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 ceramic substrates.
1. Comparison of plastic and ceramic materials
Plastics, especially epoxy resins, still occupy the dominant position in the entire electronic market until now due to their relatively good economy, but many special fields such as high temperature, mismatch in linear expansion coefficient, air tightness, stability, mechanical properties, etc. are obviously Not suitable. Even adding a large amount of organic bromide to epoxy resin 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, high thermal stability, and high melting point. These properties are very much needed in the design and manufacture of electronic circuits. Therefore, ceramics are widely used as substrate materials for different thick films, films, or circuits, and can also be used as insulators. They are used as thermal conduction paths and circuits in circuits with severe thermal performance requirements. To manufacture various electronic components.
2. Comparison of various ceramic materials
2.1Al2O3
So far, aluminum oxide substrate is the most commonly used substrate material in the electronics industry, because of its mechanical, thermal, and electrical properties compared to most other oxide ceramics, it has high strength and chemical stability, and is rich in raw materials. Various technical manufacturing and different shapes.
2.2BeO
It has a higher thermal conductivity than metal aluminum and is used in applications requiring high thermal conductivity, but the temperature drops rapidly after 300 ℃.
The most important thing is that its toxicity limits its own development.
2.3AlN
There are two very important properties of AlN worth noting: one is high thermal conductivity, and the other is the expansion coefficient matching Si. The disadvantage is that even if there is a very thin oxide layer on the surface, it will affect the thermal conductivity. Only by strictly controlling the materials and processes can we produce an AlN substrate with good consistency. At present, large-scale AlN production technology is still immature in China. Compared with Al2O3, AlN prices are relatively high, which is also a bottleneck restricting its development. Based on the above reasons, it can be known that due to its superior comprehensive performance, alumina ceramics are still in a dominant position in microelectronics, power electronics, hybrid microelectronics, power modules and other fields and are widely used.
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 ceramic substrates.
1. Comparison of plastic and ceramic materials
Plastics, especially epoxy resins, still occupy the dominant position in the entire electronic market until now due to their relatively good economy, but many special fields such as high temperature, mismatch in linear expansion coefficient, air tightness, stability, mechanical properties, etc. are obviously Not suitable. Even adding a large amount of organic bromide to epoxy resin 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, high thermal stability, and high melting point. These properties are very much needed in the design and manufacture of electronic circuits. Therefore, ceramics are widely used as substrate materials for different thick films, films, or circuits, and can also be used as insulators. They are used as thermal conduction paths and circuits in circuits with severe thermal performance requirements. To manufacture various electronic components.
2. Comparison of various ceramic materials
2.1Al2O3
So far, aluminum oxide substrate is the most commonly used substrate material in the electronics industry, because of its mechanical, thermal, and electrical properties compared to most other oxide ceramics, it has high strength and chemical stability, and a rich source of raw materials. Various technical manufacturing and different shapes.
2.2BeO
It has a higher thermal conductivity than metal aluminum and is used in applications requiring high thermal conductivity, but the temperature drops rapidly after 300 ℃.
The most important thing is that its toxicity limits its own development.
2.3AlN
There are two very important properties of AlN worth noting: one is high thermal conductivity, and the other is the expansion coefficient matching Si. The disadvantage is that even if there is a very thin oxide layer on the surface, it will affect the thermal conductivity. Only by strictly controlling the materials and processes can we produce an AlN substrate with good consistency. At present, large-scale AlN production technology is still immature in China. Compared with Al2O3, AlN prices are relatively high, which is also a bottleneck restricting its development. Based on the above reasons, it can be known that due to its superior comprehensive performance, alumina ceramics are still in a dominant position in microelectronics, power electronics, hybrid microelectronics, power modules and other fields and are widely used.
3. Manufacturing of ceramic substrates
It is very difficult to manufacture high-purity ceramic substrates. Most of the ceramics have high melting point 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 Connection, making the final product easy to machine. The preparation process of Al2O3, BeO and AlN substrates is very similar. The base material is ground to a powder diameter of a few microns, mixed with different glass fluxes and adhesives (including powdered MgO and CaO), and also added to the mixture Some organic adhesives and different plasticizers are ball milled to prevent agglomeration and make the ingredients uniform, forming green ceramic sheets, and finally sintering at high temperature. At present, ceramic molding mainly has the following methods:
Roller rolling sprays the slurry onto a flat surface and partially dries it to form flakes with a viscosity like putty. The flakes are then fed into a pair of large parallel rollers and rolled to obtain green ceramic sheets of uniform thickness.
