Lead-free solder wettability of oxidized-aluminum enhanced by Ar-H₂ plasmas for flip-chip bumping

Abstract

Purpose

An investigation has been performed on the improved solder wettability of oxidized aluminum (Al) with lead-free solder (96.5Sn-3.5Ag) using Ar-H₂ plasmas. The lead-free solder wettability was raised from 62.2 per cent wetting for Al oxidized in air at 250 C for 4 h to 98.4 per cent wetting of oxidized Al modified by Ar-H₂ plasmas at a certain H₂ flow rate. This study aims to gain insight on the surface characteristics of Al affecting the solder wettability with a liquid lead-free solder.

Design/methodology/approach

Ar-H₂ plasmas at certain H₂ flow rates are intended to reduce Al oxides on the surfaces of oxidized Al substrates both by physical bombardments via Ar plasmas and chemical reductions with H₂ plasmas, while Al substrates are exposed in Ar-H₂ plasmas to improve the solder wettability with a liquid lead-free solder.

Findings

Surface characteristics of oxidized Al substrates have been identified to play key roles for enhanced lead-free solder wettability using Ar-H₂ plasmas. A decrease in polar surface free energy and an increase in dispersive surface free energy on the surfaces of oxidized Al substrates are exploited to advance the lead-free solder wettability. Decreased composition ratios of O to Al, detected by X-ray photoelectron spectroscopy (XPS) for oxidized Al substrates, are crucial for improved lead-free solder wettability.

Originality/value

XPS is typically used to analyze the surface compositions of Al oxides. To provide a rapid and non-expansive method to identify the surfaces of Al substrates prior to soldering to assure lead-free solder wettability, this study proposes a measurable skill, a so-called sessile drop test method, to investigate surface free energies such as total, polar and dispersive surface free energy on the surfaces of Al substrates, to illuminate how the lead-free solder wettability of oxidized Al is improved by Ar-H₂ plasmas.

Citation

Lin, Y.-S., Lin, S.-M., Li, J.-Y. and Liao, M.-C. (2020), "Lead-free solder wettability of oxidized-aluminum enhanced by Ar-H₂ plasmas for flip-chip bumping", Soldering & Surface Mount Technology, Vol. 30 No. 1, pp. 42-48. https://doi.org/10.1108/SSMT-06-2017-0018