price is relatively expensive, its annual production growth rate is more than 4 times higher than that of Al2O3 ceramics, and it can replace BeO and some non-oxide ceramics in the future. Therefore, it is the general trend to use AlN ceramics as insulating and thermally conductive substrates, but it is only a matter of time and cost performance.
4.3.2 Performance comparison between direct aluminum (DAB) ceramic substrate and direct copper ceramic substrate (DBC)
As an insulating carrier, the direct aluminum substrate has made great progress in the application of electronic circuits. This technology draws on the technology of direct copper ceramic substrate. This new type of direct Al-coated substrate shows good properties both theoretically and experimentally. Although its properties are similar in many respects to direct Cu substrates. For the direct-coated Cu substrate, the thermal expansion coefficient of 96 alumina ceramic substrate is 6.0′10-6/°C at room temperature because the expansion coefficient of metallic copper is 17.0′10-6/°C at room temperature. When the temperature is high (greater than 1000 ℃), the interface will form a relatively hard product CuAlO2, so the internal stress of the aluminum oxide substrate coated with copper is relatively large, and the thermal shock resistance is relatively poor, and it is often damaged due to fatigue in use.
Compared with copper, aluminum has a lower melting point, low price and good plasticity. The melting point of pure aluminum is only 660°C, and the expansion coefficient of pure aluminum is 23.0' 10-6/°C at room temperature. Metal aluminum and aluminum oxide The bonding of the ceramic substrate is physically wet, there is no chemical reaction on the interface, and the excellent plasticity of pure aluminum can effectively relieve the thermal stress caused by the different thermal expansion coefficients of the interface. Thermal shock resistance. This is incomparable to the direct deposition of Cu substrates, and the peel strength between metal aluminum and alumina ceramics is also relatively large.
Directly coated aluminum substrate as a substrate is especially suitable for power electronic circuits. The performance of direct aluminum coated substrate is different from that of direct copper coated substrate. The former has better stability under high temperature cycles. Chips with direct aluminum cladding also show better stability than direct copper cladding. With its high thermal shock resistance and low weight, the direct-clad aluminum substrate is expected to develop better performance in the future to meet higher demands.
4.3.3 Development Trend of Aluminum-clad Ceramic Substrates
Aluminum-clad ceramic substrates (DAB) are used for insulating carriers, especially power electronic circuits, due to their unique properties. This new material is similar to direct copper substrate (DBC) in many aspects, and it has remarkable thermal shock resistance and thermal stability, which is very obvious for improving the stability of devices operating at extreme temperatures. Power device modules made of Al-Al2O3 substrates and Al-AlN substrates have been successfully used in the Japanese automobile industry. DAB substrates have great potential for devices with special requirements for high reliability, which makes them ideal for optimizing power electronic systems, automation, aerospace, and more.
4.4 DPC (Direct Plate Copper)
DPC is also known as direct copper plating substrate. The DPC substrate process is taken as an example: first, the ceramic substrate is pre-treated and cleaned, and the professional thin-film manufacturing technology-vacuum coating method is used to sputter and bond the copper metal composite layer on the ceramic substrate, and then use yellow light lithography. The photoresist is re-exposed, developed, etched, and film-removed to complete the circuit fabrication. Finally, the thickness of the circuit is increased by electroplating/electroless deposition. After the photoresist is removed, the metallization circuit is completed. The detailed DPC production flow chart As shown below.
5. Ceramic base
5.1 Thermal conductivity
Thermal conductivity represents the ability of the substrate material to directly conduct heat energy. The higher the value, the better the heat dissipation capacity. The main function of the heat dissipation substrate in the LED field is how to effectively conduct heat energy from the LED chip to the system to dissipate heat, so as to reduce the temperature of the LED chip, increase the luminous efficiency and prolong the life of the LED. It has become one of the important evaluation items in the industry when choosing a heat dissipation substrate. Looking at Table 1, it can be clearly seen from the comparison of the four ceramic heat dissipation substrates that although the thermal conductivity of Al2O3 material is about 20~24, LTCC adds 30%~50% glass material to reduce its sintering temperature, so that the Its thermal conductivity is reduced to about 2~3W/mK; while HTCC generally has a co-firing temperature slightly lower than the sintering temperature of pure Al2O3 substrates, so its low material density makes the thermal conductivity of Al2O3 substrates about 16~17W/ between mK. Generally speaking, the heat dissipation effect of LTCC and HTCC is not as good as that of DBC and DPC heat dissipation substrates.
5.2 Operating ambient temperature
The operating ambient temperature mainly refers to the maximum process temperature used in the production process of the product. In terms of a production process, the higher the temperature used, the higher the relative manufacturing cost, and the yield rate is not easy to control. The HTCC process itself is between 1300 and 1600°C due to the different components of the ceramic powder materials, while the LTCC/DBC process temperature is also between 850 and 1000°C. In addition, HTCC and LTCC must be laminated and then sintered after the process, so that each layer will have a shrinkage ratio problem. To solve this problem, the relevant industry is also trying to find a solution. On the other hand, DBC has very strict requirements on the accuracy of process temperature. It must be in the extremely stable temperature range of 1065~1085°C to make the copper layer smelted into a eutectic melt, which is closely combined with the ceramic substrate. The temperature is not stable enough, which will inevitably lead to the phenomenon of low yield. Considering the process temperature and margin, the process temperature of DPC only needs to be about 250~350℃ to complete the fabrication of the heat dissipation substrate, which completely avoids the damage or dimensional variation caused by high temperature to the material, and also eliminates the The problem of high manufacturing cost.
