Organization And Performance Characterization Of Low Temperature Sintered NaNo Silver Paste

Electronic interconnecting joint served as signal transduction, cooling channel,mechanical support and environment protection in electronic device, play a vital role inthe performance of the entire electronic circuit and components. Besides solder andpolymer materials such as thermal adhesives and conductive adhesives, the lowtemperature joining materials are the most promising joining materials in recentdevelopment. And nano silver paste has been widely studied for its better properties aslow temperature joining materials. However, how to lower the sintering temperature,reduce the cracking possibilities and increase density and thermal conductivity of thesintered nano silver paste are important research contents currently.In this paper, the concept of mixed-size nano silver paste was proposed. Awater-based chemical reduction method using sodium citrate dihydrate and silver nitratewas proposed to prepare large size silver nanoparticles of a diameter about80nm. Themixed solution of sodium citrate dihydrate and ferrous sulfate heptahydrate was used toreduce the silver nitrate, and small size silver nanoparticles of a diameter around20nmcan be obtained. The mixed-size silver paste was prepared from mixing the large andsmall size silver nanoparticles at a ratio of1:9. And this paste can be sintered under airatmosphere at150℃without extra pressure.The experiment and analysis of the composition of nano silver paste found that thedispersants were mainly from the small size silver nanoparticles’ formulation. Thesedispersants started to decompose at150℃, but the decomposition was not completelyeven they were heated to400℃. By observing the organization and morphology of thesilver nanoparticles before and after mixing and sintering, it was noted that the largesize silver nanoparticles were surrounded by the small ones in the distribution ofmixed-size nano silver paste. With the heating temperature rising, the sintering neckbegan to form and the grain coarsed and grew up. When it was heated above230℃,the sintering tissue changed from pine-like structure to3D network and the organizationdensified significantly due to the recrystallization. After studied the performance ofsintered nano silver paste, it found that as the sintering temperature went up, the bodydensity and hardness increased. Especially when it was heated at the decompositiontemperature of the dispersants and the recrystallization temperature, the value of density and hardness demonstrated a significantly increasing trend. Meanwhile, the higher thesintering temperature was, the larger the data of thermal conductivity was. The value ofthe thermal conductivity of the nano silver paste sintered at280℃could be as high as144W/（m·K）. The analysis of the thermal expansion behavior of the silver paste thatsintered at150℃,200℃,250℃respectively indicated that the coefficients of thethermal expansion of all these sintered nano silver paste were similar and quite close tothe value of silver block when they were heated above100℃. Additionally, thethermal expansion behavior of the nano silver paste sintered above230℃presented arelatively stable state as the shrinkage induced by sintering process had less impact onthe thermal expansion behavior.