Interfacial reactions and microstructures of Sn-0.7Cu-xZn solders with ni-P UBM during thermal aging

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 150°C for 1000 h, (Cu,Ni) 6Sn 5 intermetallic compounds (IMCs) are formed at the Sn-0.7Cu/UBM interface, whereas Zn is incorporated into IMCs to form (Cu,Ni,Zn) 6Sn 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) 6Sn 5 particles are coarsened during aging, while (Cu,Ni,Zn) 6Sn 5 particles in the Zn-added solders are less coarsened and remain much smaller than (Cu,Ni) 6Sn 5. The growth rate of (Cu,Ni) 6Sn 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. © 2009 TMS.