Alloying design of Sn-Ag-Cu solders for the improvement in drop test performance

In addition to a reduced Ag content, it has been demonstrated that significant improvement of drop test performance of Sn-Ag-Cu solder joints can be achieved by alloying with Mn and Ti. This study aims to investigate the effects of Mn and Ti additives on the microstructure and solidification behavior of Sn-1.0Ag-0.5Cu alloys, as well as mechanical properties and thus to explain how the alloying elements affect drop test results. Results show that alloying of Mn and Ti results in coarse eutectic structure and greater amount of pro eutectic Sn of which the size was refined. The microstructural changes leads to a reduction in elastic modulus, which plays an important role for the enhancement of drop impact reliability. However, there exists an optimal value for the alloying content, since excess addition of Mn or Ti gives rise to the formation of massive intermetallic compounds (IMCs), MnSn₂ and Ti₂Sn₃. Those heterogeneous IMCs are harder than the inherent IMCs, Ag₃Sn and Cu₆Sn₅, according to the nanoindentation results and may cause the degradation in drop performance.