Enhanced solder wettability of oxidized-copper with lead-free solder via Ar-H_2 plasmas for flip-chip bumping: the effects of H_2 flow rates

摘要

Purpose - The purpose of this paper is to investigate the effects of H_2 flow rate on improving the solder wettability of oxidized-copper with liquid lead-free solder (96.5Sn-3Ag-0.5Cu) by Ar-H_2 plasmas. The aim was to improve the solder wettability of oxidized copper from 0 per cent wetting of copper oxidized in air at 260℃ for 1 hour to 100 per cent wetting of oxidized-copper modified by Ar-H_2 plasmas at certain H_2 flow rates and to find correlations between the surface characteristics of copper and the solder wettability with liquid lead-free solder. Design/methodology/approach - To reduce the copper oxides on the surfaces of oxidized-copper for improving solder wettability with liquid leadfree solder, this study attempted to apply Ar-H_2 plasmas to ablate the copper oxides from the surfaces of oxidized-copper by the physical bombardment of the Ar plasmas and to reduce the surfaces of oxidized-copper by the chemical reaction of H_2 plasmas with the surfaces of oxidized-copper. Findings - The solder wettability of oxidized-copper was found to be highly dependent on the surface characteristics of the copper. The values of polar surface free energy and dispersive surface free energy on the surfaces of oxidized-copper modified by Ar-H_2 plasmas were close to those values of solid lead-free solder, which resulted in improved solder wettability with liquid lead-free solder. Auger spectra indicated that the Ar-H_2 plasma modification was used to remove the copper oxides from the surfaces of oxidized-copper. Originality/value - The surface characterization of copper surfaces is typically determined by expensive surface analysis tool such as Auger Electron Spectroscopy (AES). This paper reports the results of a study of a promising technique called the sessile drop test method, for examining the surface free energies such as total surface free energy, polar surface free energy and dispersive surface free energy on the surfaces of copper to clarify how the solder wettability of oxidized-copper with liquid lead-free solder was enhanced by Ar-H_2 plasmas.