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(150°C, 500hr). 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 3wt% 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 3wt% transformed IMC from Ni3Sn4 to Ni3Sn4/Ni5Zn21. Only Ni5Zn21 were found for 7Zn specimen. The solder of lwt% 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.