Mechanical Property Evolution in SAC+Bi Lead Free Solders Subjected to Various Thermal Exposures

Abstract

Lead free solder materials are widely used as interconnects in electronic assemblies. They are often subjected to different thermal exposures such as isothermal aging and/or thermal cycling during their service life in various applications such as well drilling, automotive power electronics, aerospace engines, and geothermal energy. Therefore, the evolutions of mechanical properties of lead-free solder materials subjected to different thermal exposure profiles are of great importance and need to be investigated.In our previous work, mechanical behavior evolutions of SAC305 and SAC+3%Bi lead free solder materials were reported under different thermal exposure profiles for up to 5 days of exposure. In the present work, we have extended this work to investigate the evolution of additional SAC+Bi lead free solder alloys for up to 20 days of thermal exposure. In particular, SAC+Bi alloys with 1%, and 2% of Bi have been studied. Several thermal exposure profiles between T = -40°C and T = 125°C were investigated: (1) 150 minutes cycles with 45 minute ramps and 30 minute dwells (slow thermal cycling), (2) air-to-air thermal shock exposures with 30 minute dwells and near instantaneous ramps (thermal shock), (3) 90 minute cycles with 45 minute ramps and 0 minute dwells (thermal ramping), and (4) isothermal aging at the high temperature extreme (aging).Stress-strain tests were performed on the uniaxial test specimens to investigate the change in mechanical properties which includes effective elastic modulus (E) and ultimate tensile strength (UTS). The test results were compared with each other in order to understand the effect of different thermal exposure profiles on the mechanical properties of SAC+Bi solder materials. It was observed that the more the exposure time per cycle, the degradations in mechanical properties are higher. The degradation was greatest for the thermal cycling with the longest ramp period (slow thermal cycling). After 20 days of isothermal aging, the UTS and elastic modulus were reduced by 10%, and 30%, respectively for SAC+2%Bi; whereas in case of slow thermal cycling, these properties were reduced by 12%, and 35%, respectively. Also, the degradation of mechanical properties during thermal shock, and slow thermal ramping was higher than pure aging after 20 days.