Ceramics are often called inorganic non-metallic materials. It can be seen that people directly position ceramics as the opposite of metals. After all, the performance of the two is very different. However, the respective advantages of the two are too prominent, so in many cases it is necessary to combine ceramics and metals to show their respective strengths, thus giving birth to a very important technology—Ceramic metallization technology. Ceramic metallization has been a hot topic for many years, and scholars at home and abroad have conducted in-depth research on it.
Especially with the advent of the 5G era, the power of semiconductor chips continues to increase, the development trend of lightweight and high integration becomes increasingly obvious, and the importance of heat dissipation issues becomes more and more prominent. This is undoubtedly More stringent requirements have been put forward for packaging heat dissipation materials. In the packaging structure of power electronic components, the packaging substrate serves as a key link between the top and bottom and maintains the conduction of internal and external circuits. It also has functions such as heat dissipation and mechanical support. As an emerging electronic heat dissipation packaging material, ceramics have high thermal conductivity, insulation, heat resistance, strength, and a thermal expansion coefficient that matches the chip. It is an ideal packaging heat dissipation material for power electronic components.
Ceramics are used in circuits and must first be metallized span>, that is, a metal film that is firmly bonded to the ceramic and is not easily melted is deposited on the surface of the ceramic to make it conductive, and then connected to a metal lead or other metal conductive layer using a welding process to become one.
The most important step in the ceramic-to-metal sealing process is metallization , its quality affects the final sealing effect.
Difficulties in welding ceramics and metals
1. The linear expansion coefficient of ceramics is small, while the linear expansion coefficient of metal is relatively large, causing joints to crack. Generally, the thermal stress problem of the metal intermediate layer must be well handled.
2. The thermal conductivity of ceramic itself is low and its thermal shock resistance is weak. During welding, the temperature gradient in and around the welding part should be reduced as much as possible, and the cooling rate should be controlled after welding.
3. Most ceramics have poor conductivity or even no conductivity, making it difficult to use electric welding.
4. Since ceramic materials have stable electronic coordination, it is unlikely that metal and ceramics will connect. Ceramics need to be metallized or soldered with active filler metal.
5. Since ceramic materials are mostly covalent crystals, they are not prone to deformation and brittle fracture often occurs. At present, most use the intermediate layer to reduce the welding temperature and use the indirect diffusion method for welding.
6. The structural design of ceramic and metal welding is different from ordinary welding. It is usually divided into flat sealing structure, enveloping structure, pin sealing structure and counter-sealing structure. Among them, the encapsulating structure effect Best of all, these joint structures are very demanding to make.
Ceramic metallization mechanism
The mechanism of ceramic metallization is relatively complex, involving several chemical and physical reactions, plastic flow of substances, particle rearrangement, etc. Various substances such as oxides and non-metal oxides in the metallization layer undergo different chemical reactions and substance diffusion and migration during different sintering stages. As the temperature increases, various substances react to form intermediate compounds. When a common melting point is reached, a liquid phase is formed. The liquid glass phase has a certain viscosity and produces plastic flow at the same time. Afterwards, the particles develop under the action of capillaries.Rearrangement occurs, atoms or molecules diffuse and migrate driven by surface energy, the grains grow, the pores gradually shrink and disappear, and the metallization layer becomes densified.
Ceramic metallization process
The process flow of ceramic metallization includes:
Step one: matrix pretreatment. Use diamond abrasive paste to polish the pressureless sintered ceramic until it is optically smooth to ensure that the surface roughness is ≤1.6m. Put the substrate into acetone and alcohol and clean it with ultrasonic waves at room temperature for 20 minutes.
Step 2: Preparation of metallization slurry. Weigh the raw materials according to the metallization formula, ball-mill for a certain period of time to make a metallization slurry of a certain viscosity.
Step 3: Paint and dry. Use screen printing technology to apply slurry on the ceramic substrate. The thickness of the slurry should be appropriate. If it is too thin, the solder will easily flow into the metallization layer. If it is too thick, it will not be conducive to component migration. Then the slurry will be placed in the oven. dry.
Step 4: Heat treatment. The dried substrate is placed in a reducing atmosphere and sintered to form a metallized layer.
Specific methods of ceramic metallization
The commonly used preparation methods for ceramic metallization mainly include Mo-Mn method, activated Mo-Mn method, active metal brazing method, direct copper coating method (DBC), and magnetron sputtering method.
