Studies on Conductive Copper Paste with Low Curing Temperature

• Main Authors: Cheng-RuKuo, 郭承儒
• Other Authors: Wen-Hsi Lee
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/65n7y9

碩士 === 國立成功大學 === 電機工程學系 === 103 === In technology of manufacturing the electrically conductive paste, the electrically conductive metallic powder is the key. For the electrically conductive paste, in addition to gold and silver powder that is often applied as the electrically conductive powder, other metallic powder with lower price like copper, nickel, or aluminum powder play a secondary role. Among those metals, silver is applied most extensively. In recent years, since the price of the precious metals soars, the cost of the electrically conductive pastes raises as well. Therefore, it has become a trend to replace the electrical paste made of the precious metals by the low-cost electrically conductive metallic powder in the future. This paper discusses utilization of Gal Tiffany displacement reaction to make and prepare silver-coated copper powder, which is then made as metallic powder with low sintering temperature and low electrically conductivity rate. The nano-silver on the surface is employed as the adhesive to contact the metallic copper powder, so that the contact resistance value can be reduced. Then, the copper powder is sintered under 300℃ in no reducing atmosphere, while the nano-silver on the surface is sintered until it is in the melting status to cover the copper powder before it oxidize. Such methods can prevent oxidation of the copper powder, have the copper powder fill the gaps and holes, extensively raise the electrically conductivity rate, and significantly decrease the sintering temperature. To sum up, by exerting the chemical substitution reaction to separate out the silver for growing on the copper particles, the conductive paste made by covering the copper powder with silver is characterized with the following advantages: 1. the overall conductivity raises; 2. the copper inside will not oxidize; 3. the cost is lower than the silver's; 4. good electromigration resistance; 5. after copper is covered by the nano-silver, it can be sintered under low temperature without getting oxidized. The copper paste's conductivity will increase as the sintering temperature raises. On the other side, the conductivity of the copper paste sintered with low temperature reduces remarkably due to partial 「Non-conductive resin」, as shown in Table 1. This research aims to raise the conductivity of the copper paste sintered with low temperature, as well as overcoming the difficulty of overcoming the restriction of making copper paste merely in nitrogen. It is thus expected to obtain super low temperature, high conductivity, and low stress print-based copper electrode sintered in air at low temperature. In our experiment, we tested the mole ratio of four kinds of silver nitrate and metallic copper powder, respectively: a. 0.0058 : 0.0786, b. 0.0117 : 0.0786, c. 0.0176 : 0.0786, d. 0.0235 : 0.0786. When the mole ratio of silver nitrate and the metallic copper powder is 0.0235 : 0.0786, nano-silver's coverage rate achieves the highest; that is, the nano-silver covering structure is formed with the coverage rate more than 90%. In this experiment, two solution are used, including deionized water and EG. The former reacts too fast upon changing the solution, unable to produce an even nano-silver covered metallic copper powder surface. In contrast, the latter can slow down the displacement reaction rate, and it is characterized with the effect of dispersant without adding any dispersant, so that it is not very influential on the silver covered copper powder in our experiment. The best reaction time of the whole experiment is around 90 minutes, which is sufficient enough for the silver particles to reduction reaction, and form the nano-silver structure on the surface of the metallic copper powder. In addition, from the analysis of the surface content and SEM results, it is learned that following the displacement reaction of silver ion and copper ion, the nano-silver coverage proportion reaches more than 90%. In other words, the copper particles are covered by the complete nano-silver structure. Then, the nano-silver covered copper powder with the optimal coverage rate is mixed as the silver-covered copper paste, printed on the oxidized aluminum for sintering. Under the optimal experimental conditions, the sheet resistance value 5.7x10-3Ω/□ and the resistance rate 5.468x10-5Ω·cm are obtained. Contrary to other silver paste sintered with low temperature in the market, our experiment successfully obtains the silver-covered copper paste with high conductivity through low-temperature, low-cost, and low-stress printing process.