Creep deformation of 96.5Sn-3.5Ag solder joints in a flip chip package

As part of an investigation of using eutectic 96.5Sn-3.5Ag solder for flip chip interconnection, this paper presents the results on mechanical characterization of 96.5Sn-3.5Ag eutectic solder and solder joints. Constant-load creep tests of 96.5Sn-3.5Ag solder alloy were performed at high homologous temperatures from 25°C to 180°C. Single lap shear tests were performed on joined flip chip packages with an area array of 96.5Sn-3.5Ag solder bumps-33×33 bumps per chip. Tensile creep tests were performed on bulk 96.5Sn-3.5Ag solder specimens. The steady-state strain-rates span 7 orders of magnitude ranging from 10^-9 to 10^-2 (1/s). The apparent activation energy for creep was found to be 0.57 ev. The stress exponent in the power-law creep equation is ≈10 which is unusually high compared to that of many other metals. The underlying controlling mechanisms is identified as low-temperature dislocation climb