A new understanding on the corrosion failure of Sn-based lead-free solder through the EIS monitoring and interpretation

• Corrosion evolution of Sn-based lead-free solder was investigated. • Oxide film plays one important role in controlling the corrosion failure of solder. • Oxide film resistance is mainly related with element oxygen affinity in solder. • Methods for developing high corrosion resistant Sn-based solder were proposed. The corrosion evolution of Sn-based lead-free solder has been consistently attributed to galvanic corrosion with a dissolution of β -Sn phase in numerous studies. However, through monitoring the electrochemical impedance spectroscopy (EIS) data of solder during degradation, it has been found that the naturally formed nano-scaled protective oxide layers on solders can control the evoking of degradation and lead a totally different degradation pattern of the solder alloy. Moreover, by interpreting the EIS data, it was found that the oxide film could exhibit two distinct degradation patterns controlled by the changes in substrate composition. One pattern is characterized by the pure Sn solder, which demonstrates a thickened oxide film with a uniform structure that contributes to enhanced corrosion resistance during immersion in NaCl electrolyte, indicating superior corrosion protection. The other pattern is characterized by the SnAg solder, where corrosion attack initiates at the Ag 3 Sn phase under the NaCl electrolyte due to the formation of an uneven oxide film on this phase. The initial structure and physicochemical properties of the oxide films have been demonstrated to exert control over two distinct degradation patterns, which are further determined by the disparity in oxygen affinities between the alloy elements and substrate Sn. Such understanding is crucial for the development of corrosion-resistant lead-free solders with protective oxide film, i.e., the alloy elements promote both passive film formation and acceptable metal-Sn bonding would be selected.