Jinghui Industry Ltd.
This article includes the definition of Metallized Ceramics, manufacturing process of Metallized ceramics, the types of Ceramic metallization methods, the factors affecting metallized ceramics,quality assurance and its applications, you will discover insight from following:
Metallized ceramics refers to a layer of metal film is deposited onto the specific surface of engineered ceramics, and then curing in high-temperature reduction atmosphere (hydrogen or nitrogen) furnace, so that the metal film will tightly attach to the surface of the ceramic components, refer to figure 1 .
Figure 1: Metallized ceramics
After metallizing process, the ceramic surface offers the characteristics of metal, can be achieved effective connection between ceramic and metal by means of brazing.
As a typical inorganic non-metallic material, Advanced ceramics have been widely used in various high voltage, high current and high-pressure electrical & electronic vacuum devices, new energy vehicles, semiconductor packages and IGBT modules because of their excellent electrical, physical & chemical properties, mechanical properties, thermal properties and optical properties. In these practical applications, it often involves the joint of ceramics and metal parts in different materials, such as stainless steel, oxygen-free copper, Kovar and so on. Since the thermal expansion coefficient of ceramic and metal material has huge difference;In the meantime, the two materials naturally have poor wetting effect; And in these fields, the sealing surface of ceramic and metal parts has strict sealing strength (tensile strength) and air tightness requirements after brazing, thus they can not be directly and simply connected. So ceramic metallization technology was born.
1. high thermal conductivity: The heat generated by the chip can directly transfer to the Ceramic parts without an insulating layer, bringing out more ideal heat dissipation.
2. Ideal thermal expansion coefficient:
The thermal expansion coefficient of both advanced ceramics and chips is similar, and it will not cause too much deformation when the temperature difference changes, resulting in problems such as circuit desoldering and internal stress at connection section.
3. Low dielectric constant : The dielectric constant of the Ceramic material itself makes the signal loss smaller, so the Technical ceramic materials are widely used in communication equipment and signal transmission.
4. High bonding force : High bonding strength of metal layer and Ceramic Substrate of Ceramic circuit board products, up to 45MPa (greater than the strength of 1mm thick ceramic parts themselves)
5. High operating temperature: Ceramics can withstand high and low temperature cycles with large fluctuations, and can even operate at a high operating temperature of 800 degrees for a long time.
6. High electrical insulation: Industrial ceramics themselves are insulating materials that can withstand high breakdown voltages, especially Ceramic Insulators after glazing, and can even be applied in fields with voltages above 100KV.
7. Chemical stability: The ceramic body has better chemical stability, and will not react with most of the strong acids and bases, and will not be oxidized in the high temperature environment.
What is the mechanism of ceramic metallization? The mechanism of ceramic metallization takes advantage of the different chemical reactions and diffusion migration of various substances in advanced ceramics and metallized layers at different sintering stages, such as oxides and nonmetallic oxides. As the temperature rises, the liquid phase is formed when all substances react to form intermediate compounds and reach the common melting point. The liquid glass phase has a certain viscosity and produces a plastic flow at the same time. Afterwards, the glass particles are rearranged under the action of capillaries, and the atoms or molecules are diffused and migrated under the drive of surface energy. The pores gradually shrink and disappear with the increase of grain size, thus realizing the densification of the metallized layer. Refer to figure 2 below:
Figure 2: Schematic of Ceramic Metallization Structure
1. Mo-Mn method:
The MO-Mn method is based on refractory metal powder Mo, and then dope a small amount of low-melting point Mn metallization formula, add a binder coating to the Al2O3 ceramic surface, and then sintering to form a metallized layer. The disadvantages of the traditional Mo-Mn method are high firing temperature, high energy consumption, and low sealing strength due to the absence of activator in the formula.
2. Activation Mo-Mn method:
The activated Mo-Mn method is an improvement based on the traditional one. The main directions of improvement are: adding activators and replacing metal powder with molybdenum and manganese oxides or salts. Both of these improvements are designed to reduce the metallizing temperature. The disadvantage of activated Mo-Mn method is that the process is complex and the cost is high, but it can greatly improve the wettability, and significantly improve the bond strength between ceramics and metals, so the activated Mo-Mn method is still the most widely used process.