The casting slurry is coated with a sharp blade on a moving belt to form a sheet. Compared with other processes, this is a low-pressure process.
Powder pressing The powder is sintered in the hard mold cavity and a large pressure (about 138MPa) is applied. Although the uneven pressure may cause excessive warpage, the sintered parts produced by this process are very dense and the tolerance is small.
This process of isostatic powder compaction uses a mold surrounded by water or water and a pressure of up to 69 MPa. This pressure is more uniform and the parts made are less warped.
Extrusion of slurry through a die The slurry used in this process has a low viscosity and it is difficult to obtain a small tolerance, 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 an earlier developed technology, but due to the high sintering temperature, the choice of electrode materials is limited, and the manufacturing cost is relatively expensive. These factors promote the development of LTCC. Although LTCC reduces the co-firing temperature to about 850 ° C, the disadvantage is that the dimensional accuracy and product strength are not easy to control. The DBC and DPC are mature and energy-produced professional technologies that have only been developed in China in recent years. DBC uses high temperature heating to combine Al2O3 and Cu plates. Its technical bottleneck is that it is not easy to solve the micropores between Al2O3 and Cu plates. Problem, which makes the mass production energy and yield of the product face greater challenges, and DPC technology uses direct copper plating technology to deposit Cu on the Al2O3 substrate, its process combines materials and thin film process technology, 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 multilayer ceramic substrate. This technology must first mix inorganic alumina powder with about 30% ~ 50% of glass material with organic binder to make it evenly mixed into a slurry slurry. Use a scraper to scrape the slurry into a sheet, and then form a thin piece of green embryo through a drying process, and then drill through holes according to the design of each layer as a signal transmission of each layer. LTCC internal circuit Then use screen printing technology to make holes and print lines on the green embryo respectively. The internal and external electrodes can use silver, copper, gold and other metals respectively. Finally, each layer is laminated and placed at 850 ~ 900 ℃ It can be completed by sintering in the sintering furnace.
4.2HTCC (High-Temperature Co-fired Ceramic)
HTCC is also called high-temperature co-fired multilayer ceramics. The manufacturing process is very similar to LTCC. The main difference is that HTCC's ceramic powder is not added to glass materials. Therefore, HTCC must be dried and hardened at a high temperature of 1300 ~ 1600 ℃. The embryo is then drilled, and the via hole is also drilled to fill the hole and the printed circuit with screen printing technology. Due to the high co-firing temperature, the choice of metal conductor materials is limited. The main material is a higher melting point but conductive Tungsten, molybdenum, manganese, etc., which have poor properties, are finally laminated and sintered.
4.3DBC (Direct Bonded Copper)
Direct copper deposition technology uses copper's oxygen-containing eutectic solution to directly deposit copper on the ceramic. The basic principle is to introduce an appropriate amount of oxygen between copper and ceramic before or during the deposition process, at 1065 ℃ ~ 1083 In the range of ℃, copper and oxygen form a Cu-O eutectic solution. The DBC technology uses this eutectic solution to chemically react with the ceramic substrate to generate CuAlO2 or CuAl2O4 phase, and on the other hand, infiltrate the copper foil to achieve the combination of the ceramic substrate and the copper plate.
Direct copper-clad ceramic substrates are widely used because they have the advantages of excellent electrical and thermal conductivity of copper, high mechanical strength and low dielectric loss of ceramics. In the past few decades, copper-clad substrates have made great contributions to power electronics packaging, which is mainly due to the following performance characteristics of direct copper-clad substrates:
Good thermal performance;
Capacitance performance
High insulation performance;
Si matched thermal expansion coefficient;
Superior electrical performance and strong current carrying capacity.
The initial research of direct copper-clad ceramic substrate was developed to solve the large current and heat dissipation, and later applied to the metallization of AlN ceramic. 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 binding force, anti-corrosion;
Excellent thermal cycling performance, with 50,000 cycles and high reliability;
Like PCB board (or IMS substrate), various patterns can be etched; no pollution, no pollution;
The use temperature is wide -55 ℃ ～ 850 ℃; the thermal expansion coefficient is close to silicon, which simplifies the production process of power module.
