SAC焊料中的Ag3Sn在服役过程中会发生选择性氧化，生成纯Ag[10,16]。纯Ag与焊料中的Sn基体形成Ag/Sn电偶。在外加电压的作用下，可能会导致焊料的腐蚀行为发生改变。Zhong等[17]研究表明，在NaCl溶液条件下，当外加电位比较高时，SAC焊料发生腐蚀和电化学迁移，生成的枝晶中含有Ag，该研究表明，SAC焊料中的Ag3Sn发生选择性氧化，导致焊料表面出现纯Ag，在高电位作用下，Ag发生腐蚀，溶出的金属离子在阴极还原形成Ag枝晶。Ag的电极电位比Sn的高，即在热力学上，Ag比Sn更不容易发生腐蚀[13]。在无外加电位条件下，Ag与Sn形成电偶时，Sn作为阳极优先发生腐蚀[18]。对于外加电位的影响，文献中未有详细报道。此外，浸银(ImAg)是一种常用的印刷电路板(PCB)表面处理工艺，焊料中的Sn容易与Ag形成腐蚀电偶[19]，电偶腐蚀是一种重要的腐蚀形式[20~22]。因此，研究外加电位下对Ag/Sn电偶的腐蚀行为，揭示其腐蚀机理，对于电子器件的可靠性研究具有重要价值。
微量的Cl-常见于电子器件的服役环境中，因此，本文以0.01 M NaCl为介质，首先通过开路电位和极化曲线测试揭示纯Ag和纯Sn的基本腐蚀行为，在此基础上，采用双极性电化学技术研究Ag/Sn电偶在不同外加电位下的腐蚀行为，并对测试后的样品进行光学显微镜、扫描电子显微镜/能谱(SEM/EDS)表征，分析外加电位对Ag/Sn电偶的腐蚀行为的影响。
1 实验方法
实验材料为纯金属Ag和Sn，纯度均为99.99%。对于极化曲线测试实验，样品的尺寸为12 mm × 12 mm × 3 mm，样品的背面通过焊接与导线连接，再封入环氧树脂中作为工作电极，表面露出作为测试面，控制暴露尺寸为10 mm × 10 mm。在进行电化学实验之前，所有的工作电极用水磨砂纸逐级打磨至2000#，接着使用2.5 μm金刚石抛光膏进行机械抛光，最后依次使用去离子水、乙醇和丙酮清洗样品表面并用冷风吹干。实验溶液为0.01 mol/L NaCl。
采用CHI660E型电化学工作站进行极化曲线测试，测试温度为25℃。实验装置为标准的三电极体系，待测样品作为工作电极，Pt片电极作为对电极，饱和甘汞电极(SCE)作为参比电极。在进行极化曲线测试之前，先将样品浸入实验溶液中，在开路状态下稳定1800 s，记录样品的开路电位(OCP)随时间的变化。随后，从-0.25 VOCP开始，以10 mV/min的速率向阳极方向对样品进行动电位扫描，终止电位为1.5 VSCE。为了保证数据的可重复性，每组极化曲线都进行3组平行实验。
图1
图1
双极性电化学测试装置和样品示意图
Fig.1
Schematic of bipolar electrochemistry test setup (a), top view (b) and side view (c) of bipolar electrode, ‘E’ represents the potential
电化学实验结束后，用ZEISS Axiolab 5光学显微镜观察样品宏观腐蚀形貌，并采用自带能谱分析(EDS)的ZEISS Gemini 300扫描电子显微镜(SEM)进行腐蚀形貌观察及表征。由于光学显微镜所能拍摄的范围有限，对于双极性电化学测试后的样品，其光镜形貌照片是将各个区域的照片拼接后得到的，以展示样品在系列电位下的腐蚀全貌。
2 结果与讨论
2.1 Ag和Sn的开路电位
图2
图2
Ag和Sn在0.01 mol/L NaCl溶液中的OCP
Fig.2
OCP of Ag and Sn in 0.01 mol/L NaCl solution
2.2 Ag和Sn的极化曲线
Ag和Sn在0.01 mol/L NaCl溶液中的动电位极化曲线如图3所示。在NaCl溶液中，随着电位升高，Ag的阳极电流密度迅速增大，电位高于0.25 VSCE后，电流密度保持稳定，约在
