High Temperature Aging Effects in SAC and SAC+X Lead Free Solders

Abstract

Lead free solders are common as interconnects in electronic packaging due to their relatively high melting point, attractive mechanical properties, good thermal cycling reliability, and environmentally friendly chemical properties. The mechanical behavior and reliability of a lead free solder is highly dependent on the operating temperature. Previous investigations on mechanical characterization of lead free solders have mainly emphasized stress-strain and creep testing at temperatures up to T = 125 °C. However, electronic devices sometimes experience harsh environment applications including well drilling, geothermal energy, automotive power electronics, and aerospace engines, where solders are exposed to very high temperatures from T = 125-200 °C. Knowledge on the mechanical properties of lead free solders at elevated temperatures is limited. In our prior work presented at ECTC 2018, we investigated the mechanical behavior of several SAC and SAC+X lead free solder alloys including SAC305 (96.5Sn-3.0Ag-0.5Cu), SAC_Q (SAC+Bi), and Innolot (SAC+Bi+Ni+Sb) at extreme high temperatures up to 200 °C. In the current study, we have extended our prior work to consider extreme high temperature aging effects in lead free solders (SAC305 and SAC_Q). Before testing, the solder uniaxial specimens were aged (preconditioned) at the extreme high temperature of either T = 125 °C or T = 200 °C. At each of these aging temperatures, several durations of aging were considered including 0, 1, 5, and 20 days. Stress-strain and creep tests were then performed on the aged specimens. Using the measured data, the evolutions of the stress-strain and creep behaviors were determined as a function of aging temperature and aging time, and models describing the evolution of the mechanical properties with extreme aging were established. Microstructural evolution of the solder alloys during extreme high temperature aging has also been explored. In particular, aging induced coarsening of the IMCs has been studied using Scanning Electron Microscopy (SEM), and correlated to our material property evolution findings.