Interfacial reactions of lead-free Sn-Zn based solders on Cu and Cu plated electroless Ni-P/Au layer under aging at 150°C

The interfacial reactions of Sn-Zn based solder on Cu and Cu/Ni-P/ Cu-plating substrates under aging at 150 °C were investigated in this study. The compositions of solders investigated were Sn-9Zn, Sn-8.55Zn-0.45Al, and Sn-8.55Zn-0.45Al-0.5Ag solders in weight percent. The experimental results indicated that the Cu substrate formed Cu ₅Zn₈ with the Sn-9Zn solder and Al-Cu-Zn compound with Al-containing solders. However, it was detected that Cu ₆Sn₅ formed at the Sn-9Zn/Cu interface and Cu₅Zn₈ formed at the Al-containing solders/Cu interface after aging for 1000 h. When it contacted with the Cu/Ni-P/Au substrate, the Sn-9Zn solder formed Au-Zn compound, and the Al-containing solders formed Al-Cu-Zn compound at the interface. After a long aging time, the intermetallic compounds existing between solders and the Cu/Ni-P/Au metallization layers almost did not grow. It was found that the interdiffusion between solders and Cu/Ni-P/Au was slower than that with Cu under aging. Furthermore, the addition of Ag to Sn-Zn solder resulted in the formation of AgZn₃ particles at the interface.