Foreword
The metallization paste is usually composed of gold conductive phase, binder phase and organic carrier, which are evenly mixed in different proportions to make metal paste, which is coated to the surface of the ceramic substrate through screen printing, spray gun spraying or other processes, and the metal and ceramics are sealed after high-temperature co-firing.
Conductive phase
Conductive phase refers to the conductive particles in the metal paste, and the metal paste can be divided into *** paste and base metal paste according to the different conductive phases. Slurry refers to a type of conductive paste with Au, Ag, etc. and its alloy as the conductive phase, which is more widely used in thick film elements and their circuits. The physical and chemical properties of the slurry are relatively stable, but its development and application are limited due to its high price. In order to find materials to replace the best slurry, people began to study base metal materials. Base metal slurry mainly includes copper paste, nickel paste, molybdenum-manganese paste and tungsten paste.
CU conductive paste
Metal pastes using Cu as the conductive phase have many advantages, such as good conductivity, can be used for multi-layer structures, are easy to print, and have low production costs. Table 1-3 is the composition of various Cu slurries, the Cu content is between 73 80wt%, and contains trace amounts of Mg, Ag, Mn and other substances.
The Cu slurry is usually printed on an Al2O3 ceramic substrate or a thick film dielectric layer, and is fired in an N2 atmosphere after drying, but the content of O2 in the firing atmosphere will have a great impact on the substances in the slurry.
NI conductive paste
Ni conductive paste is usually printed on the surface of glass substrates, and the sintering atmosphere is generally atmospheric or N2, and sometimes sintered in a reducing atmosphere, and the sintering temperature is generally about 600°C. However, the solderability and conductivity of Ni are not as good as those of Cu, so it is necessary to dip into tin, and the solder zone needs to be ground or covered with conductors such as AG or AG-PB. Table 1-4 lists the characteristics of NI and AL conductive pastes.
Molybdenum-manganese (Mo-Mn) conductive paste
MO-MN slurry is widely used in the sealing process of ceramics and metals, and the method of metallizing ceramics using MO-MN slurry is often called MO-MN method. The MO-MN method is to mix the mixed powder of metal MO or MO and MN with an organic carrier into a metal paste, and then the slurry is coated on the surface of the ceramic green billet, and then sintered at high temperature in wet hydrogen or inert atmosphere, and brazing is carried out after Ni plating on the surface of the sintered metal layer to realize the sealing of metal and ceramic.
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Based on the research of many scholars, the sealing mechanism of ceramics and metals mainly includes the following two aspects:
(1) Migration of glass phase. During high-temperature sintering, the glass phase in the ceramic matrix and the glass phase in the metal paste migrate to each other, wetting the metal particles and ceramics respectively, and sealing the metal and ceramics together during cooling.
2) Chemical reactions. The sintering additives in the ceramic matrix and the inorganic oxides in the metal paste will react chemically to form new compounds, which is conducive to improving the sealing strength of ceramics and metals.
Tungsten (W) conductive paste
Due to the high melting point of the metal W reaching 3140 C, and the high electrothermal conversion efficiency, tungsten is the preferred raw material for many electric heating materials in the preparation of electric heating materials. Compared with the use of Mo-MN paste to metallize Al2O3 ceramics, there are relatively few studies on the use of tungsten paste.
6] Lin B, Zhou Xiaohua, Luo Wenzhong, et al. Study on stability of tungsten slurry[J].Electronic Components & Materials,2012,31(8):62-65
7] Jiang Wenjuan, Meng Fantao, Pan Guangshen, et al. Study on degreasing process of tungsten metallized alumina ceramic substrate[J].Bulletin of the Chinese Ceramic Journal, 2013,32(11):2366-2369
8] Shang Hua, Duan Bing, Mao Jinfeng, et al. Study on the formulation and batch production process of BEO high thermal conductivity ceramic metallization paste[J].Electronic Components and Materials, 2017,36(6):37-42
9] Xia Qingshui, Li Haibo, Cao Kun. Research on metallization process of multi-layer co-fired aluminum nitride ceramics[J].Journal of Electronics and Packaging,2009,9(11):34-36
Through the continuous research of many scholars on W slurry, it is concluded that W slurry has the following characteristics:
(1) The linear expansion coefficient is small, and its value is 46 10-6 C, the coefficient of expansion of alumina ceramics is about
6 10-6 C, which is close to the expansion coefficient of tungsten slurry, so the bonding strength of metal and ceramic is higher during sintering.
(2) The viscosity of tungsten slurry can be adjusted according to the different proportions of powder and organic carrier
Silk screen printing requirements under the same conditions;
(3) The particle size of tungsten particles in tungsten slurry is generally < 2 m, and it has good dispersion;
(4) It can be used for co-firing of metals and ceramics at higher temperatures;
(5) Suitable for 1300 1700 C high temperature
References:
Preparation of cermet heating element and regulation of temperature coefficient of resistance
Master of Engineering, Wuhan University of Technology**.
Author's name: Zhu Feng.