A Study of Deformation Mechanism During Nanoindentation Creep in Tin-Based Solder Balls

As the shrinkage and integration of devices, the creep behavior of tin-based alloys becomes important with microscales. In this paper, the behavior of creep deformation in solder alloys during a nanoindentation test was examined. Nanoindentation creep test was carried out for tin-based solder balls. Obtained results summarized as follows: (i) The stress exponent for power-law creep estimated can be evaluated from the evolution of hardness. These values obtained in the early stage corresponds with that of bulk within the range of high strain rate. (ii) The stress sensitivity decreases after stress relaxation in nanoindentation creep tests. The saturated value is 1 in three solder balls. (iii) The morphology of indented surface consists of three parts: initial indentation, power-law creep, and granular surface. It suggests that the transition from power-law creep to diffusion creep takes place. (iv) Finite element method analysis reveals stress and strain concentration appears in the vicinity of the tip. Strain field remains self-similar as the indentation proceeds. (v) The gradient of triaxial stresses below the tip in a nanoindentation test accelerates the creep strain rate due to the diffusive flow, relatively.