Suliman Dregia (Committee Member), Sharmila Mukhopadhyay (Committee Member), Thomas Reitz (Committee Member), Raghavan Srinivasan (Committee Member), Daniel Young (Advisor)
Doctor of Philosophy (PhD)
The principal objective in this research was to develop a die attach method to facilitate the use of wide band gap semiconductors in high-temperature environments, and study fundamental mass transport in nano-initiated die attach microstructure at high temperature. Low temperature sintering silver-nano-pastes have been demonstrated as an effective method for die attach by other groups. Fabrication and optimization of the die attach method in this work was accomplished via tape casting. A novel binder system and a bimodal particle distribution were utilized. The tape casting method allowed die attach materials to be pre-stressed. Fundamental understanding of creep deformation process and creep analysis of the resulting silver microstructure was investigated for isothermal stability over extended time.
The binder system played a significant role on the level of residual char left within the sintered microstructure. Of the binders investigated, polypropylene carbonate (PPC) resulted in the smallest amount of char residue at 0.6%. Decreasing the organic concentration of PPC resulted in a tradeoff of lower green strength with increased shear strength. Tapes formulated with 10wt.% of PPC binder resulted in viable tapes having approximately 54MPa's of shear strength. Bimodal distribution of nanopowder with micron powder reduced the need for nanopowder by 70%. Micron powder (1-3um) resulted in comparable shear strengths with the nanotapes. Shear strength decreased as particle size was increased above 5um. Pre-stressing allowed for the decoupling of die-material fabrication pressure from the assembly pressure. Tapes pre-stressed to 212MPa showed a significant improvement in green microstructure and shear strength over un-stressed tapes. Relaxation during isothermal annealing of curved bimorph strips monitored by laser beam deflection was used to measure changes in stress and showed a logarithmic decay over time. Thermal cycling of the bimorph strips indicated the material exhibited the Bauschinger effect. In this work, low temperature sintering with silver nanotape has been demonstrated as an effective method to attach a bare die. Optimization of the tape by means of binder, organic concentration, bimodal dispersion, and pre-stressing resulted in a state of the art method capable of shear strengths greater than 60MPa's. Post sintering, the microstructure exhibited recrystallization and grain growth due to surface diffusion over prolonged thermal exposure. According to this work, die attach of purely nano-silver forms an effective bond at low temperatures but prolonged thermal exposure at high temperatures results in creep and abnormal grain growth.
Department or Program
Ph.D. in Engineering
Year Degree Awarded
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