A simple process for electrodeposition of Sn-rich, Au–Sn solder films

Abstract

The Sn-rich eutectic alloy (90 wt% Sn) in the Au–Sn system offers a potentially cheaper alternative to the Au-rich eutectic alloy (20 wt% Sn) for optoelectronic and microelectromechanical systems device packaging and may be applicable as a Pb-free solder for microelectronic packaging. A simple electrodeposition method was utilized to fabricate Sn-rich, Au–Sn solder films, including the eutectic composition for this purpose. The electrolyte consisted of a solution of Sn chloride and ammonium citrate. Gold was added to the electrolyte in the form of either a Au nanoparticle (<20 nm) suspension, prepared with Na citrate, or by directly adding Au powder (500–800 nm particles). The resultant suspensions were used to electrodeposit eutectic and near-eutectic alloy films. Uniform thicknesses and compositions were obtained with the latter approach, i.e., direct addition of Au powder. Gold content in the deposits increased with increasing Au particle loading in the electrolyte and increasing current density. Room temperature aging led to the formation of AuSn₄ at the Au particle-Sn matrix interface. Reflow of deposits with near-eutectic compositions resulted in the formation of the two eutectic phases, Sn and AuSn₄.