5.3 Process capability
Process capability mainly indicates which process technology is used to complete the metal circuits of various heat-dissipating substrates. Since the method of circuit manufacturing/forming directly affects the characteristics of circuit accuracy, surface roughness plating, alignment accuracy... Under the requirements of fine lines with small power and small size, process resolution has become one of the important items that must be considered. Both LTCC and HTCC use thick film printing technology to complete the circuit production. Thick film printing itself is limited by the screen tension problem. Generally speaking, the surface of the circuit is relatively rough, and it is easy to cause inaccurate alignment and excessive progressive tolerance. etc. phenomenon. In addition, in the multi-layer ceramic lamination and sintering process, there is also the problem of shrinkage ratio that needs to be considered, which makes the process resolution more limited. Although DBC uses lithography to prepare metal lines, due to the limitation of process capability, the lower limit of metal copper thickness is about 150~300um, which makes the upper limit of its metal line resolution only between 150~300um ( with an aspect ratio of 1:1). The DPC is produced by the thin film process, using vacuum coating and yellow light lithography to make the circuit, so that the circuit on the substrate can be more accurate, the surface flatness is high, and then the thickness of the circuit is increased by electroplating/electrochemical plating deposition method, DPC The thickness of the metal circuit can be designed according to the actual requirements of the product (metal thickness and circuit resolution). Generally speaking, the resolution of DPC metal lines is about 10~50um under the principle of metal line aspect ratio of 1:1. Therefore, DPC eliminates the sintering shrinkage ratio of LTCC/HTCC and the screen opening problem of thick film technology.
5.4. Application of ceramic heat dissipation substrate
The appearance of ceramic heat dissipation substrates will vary according to different needs and applications. On the other hand, various ceramic substrates can also be basically distinguished according to the different manufacturing methods of the products. In the application of LTCC heat dissipation substrates to LED products, most of them are large-size high-power and small-size low-power products. Basically, the appearance is mostly concave cup shape, and according to the needs of the client, it can be made with lead frame and without lead frame. The shape of the concave cup is mainly designed for the packaging process using a simpler dispensing method, and the edge of the concave cup is used as the path for light reflection. However, the LTCC itself is limited by process factors, making the product difficult to prepare. In addition, the thick film is used to make the circuit, so that the circuit accuracy is not enough to meet the high power and small size LED products. HTCC, which is similar in process and appearance to LTCC, has not been widely used in the LED heat dissipation substrate, mainly because HTCC adopts high temperature drying and hardening at 1300~1600 °C, which increases the production cost, and the relative cost of HTCC substrate is also high. Therefore, for the LED industry that is striving to move towards a low-cost trend, it faces a more severe test HTCC.
On the other hand, DBC and DPC are different from LTCC/HTCC not only in appearance, but also in the packaging method of LED products. DBC/DPC are all flat heat dissipation substrates, and flat heat dissipation substrates can be customized according to customer requirements. The metal circuit is processed by chemical preparation, and then cut into small-sized products according to customer needs. to improve the luminous efficiency of LEDs. However, due to the limitation of process capability of DBC products, the upper limit of line resolution is only 150~300um. If you want to make thin line products, you must use grinding method to reduce the thickness of copper layer, but it makes the surface flatness difficult to control and difficult to control. The additional cost and other issues make DBC products difficult to apply to the requirements of high circuit accuracy and high flatness in the eutectic/multi-crystal process. DPC uses thin film lithography to prepare metal circuit processing, which has the characteristics of high circuit accuracy and high surface flatness. Thereby improving the efficiency of heat dissipation.
After the above-mentioned production process, characteristic comparison, and application scope description of various ceramic substrates, individual differences can be clearly compared. Among them, LTCC heat dissipation substrates have been widely used in the LED industry, but in order to reduce the sintering temperature, LTCC adds glass material to the material, which reduces the overall thermal conductivity to 2~3W/mK, which is higher than other ceramic substrates. even lower. Furthermore, LTCC uses the screen printing method to print the circuit, so that the circuit itself has the problem of insufficient wire diameter and screen opening, resulting in insufficient circuit accuracy and poor surface flatness. There is a problem of substrate shrinkage ratio to be considered, which does not meet the requirements of high power and small size. Therefore, the application in the LED industry is currently dominated by high-power, large-size, or low-power products. However, HTCC, which is similar to LTCC process, is dried and hardened at high temperature of 1300~1600 °C, which makes the production cost relatively high, and it is rarely used in the LED industry due to cost considerations. Power and small size LED products. On the other hand, in order to have good adhesion between the copper layer of DBC and the ceramic substrate, it must be smelted at a high temperature of 1065~1085°C, the manufacturing cost is high, and the problem of micro-porosity between the substrate and the Cu plate is difficult to solve, which makes the DBC product production capacity. and yield have been greatly tested; in addition, special treatment methods must be used to reduce the thickness of the copper layer to make thin circuits, but the problem of poor surface flatness is caused. The LED products are relatively strict. On the contrary, DPC products use thin-film vacuum sputtering to coat thin copper, and then use yellow light lithography to complete the circuit, so the width of the wire diameter is 10~50um, or even thinner, and the surface flatness is high (<0.3um) , The line alignment accuracy error value is only +/- 1%, which completely avoids problems such as shrinkage ratio, screen opening, surface flatness, high manufacturing cost, etc. Although ceramic substrates such as LTCC, HTCC, DBC, and DPC have been widely used and studied, however, in the field of high-power LED ceramic heat dissipation, DPC can be said to be the most suitable for the development needs of high-power LED according to the current development trend. Ceramic heat sink substrate.