图3
图3
Ag和Sn在0.01 mol/L NaCl溶液中的动电位极化曲线
Fig.3
Polarization curves of Ag and Sn in 0.01 mol/L NaCl solution
Ag在0.01 mol/L NaCl溶液中极化曲线测试后的形貌和SEM/EDS表征结果如图4所示。光镜和SEM结果(图4a和b)显示，Ag表面有腐蚀产物覆盖，主要成分是AgCl (图4c和d)。腐蚀产物对基体粘附力比较弱，水流冲洗后部分产物发生脱落。产物膜呈现双层结构，外层疏松多孔(图4e)，内层较为致密(图4f)，二者的EDS成分结果有所区别，可能是因为厚度不同，导致基体Ag的信号强度不一样(图4h和i)。产物膜下方是Ag基体，表面因为腐蚀而变得不平整(图4g)，Cl含量很少(图4j)。上述结果表明，Ag在NaCl溶液中极化曲线测试时形成AgCl产物膜覆盖在基体表面，阻碍了离子的扩散。在极化曲线上表现为电位高于0.25 VSCE后，Ag的电流密度基本不变，即达到了极限扩散电流密度。
图4
图4
Ag在0.01 mol/L NaCl溶液中极化曲线测试后的形貌和SEM和EDS表征
Fig.4
Optical image (a), SEM image (b), element distribution maps (c, d) and magnified views (e‒g) of Fig.4b, EDS results of points in Fig.4e-g (h‒j) of Ag after polarization in 0.01 mol/L NaCl solution
图5
图5
Sn在0.01 mol/L NaCl溶液中极化曲线测试后的形貌和SEM/EDS表征
Fig.5
Optical image (a), SEM images (b, f), element distribution maps of Fig.5b (c‒e) and EDS result of points in Fig.5f (g, h) of Sn after polarization in 0.01 mol/L NaCl solution
反映在极化曲线上是高电位下，Sn的电流密度较高。
2.3 Ag/Sn电偶的双极性电化学测试
图6
图6
Ag和Sn在0.01 mol/L NaCl溶液中经过10 min双极性电化学测试后的光镜形貌和SEM和EDS结果
Fig.6
SEM images and EDS results of the Ag (a1-a3) of zones 1, 2, and 3 in Fig.6a and Sn (b1-b3) of zones 1, 2, and 3 in Fig.6b after the BPE test for 10 min in 0.01 mol/L NaCl solution
2.4 讨论
AgCl在水溶液中的溶解度较低(Ksp(AgCl) =
电位降低，Sn进入免蚀状态。当电位进一步降低时，SnH4生成：
导致Sn发生阴极腐蚀。
因此，在NaCl溶液中，Ag/Sn电偶中的Sn在高电位和
3 结论
本文针对SAC焊点材料的腐蚀，采用单金属Ag和Sn，结合开路电位和极化曲线测试、双极性电化学测试方法研究外加电位下Ag/Sn电偶的腐蚀行为，得到以下结论：
(1) Sn在NaCl溶液中的OCP低于Ag，表明Sn的热力学腐蚀倾向高于Ag。
(2) 在NaCl溶液中，Sn的腐蚀电流始终大于Ag，Ag表面生成较为致密的AgCl膜，阻碍基体溶解。
(3) 双极性电化学测试能够同时揭示Ag/Sn电偶在系列外加电位下的腐蚀形貌，具有高通量的特性。Sn在低电位下发生阴极腐蚀，在高电位下，NaCl溶液中，由于
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