3. Silver paste sintering method:
The silver method is to apply a layer of Ag paste on the ceramic surface, composed of Ag salt flux and adhesive, and then sintering at high temperature to reduce Ag ions to elemental Ag, Ag layer can be reduced by triethanolamine silver carbonate or by adding silver nitrate to ammonia and then reduced by formaldehyde or formic acid.
6Ag2CO3+N(CH2CH2OH)3=N(CH2CHOOH)3+12Ag↓+6CO2+3H2O
AgNO3+N H4OH=AgOH+NH4NO3 , 2AgOH=Ag2O+H2O
Ag2O+H2C=O=2Ag+HCOOH Or Ag2O+HCOOH=2Ag+CO2↑+H2O
Due to the strong diffusion of silver ions, it is not appropriate to use the silver paste sintering method for electrical appliances used in strong electric fields, because the electrical properties will deteriorate rapidly under high temperature, high humidity and direct current electric fields. At present, the main problems of Ag layer sintered by sintering method are thin, discontinuous, non-uniform and poor corrosion resistance.
4. Active Metal Brazing (AMB method):
Active metal brazing is also a more widely used ceramic-to- metal sealing process, it is 10 years later than the development of Mo-Mn method, characterized by fewer processes, shorter cycle, good welding reliability, and suitable for a variety of different ceramic materials. The ceramic-metal sealing can be completed with only one heating process. Brazing alloys contain added Ti, Zr, Hf and Ta active elements, the added active elements react with Al2O3 to form a reaction layer with metal characteristics at the interface, this method can be easily adapted to large-scale production, compared with molybdenum - manganese process, this method is relatively simple and economical.
The disadvantage of the active metal brazing method is that the active filler metal is single, which causes its application to be limited to a certain extent, and it is not suitable for continuous production, but only suitable for large, single piece production or small batch production.
5.Direct Bond Cooper (DBC method):
DBC is a metallization method of bonding copper foil on a ceramic surface (mainly Al2O3 and AlN), which is a new process developed with the rise of chip on board (COB) packaging technology. 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 base and copper foil to form CuAlO2 or Cu(AlO2)2, and realize the bonding between copper foil and ceramic matrix under the action of intermediate phase.
6. Magnetron sputtering:
It is a kind of physical vapor deposition, which is to deposit multilayer film on the substrate by magnetic control technology, refer to the figure 3
Figure 3: The diagram of Magnetron vacuum sputtering
which has the advantages that other deposition technologies do not have, with better adhesion, less pollution and improve the crystallinity of the deposited sample, to obtain high-quality film. The metallization layer obtained by this method is very thin, which can ensure the accuracy of the dimension of the part. The DPC process supports PTH (electroplated through hole) /Vias (through hole). High-density assembly is possible - line/pitch (L/S) resolution can reach 20μm, thus achieving lightweight, miniaturization, and integration of devices.
Chapter 6: Metallization process
1. Pretreatment of ceramic base:
The sintered ceramics were ground to optical smoothness with diamond abrasive paste to ensure that the surface roughness was ≤1.6µm. The ceramic bases were placed in acetone and alcohol, and ultrasonically cleaned at room temperature for 20 minutes.
2. Metallization paste preparation:
The raw materials are weighed according to the metallization formula, and the metallization slurry with a certain viscosity is made after ball milling for a certain period of time.
3. Coating, drying:
Apply the paste on the Ceramic Insulator or ceramic substrate by screen printing, pad printing or brush coating technology. The paste thickness should be appropriate. If the paste is too thin, the solder will easily flow into the metallization layer. If it is too thick, it is not conducive to the migration of components.
4. Heat treatment: The dried substrate is sintered in a reducing atmosphere to form a metallized layer.
5. Enamelling: The glaze spray process is used to evenly spray a layer of glaze on the non-metallized outer surface, and then glaze through medium temperature.
