Enhanced lead-free solder wettability of oxidized-nickel by Ar–H2 plasmas for flip chip bumping

Abstract

Solder wettability (SW) of oxidized-nickel (ON) with liquid lead-free solder (LFS) (96.5Sn–3.5Ag) is notably improved by Ar–H2 plasmas from 0 % wetting of nickel substrate oxidized in air gases at 200 °C for 2 h to 100 % wetting of ON substrate treated by Ar–H2 plasmas at certain plasma settings. The SW of ON substrates with LFS is dependent on the surface characteristics of ON substrates. The enhanced SW of ON substrates with LFS by Ar–H2 plasmas is mostly due to a decrease in polar surface free energy and an increase in dispersive surface free energy on the surfaces of ON substrates. The ratios of the compositions of O to Ni are critical to the SW of ON substrate with LFS. The atomic compositions of Ni and O on ON substrates are analyzed by an energy-dispersive X-ray spectroscope. The nickel oxides NiO and Ni2O3 on the surfaces of ON substrate are identified by an X-ray photoelectron spectroscope.