Influence of Fe and Co Nanoparticle Flux Doping on Stress Relaxation Behaviour of Sn-3.5Ag Solder Joints

The mechanical integrity of lead-free solder joints under thermal and mechanical loading remains a critical challenge in electronic packaging. This study examines the influence of iron (Fe) and cobalt (Co) nanoparticle (NP) doping via flux on the stress relaxation behavior of eutectic Sn-3.5Ag solder joints. Lap joint specimens were fabricated using NPdoped fluxes with concentrations in the range of 0.1 to $1.0 \mathrm{wt}. \%$ and tested at room temperature, $100^{\circ} \mathrm{C}$ and $150^{\circ} \mathrm{C}$ in both asreflowed and thermally aged states. Stress relaxation tests (SRTs) were conducted under constant displacement to evaluate time and temperature dependent deformation behavior of solder joints. Results show that Fe-NP doping led to increased shear strength while simultaneously a significantly higher relaxation rates and reduced stress exponent (n) values were obtained. In contrast, Co-NP doping exhibited more complex behavior: while a doping level of $0.1 \mathrm{wt}. \%$ enhanced shear strength and reduced stress relaxation rate, higher concentrations mostly showed negligible effects. Furthermore, the obtained data revealed a considerable effect of experimental parameters on the stress relaxation response of the samples. Accordingly, the relationship between the initial load, strain range and the effective solder joint area on the degree of stress relaxation behavior of solder joints are discussed.