6. Secondary metallization:
The most typical secondary metallization is nickel plating on the underlying Mo/Mn or W metallization, the purpose of following plating is to improve the fluidity and wettability of the solder in the subsequent brazing process, and to prevent the underlying metallization from being oxidized, so that the welding strength can`t be decreased.
1. The Mo-Mn method mainly includes molybdenum, manganese, tungsten, nickel, silver and gold.
2. DBC method mainly includes oxygen-free copper.
3. Materials of other metallization methods include Palladium (Pa), Platinum (Pt), Titanium (Ti), Aluminum (AL) Selected metal alloys may also be used.
1. Ceramic itself reliability:
The influence of ceramic on the quality of metallization is mainly its internal quality and surface characteristics.
First, from the microscopic aspect of the analysis, in other word, the size of the ceramic grain, if the ceramic grain is getting properly large, the brazing force will increase with the size of the grain; However, if the grain is too large, the bonding strength will be reduced accordingly, and the optimal grain size of high purity alumina is about 30um.
Second, from the macro analysis, and the finish and appearance quality of the ceramic surface, in order to ensure the adhesion and sealing strength of the metallized layer, the metallized surface of the ceramic needs to be precision grinding, and the ceramic components` roughness needs to be ensured at Ra0.8.
2. Formulation of metallization paste:
Three activators, MnO, Al2O3 and SiO2, must be added to the metallization paste. MnO mainly reduces the viscosity of the glass phase, Al2O3 can improve the brazing strength of metallization, and SiO2 can improve the wettability of brazing. After the addition of these activators, the paste will obviously exhibits the following three characteristics:
a. The thermal expansion coefficient of the glass phase is reduced;
b. The sintering temperature of metallization will be reduced; c. The infiltrating characteristics of Mo will be improved.
If nano powders are introduced into the preparation of the metallized paste, the density of the metallized layer, the metallizing temperature, and the brazing strength will be further optimized.
3. Metallizing temperature and holding time:
Another factor affecting the quality of metallization is the metallizing temperature with the holding time. The metallizing temperature is divided into four grades: ultra-high temperature, high temperature, medium and low temperature. The temperature of ultra-high temperature is above 1600 ° C, the high temperature is 1450~1600 ° C, the medium temperature is 1300~1450 ° C, and the low temperature is below 1300 ° C.Different products in different materials need to use the appropriate metallization temperature. Too high temperature will weaken the adhesion of the metallization layer, and even the metallization layer will fall off from the ceramic surface, resulting in weakened brazing strength and even sealing failure; On the contrary, the temperature is too low, the diffusion of the glass phase will weaken the migration, and the adhesion of the metallized layer will also be weakened.
4. Microstructure of the metallized layer:
The metallizing process determines the microstructure of the metallization, and its microstructure has a direct influence on the reliability of its brazed parts. The key is to ensure that the metallized layer can be continuously and densely covered on the ceramic body with ideal adhesion. If the microstructure of each level of the metallized layer is distinct, and no uniform brittle metal compounds are observed at any interface, such a metallized layer will reduce the record of brittleness and crack growth; With fewer tight cracks at each level, solder penetration can be reduced.
5. Influence of metallized powder particle size and reasonable grading:
The particle size of metallization metal powder is closely related to the coating quality and sintering effect. The metal powder is too fine, the surface energy is large, the activity is stronger, and the agglomeration is easy to form, which will affect the flatness of the coating; If the powder is too coarse, the surface energy will be reduced, resulting in the need to increase the sintering temperature, which will affect the sintering quality. In addition, if the particle size distribution of various powders is relatively concentrated, there will be an "arch bridge effect" between the powders during the metallization process, resulting in an increase in the porosity of the metal layer, which will adversely affect the air tightness of the vacuum arc extinguishing chamber.
6. The effect of coating mode:
The uniform distribution of each component of the metallized paste and the good coating performance directly affect the quality of the metallized layer. Coating can be manual pen coating, mechanical coating, spray gun spraying and screen printing and other methods, for a small number of products with inconsistent sizes should be used pen coating, for the large-scale production of the same product, it is appropriate to use screen printing. Screen printing allows precise and uniform thickness control. The thickness of the paste is usually 50~80 micros.
