目的 探究焊锡膏活性剂中柠檬酸（CA）浓度变化对锡铟合金焊料耐蚀性的影响规律,从而优化焊锡膏配方,防止焊锡膏产品在储存过程中因腐蚀“发干”而失效。方法 采用电化学测试（OCP、EIS、PDP和Mott-Schottky）和物理表征方法（SEM、EDS、超景深显微镜、EPMA、XPS）研究了Sn8In合金在不同浓度柠檬酸水溶液中的腐蚀行为。结果 在初期浸泡阶段,随CA浓度的增加,Sn8In合金的耐蚀性先增加后下降,在浓度9%（质量分数）时出现最大的Rct值（926.07 Ω·cm2）,同时Mott-Schottky测试显示所有体系浓度中的Sn8In合金表面均生成了n型半导体膜;而在长期浸泡中,Sn8In合金虽然随浓度的增加并未出现最大值拐点,但从耐久性和稳定性方面考虑,CA浓度为9%时的Sn8In合金表现最为突出。物理表征结果表明,Sn8In合金表面所生成的保护物质主要为SnO2和In2O3氧化膜,而CA浓度的改变会造成保护膜完整性及成分分布的差异。CA浓度太低表面无法形成产物膜层,而浓度过大时保护膜层裂纹增多,完整性被破坏。结论 Sn8In合金表面耐蚀性随CA浓度的增大存在先增大后降低的趋势,即活性剂中CA的含量存在最优值9%。
Abstract
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.
关键词
锡铟合金 /
腐蚀 /
柠檬酸
Key words
SnIn alloy /
corrosion /
citric acid
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基金
天津市自然科学基金面上项目（24JCYBJC00900）; 云南省科技厅科技计划项目（202301BC07 0001-021）; 中央引导地方科技发展资金项目（24ZYCGCG00520）
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