Ceramic meets metallization

Ceramics, often referred to as inorganic non-metallic materials, can be seen to be positioned directly on the opposite side of metals - after all, their properties are very different. But their respective advantages are so outstanding that in many cases it is necessary to combine ceramics and metals to show their respective advantages, thus giving birth to a very important technology - ceramic metallization technology. For many years, ceramic metallization has been a hot topic, 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 is increasing, the development trend of light weight and high integration is becoming more and more obvious, and the importance of heat dissipation is becoming more and more prominent, which undoubtedly puts forward more stringent requirements for packaging heat dissipation materials. In the package structure of power electronic components, the package substrate serves as a key link to carry the top and bottom and keep the internal and external circuits conductive, with the functions of heat dissipation and mechanical support. As a new type of electronic heat dissipation packaging material, ceramic has high thermal conductivity, insulation, heat resistance, strength and thermal expansion coefficient matched with the chip, making it an ideal packaging material for power electronic components.
When ceramics are used in circuits, they must first be metallized, i.e., coated with a metal film that is firmly bonded to the ceramic and does not melt easily to make it conductive, and then connected to metal leads or other metal conductive layers as one by soldering technology.
Metallization is the most important step in the ceramic-metal sealing process, which affects the final sealing effect.
I: Difficulties in welding ceramic to metal
1. The coefficient of linear expansion of ceramics is small, while the coefficient of linear expansion of metals is relatively large, leading to cracking of the joint. Generally have to deal with the thermal stress of the metal interlayer.
2, the ceramic thermal conductivity is low, the thermal shock resistance is weak. When welding, minimize the temperature gradient in and around the welding position and control the cooling rate after welding.
3, most ceramics have poor or even no electrical conductivity, so it is difficult to use electric welding. Because ceramic materials have a stable electronic coordination, it is impossible to connect the metal and ceramic. Need ceramic metallization or active solder brazing.
4. Since ceramic materials are mostly covalent crystals, they are not easily deformed and often undergo brittle fracture. At present, the intermediate layer is mostly used to reduce the welding temperature and the welding is done by indirect diffusion method.
5, ceramic-metal welding structure design is different from ordinary welding, usually divided into flat sealing structure, sleeve sealing structure, pin sealing structure and relative sealing structure, of which the sleeve sealing structure is the most effective, these joints structure manufacturing requirements are high.
II: Ceramic metallization treatment
The mechanism of ceramic metallization is complex and involves several chemical and physical reactions, plastic flow of substances, particle rearrangement, etc. Various substances such as oxides and non-metallic oxides in the metallized layer undergo different chemical reactions and diffusion migration at different sintering stages. As the temperature increases, all substances react to generate intermediate compounds and form a liquid phase when the common melting point is reached. The liquid glass has a certain viscosity and at the same time generates plastic flow. Afterwards, the particles are rearranged by capillary action and the atoms or molecules are driven by surface energy to diffuse and migrate. The grains grow and the pores gradually shrink and disappear, thus achieving the densification of the metallized layer.
III: Ceramic metallization process
The process flow of ceramic metallization includes.
Step 1: Substrate pretreatment. Diamond grinding paste is used to polish the pressureless sintered ceramic until it is optically smooth to ensure the surface roughness ≤ 1.6µm. The substrate is put into acetone and alcohol, and ultrasonic cleaning at room temperature for 20min. step 2: Metallization slurry preparation. According to the metallization formula weighing raw materials, ball mill a certain time to make a certain viscosity of the metallization slurry. Step 3: Coating and drying. Use screen printing technology in the ceramic substrate coated with slurry, slurry thickness should be appropriate, too thin solder easy to flow into the metallization layer, too thick is not conducive to component migration, and then the slurry substrate in the oven drying. Step 4: Heat treatment. The dried substrate into the reducing atmosphere sintered to form a metallized layer. 
IV: Specific methods of ceramic metallization
Ceramic metallization commonly used preparation methods are mainly Mo-Mn method, activated Mo-Mn method, active metal brazing method, direct copper cladding method (DBC), magnetron sputtering method.
