Relationship between morphologies and orientations of Cu6Sn5 grains in Sn3.0Ag0.5Cu solder joints on different Cu pads

Abstract

The morphologies and orientations of Cu6Sn5 intermetallic compounds in the Sn3.0Ag0.5Cu solder joints both on polycrystalline and single crystal Cu pads under different peak reflow temperatures and times above liquids were investigated. The relationship between Cu6Sn5 grain orientations and morphologies was clarified. At the interface of Sn3.0Ag0.5Cu/polycrystalline Cu pad, scalloped Cu6Sn5 intermetallic compounds formed at 250°C and roof shape Cu6Sn5 formed at 300°C. Both scalloped Cu6Sn5 and roof shape Cu6Sn5 had a preferred orientation of {0001} plane being parallel to polycrystalline Cu pad surface. Besides, the percentage of large angle grain boundaries increased as the peak reflow temperature rose. At the interface of Sn3.0Ag0.5Cu/(111) single crystal Cu pad, the Cu6Sn5 intermetallic compounds were mainly scallop-type at 250°C and were prism type at 300°C. The prismatic Cu6Sn5 grains grew along the three preferred directions with the inter-angles of 60° on (111) single crystal Cu pad while along two perpendicular directions on (100) single crystal Cu pad. The orientation relationship between Cu6Sn5 grains and the single crystal Cu pads was investigated by electron backscatter diffraction technology. In addition, two types of hollowed Cu6Sn5 intermetallic compounds were found inside the joints of polycrystalline Cu pads. The long hexagonal Cu6Sn5 strips were observed in the joints reflowing at 250°C while the hollowed Cu6Sn5 strips with the ‘▪’ shape cross-sections appeared at 300°C, which was attributed to the different grain growth rates of different Cu6Sn5 crystal faces.