Compression Creep Behavior of the 95.5Sn-(4.3, 3.9, 3.8)Ag-(0.2, 0.6, 0.7)Cu Solders

Abstract

ABSTRACTThe compression creep properties were evaluated for the Pb-free solders 95.5Sn-4.3Ag-0.2Cu (wt.%), 95.5Sn-3.9Ag-0.6Cu, and 95.5Sn-3.8Ag-0.7Cu to determine the effects of small composition differences on time-dependent deformation. The test temperatures were -25°C, 25°C, 75°C, 125°C, and 160°C. The nominal applied stresses were in the range of 2 – 45 MPa. Samples were tested in the as-fabricated condition as well as post-aged at 125°C for 24 hours. Negative creep was recorded for all three alloy compositions. However, the extent of this phenomenon was sensitive to alloy composition and the aging treatment. Creep deformation resulted in the formation of coarsened-particle boundaries within the eutectic regions of the microstructure. The boundaries were comprised of Cu6Sn5and, to a lesser extent, Ag3Sn particles. The minimum creep rate kinetics were evaluated for these solders. The sinh term exponent, n, was 4 – 6 for the Sn-Ag-0.2Cu and Sn-Ag-0.6Cu solders and 1 – 2 for the Sn-Ag-0.7Cu alloy. The apparent activation energy (ΔH) values were in the range of 30 – 70 kJ/mol for all alloys, indicating that a short-circuit or fast-diffusion mechanism controlled creep deformation. The aging treatment did not consistently alter the rate kinetics parameters amongst the alloys. Separating the minimum creep rate data into the low and high temperature regimes, [-25°C, 75°C] and [75°C, 160°C], respectively, showed that bulk diffusion contributed to creep in the higher temperature regime. The ΔH values for the low temperature regime, which indicated that creep was dominated by a fast-diffusion mechanism, were sensitive to solder composition.