Comparative Study of Drop Reliability between Cu and Ni(P)/Au UBM with Sn-3.5Ag-xZn Solders

The performance of drop reliability was measured with Sn-3.5Ag-xZn solder joint on Cu and electroless Ni(P)/Au metallization after reflow and thermal aging (150degC, 500 hr). The content of Zn was 0, 1, 3 and 7 wt%. And interfacial reactions were investigated. The presence of Zn affected the type and morphology of IMC and showed the benefits on the drop reliability. In the case of Cu pad, Zn addition of more than 3 wt% suppressed growth of Cu₆Sn₅ and introduced Ag₅Zn₈/Cu₅Zn₈ layer, which increased drop resistance by about two times. During thermal aging, Cu₆Sn₅ and Ag₃Sn precipitated and grew in the interfacial IMC layer, which decreased the beneficial effect of Zn. In the case of Ni(P)/Au, Zn addition of 1 and 3 wt% transformed IMC from Ni₃Sn₄ to Ni₃Sn₄/Ni₅Zn₂₁. Only Ni₅Zn₂₁ were found for 7Zn specimen. The solder of 1 wt% Zn showed drop resistance about six times than that of Sn-3.5Ag. The improvement of drop resistance resulted from suppression of Ni₃P and Ni₃SnP growth. As the growth of Ni₃P and Ni₃SnP layer was effectively suppressed by zinc, the drop resistance increased substantially.