What Technology Enables Ceramics and Metals to Achieve A “Strong Combination”?

Ceramics are often referred to as inorganic non-metallic materials. It can be seen that people directly position ceramics on the opposite side of metal. After all, the performance of the two is worlds apart. But the advantages of the two are too prominent, so in many cases, it is necessary to combine ceramics and metals, each showing their strengths, so the technology-ceramic metalization technology was born.

Especially with the advent of the 5G era, the power of semiconductor chips continues to increase. The development trend of lightness and high integration is becoming more and more obvious, and the importance of heat dissipation is also becoming more and more prominent. This undoubtedly puts more stringent requirements on package heat dissipation materials. In the packaging structure of power electronic components, the packaging substrate serves as a key link for connecting the upper and lower parts and keeping the internal and external circuits connected and has functions such as heat dissipation and mechanical support. As an emerging electronic heat dissipation packaging material, ceramic has many advantages high thermal conductivity, insulation, heat resistance, strength, and thermal expansion coefficient matching the chip, so it is an ideal packaging heat dissipation material for power electronic components.

Ceramics used in circuits must first be metalized. A layer of a metal film should be applied to the surface of the ceramic. And to make it conductive, then welded with metal leads or other metal conductive layers. To be connected and become one.

The most important step in the ceramic-metal sealing process is metalization, and 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 the joints to crack. Generally, the thermal stress of the metal intermediate layer should be handled well.
2. The ceramic itself has low thermal conductivity and weak thermal shock resistance. When welding, it is important to minimize the temperature and control the cooling rate after welding.
3. Most ceramics have poor conductivity or even non-conductivity. It is difficult to use electric welding.
4. Due to the stable electronic coordination of ceramic materials, the connection between metal and ceramic is unlikely. Need to metalize ceramics or braze with active solder.
5. Since ceramic materials are mostly covalent crystals, they are not easily deformed, and brittle fractures often occur. At present, the intermediate layer is mostly used to reduce the welding temperature, and the indirect diffusion method is used for welding.
6. The structure design of ceramic and metal welding is different from ordinary welding. It is usually divided into flat seal structure, sleeve structure, pin seal structure, and double seal structure. The sleeve structure has the best effect, and the production requirements of these joint structures are very high.

The Mechanism of the Ceramic metalization
The mechanism of ceramic metalization is relatively complex, involving several chemical and physical reactions, the plastic flow of substances, and particle rearrangement. Various substances such as oxides and non-metal oxides in the metalized layer undergo different chemical reactions and substance diffusion and migration in different sintering stages. As the temperature rises, each substance reacts to form an intermediate compound, and when it reaches a common melting point, it forms a liquid phase. The liquid glass phase has a certain viscosity and at the same time produces plastic flow. After that, the particles are rearranged under the action of the capillary. Driven by energy, atoms or molecules undergo diffusion and migration, crystal grains grow, pores gradually shrink and disappear, and the metalization layer is densified.

The Process of Ceramic Metalization
1. Substrate pretreatment. 
2. Preparation of metalization slurry. 
3. Coating and drying.
4. Heat treatment. 

The specific method of ceramic metalization
1. Mo-Mn method
2. Activation Mo-Mn method
3. Active metal brazing
4. Direct Bonded Copper (DBC)
5. Magnetron sputtering

Influencing factors of ceramic metalization
1. Metalization formula
2. Metalization temperature and holding time
3. Microstructure of metalization layer
4. Other factors

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Do you know the five major applications of silicon nitride ceramics?

Silicon Nitride Ceramic Components

Silicon nitride ceramics have high temperature resistance, corrosion resistance, wear resistance, and unique electrical properties and are considered one of the most promising high-temperature structural ceramic materials.

Aircraft engine
The mechanical structure of the future aero engine will be simpler than the existing type, with fewer components, and operating at a higher turbine inlet temperature and component load. Its reliability and component life will also be improved. Turbine materials must meet the requirements in terms of tensile strength, creep resistance, high temperature corrosion resistance, and impact damage resistance. The use of ceramic materials with better thermal performance can reduce the amount of cooling air required and significantly increase the gas temperature.

In the past, large aircraft engines mainly used nickel-based high-temperature materials, and silicon nitride materials have higher strength, better creep strength, and oxidation resistance than nickel-based heat-resistant alloys at temperatures above 1000°C and have a small specific gravity. It is only 40% of the heat-resistant alloy, which can meet future aviation engines’ requirements to reduce weight and fuel consumption.

Mechanical industry
Silicon nitride ceramics can be used as turbine blades, mechanical seal rings, high-temperature bearings, high-speed cutting tools, permanent molds, etc. in the machinery industry. Many devices in the traditional machinery industry use metal materials. Since the metal will be corroded, the reliability and service life of these devices is greatly affected. Silicon nitride ceramic materials have excellent wear resistance, corrosion resistance, and high temperature thermal shock resistance. Can replace metal materials in the field of machinery industry.

For example, silicon nitride can be used to manufacture ball bearings with lightweight and high rigidity. It has higher precision than metal bearings, generates less heat, and can operate in higher temperatures and corrosive media. The steam nozzle made of silicon nitride ceramics has the characteristics of wear resistance and heat resistance, and no obvious damage after being used in a 650°C boiler for a few months, while other heat resistance and corrosion resistant alloy steel nozzles can only be used for 1-2 months under the same conditions.

Superfine grinding
Silicon nitride ceramics are covalent compounds whose bonding mainly relies on covalent bonds between atoms, and the prepared material itself has high hardness and wear resistance. Silicon nitride has high hardness, second only to a few super hard materials such as diamond and cubic boron nitride, and has low friction coefficient and self-lubricating properties. In the ultrafine powder and food processing industries, silicon nitride ceramic grinding balls’ performance is higher than traditional grinding balls, with higher hardness and superior wear resistance.

Ceramic substrate
With the rapid development of information technology, integrating integrated circuits is getting higher and higher, and the wiring density is getting higher and higher. If the electronic package substrate cannot dissipate heat in time, a large amount of heat will accumulate on the integrated circuit, which will eventually lead to its failure and damage. Therefore, the thermal conductivity of the substrate is extremely important. Silicon nitride ceramics are structural ceramic materials with the best comprehensive performance. The theoretical thermal conductivity of single crystal Si3N4 can reach more than 400W· M-1, which has the potential to become a high thermal conductivity substrate. Due to its excellent mechanical properties and high thermal conductivity potential, Si3N4 ceramics are expected to make up for the shortage of existing aluminum oxide, aluminum nitride, and other substrate materials have a great market prospect in the application of electronic packaging substrate.

Metallurgical field
Silicon nitride ceramic materials have excellent chemical stability and excellent mechanical properties. They can be used as components on thermal equipment such as crucibles, burners, aluminum electrolytic cell linings, etc. in the metallurgical industry. Silicon nitride ceramics have good oxidation resistance. The oxidation resistance temperature can be as high as 1400°C. It is stable in a dry oxidizing atmosphere below 1400°C, and the use temperature can reach 1300°C. And silicon nitride material can be used in the environment of rapid cold and rapid heat, so it also has a very wide range of applications in the metallurgical industry.