Wettability and interfacial reactions of Sn-based Pb-free solders with Cu–xZn alloy under bump metallurgies

Abstract Under bump metallurgies (UBMs) made of Cu–10Zn and Cu–20Zn (numbers are all in weight percent unless specified otherwise) are proposed. After a reaction with Sn–0.7Cu and Sn–3.8Ag–0.7Cu solders, the wettability and interfacial reactions during reflow and aging were compared with those of a pure Cu UBM. The wettability of both solders on Cu–10Zn and Cu–20Zn decreased by about 30% even though they were reflowed under the same conditions. The total thickness of the intermetallic compounds (IMCs: Cu 6 Sn 5 plus Cu 3 Sn) at the interface during reflow was significantly reduced by the reaction with the Cu–Zn UBMs, and this reduction is attributed to the large reduction of the growth of Cu 3 Sn IMCs in both solder joints. Moreover, when the Zn content was increased to 20 wt.% in the Cu– x Zn UBM, the growth of Cu 6 Sn 5 as well as Cu 3 Sn was retarded during aging, and thus the thickness of the total IMCs was significantly reduced. In addition, the first-forming IMC on the Cu–Zn UBMs during the reflow was calculated thermodynamically and compared with the experimental results.