Shear deformation in Sn-3.5Ag and Sn-3.6Ag-1.0Cu solder joints subjected to asymmetric four-point bend tests

The shear deformation behavior of two lead-free solder compositions, Sn-3.5 Ag (wt.%) and Sn-3.6Ag-1.0Cu (wt.%), both on copper substrates, was studied using an asymmetric four point bend technique. Four test joints were obtai ned from one master specimen of each composition, and each joint was subjec t to progressive loading, up to the maximum shear strength of the joint. On e unstressed bar from each composition was retained as a reference. Each sa mple was metallographically polished and lightly etched, and examined in a field emission scanning electron microscope (SEM) before shearing. Sheared joints were then re-examined in the SEM with no additional surface treatmen t. Compared with the traditional ring and plug method, the asymmetric four- point bend (AFPB) technique subjects the joints to a condition of pure shea r, while providing an opportunity for unambiguous observation of microstruc tural features before and after shearing, without an intervening mechanical sectioning step. Shear banding in the Sn-rich matrix and crack nucleation in the vicinity of the intermetallic interface were observed at low displac ements in the binary alloy. Evidence of non-homogeneous plastic flow in the matrix was seen at higher shear loadings. No evidence of brittle fracture was observed in the Sn-3.6Ag-1.0Cu alloy, with elastic deformation at low s tress levels giving rise to plastic deformation at higher loading values. R esults show that the AFPB technique is a viable approach to the study of sh ear loading on solder joints.