Due to the characteristics of the directly copper-clad ceramic substrate, it has the irreplaceable characteristics of the PCB substrate. The thermal expansion coefficient of DBC is close to that of silicon chips, which can save the Mo layer of the transition layer, save labor, save materials, and reduce costs. Because the direct copper-clad ceramic substrate does not add any brazing ingredients, this reduces the solder layer, reduces the thermal resistance, and reduces the holes. Improve yield, and 0.3mm thick copper foil line width is only 10% of ordinary printed circuit board under the same current carrying capacity; its excellent thermal conductivity makes the chip package very compact, which greatly improves the power density and improves the system And device reliability.
In order to improve the thermal conductivity of the substrate, the thickness of the substrate is generally reduced. The ultra-thin (0.25mm) DBC board can replace BeO, and the thickness of the direct copper deposit can reach 0.65mm, so that the direct copper-clad ceramic substrate can carry a larger The current and temperature increase are not obvious. 100A current continuously passes through a 1mm wide 0.3mm thick copper body with a temperature rise of about 17 ℃; 100A current continuously flows through a 2mm wide 0.3mm thick copper body with a temperature rise of only about 5 ℃. Compared with brazing and Mo-Mn method, DBC has very low thermal resistance characteristics. Take the thermal resistance of 10 & TImes; 10mmDBC board as an example:
The thermal resistance of the 0.63mm thickness ceramic substrate DBC is 0.31K / W, the thermal resistance of the 0.38mm thickness ceramic substrate DBC is 0.19K / W, and the thermal resistance of the 0.25mm thickness ceramic substrate DBC is 0.14K / W.
Alumina ceramics have the highest resistance and high insulation withstand voltage, so as to ensure personal safety and equipment protection capabilities; in addition, DBC substrates can realize new packaging and assembly methods, making products highly integrated and shrinking in size.
4.3.1 Development trend of direct copper-clad ceramic substrates
In high-power, 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 dissipate heat to meet the requirements when operating high-power, high-density packaged devices. BeO ceramics have the best thermal conductivity, but are basically eliminated due to environmental issues. SiC ceramics have unstable bonding after metallization. When used as an insulating substrate, they cause changes in thermal conductivity and dielectric constant. AlN ceramics have high thermal conductivity and are suitable for high-power semiconductor substrates. Natural cooling can be achieved during the heat dissipation process. At the same time, they have good mechanical strength and excellent electrical properties. Although the domestic manufacturing technology still needs to be improved and the price is relatively expensive, its annual output growth rate is more than 4 times higher than that of Al2O3 ceramics. It can replace BeO and some non-oxide ceramics in the future. Therefore, it is a general trend to use AlN ceramic as an insulating and thermally conductive substrate, but there are only problems of time and cost performance.
4.3.2 Performance comparison between direct aluminum-plated (DAB) ceramic substrate and direct copper-plated ceramic substrate (DBC)
The direct application of aluminum substrate as an insulating carrier has made great progress in the application of electronic circuits. Is this technology borrowed? Direct copper-clad ceramic substrate technology. This new type of direct Al-plated substrate shows good characteristics in theory and experiment. Although its characteristics are similar in many ways to direct Cu substrates. For the direct Cu substrate, the expansion coefficient of the metallic copper is 17.0 ′10 -6 / ° C at room temperature, and the thermal expansion coefficient of the 96 alumina ceramic substrate is 6.0′10-6 / ° C at room temperature. The temperature is higher (greater than 1000 ℃), the interface will form a relatively hard product CuAlO2, so the internal stress of the copper-coated aluminum oxide substrate is large, the thermal shock resistance is relatively poor, and it is often damaged by fatigue during use.
Compared with copper, aluminum has a lower melting point, lower price and good plasticity. The melting point of pure aluminum is only 660 ℃, the expansion coefficient of pure aluminum is 23.0 ′ 10-6 / ℃ at room temperature, metal aluminum and alumina The coating 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. The research also confirmed that the Al / Al2O3 ceramic substrate has very good Thermal shock resistance. This is unmatched by directly applying Cu substrates, and the peel strength between aluminum metal and alumina ceramics is also large.
The direct aluminum-clad substrate as a substrate is particularly suitable for power electronic circuits. The performance of the direct aluminum-clad substrate is different from the performance of the direct copper-clad substrate. The former has better stability under high temperature cycling. Chips with direct aluminum substrates also exhibit better stability than direct copper substrates. With its high thermal shock resistance and low weight, direct aluminum-clad substrates are expected to develop better performance in the future to meet higher demands.
