Microscopic investigation on sintering mechanism of electronic silver paste and its effect on electrical conductivity of sintered electrodes
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In view of extensive application of silver (Ag) paste metallization in the manufacture of electronic devices, it is of great significance to accurately grasp the factors affecting the Ag paste electrode conductivity and the relevant mechanism. Although so far many progresses have been made in the improvement of Ag paste technology, there is still a lack of comprehensive understanding of the microscopic mechanism about Ag paste sintering. Herein, this article tries to relate the conductivity of Ag paste electrodes sintered at different temperatures to those factors, including the morphology, grain size and aggregation state of Ag powder used, as well as the glass frit additive by investigating the microstructure evolution of silver powder (paste) in the sintering process. The results show that these factors determine the grain growth rate and the structure densification during the Ag paste sintering, and thus make an important influence on the electrode conductivity. In general, the larger the grain size and the denser the structure, the greater the electrode conductivity. Therefore, the high electrode conductivity can be achieved by using the flake shaped or spherical Ag powder with a nano-assembly structure. According to the microscopic observations, we suggest that the sintering process should include three stages at temperatures from 300 to 800 °C, i.e. the merging of nanocrystallites in assemblies, interconnection of separated particles, and sintering completion. The influence of glass frit additive on Ag powder sintering is attributed to the dissolution of Ag grain surface by glass melt. This effect may help to increase the grain growth rate and structure densification degree. However, the entry of glass phase matter into Ag grain boundaries would impede the electron transport between grains. Therefore, with increasing the amount of glass phase additive in the Ag paste, the electrode conductivity increased first and then decreased rapidly.
This study was partly supported by the Shanghai Science and Technology Committee Project (No. 17DZ1201102). We also acknowledge the support from the National Natural Science Foundation of China (Grant No. 51202272) and Shanghai Municipal Science and Technology Commission (Grant No. 16DZ2260600).
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