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 6Sn 5 plus Cu 3Sn) 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 3Sn IMCs in both solder joints. Moreover, when the Zn content was increased to 20 wt.% in the Cu– xZn UBM, the growth of Cu 6Sn 5 as well as Cu 3Sn 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.