Assessment of microstructure and shear strength for low melting point tin-free alloys on Cu

This study explores and compares two new alloys, namely 50In-50Bi and 17Sn-26In-57Bi (wt%), with melting peak temperatures of 95 and 82 °C, which are substantially lower than the melting points of lead-free solders commonly used for low-temperature applications. The microstructures and intermetallic compounds (IMCs) of the new alloys, which were reflowed on Cu substrates, were studied. Notable Bi₃In₅ and BiIn₂ compounds were formed in the reflowed 50In-50Bi alloy. The IMCs detected at the interfaces of 50In-50Bi and 17Sn-26In-57Bi were Cu₁₁(In, Bi)₉ and Cu₆(In, Sn)₅, respectively. Shear tests were systematically performed on organic solderability preservative (OSP) Cu substrates with 300-μm openings. Investigation of the fracture surfaces of both alloys indicated that the 17Sn-26In-57Bi/OSP Cu system had the higher shear strength and the more ductile joints.