Compression stress–strain and creep properties of the 52In–48Sn and 97In–3Ag low-temperature Pb-free solders

Lead (Pb)-free, low melting temperature solders are required for step-soldering processes used to assemble micro-electrical mechanical system (MEMS) and optoelectronic (OE) devices. Stress–strain and creep studies, which provide solder mechanical properties for unified creep-plasticity (UCP) predictive models, were performed on the Pb-free 97In–3Ag (wt.%) and 58In–42Sn solders and counterpart Pb-bearing 80In–15Pb–5Ag and 70In–15Sn–9.6Pb–5.4Cd alloys. Stress–strain tests were performed at 4.4 × 10⁻⁵ s⁻¹ and 8.8 × 10⁻⁴ s⁻¹. Stress–strain and creep tests were performed at −25, 25, 75, and 100°C or 125°C. The samples were evaluated in the as-fabricated and post-annealed conditions. The In–Ag solder had yield stress values of 0.5–8.5 MPa. The values of ΔH for steady-state creep were 99 ± 14 kJ/mol and 46 ± 11 kJ/mol, indicating that bulk diffusion controlled creep in the as-fabricated samples (former) and fast-diffusion controlled creep in the annealed samples (latter). The In–Sn yield stresses were 1.0–22 MPa and were not dependent on an annealed condition. The steady-state creep ΔH values were 55 ± 11 kJ/mol and 48 ± 13 kJ/mol for the as-fabricated and annealed samples, respectively, indicating the fast-diffusion controlled creep for the two conditions. The UCP constitutive models were derived for the In–Ag solder in the as-fabricated and annealed conditions.