Transformation-Induced Plasticity in Sn-In Solder Joints

The research reported here concerns the contribution of transformation- induced plasticity (TRIP) to the shear deformation of Sn-x wt.%In solders with Cu or Ni metallization, where the In content (x) ranges from 9 wt.% to 15 wt.%. In this concentration range the high-temperature γ-phase (hexagonal structure) transforms to the low-temperature β-phase (β-Sn structure) on cooling, and the transformation can be martensitic. The results show that Sn-9In and Sn-11In solder joints do exhibit TRIP that significantly enhances their ductility when tested at temperatures between the deformation-induced martensite temperature (M d) and the stress-induced martensite temperature (M y). For Sn-9In, M d ≈ 105°C, and the TRIP effect is optimal near M y ≈ 80°C, where the total elongation reaches ~100% when the substrate metallization is Cu. The TRIP elongation is less spectacular with Ni metallization because of weakness at the solder–substrate interface. Sn-11 wt.%In joints also show extensive TRIP effect, with an M d temperature near 60°C, and an M y of 35°C or less. The total elongation of 11 wt.% In joints on Cu reaches 350% at 35°C. Sn-15 wt.%In joints with Cu metallization also have excellent ductility at low temperature, with total elongation of ~50% at 35°C. In this case, however, the excellent ductility is due to the fine-grained, two-phase microstructure of the solder rather than any TRIP effect.