Rapid Low Temperature Sintering Mechanism Of Ag Nanoparticle Paste And Properties Of The Joint

With the increase of power density in electronic packaging systems, heat dissipation becomes an important factor that affects their reliability and service life. Ag nanoparticle pastes have been extensively studied because of the characters of forming interconnects at low sintering temperature, but serving at high temperature and possessing good performance in heat conduction. However, the application of Ag nanoparticle paste is still restricted by the long sintering time (20-30min), the high sintering temperature (>250oC) and the applied sintering pressure. In the aspect of sintering mechanism, the behavior of organic shells which are the very important composition in Ag nanoparticle paste is always discussed and analyzed separately from the sintering process of Ag nanoparticles, which is inconsistent with the actual situation. Besides, up to now, the characteristics of Ag nanoparticle paste forming interconnects at low sintering temperature and serving at high temperature have not been verified. In view of limited studies reported previously, the effect of organic shells on the sintering process of Ag nanoparticles was investigated in this study. The decomposition and evolution behaviors of organics during sintering process were revealed. The effects of organic shells on microstructures and physical performances of sintered samples were analyzed. By optimizing the content of organic shells, thermal conductivity of sintered Ag nanoparticle paste as well as the ability of interconnect process were improved. The stability of sintered Ag nanoparticle paste at high temperatures was also studied.In this dissertation, the water-based Ag nanoparticle paste was prepared with a chemical reduction method. The sintering temperature dropped significantly because there was no additional organic polymer in the Ag nanoparticle paste. The pressureless sintering process was realized at low sintering temperature. The microstructure of sintered Ag nanoparticle paste appeared to be pinecone-like, composing of two-dimensional chains. Based on the discussion above, the content of organic shells was controlled by using different flocculants with certain concentrations. The content of organic shells was reduced under the condition of preventing Ag nanoparticles from aggregation. Ag nanoparticles with less organic shells would grow in three-dimensional directions, since there were more opportunities for Ag nanoparticles to contact with each other after the content of organic shells was reduced. The net-like sintered microstructure was formed. A plenty of twins were observed in sintered samples by using TEM. The sintered Ag nanoparticle paste possessed an extremely low thermal resistivity because the scattering effect of twin boundaries to electrons is very limited, of which value is about one order of magnitude lower than that of conventional high-angle grain boundaries. Moreover, twins which penetrated through grains can introduce low energy segments into the random high-angle grain boundaries, forming low-Σ CSL structures where the atomic arrangement is more ordered than those of high-angle grain boundaries. Therefore, they resulted in a low thermal resistance of sintered materials. Such sintered materials overcame the intrinsic drawback that metals with nanosized grains generally exhibit a signifcantly reduced thermal conductivity because of the grain boundary scattering effect. The thermal conductivity of sintered Ag nanoparticle paste was up to229W/m·K. By optimizing the content of organic shells, the rapid pressureless bonding process was achieved at the temperature range of150-200oC. The sintering time was shortened to30-120s. The shear strength of sintered joints reached16-34MPa.High temperature service reliability of sintered joints using Ag nanoparticle paste was investigated by testing the change in size of sintered Ag nanoparticle paste with the increase of temperature. The results showed that the coefficient of thermal expansion of sintered Ag nanoparticle paste in the temperature range of30-150oC was slightly lower than that of bulk Ag because there were nano-scaled or submicron-scaled voids distributing uniformly in sintered samples. These voids contributed to the stress release as cushion. The sample began to contract when the testing temperature was higher than its sintering temperature, which was resulted from the unstable state of the microstructure sintered at low temperature. With the increase of testing temperature, the sintering driving force increased, and the microstructure evolved to more stable state, which resulted in the decrease of porosity and shrinkage of the sample. This finding makes the reliability assessment of sintered joints using Ag nanoparticle paste to be complicated. It needs further study on the nonlinear variation relationship between sintered microstructures of Ag nanoparticle paste and their physical properties.