Enhanced Wettability of Oxidized Copper with Lead-Free Solder by Ar-H2 Plasmas for Flip-Chip Bumping

Enhanced solder wettability (SW) of oxidized-Cu (OC) with 96.5Sn-3Ag-0.5Cu lead-free solder (LFS) by Ar-H2 plasmas was investigated. The SW of OC was significantly improved from 0% wetting of Cu oxidized in air at 260°C for 1 h to 100% wetting of OC modified by Ar-H2 plasmas for 10 min. The SW of Cu was found to be highly dependent on the surface characteristics of Cu. By decreasing the total surface energy (TSE), decreasing the polar surface energy (PSE), and increasing the dispersive surface energy (DSE) on the surfaces of OC modified by Ar-H2 plasmas, the SW with LFS improves. X-ray photoelectron spectroscopy (XPS) indicates that Ar-H2 plasma treatment is used to remove the copper oxides CuO and Cu2O from the OC surfaces. The ratio of the total amount of Cu2O to CuO was found to be a good indication of how the copper oxides CuO and Cu2O affect the PSE, DSE, and SW of Cu.