Compression Stress–Strain Behavior of Sn-Ag-Cu Solders

Abstract

New Pb-free alloys that are variations of the Sn-Ag-Cu (SAC) ternary system, having reduced Ag content, are being developed to address the poor shock load survivability of current SAC305, SAC396, and SAC405 compositions. However, the thermal mechanical fatigue properties must be determined for the new alloys in order to develop constitutive models for predicting solder joint fatigue. A long-term study was initiated to investigate the time-independent (stress–strain) and time-dependent (creep) deformation properties of the alloy 98.5Sn-1.0Ag-0.5Cu (wt.% SAC105). The compression stress–strain properties, which are reported herein, were obtained for the solder in as-cast and aged conditions. The test temperatures were −25°C, 25°C, 75°C, 125°C, and 160°C and the strain rates were 4.2 × 10−5 s−1 and 8.3 × 10−4 s−1. The SAC105 performance was compared with that of the 95.5Sn-3.9Ag-0.6Cu (SAC396) solder. Like the SAC396 solder, the SAC105 microstructure exhibited only small microstructural changes after deformation. The stress–strain curves showed work-hardening behavior that diminished with increased temperature to a degree that indicated dynamic recrystallization activity. The aging treatment had a small effect on the stress–strain curves, increasing the degree of work hardening. The yield stresses of SAC105 were significantly less than those of SAC396. The aging treatment caused a small drop in yield stress, as is observed with the SAC396 material. The static modulus values of SAC105 were lower than those of SAC396 and exhibited both temperature and aging treatment dependencies that differed from those of the SAC396 material. These trends clearly show that the stress–strain behavior of Sn-Ag-Cu solders is sensitive to the specific, individual composition.