1、Mo-Mn method
Mo-Mn method is a refractory metal powder Mo-based, and then add a small amount of low melting point Mn metallization formula, add a binder coated to the surface of Al2O3 ceramics, and then sintered to form a metallization layer. The disadvantages of the traditional Mo-Mn method are high sintering temperature, high energy consumption, and no activator participation in the formulation resulting in low sealing strength.
2、Activated Mo-Mn method
Activated Mo-Mn method is an improvement on the basis of the traditional Mo-Mn method, the main directions of improvement are: adding activator and using molybdenum, manganese oxides or salts instead of metal powder. Both types of improvement methods are designed to reduce the metallization temperature. The disadvantage of the activated Mo-Mn method is that the process is complex and costly, but its bonding is firm and can greatly improve the wettability, so it is still the earliest invented, most mature and most widely used process in the ceramic-metal sealing process.
3、Activated metal brazing method
Active metal brazing method is also a more widely used ceramic - metal sealing process, it is 10 years later than the development of Mo - Mn method, characterized by fewer processes, ceramic - metal sealing only need a heating process can be completed. 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 properties at the interface, and this method can be easily adapted to mass production, which is relatively simple and economical compared to the Mo-Mn process.
The disadvantage of the active metal brazing method is that the active brazing material is single, resulting in its application is somewhat limited, and is not suitable for continuous production, only suitable for large, single production or small batch production.
4, direct copper method (Directbondedcopper, DBC)
DBC is in the ceramic surface (mainly Al2O3 and AlN) bonded copper foil a metallization method, it is with the rise of chip-on-board (COB) packaging technology and the development of a new process. The basic principle is to introduce oxygen between Cu and ceramic, and then form Cu/O eutectic liquid phase at 1065~1083℃, and then react with ceramic substrate and copper foil to generate CuAlO2 or Cu(AlO2)2, and realize the bonding of copper foil and substrate under the action of intermediate phase.
5、Magnetron sputtering method
Magnetron sputtering is a kind of physical vapor deposition, which is the deposition of multilayer film on the substrate by magnetron technique, and has advantages over other deposition techniques, such as better adhesion, less contamination and improved crystallinity of the deposited sample to obtain a high quality film.
The metallization layer obtained by this method is very thin and can ensure the accuracy of the part dimensions, but it is not suitable for the implementation of metallization of ceramics that are not resistant to high temperatures (such as piezoelectric ceramics as well as single crystal).
Five: the influence of ceramic metallization factors
1, metallization formula
This is to achieve the premise of ceramic metallization, the need to make a careful, scientific design of its formula.
2, metallization temperature and holding time
Another key factor affecting the metallization of ceramics is the metallization sintering temperature and holding time. Metallization temperature can be divided into the following four processes: more than 1600 ℃ ultra-high temperature, 1450 ~ 1600 ℃ high temperature, 1300 ~ 1450 ℃ medium temperature, 1300 ℃ below the low temperature. A proper 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 is poor and the metallization layer will easily fall off from the ceramic, leading to seal failure.
3、Metallization layer microstructure
The metallization process determines the microstructure of the metallized layer, and the microstructure directly affects the final performance of the weld body. In order to obtain good welding performance, the metallization layer should be a dense film with high bonding strength. If each region in the microstructure of the metallized layer is different and no continuous brittle metal compounds are observed at any interface, the probability of brittleness and crack extension will be reduced and the interface is tight with few cracks, which is conducive to reducing solder penetration, indicating that the metallized layer has good denseness and relatively high bond strength.
4、Other factors
There are also many factors affecting the degree of ceramic metallization, such as the impact of powder particle size and reasonable grading, the powder is too fine, the surface energy is large, easy to form agglomerates, will affect the flatness of the coating; if the powder is too coarse, the surface energy is reduced, resulting in higher sintering temperature, affecting the sintering quality. In addition, the coating method and coating thickness have a great impact on ceramic metallization.
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