Ni/Cu under-bump metallization (UBM) for flip-chip application is widely used in electronics packaging. The intermetallic compounds (IMCs) formed at the interface between Sn-Pb solder and Ni/Cu UBM during reflowing are mainly (Ni, Cu)(3)Sn-4 and (Cu, Ni)(6)Sn-5. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) analyses were employed to analyze the morphology and structure in intermetallic compounds. The elemental distribution near the interfacial region was evaluated by an electron probe microanalyzer (EPMA). From the morphology of IMCs observed by SEM and cross-sectional EM images of Sn-Pb solder joint, it is revealed that the growth of (Cu1-y, Ni-y)(6)Sn-5 IMC is accompanied by a decreasing thickness of (Ni1-x, Cu-x)(3)Sn-4 IMC between (Cu1-y, Ni-y)(6)Sn-5 and Ni/Cu layer. This phenomenon was further evidenced by the cross-sectional SEM images of Sn-Pb solder/Ni layer interface after multiple reflow cycles. Based on the observation and characterization by FE-SEM, TEM, and EPMA, the evolution of the phase transformation between (Cu1-y, Ni-y)(6)Sn-5 and (Ni1-x, Cu-x)(3)Sn-4 IMCs is proposed. (C) 2004 Elsevier B.V. All rights reserved.