| To manufacture electronic paste, manufacturing of conductive metal powder is the key technology. For conductive paste, the conductive phase is mainly made up of powder from precious metals such as platinum, palladium, gold and silver and silver-based conductive paste is most widely used among these. In recent years, due to the sharp rise of the prices of precious metals, the cost for electronic paste increases. In addition, silver migration is the inherent drawback of silver-based paste and hence cannot meet the requirements of high performance electronic components. Therefore, reducing the cost, searching for new conductive powder with excellent properties and using less expensive metals in place of precious ones to manufacture electronic paste have become the major development trends for electronic paste. In this context, the research topic for this thesis has been proposed.In this study an invented and patented replacement-reduction method is used to prepare silver-coated copper powder and the thermodynamics of the silver-coating process is analysed. The silver-coating process and silver-coating layer structure are optimised. Using silver-coated copper powder as the conductive phase, polymer silver-coated copper powder conductive paste is prepared. Properties of silver-coated copper powder paste are analysed and for the first time a study is made with respect to the properties of silver-coated copper powder paste against silver migration. With the work presented in this thesis the technology is commercialised.The major findings of the research are as follows:1. Through analysis of the E-pH charts for Cu-Ag-NH3-H2O series and calculations and thermodynamics analysis of the silver-coating process, a theoretical base for manufacturing silver-coated copper powder is provided.(1) In a solution containing NH3, the stability regions for Cu and Ag oxidants are reduced. The stability regions for the water dissolvables of Cu and Ag in the solution are greatly broadened and the stability of the solution is enhanced. Therefore it is advantageous to prepare silver-coated copper powder in water solutions containing NH3.(2) In ammoniac solutions the potential differences of copper and silver increase; the thermodynamics trend of the reaction increases and the alkaline condition is more favourable for the oxidation-reduction reaction to proceed.(3) For replacement and reduction reactions, reductants must be added so that silver-coated copper powder with a silver layer of a certain thickness can be obtained.2. Silver-coated copper powder is prepared using the invented and patented replacement-reduction method. The dispersion of flake copper powder in water solution, the effects of composition of the coating fluid and of processing conditions on the coverage of silver-coated copper powder and the properties are studied. Characterisation is made of the colour, apparent density, compact resistance and surface morphology for silver-coated copper powder. Optimal coating liquid composition and process conditions when manufacturing silver coated copper powder are obtained, i.e., AgNO3:2.4-14.2g/L, ammonia solution:0.8g/L, formaldehyde:1-3g/L, hydrazine hydrate:1-4g/L, composite dispersing agent:1.0g/L, PH value:11, coating liquid temperature:60℃, solid to liquid ratio:1:15-1:20, stirring speed:1000 rpm, drying of silver coated copper powder:50℃for a time period of 30 minutes.3. Using the optimal silver-coated copper powder composition and processing conditions, silver-coated powders of different morphology and contents are prepared. By characterising the colour, compact resistance, surface morphology, ratio surface and anti-oxidation, the changes in properties of silver-coated copper powder before and after silver-coating are studied and the following results are obtained.(1) The shape of silver coated copper powder, apparent density and tap density are mainly dependent on the properties of the copper powder as raw material while the silver coating layer does not have a significant impact on these properties.(2) The silver coating layer has some impact on the distribution of the size for the silver coated copper powder. Size D50 of flake shaped silver coated copper powder decreases with the increase of the silver content. When the silver content exceeds 20%, its D50 starts to increase.(3) Silver coat layer has large impact on the compact resistance of silver-coated copper powder and compact resistance of silver-coated copper powder will decrease with the increase of silver contents. When the silver content exceeds 20%, the compact resistance decreases slowly and tends to be stable.(4) Silver coat layer has the following impacts on the ratio surface of silver-coated copper powder:when the silver content is low in silver-coated copper powder, the ratio surface for flake silver-coated copper powder increases with increasing silver content. When Ag content exceeds 5%, the ratio surface of silver-coated copper powder decreases with the increase of the coating silver content.(5) Being left in the air, compact resistance of silver coated copper powder will increase. Regarding the silver content, with low silver content, the changes are greater than with high silver content. Considering the exposure time, the change is larger during the first 6 days and the rate of change in compact resistance for silver-coated copper powder is all greater than 10%. When silver content exceeds 20%, the rate of change in compact resistance for silver-coated copper powder is less than 10%. After 27 days, the rate of change in compact resistance for silver-coated copper powder tends to be stable.(6) Oxidation starts at different temperatures for silver-coated copper powder with different silver contents. The one with low silver content starts the oxidation at a high temperature.(7) The coated layer on silver-coated copper powder is found by XRD analysis to be metal silver and copper and there exist no other oxidants or elements.