Cu6Sn5 Morphology Transition and Its Effect on Mechanical Properties of Eutectic Sn-Ag Solder Joints

The morphologies of Cu6Sn5 grains formed at the interface between Sn-3.5Ag (wt.% unless otherwise specified) and Cu substrates were studied in this work. Reflow experiments were performed for 60 s at peak temperatures of 513 K, 533 K, 543 K, and 553 K. Two morphologies of interfacial Cu6Sn5 grains were observed in wetting reactions: prism type, above 543 K, and scallop type, below 533 K. During aging, the two morphologies gradually transitioned to layer type. These three morphologies could be transformed into each other as long as the corresponding condition changed. The morphology transition of Cu6Sn5 in the wetting reaction was explained by the change in Jackson's parameter with temperature. In addition, the effect of the Cu content in molten solder on interfacial Cu6Sn5 grains was examined. Significant differences in shear strength were observed for solder joints with different interfacial Cu6Sn5 morphologies in the case of a lower shear height. Joint strength is discussed in terms of the microstructure of the solder matrix and the morphology of interfacial Cu6Sn5 grains.