With the rapid development of the electronics industry, the demands for new environment-friendly lead-free solders have become increasingly urgent. SnIn alloy, owing to its lower melting point compared with traditional tin-lead alloys, enables safe and non-destructive soldering of low-melting-point precision materials, ensuring excellent soldering quality and product reliability, thus emerging as a highly promising substitute for lead-free solders. However, in practical applications, solder alloys are often mixed with flux to form solder paste. The active agents within the solder paste can corrode the solder alloy, leading to the "drying out" and failure of the solder paste during storage, which severely affects the production and quality of electronic products. Therefore, this study aims to thoroughly investigate the influence of citric acid (CA) concentration (one commonly used weak acid components in active agents) on the corrosion resistance of Sn8In alloy. This research employed a variety of advanced experimental methods, such as electrochemical tests and physical characterization techniques, in a comprehensive way. In terms of electrochemical tests, open circuit potential (OCP), potentiodynamic polarization curves (PDP), electrochemical impedance spectroscopy (EIS), and Mott-Schottky tests were conducted to accurately obtain the corrosion thermodynamics and kinetics parameters of the alloy in CA solutions at various concentrations. For physical characterization, Extended Depth of Field Microscope (EDFM), Scanning Electron Microscopy (SEM), Electron Probe X-ray Microanalysis (EPMA), X-ray Photoelectron Spectroscopy (XPS) were utilized to observe the surface morphology, elemental distribution, and phase composition of the alloy after immersion for 7 days.
The results demonstrated that during the initial immersion stage, the corrosion resistance of Sn8In alloy exhibited a distinct trend of first increase and then decrease with the increase of the CA concentration. When the CA concentration reached 9wt.%, the charge transfer resistance Rct reached its maximum of 926.07 Ω·cm2, indicating the optimal corrosion resistance performance of the alloy. Mott-Schottky tests confirmed that an n-type semiconductor film was formed on the surface of Sn8In alloy in all concentration systems, which significantly influenced the alloy's corrosion resistance. During the long-term immersion process, although the corrosion resistance of the alloy did not show an obvious inflection point with the increase of the concentration, considering long-term and stable performance comprehensively, the Sn8In alloy in 9wt.% CA solution performed the best. SEM and EPMA analysis results indicated that the protective substances formed on the surface of Sn8In alloy were mainly SnO2 and In2O3 oxide films, and different CA concentrations led to significant differences in the composition and microstructure of the protective films. In a low-concentration CA environment, the oxide film on the alloy surface was loose and porous with numerous defects, failing to effectively block corrosive media from penetrating to the metal matrix. When the CA concentration was 9wt.%, the oxide film had a dense and uniform structure, with SnO2 and In2O3 evenly distributed, tightly covering the alloy surface and thus significantly enhancing corrosion resistance. When the CA concentration was too high, the increase in cracks led to the peeling off of the coating layer, severely damaging the integrity of the protective film layer, and resulting in a sharp decline in corrosion resistance.
In conclusion, as the CA concentration gradually increases, the corrosion resistance of Sn8In alloy surface improves due to the increased content of protective oxide SnO2. However, an excessively high CA concentration will trigger pitting corrosion and damage the protective film layer. Therefore, Sn8In alloy exhibits high corrosion resistance, long-term stability, and reliability in 9wt.% CA solution. These findings not only provide a theoretical basis for a deeper understanding of the corrosion mechanism of SnIn alloy in citric acid-containing environments but also offer crucial technical support for optimizing solder paste formulations, solving the "drying out" failure problem of solder paste, improving the soldering quality and production efficiency of electronic products. It holds significant theoretical and practical implications for promoting the widespread application of lead-free solders in the electronics industry.
Key words
SnIn alloy /
corrosion /
citric acid
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Funding
Tianjin Natural Science Foundation General Project (24JCYBJC00900); Yunnan Fundamental Research Projects (202301BC070001-021); Central Guidance for Local Scientific and Technology Development Funds (24ZYCGCG00520)
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