Abstract
Cu is widely used in today's electronics packaging, and electroless Ni-P (EN)/Cu is popularly adopted in the under bump metallurgy (UBM) for flip chip application. The diffusion behavior of Cu in the metallization layer is an important issue. In this study, a joint in the Cu/EN/Pb-Sn structure with different EN thickness annealed at 240oC were employed to investigate the diffusion behavior of Cu for various annealing time. The concentration-distance profiles at the interface of EN/solders were evaluated by EPMA (Electron Probe Microanalyzer). The interdiffusion flux and effective interdiffusion coefficient of species Cu, Ni and P were evaluated with the aid of corresponding concentration profile. Ni3P precipitated layer and Ni3Sn4 intermetallic compound were observed near the interface of EN and Sn-Pb solder during annealing. According to detailed quantitative analysis by EPMA, the concentration of Cu in Ni3P and Ni3Sn4 intermetallic compound was around 0.5at% and 1.0at%, respectively, for the Cu/EN(7.5 □m)/Sn-Pb solder joint, while it was 0.2-0.3at% and 0.8-0.9at%, respectively, for the Cu/EN(15 □m)/Sn-Pb solder joint. With respect to the evaluated Cu concentration variation across the joint assembly, Ni3P can be regarded as diffusion barriers to prevent extra Cu diffusion through the EN layer to the solder.Another isothermal interdiffusion experiments were also carried out at 240oC for different time with solid-solid diffusion couples assembled in the Cu/electroless-Ni (Ni-10wt%P) and Cu/electroless Ni (Ni-10wt%P)/Sn-37Pb joint. The diffusion structure and concentration profile were examined by SEM and EPMA analysis. The interdiffusion fluxes of Cu, Ni and P were calculated from the concentration profile with the aid of Matano plane evaluation. The values of, and decreased with increasing annealing time. The average effective interdiffusion coefficients in the order of 10-14 cm2/s were also evaluated within the diffusion zone. Cu dissolved in intermetallic compound (IMC) Ni3Sn4 and Ni3P precipitate after annealing in Cu/electroless Ni/Sn-37Pb system was about 0.25 at% and 0.5 at%. For the short period of annealing, it appears that the presence of EN with Sn-Pb soldering reaction assisted the diffusion of Cu through the EN layer time.