Undercooling, microstructures and hardness of Sn-rich Pb-free solders on Cu-xZn alloy under bump metallurgies

Abstract

The undercooling, microstructures and hardness of Sn-rich Pb-free solders changed by a reaction with Cu-xZn alloy under bump metallurgy (UBM) were investigated and compared to those of solders with Cu UBM. The investigation was based on four types of Sn-rich solders (pure Sn, Sn-0.7Cu, Sn-3.5Ag and Sn-3.8Ag-0.7Cu, where the numbers are all in mass percent unless specified otherwise) and three types of UBMs (pure Cu, Cu-10Zn and Cu-20Zn). The undercooling of the Sn-rich solders was reduced significantly by the reaction with the Cu-xZn UBMs. A decrease of 21-27°C in the undercooling was obtained, whereas a decrease of only 10-16°C was obtained by the reaction with Cu UBMs. In the Sn-rich solders after the reactions with the Cu-xZn UBMs, there was a barely perceptible large growth of primary intermetallic compound phases, such as Cu6Sn5 and Ag3Sn; moreover, there were a large increase in the volume fraction of the eutectic phases and a coarsening of ß-Sn dendrites. In addition, the Sn-rich solders with Cu-xZn UBMs showed a large increase in hardness. These changes in the undercooling, microstructures and hardness are discussed in terms of the compositions of Sn-rich solders changed by the interfacial reactions with Cu-xZn UBMs. © 2009 The Japan Institute of Metals.