1. Mo-Mn method
The Mo-Mn method is based on the refractory metal powder Mo, and then adds a small amount of low melting point Mn to the metallization formula, adding a binder to coat the surface of the Al2O3 ceramic, and then sintering to form a metal chemical layer. The disadvantages of the traditional Mo-Mn method are high sintering temperature, high energy consumption, and the lack of activator in the formula, resulting in low sealing strength.
2. Activated Mo-Mn method
The activated Mo-Mn method is an improvement based on the traditional Mo-Mn method. The main directions for improvement are: adding activators and replacing metals with oxides or salts of molybdenum and manganese. pink. Both types of improvement methods are aimed at reducing the metallization temperature.
The disadvantages of the activated Mo-Mn method are that the process is complex and the cost is high, but its combination is strong and can greatly improve the wettability, so it is still the earliest invention in the ceramic-metal sealing process. The most mature and widely used process.
3. Active metal brazing method
The active metal brazing method is also a widely used ceramic-metal sealing process. It was developed 10 years later than the Mo-Mn method. It is characterized by fewer processes and ceramic-metal sealing. Sealing only requires one heating process to complete. The brazing alloy contains active elements such as Ti, Zr, Hf and Ta. The added active elements react with Al2O3 to form a reaction layer with metallic characteristics at the interface. This method can be easily adapted to mass production, with molybdenum- Compared with the manganese process, this method is relatively simple and economical.
The disadvantage of the active metal brazing method is that the active solder is single, which results in certain limitations in its application. It is not suitable for continuous production and is only suitable for large parts, single-piece production or small batch production.
4. Directbondedcopper (DBC)
DBC is a metallization method for bonding copper foil on a ceramic surface (mainly Al2O3 and AlN). It was developed with the rise of chip-on-board (COB) packaging technology a new type of technology. The basic principle is to introduce oxygen element between Cu and ceramics, and then form a Cu/O eutectic liquid phase at 1065-1083°C, which then reacts with the ceramic matrix and copper foil to generate CuAlO2 or Cu(AlO2)2, and The bonding between the copper foil and the matrix is achieved under the action of the intermediate phase.
5. Magnetron sputtering method
Magnetron sputtering is a type of physical vapor deposition. It uses magnetron technology to deposit multi-layer films on a substrate. It has advantages over other deposition technologies, such as better Adhesion, less contamination and improved crystallinity of the deposited sample, resulting in high quality films.
The metallization layer obtained by this method is very thin and can ensure the accuracy of the part size. However, it is not suitable for metallizing ceramics that are not resistant to high temperatures (such as piezoelectric ceramics and single crystals).
Influencing factors of ceramic metallization
1. Metalization formula
This is the prerequisite for realizing ceramic metallization, which requires careful and scientific design of its formula.
2. Metalization temperature and holding time
Another key factor affecting ceramic metallization is the metallization sintering temperature and holding time. The metallization temperature can be divided into the following four processes: those with temperatures above 1600°C are extremely high temperature, 1450~1600°C are high temperature, 1300~1450°C are medium temperature, and those below 1300°C are low temperature. Appropriate sintering temperature is necessary. If the temperature is too low, the glass phase will not diffuse and migrate. If the temperature is too high, the metallization strength will be poor. The metallization layer will easily fall off from the ceramic, causing sealing failure.
3. Microstructure of metallized layer
The metallization process determines the microstructure of the metallization layer, and the microstructure directly affects the final performance of the welded body. To obtain good welding performance, first the metallization layer should be a dense film with high bonding strength. If the microstructure of the metallization layer has clear layers in each area, and no continuous brittle metal compounds are observed at any interface, it will reduce the probability of brittleness and crack propagation, and the interface will be tight with fewer cracks, which will help reduce solder penetration. This shows that the metallization layer is dense and the bonding strength is relatively high.
4. Other factors
There are many factors that affect the degree of ceramic metallization that need to be paid attention to, such as the influence of powder particle size and reasonable gradation. If the powder is too fine and has large surface energy, it is easy to form agglomeration, which will affect the coating. The flatness of the powder; if the powder is too coarse, the surface energy will be reduced, which will lead to an increase in the sintering temperature and affect the sintering quality. In addition, the coating method and coating thickness will also have a great impact on ceramic metallization.
