Creep and Microstructure Evolutions in SAC305 Lead Free Solder Subjected to Different Thermal Exposure Profiles

Abstract

Solder joints in electronic assemblies are frequently exposed to thermal cycling environments in their service life which causes microstructural evolution and material property degradation. In our prior study, a comparison of material property degradations of SAC305 lead-free solder material in terms of effective elastic modulus (E) and ultimate tensile strength (UTS) was made for isothermal aging and three different thermal cycling exposures. In our present work, our previous investigations have been extended to examine the evolution of creep behaviors of SAC305 solder subjected to isothermal aging, slow thermal ramping, and slow thermal cycling exposures. For creep testing, rectangular cross-section uniaxial test specimens were prepared using reflow solidification under a controlled temperature profile. After formation, the samples were subjected to either thermal aging or thermal cycling pre-conditioning exposures for various durations under stress-free conditions (no load). After the preconditioning was completed, creep testing at room temperature was performed on the thermally exposed samples.The three thermal profiles considered were (1) isothermal aging at 125 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C, (2) slow thermal cycling from -40 to +125 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C with 45 minutes ramps and 30 minutes dwells, and (3) thermal ramping from -40 to +125 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C with 45 minutes ramps and no dwells. The thermal exposures times considered with the three profiles were 0, 1, 2, 5, and 20 days, and creep testing was performed on the exposed samples at three different stress levels (10, 12, 15 MPa). For each type of thermal exposure, the evolution (increase) of the secondary creep strain rate with exposure time was characterized. The evolutions were then compared. Furthermore, a microstructure study was performed using Scanning Electron Microscopy (SEM) to understand the physics behind the thermal cycling induced degradations in the creep strain rate. For long exposures, it was found that the creep rate degradation and microstructural evolution were the highest for thermal cycling with a slow ramp rate. This effect was exacerbated for higher creep stress levels. We are currently exploring the use of SAC+Bi alloys to reduce the creep rate degradations occurring in lead free solders during thermal exposures.