Silver powder and silver conductive pastes

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As silver has optimal electrical conductivity at normal temperature, the best thermal conductivity, the strongest reflection characteristics, photographic imaging properties and anti-inflammatory properties as well as good chemical stability (the cheapest metal which has no oxidation at high temperature), it is widely used in modern industry. With the development of electronic industry, silver will become a kind of indispensable material in the electronic industry due to its good electrical conductivity and thermal conductivity. Currently the application of silver on electronic industry has become its major usage. However, silver also has some shortage in the electronic industry, which mainly reflects in three aspects: it has poor corrosion resistance to soldering tin as well as its silver ion migration and vulcanization. Therefore, platinum and palladium should be added in some cases to reduce its shortage. The application of silver on electronic industry can be divided into two aspects which are micro-electronics (which has small power and low voltage) and electrical power (which has high power and high voltage). With the light, small and thin trend of civil electrical development, the application of silver on micro-electronics will become its main aspect. The application form of silver on the micro-electronics industry is spreading. According to the light, small and thin condition of the electron machines and the cost requirements, currently the main techniques including thick film coating technology, electroplating technology and other physical techniques (such as chemical vapor deposition and sputtering) are used to realize the spreading, among which the thick film coating technology has become the main way to realize the conductive film layer as it is economic, easy for quantitative production, suitable for all kinds of base materials and has simple film forming conditions.

In the electronic industry, the distinction between thick film and thin film is its different film forming way rather than the film thickness. Thick-film technology includes printing and sintering film formation process, in which the silver conductive paste is an important material. Thick film pastes started in the United States in 1930’s when the way of electrode formation on BaTiO3 single plate capacitor substrate is associated with the ceramic glaze process in history, in which using the “paste” or printing ink that has leading change features formed by processing the mixture of glass powder used as bonding phase as well as silver powder and carrier (the organic polymer and solvent), thus the thick film pastes will be generated by forming guiding electrolemma in the ceramics through printing and sintering methods.

Thick film pastes can be divided into three categories which are conductor, resistance and medium. Among them, the conductor paste is most important and has the biggest usage. However, the main body of conductor paste is silver conductor paste which is composed by three parts of silver powder, bonding phase and organic carrier. With the development of microelectronic industry, the thick film paste is also continuously developed and breaks through its original basic concept. Currently there are three kinds of “printing ink” materials that use silver powder as main functional material, besides, the “printing ink state” silver conductor materials are generally called silver conductor pastes. Among the three kinds of silver conductor pastes, the conductive silver   paste used for printing occupies the main position, which can be divided into two categories: a. polymer conductive silver paste (it can be dried or solidified into film as well as using its organic polymer as bonding phase); b. sintered conductive silver paste (it can be sintered into film, besides, when the temperature is more than 500 degrees Celsius, the glass powder or oxide will be used as the bonding phase).

Silver powder is classified by its grain size, in which the average grain size less than 0.1μm(100nm) is called nanometer silver powder; Dav (the average grain size) more than 0.1μm and less than 10.0μm is called silver micro powder; Dav (the average grain size) more than 10.0μm is called thick silver powder. There are many preparation methods of powder, for example, the preparation method of silver powder includes physical method (plasma and atomization method) and chemical method (silver nitrate thermal decomposition method and liquid phase reduction). As silver is precious metal and easy to reduce into elemental state, the liquid phase reduction is the leading method in producing silver powder, in which the silver reduced powder that has the average grain size in the range of 0.1-10.0μm is generated by washing and drying the silver powder formed with the deposition effect before dissolving the silver salt (including silver nitrate and so on) and adding chemical reductant (such as hydrazine hydrate and so on) into the water. The preparation of different physical and chemical properties silver micro powders (including the particle morphology, dispersion degree, the average grain size and size distribution, specific area, apparent density, tap density, grain size, and crystallinity, etc) depends on the choice of reducing agent, the control of reaction condition and the use of interfacial agent. Polished silver powder and silver flake can be formed by conducting machining process (ball mill and so on) on the reduced silver powder.

The three kinds of silver conductor pastes (silver pastes for short) need different categories of silver powder or the silver combination as the conductive filter, even the different formula in every kind needs different silver powder as the conductive functional materials. Its aim is to make the fullest use of silver electrical conductivity and thermal conductivity by using the least silver powder under the confirmed formula or the film forming technology conditions, which is related to the film performance optimizing and cost.