7. The effect of coating thickness: Different metallization formulations and preparation methods, the thickness of the metallization layer is different.
The types of metallized ceramics mainly include metallized ceramic structural parts and metallized Ceramic Substrates:
1. Metallized ceramic structural parts
They mainly play a role of protection, hermetic, support, insulation, connection, heat dissipation and other functions. The main materials used include aluminum oxide (Al2O3), zirconia toughened aluminum oxide (ZTA), zirconia (ZrO2), aluminum nitride (AlN), beryllium oxide(BeO), Boron nitride (BN) and silicon carbide (SiC).
2. Metallized Ceramic Substrate
In the application, it is mainly used as a circuit carrier to assist chip heat dissipation and insulation. The main materials include alumina, aluminum nitride, silicon nitride and beryllium oxide.
The typical quality assessment and corresponding measurement methods for ceramic metallized products are described as follows:
1. Visual Inspection:
It mainly includes crack, chips, metallization appearance, impurities inspection on surface of ceramic base. Ensure that the ceramic surface is smooth and free from abnormalities. The XRF, microscope and naked eyes are often used as inspection tools.
2. Metallization thickness measurement: Ensure that the metallization thickness within specification with non-destructive coating thickness gauge as inspection tool.
3. Resistance measurement: Using Ohmmeter to check the electrical resistance to ensure conducting electricity within specification
4. Metallization layer quality inspection:
Using scanning electron microscopy (SEM) to observe the surface morphology and cross-sectional microstructure of metallization layer, and whether the interface is permeated to ensure the metallization layer.
5. Bond strength test: Using electronic universal testing machine to check the adhesion strength of brazed sub-assembly.
6. Hermeticity test: Using Helium mass spectrometer to check air tightness of sub-assembly to ensure the gas tightness /leak rate of metallized ceramics.
7. Reliability and lifetime test:
It mainly includes thermal cycling test, Temperature coefficient of resistance, High-temperature aging test, Humidity and moisture resistance test, Thermal shock test, Mechanical stress test, Accelerated aging test, Electrical performance test and Failure analysis
1. Thermal expansion mismatch: It`s critical challenge for metallized ceramics. 2.Metallization adhesion issues 3. Cost considerations 4. Metallization failure mechanisms
Metallized ceramics are widely used in many modern industries:
High power and high frequency applications
1. Power electronics 2. Microwave devices 3. RF amplifers Electronic components and devices 1. Intergrated circuits 2. Resistors and capacitors 3. Sensors and transducers Hemetical packaging and sealing 1.Vacuum tubes and electron tubes 2. Optoelectronic devices 3. Medical implants and devices
Chapter 13: Future trends and Advancements
At present, the research of metallized ceramic technology includes the quality control of ceramic matrix, physical properties, microstructure, metallization mechanism, the development and popularization of new technology. Among them, there are more researches on the quality control, physical properties, structure and new process of ceramic matrix, but less on the metallization mechanism and applications. Considering that there is still no consensus in many aspects of the mechanism of metallization in the academic community, but the mechanism of metallization is the basis for improving the level of metallization and improving the sealing of ceramics and metals, the future will inevitably focus on the research of metallization mechanism.
With the dynamically improvement of the power and integration of electronic components, more stringent requirements are also put forward for packaging and cooling materials. Therefore, the traditional metal cooling materials and hermetical components gradually cannot meet the requirements of electronic devices. As an emerging thermal conductivity and sealing material, advanced ceramics not only have a thermal expansion coefficient close to the chip, but also have excellent mechanical strength, insulation strength, high temperature resistance, corrosion resistance and other characteristics, which is the most ideal material choice for power electronic device packaging and heat dissipation. The deeper research on the metallization mechanism of ceramics, as well as the exploration and development of new processes, is the basis for improving the metal and ceramic sealing surface, which will further expand the application field,
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