Research and development of copper paste for nitrogen-protected medium-low temperature varistor

     Varistor is a class of resistance value with the voltage changes with the nonlinear characteristics of electronic components, above the critical voltage, such components can instantaneously through the large current to avoid the rapid rise of voltage, its main use power supply, communication equipment, power line intelligent communication equipment, lightning protection equipment. Zinc oxide varistors are the most widely used varistors. It is based on zinc oxide material, adding a variety of metal oxides in the raw material in the air atmosphere through high temperature sintering of ceramic semiconductor components. The reliability factors that determine the varistor: there are mainly varistor matrix, electrode material and outer envelope resin, when the varistor matrix and outer envelope resin are determined, the most critical factor affecting the reliability of the varistor is the electrode of the varistor.

     At present, the electrode material of the commonly used zinc oxide varistor is the precious metal silver electrode [2], and the sintering temperature of the silver electrode slurry is between 600 and 650℃, at this temperature, the performance of the varistor is easy to deteriorate, and the process conditions are harsh, and the cost of the entire silver electrode is about 30% of the total cost of the varistor. Therefore, the development of low-cost electrode materials is a very important direction in the research and development of varistors.

     As a conductive material second only to silver, copper has been used in many fields to prepare electrodes for electronic components, such as MLCC multi-layer ceramic capacitor end electrode and chip ceramic capacitor surface electrode, alumina ceramic conductor wiring, ring strontium titanate varistor electrode. In the field of zinc oxide varistor, many companies have begun to prepare copper electrodes, but because copper has a higher melting point than silver, the sintering temperature is generally above 600 ° C, and a nitrogen atmosphere is required. Under such conditions, the DC three parameters of most zinc oxide varistors are bad: leakage current increases, pressure sensitive voltage decreases, and the coefficient decreases. In serious cases, the pressure sensitive characteristics are lost, which seriously hinders the application of copper electrodes in this field.

     By analyzing the influence of different sintering temperature on DC three parameters of zinc oxide varistor under nitrogen atmosphere, the optimum copper burning temperature range is determined. The effect of particle size of copper powder and formula of glass powder on sintering activity of copper paste was studied by many experiments, and the process of copper paste under 99.999% nitrogen protection was studied. The stable low temperature copper paste under nitrogen protection was obtained, which can be used in zinc oxide varistor chips with excellent copper electrode properties.