Growth behavior of prismatic Cu6Sn5 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 after the joints solidification in air: prism type, above 543 K, and scallop type, below 533 K. The crystal planes of the faceted surfaces of these prismatic grains were confirmed by electron backscattering diffraction (EBSD). To study the growth behavior of these prismatic grains, the morphologies of Cu6Sn5 grains formed at the interface during soldering were obtained by an experiment. The results show that all the interfacial grains formed during soldering displayed a similar scallop type, indicating that the prismatic grains were formed during solder solidification. In addition, 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.