A Study Of Mechanical Property For Nano-Silver Sintered Lap Shear Joint

As a novel lead-free interconnection material, nano-silver paste has potential to be applied in high power electronic packaging, which will replace traditional solder and silver epoxy, because of its high melting point, low sintering temperature, high electrical/thermal conductivity and excellent mechanical reliability. However, research on mechanical property and reliability of sintered nano-silver bonding joints is insufficient. This paper mainly discussed the mechanical properties and reliability of the sintered joints under different loading conditions through designed sintered nano-silver lap shear structure.Through analyzing the impact of ambient temperature and loading rate on the shear behavior of nano-silver joint, the fundamental mechanical performances of nano-silver joint were studied. The tests showed that the sintered nano-silver joints were highly dependent on shear strain rate and temperature. The shear strength of the lap shear joint decreased with the decreasing shear strain rate and increasing ambient temperature, and the lower stress rate and higher temperature significantly contributed to larger elongation of the silver joints. From the observation of the fracture section by SEM, it showed that plastic strain occurred among the silver particles, and plastic flow of paste layer was improved by higher temperature or lower loading rate. The stress distribution of paste layer was discussed by interface stress model under loaded force, which matched the result of finite element analysis (FEA).A number of creep tests were carried out to study the effect of stress level and testing temperature on the creep behavior of sintered nano-silver joints in a systematic manner. It was found that the creep shear strain rate increased as applied stress level and ambient temperature increasing. With the modified Arrhenius power-law model, the tested results fitted well with the predicted results. The theory of damage localization was applied in creep damage description and makes a consistent result.The effect of mean stress, stress amplitude and environment temperature on the lap reliability were observed by stress controlled and strain controlled cyclic shear experiments, respectively. To study the interaction of creep and ratcheting, the stress was held at the peak value. The results showed that the peak stress dwelling accelerated accumulation of ratcheting strain, but unloading of the cyclic stress decreased the creep shear strain rate. Based on the isothermal mechanical fatigue test, a temperature-related fatigue life prediction model was proposed, which predicted the fatigue life of sintered nano-silver joints precisely.