4.3.3 Development trend of aluminum-clad ceramic substrate
Aluminum-clad ceramic substrates (DAB) are used in insulating carriers with their unique properties, especially power electronic circuits. This new material has similarities with the direct copper-clad substrate (DBC) in many respects, and it has significant thermal shock resistance and thermal stability performance, which is very obvious for improving the stability of the working device at extreme temperatures. The power device module made of Al-Al2O3 substrate and Al-AlN substrate has been successfully applied in the Japanese automobile industry. The DAB substrate has great potential in devices with special requirements for high reliability, which makes it very suitable for optimizing power electronic systems, automation, aviation and aerospace.
4.4 DPC (Direct Plate Copper)
DPC is also known as direct copper-plated substrate. The DPC substrate process is an example: first, the ceramic substrate is pre-treated and cleaned, and the copper substrate is sputtered and combined with the copper metal composite layer on the ceramic substrate using professional thin film manufacturing technology-vacuum coating method, and then the yellow light micro-shadow The photoresist is re-exposed, developed, etched, and film-removed to complete the circuit production. Finally, the thickness of the circuit is increased by electroplating / electroless plating deposition. After the photoresist is removed, the metallization circuit is completed. Detailed DPC production flowchart As shown below.
5. Characteristics of ceramic substrate
5.1 Thermal conductivity
Thermal conductivity represents the ability of the substrate material itself to directly conduct thermal energy. The higher the value, the better the heat dissipation capability. The most important role 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 to reduce the temperature of the LED chip, increase luminous efficiency and extend the life of the LED. Therefore, the advantages and disadvantages of the heat conduction effect of the heat dissipation substrate It has become one of the most important evaluation items in the industry when choosing a heat-dissipating substrate. Looking at Table 1, it can be clearly seen from the comparison of the four ceramic heat dissipation substrates, although the thermal conductivity of Al2O3 material is about 20 ~ 24, LTCC added 30% ~ 50% glass material to reduce its sintering temperature, so that Its thermal conductivity is reduced to about 2 ~ 3W / mK; and HTCC because its common co-firing temperature is slightly lower than the sintering temperature of pure Al2O3 substrate, so its low thermal conductivity coefficient Al2O3 substrate is about 16 ~ 17W / due to the low material density 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 environment temperature mainly refers to the highest 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 is not easy to control. The HTCC process itself is due to the different composition of the ceramic powder material, and its process temperature is about 1300 ~ 1600 ℃, while the LTCC / DBC process temperature is also about 850 ~ 1000 ℃. In addition, HTCC and LTCC must be laminated after the process and then sintered, so that each layer will have a shrinkage ratio problem. To solve this problem, related companies are also working hard to find a solution. On the other hand, DBC has very strict requirements on the accuracy of the process temperature. It must be in the extremely stable temperature range of 1065 ~ 1085 ℃, so that the copper layer can be smelted into a eutectic melt, which is closely combined with the ceramic substrate. If the temperature is not stable enough, it will inevitably cause the phenomenon of low yield. In consideration of the process temperature and margin, the process temperature of the DPC only needs to be about 250 ~ 350 ℃ to complete the manufacture of the heat dissipation substrate, which completely avoids the damage or dimensional variation caused by the high temperature to the material, and also excludes The problem of high manufacturing costs.
5.3 Process capability
The process capability is mainly to indicate the process technology used to complete the metal circuit of various heat dissipation substrates. Since the method of circuit manufacturing / forming directly affects the characteristics of circuit accuracy, rough surface plating, alignment accuracy ... With the demand for fine lines with small power, 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 manufacturing. Thick film printing itself is limited by the screen tension problem. In general, the circuit surface is rough, and it is easy to cause registration inaccuracies and progressive tolerances. And other phenomena. In addition, the multilayer ceramic sintering process also has the problem of shrinkage ratio, which makes its process resolution limited. Although DBC uses the lithography process to prepare metal lines, the lower limit of the thickness of the metal copper is about 150-300um due to the limitation of its process capability, which makes the upper limit of the resolution of the metal circuit only 150-300um ( With the aspect ratio of 1: 1 as the standard). The DPC is made by a thin film process, which uses vacuum coating and yellow light lithography process to make the circuit, so that the circuit on the substrate can be more accurate, the surface smoothness is high, and then the thickness of the circuit is increased by electroplating / electrochemical plating deposition. The thickness of the metal line can be designed according to the actual needs of the product (metal thickness and line resolution). In general, the resolution of DPC metal lines is about 10-50um under the principle that the aspect ratio of metal lines is 1: 1. Therefore, DPC eliminates the sintering shrinkage ratio of LTCC / HTCC and the problem of screen opening 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 differentiated according to the manufacturing method of the product. The application of LTCC heat dissipation substrates in LED products is mainly based on large-size high-power and small-size low-power products. Basically, the appearance is mostly concave cup-shaped, and according to the needs of users, it can be produced with lead frame & without lead frame. A heat dissipation substrate, the shape of the concave cup is mainly designed for the packaging process using a simple dispensing method, and the edge of the concave cup is used as the path of light reflection, but the LTCC itself is limited by the process factors, making the product difficult to prepare It is made into small size, and the thick film is used to make the circuit, so that the accuracy of the circuit is not enough to meet the high power and small size of LED products. The HTCC, which is similar to the LTCC process and appearance, has not been widely used in the LED heat dissipation substrate. The main reason is that HTCC adopts 1300 ~ 1600 ℃ high temperature drying and hardening, which increases the production cost and the relative HTCC substrate cost is also high. Therefore, for striving to move towards the low-cost LED industry, it faces a severe test HTCC.
