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

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: Cu6Sn5 plus Cu3Sn) 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 Cu3Sn IMCs in both solder joints. Moreover, when the Zn content was increased to 20wt.% in the Cu-xZn UBM, the growth of Cu6Sn5 as well as Cu3Sn 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. (C) 2008 Elsevier B.V. All rights reserved.