The formation and growth of intermetallic compounds in Sn-Zn and Sn-Zn-Al solder with Ni/Au surface finish bond pad

In this study, we used microstructure evolution and electron microprobe analysis (EPMA) to investigate the interfacial reactions in Sn-Zn and Sn-Zn-Al solder balls with Au/Ni surface finish ball-grid-array (BGA) bond pad over a period of isothermal aging at 150°C. During reflow, Au dissolved into the solder balls and reacted with Zn to form γ-Au₃Zn₇ and γ₂-AuZn₃. As aging progressed, γ and γ₂ transformed into γ₃-AuZn₄. Finally, Zn precipitated out next to γ₃-AuZn₄. The Zn reacted with the Ni layer to form Ni₅Zn₂₁. A thin layer (Al, Au, Zn) intermetallic compound (IMC) formed at the interface of the Sn-Zn-Al solder balls, inhibiting the reaction of Ni with Zn. Even after 50 days of aging, no Ni₅Zn₂₁ was observed. Instead, fine (Al, Au, Zn) particles similar to Al₂ (Au, Zn) in composition formed and remained stable in the solder. The lower ball shear strength corresponded with the brittle fracture morphology in Sn-Zn-Al solder ball samples.