(8) The ideal sedimentation structure of silver coating layer for oxidation resistance is characterised by:flake-shaped silver is sedimented on the copper surface such that the flakes are joined and rest on each other at the ends. This way, a seamless and tight layer will cover completely the copper powder and isolate it from the surrounding.(9) A ball mill with wet process is used to treat the silver coated copper powder. With the same silver content, for silver coated copper powder treated by a ball mill, the area of the base copper powder covered by silver increases. At the same time, the bounding between the silver coating layer and the copper powder will get stronger. The silver coated copper powder will also become more antioxidant with the use of a ball mill.4. Using silver-coated copper powder as the conductive phase, comparative studies are carried out, with respect to viscosity, thixotropy, adhesion, square resistance, rupture strength and oxidation resistance, on the paste with different organic carriers and formulas.The following results are obtained:(1) The optimal series of organic carrier is the one with polymer being made up of polyester and polyacrylate following a weight ratio 1:1; with glycol ether ester as the solvent and with polymer content of 1 Owt%;(2) The optimal formula for the polymer silver-coated copper powder paste is:a conductive phase content of 55% for silver coated copper powder (silver content 30%) and a content of 45% for the organic carrier.(3) The printing properties are better for a silver paste with conductive phase content of 50-55% while for silver-coated copper powder paste, the better printing properties are obtained with a silver content of 55-60%.(4) Preparing the pastes using respective optimal formulas, the results are respectively: for silver paste, the square resistance is 12.8mΩ/□, rate of crease resistance 4%, adhesion requirement met, and good stability for silver paste; for silver-coated copper powder paste, 14.6mΩ/□, rate of crease resistance 5.5%, adhesion requirement met, and excellent stability.5. Using silver paste and silver-coated copper powder paste to prepare experiment electrodes, water drop method is used to observe and compare silver migration for different conductive phases, contents, geometric shapes and distances. Measurements are made on the experiment electrodes with respect to Tafel curves and DC impedance curves. Electrode silver migration with different silver contents in silver-coated copper powder paste is studied. All these lead to the conclusions:(1) The main characteristic of the migration phenomenon in water drop experiment is when external voltage is applied, ion migration will occur on electrode in the electrolyte solution. Tree-shaped sediment always emerges on the cathode and will grow towards anode. Such sediment will not start to appear once the voltage is applied as different conductive phases, different shapes of the electrodes, different voltages applied will all affect the migration speed. During the migration process flocks of sediment will generally be generated.(2) Through measurements and drawing Tafel curves and DC impedance curves for electrodes using silver paste and silver-coated copper powder paste, it is shown that the self-corrosion potential for silver-coated copper powder paste moves in a more positive direction than that of silver paste. The former has lower self-corrosion current and good corrosion resistance and slow dissolution of electrodes. Electrodes of silver-coated copper powder paste have more favourable electrochemical stability in the electrolyte solution than electrodes of silver paste. Electrodes of silver paste are more prone to corrosion and the anti-corrosiveness of electrodes of silver-coated copper powder paste is superior to that of silver paste as well. In addition, the dissolution of electrode of silver-coated copper powder paste is slower than that of silver paste.(3) The mechanism of anti-migration for silver coated copper powder is studied. The results show that the existence of copper suppresses the dissolution of the silver on the anode in silver coated copper powder and the concentration of silver ion in the electrolyte decreases. This causes the sedimentation speed on cathode to be lower and the dendrite growth to get slower and thus enhances the ability against silver migration.(4) Paste made by silver-coated copper powder is much stronger for resistance against silver migration than silver paste. The study shows such a resistance with silver-coated copper powder paste does not increase with the increase of silver content; rather, there exists an appropriate range of silver content. When the silver content is around 25% in silver-coated copper powder, the paste has the maximal resistance against silver migration.(5) For the first time, the relationship between the silver migration resistance and the coating layer structure for silver-coated copper powder is revealed. It is concluded that the most ideal coating layer structure for silver-coated copper powder is net-shaped, which is better than compact structure.6. With the achievements made through this work for silver-coated copper powder and paste, commercialization is carried out through Kunming Hendera Science and Technology Ltd., Co. A commercialisation demonstration base is established in Kunming High Tech Development Park. A production line with capability of annually producing 400 tonnes of silver-coated copper powder,2 tonnes of silver-coated copper powder paste and 100 tonnes of electromagnetic insulation paint of silver-coated copper powder is built. During 2006-2008, an accumulated production of RMB 63 million has been made.4 patents are applied and approved and 2 corporate standards are issued.6 papers are published. The achievement won first prize in China Non-ferrous Metal Industry Science and Technology award in 2009 and second prize in Yunnan Province’s Science and Technology Innovation award. A great benefit has been gained for the economy and social life. |
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