Effects of aging on SAC-Bi solder materials

Lead free solder materials are widely used in the electronic packaging industry due to environmental concerns. However, experimental testing and microstructural characterization have revealed that Sn-Ag-Cu (SAC) lead free solders exhibit evolving properties that change significantly with environmental exposures such as isothermal aging and thermal cycling. In our prior studies on aging effects, we have demonstrated that these aging effects can be partially mitigated by adding dopants to the SAC alloy composition. In this work, we have explored the mechanical behavior and aging effects for a SAC-Bi alloy (SAC_R) that has been proposed as a replacement for low silver content SAC105. Mechanical stress-strain tests have been performed on SAC_R solder samples, and then compared to those measured for SAC105. For each alloy, three different microstructures were explored using different cooling profiles as well as subsequent isothermal aging. Test specimens were initially solidified with both water quenched (WQ) and reflowed (RF) cooling profiles. In addition, some of the reflowed samples were subsequently subjected to 3 months of isothermal aging at T = 100 oC (RF + 3 Months Aging) to further coarsen the reflowed microstructure. Stress-strain tests were performed at three different strain rates and five different temperatures (15 different sets of testing conditions). From this data, the nine Anand constitutive model parameters were determined for each unique solder alloy and microstructure combination. The mechanical behaviors of SAC_R and SAC105 have been compared, and SAC_R has been shown to have improved mechanical properties as well as being highly resistant to aging induced degradations. Microstructural analysis has shown that the improved aging resistance is due to solid solution strengthening where the Bi-phases go into the solution in the β-Sn matrix during aging and enhance strength.