Nano-Scale Mechanical Responses of Sn-Ag Based Lead-free Solders

The mechanical properties of the Sn-3.5Ag lead-free solder alloys were characterized in different size scales by nanoindentation technology. Some of the principle mechanical properties, such as Mayer's hardness, reduced Young's modulus and the strain rate sensitivity index were assessed. In particular, the strain rate sensitivity index is extracted from the creep deformation of the solder alloys at the dwell time of target load based on the Miyo-Nix method. It was found that the mechanical properties of the Sn-3.5Ag solder alloy were indeed size dependent. That is, the micro-scale BGA balls showed different mechanical properties from the conventional bulk specimens. According to the strain gradient theory, the non-homogeneous microstructure of the Sn-3.5Ag alloy may be responsible for the creep deformation variation of the micro-scale BGA solder balls. In addition, the influence of aging on the mechanical properties was also discussed.