Research On Process And Mechanism Of Bonding Using Small Nanosilver Paste At Low Temperature And Pressure

Power devices based on third-generation semiconductor have been widely applied in production during the past few years and high power devices usually work at high temperature for a relatively long time, thus, traditional tin-lead solder and lead free solders are no longer appropriate for packaging of power devices. Small nanosilver paste have been prepared and strong joint has been achieved by using it between Cu and Cu. The process of sintering and the mechanism of joining was investigated as well.In this study, nanosilver with good stability has been prepared and its average dimension is about 4.76 nm. The nanosilver paste was produced by mixing ethylene glycol, deionized water and nanosilver. The study of the thermal properties of the paste demonstrated that the thermal decomposition temperature of citric acid when acting as coating layer of nanosilver was 160 ℃ which was 146 ℃ lower than its normal decomposition temperature. The effect of temperature on density, porosity,hardness and conductivity has been studied and the results showed that with the increasing of temperature, porosity decreased while the other properties studied also showed an increasing tendency.The research on effect of temperature, washing times, holding time and pressure on microstructure of the interface and mechanical property were conducted and the result showed that the density of joint went up first and dropped after with the increasing times of washing of the nanosilver, whereas it showed a rising trend with the increasing of connecting temperature, holding time and pressure. Mechanical property went up first and dropped after with the increasing of connecting temperature and washing times of nanosilver, whereas it showed an rising trend with the increasing of pressure and holding time. In addition, the mechanical property after different aging times under 150℃ and 250℃ was studied and the results showed that the lower temperature(150℃) had little influence on the shear strength of the joint, whereas the higher temperature(250℃)had a strong impact on the shear strength after aging for144 h after which the shear strength decreased rapidly to 4.9MPa.Considering three mechanisms of action among nanosilver( sintering, absorbing and vanishing), the sintering behavior of nanosilver was studied. During the heating process, organic coating layer decomposes and after that, some of the nanosilver grows together by sintering. Smaller nanosilver particle is absorbed by another particle forming new nanosilver particle which grows together with other particles by sintering. Others vanish during the formation of reticulate structure. The connectionism of the interface was studied and the results showed that there are two types of connectionism, one is based on diffusion between Ag and Cu and the other one is the stable connection formed by the crossing of the Ag and Cu lattice. These two connectionisms ensure the strength of bonding.