Even though electroless Ni-P and Sn-Ag-Cu solders Are widely used materials in, flip-chip bumping technologies, interfacial reactions of the ternary Cu-Ni-Sn system are not well understood. The growth of intermetallic compounds (IMCs) at, the under bump metallization (UBM)/solder interface can affect solder-joint reliability, so analysis of IMC phases and understanding their growth kinetics are important. In this, study, interfacial reactions between electroless Ni-P UBM and the 95.5Sn-4.0Ag-0.5Cu alloy were investigated, focusing on identification of IMC phases and IMC growth kinetics at various reflowing and aging temperatures and times. The stable ternary IMC initially formed at the interface after reflowing was the (Cu,Ni)(6)Sn-5 phase. However, during aging,the (Cu,Ni)6Sn5 phase slowly changed into the quaternary IMC composed. of Cu, Ni, Sn, and a small amount of Au. The Au atoms in the quaternary IMC originated from immersion Au plated on electroless Ni-P UBM. During further reflowing or aging, the (Ni,Cu)(3)Sn-4 IMC started forming because of the limited Cu content in the solder. Morphology, composition, and crystal structure of each IMC were identified using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Small amounts of Cu in the solder affect the types of IMC phases and the amount-of the IMC. The activation energies of (Cu,Ni)(6)Sn-5 and (Ni,CU)(3)Sn-4 IMCs were used to, estimate the growth kinetic's of IMCs. The growth of IMCs formed in aging was very slow and temperature-dependent compared to IMCs formed in reflow because of the,higher activation energies of IMCs in aging. Comparing activation energies of each IMC, growth mechanism of IMCs at electroless Ni-P/SnAgCu solder interface will be discussed.