Mechanical Characterization and Modeling of iSAC Lead-Free Solder

The Anand viscoplastic constitutive model is commonly used to represent the rate-dependent deformation behavior of lead-free solder materials. This model is characterized by nine parameters and is a well-established component within various commercial finite element analysis software packages such as ANSYS and ABAQUS. Reliability predictions for electronic assemblies are known to be highly sensitive to the input Anand parameters, so that their accurate determination by experiments is critical.In this study, a new lead-free doped solder material referred to as iSAC has been mechanically characterized. The iSAC material includes the traditional Sn-Ag-Cu (SAC) elements found in most lead-free solders, along with tailored amounts of Bi, Ni, Zn, and Ge to enhance interface characteristics, manufacturability, and assembly reliability. Cross-sectional samples of iSAC joints were first analyzed to determine their microstructure, and chemical composition analysis was performed using Energy Dispersive X-ray Spectroscopy (EDS). Miniature tensile samples of the iSAC material with controlled microstructures were then fabricated by a reflow process in glass tubes. Uniaxial tensile tests were then performed on reflowed rectangular cross-section solder specimens over a range of temperatures and strain rates. Using the stress-strain data, the mechanical properties of the iSAC material were determined and compared to other lead-free alloys. In addition, the nine Anand parameters for the iSAC solder were characterized using the measured uniaxial response at 15 different pairs of temperature and strain rate. A good correlation was obtained between the Anand model predictions and the measured experimental data. Finally, a finite element analysis (FEA) was also performed to compare the thermomechanical performances of PBGA packages with iSAC solder joints with conventional SAC305 joints. The iSAC joints were predicted to have lower strain and energy dissipation levels.