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 gamma-phase (hexagonal structure) transforms to the low-temperature beta-phase (beta-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) a parts per thousand 105A degrees C, and the TRIP effect is optimal near M (y) a parts per thousand 80A degrees 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 60A degrees C, and an M (y) of 35A degrees C or less. The total elongation of 11 wt.% In joints on Cu reaches 350% at 35A degrees C. Sn-15 wt.%In joints with Cu metallization also have excellent ductility at low temperature, with total elongation of 50% at 35A degrees 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.