Size effect on the fracture of sintered porous nano-silver joints: Experiments and Weibull analysis

Nano-silver paste is a promising alternative of high temperature solder material for the next generation electronic devices, which is composed of organic solvent and silver nanoparticles/sheets. The mechanical properties of nano-silver interconnections are essential for the bonding between chip and substrate by sintering. During sintering process, volatilization and combustion of organic solvent in nano-silver paste lead to development of abundant voids in the joint, forming a three-dimensional network structure, which has strong influence on the mechanical properties. Sintering temperature and cross section area can affect the distribution of voids in solder joint. In the current study, effects of sintering temperature (200 °C, 250 °C, 300 °C) on the tensile and shear failure strength of sintered nano-silver joints were investigated, the size effect caused by different cross section area (i.e. (0.5, 1, 2, 3) × 5 mm²) was studied. The volatilization of organic solvent in nano-silver paste is the main reason for the porosity diversity, and the difference of porosity is one of the key factors causing size effect. It also leads to dispersion of experimental data of failure strength, thus, statistical analysis was performed to the experimental data. Four universal estimators and Weibull distributions were adopted to analyze the tensile and shear failure strength of sintered nano-silver joints. Based on the Gurson-Tvergaard-Needleman model, the critical porosity equation of fracture was developed. Furthermore, the relationship between critical porosity and size effect was established considering porosity increases with the increasing of cross section area, and a modified Weibull model containing size effect was proposed, which showed good agreement compared with the experimental data.