Research On The Indentation Creep Behavior And Size Effect Of One-grain Sn Microbumps

The volume of microbump for high-density interconnects continues to shrink,and Sn microbumps may contain only one grain,which is referred to herein as One-grain-Sn microbumps.Due to the mechanical properties of anisotropy and the lack of grain boundary that acts as the coordination role in the deformation,the deformation mechanism,constitutive relationship,fracture patterns and other microbehavior of the One-grain-Sn microbumps will be different from that of the limited grain or multigrain solder joints.In addition,the working environment of Sn microbumps for interconnection is higher and the creep in high temperature will change the organization and composition of interconnected structural materials.Therefore,the creep performance of One-grain-Sn microbumps has great significance for the high-density packaging reliability assessment.The main conclusions of this paper are as follows:(1)Compared to the grain boundaries in the microtopography of the bulk Sn,no grain boundary is observed in the microtopography of Sn microbumps.The surface orientation and grain boundary analysis of Sn microbump by electron backscattering technique(EBSD)show that there is no grain boundary in the scanning region and only one grain is present.Also,the surface element analysis of Sn microbump is scanned by EDS.It is proved that Sn microbump is One-grain-Sn microbump.(2)The finite element analysis is taken to design the nanoindentation experiment.The effect of loading rate on the indentation creep curve is studied.The creep rate sensitivity index m is 0.0848 by the experimental data.The creep resistance of One-grain-Sn microbump is better than Sn alloy solder,but is not as good as SAC BGA solder joints(3)Different maximum load indentations are taken to perform hardness test and indentation creep test.The result of indentation hardness test meets Meyer empirical equation,energy correction model and strain gradient theory well,which implies the size effect,but does not fit for the sample proportional resistance model.Two micro-deformation mechanisms dominate the deformation of the indentation creep size effect: the dislocation under high load and the diffusion creep under small load.