Interfacial Reactions and Microstructures of Sn-0.7Cu-xZn Solders with Ni-P UBM During Thermal Aging

Abstract

The effects of the addition of Zn to Sn-0.7Cu solders are investigated. The study is focused on the interfacial reactions, microstructures, and mechanical properties after reaction with Ni-P under bump metallurgies (UBMs). The Zn contents in Sn-0.7Cu-xZn are varied as 0.2, 0.4, and 0.8 (in wt.% unless otherwise specified). In the reaction with Ni-P UBM during thermal aging at 150A degrees C for 1000 h, (Cu,Ni)(6)Sn(5) intermetallic compounds (IMCs) are formed at the Sn-0.7Cu/UBM interface, whereas Zn is incorporated into IMCs to form (Cu,Ni,Zn)(6)Sn(5) in the Zn-doped solders. As the Zn content increases, the interfacial IMC thickness is reduced. A total reduction of about 40% in IMC thickness was observed for the 0.8% Zn-doped Sn-Cu. The same IMC particles are also observed in the matrix of each solder. In Sn-0.7Cu, (Cu,Ni)(6)Sn(5) particles are coarsened during aging, while (Cu,Ni,Zn)(6)Sn(5) particles in the Zn-added solders are less coarsened and remain much smaller than (Cu,Ni)(6)Sn(5). The growth rate of (Cu,Ni)(6)Sn(5) during thermal aging is significantly suppressed by the addition of Zn. Consequently, after reaction with Ni-P UBM, the Zn-doped solders exhibit a thermally stable microstructure as measured by hardness and shear strength.