The Investigation Of Low Temperature Sintered Nanosilver Paste On Migration And Thermal Bending In Die-attachment

As an alternative lead-free die-attachment material in chip level interconnection for high temperature and high power application, nano-silver paste has grabbed many scientists' and engineers' attentions because of its good mechanical performance, high thermal conductivity and high melting point etc. Since most investigations of nano-silver paste were focused on sintering technology and its adhesion performance, there is a lack of the fundamental research on the reliability of nano-silver paste in high temperature or high power applications, which could provide a guideline for the application of this novel material. Therefore, the phenomenon of silver migration at high temperature in dry air and the impact of temperature cycling on its adhesion reliability had been studied in this work.The migrating rate of silver at high temperature in dry air was investigated with the variation of temperature and electric field. It is concluded that both increasing the temperature or the electric field can short the "lifetime" of silver migration. Based on Arrhenius equation, the prediction model of the "lifetime" of silver migration was proposed. And the silver was migrated from cathode to anode in the form of dendrite. It is worth noting that oxygen plays an important role in silver migration at high temperature in dry air. According to the findings, the mechanism of silver migration was proposed in the thesis.The verification of the mechanism of silver migration was carried out by taking advantage of self-designed sealed chamber, which can provide different partial pressure of oxygen. It is shown that decreasing the partial pressure of oxygen can slow down the migrating rate of silver to a great extent. Some possible ways to alleviate silver migration could be generated based on the mechanism.On the basis of curvature method for measuring residual stress, the characteristic of residual bending in the assembly, which was die-attached by nano-silver paste, was studied. The residual bending was caused by the mismatch of CTE(Coefficient of Thermal Expansion) among chip, adhesive and substrate.FEM(Finite Element Method) was also introduced to simulate the residual bending in the assembly using the software called ANSYS.The effect of cyclic temperature ranged from -40oC to 125oC on the relaxation of residual bending in the mentioned assembly was discussed in the thesis. And the results agree well with the simulation of temperature cycling. In addition, the variation of the micro-structure in the assembly after temperature cycling was also analyzed in the mean time. It is indicated that assembly with thicker bondline should be susceptible to the impact of cyclic temperature. As a result, cracks were apt to be initiated in nano-silver layer, which will impact the application of nano-silver paste.