On the other hand, DBC and DPC not only differ in appearance from LTCC / HTCC, but also have different packaging methods for LED products. DBC / DPC are flat heat dissipation substrates, and flat heat dissipation substrates can be customized The preparation of metal circuit processing, and then cut into small-size products according to customer needs, supplemented by eutectic / polycrystalline process, combined with the very well-known phosphor coating technology and high-end packaging technology casting film molding, can be significantly Improve the luminous efficiency of LED. However, DBC products are limited by process capability, so the upper limit of circuit resolution is only 150 ~ 300um. If you want to make fine circuit products, you must use grinding to reduce the thickness of the copper layer, but it makes the surface flatness difficult to control. Increasing additional costs and other issues make DBC products not easy to use in eutectic / polycrystal processes requiring high circuit accuracy and high flatness. DPC uses thin film lithography to prepare metal circuit processing, which has the characteristics of high precision and high surface flatness of the circuit. It is very suitable for the process of polycrystalline / eutectic bonding, which can greatly reduce the cross-sectional area of ​​LED products. In turn, the efficiency of heat dissipation is improved.
6 Conclusion
After the description of the production process, characteristics comparison, and application scope of the various ceramic substrates mentioned above, individual differences can be clearly compared. Among them, the LTCC heat dissipation substrate has been widely used in the LED industry, but in order to reduce the sintering temperature, LTCC has added glass materials to the material to reduce the overall thermal conductivity to 2 ~ 3W / mK, which is better than other ceramic substrates. Even lower. In addition, LTCC uses the screen printing method to print the circuit, which causes the circuit itself to have insufficient wire diameter and width, and the problem of screen printing, resulting in insufficient circuit accuracy and poor surface smoothness. 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 mainly based on high power and large size, or low power products. The HTCC, which is similar to the LTCC process, is dried and hardened at a high temperature of 1300 to 1600 ° C, which makes the production cost relatively high. Because of the few cost considerations, it is currently rarely used in the LED industry. HTCC and LTCC have the same problems and are not suitable for high temperature. LED products with small power. On the other hand, in order to make the adhesion between the copper layer of the DBC and the ceramic substrate good, it must be smelted at a high temperature of 1065 ~ 1085 ℃, the manufacturing cost is high, and the problem of micropores between the substrate and the Cu plate is not easy to solve, making the DBC product capacity And the yield is greatly tested; moreover, if you want to make a thin circuit, you must use a special treatment to thin the copper layer, but it causes the problem of poor surface flatness. If you use the product in the eutectic / polycrystal process Of LED products are relatively harsh. On the contrary, it is a DPC product, which is coated with thin copper by the vacuum sputtering method of the thin film process, and then the circuit is completed by the yellow light lithography process, so the wire diameter width is 10 ~ 50um, which can be even finer, and the surface smoothness is high (<0.3um) The accuracy value of the line alignment accuracy is only +/- 1%, which completely avoids the problems of shrinkage ratio, screen printing, surface smoothness, high manufacturing cost, etc. Although ceramic substrates such as LTCC, HTCC, DBC, and DPC have been widely used and researched, in the field of high-power LED ceramic heat dissipation, DPC can be said to be the most suitable for high-power and small-size LEDs in the current development trend Development needs of ceramic heat dissipation substrates.
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