Uniaxial and cyclic stress-strain behavior of lead-free solders at nanoscale

Abstract

Solder joints, an integral part in electrical appliances, undergo steady or fluctuating strain throughout their lifetime for which the components develop cracks and the components become susceptible to failure. Uniaxial and cyclic loading have different outcomes of damage accumulation, eventually making the electrical components susceptible to failure. In this study, the effects of two parameters (temperature and solder-alloy composition) on the uniaxial and also, for the first time, on the cyclic stress-strain behavior of lead-free solders at nanoscale were observed. A rectangular-box model of SAC (alloy of Sn, Ag and Cu), a lead-free solder, was subjected to uniaxial and cyclic (tension and compression) loading at nanoscale. The effects of uniaxial loading on SAC alloys were also observed through the simulations and the failure criterion was also observed as to get a better view on the cracks. A negative correlation between temperature and UTS (Ultimate Tensile Strength) was observed. Also, the nanoscale model was cyclically loaded under strain-controlled conditions (constant positive and negative strain limits). The study aims at determining the hysteresis loop size (area), for a stable cycle, that was calculated for a given solder alloy and varying temperature. This area represents the strain energy density dissipated per cycle, which can be correlated to the damage accumulation in the joint. In this study, most simulations were performed with SAC305. However, simulations were also performed for four SAC alloys in total (105,205,305,405) with varying silver content (1-4%) under strain-controlled cyclic loading and uniaxial loading. In addition, the effect of the temperature has also been studied by performing simulations of cyclic loading of SACN05 (N=1,2,3,4) models at six different temperatures (300, 323, 348, 373, 398 and 423K). An increase in the plastic strain range and a drop of the peak stress and loop area were found at higher temperatures.