Electrochemical migration behavior of Sn–3.0Ag–0.5Cu solder alloy under SO2 polluted thin electrolyte layers

The effect of HSO3− on the electrochemical migration (ECM) of Sn–3.0Ag–0.5Cu lead-free solder alloy under thin electrolyte layers was investigated using the thin electrolyte layer method. The results showed that the migration element of Sn–3.0Ag–0.5Cu alloy was Sn and the faster-growing branch of the dendrite resulted in a sharper tip during the dendrite growth process. Increasing the amount of HSO3− decreased the probability of ECM. Due to the hydrolysis of HSO3, the pH value of electrolyte in thin electrolyte layer shifted toward a strongly acidic environment and a lower pH condition was favorable for dendrite formation. However, HSO3− reacted with tin ions to form some insoluble compounds during the ECM process, resulting in a decrease of tin ion concentration under thin electrolyte layers. A protective film formed on the anode surface, thus blocking the anodic dissolution process. Possible reactions were proposed to explain the inhibitory effect of HSO3− on the ECM of Sn–3.0Ag–0.5Cu alloy.