Corrosion process study oJZn-30Sn high-temperature lead-free solder ' Zhenghong Wang , Gong Zhang
Chuantong Chen, and Katsuaki Suganuma
Department of Mechanical Engineering Tsinghua University, Beijing 100084, China Wangzhenghong
[email protected] Institute of Scientific and Industrial Research Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan
Abstract-The
corrosion
process
of
Zn-30Sn
high
temperature solder alloy was investigated in neutral 0.5 M NaCI solution at room temperature (25
±
0.5
0c)
by Scanning Electron
Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The surface observation by SEM indicates that the surface morphology of Zn-30Sn evolves from the dense particle structure to porous structure, and finally converts to platelet-like structure. Energy Dispersive X-ray Analysis (EDXA) shows that Zn phase is corroded preferentially due to no Sn compound was observed. The
outer
Zn(OH)2
surface by
characterized
of
XPS. as
corrosion
product
Dominant a
mixture
was
corrosion of
ZnO,
confirmed
products Zn(OH)2
as
were and
Zns(OH)sCb' H20. The corrosion process was further discussed in detail.
Keywords-lead-free solder; Zn-30Sn; XPS; corrosion process
I.
INTRODUCTION
High lead solder (lead-content over 85 wt.%) has been still widely used as high-temperature solder in the field of step soldering, automotive electronics, aerospace and other critical electronic components due to its excellent properties [1-4]. However, with the improvement of environmental awareness, it has been a main trend for electronics industry to be lead-free, resulting a strong driving-force for the development of lead free solders. Though high lead solder has been exempted temporarily by RoHS (The Restriction of the use of Certain Hazardous Substances in Electrical and Electronic Equipment), it is an urgent priority to develop alternative high-temperature solders. Until now, most of the research focused on low temperature lead-free solder [5-7], while the research on high temperature solder is relatively less. The alternatives for conventional Pb-5Sn alloy include Au-based system, Bi-Ag system and Zn-based system alloys [8-11]. Among all the alternatives, Zn-Sn based alloy system is considered as a promising candidate due to its excellent electrical/thermal properties, mechanical properties and low cost [2]. However, Zn is an active metal, which is often sensitive to the corrosion. Once the solder joint was exposed in the corrosion media, such as moisture, air pollutants and oceanic environments, they may easily be corroded, leading to the failure of connection and reduction of component service life [12, 13]. Thus, to achieve high reliability for joints, it is required that solder alloys possess high corrosion resistance.
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Heretofore, there is very few research available on the corrosion properties of Zn-Sn high-temperature alloys, while most of work has concentrated on the mechanical properties, thermal properties and interface evolution [11,14,15]. Therefore, it is an urgent to study the corrosion process of Zn Sn high-temperature solder alloys for its practical application, thereby improving its corrosion resistance based on the understanding of corrosion process. In this work, typical high-temperature Zn-Sn system alloy Zn-30Sn was selected as an experimental object and its corrosion process was investigated. Surface of Zn-30Sn samples with different immersion time was observed by Scanning Electron Microscopy (SEM), and the surface corrosion products were characterized with Energy Dispersive X-ray Analysis (EDXA), X-Ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion products evolution was further discussed. II.
EXPERIMENTAL
Zn-30Sn was prepared from pure Zn and Sn particles (purity higher than 99.9 %). The raw materials were encapsulated in a quartz tube under vacuum environment (10.4 Torr), followed with a heating process at 500 °C in a muffle furnace for 5 h. During heating process, periodic shaking was conducted every 30 min to obtain homogeneous composition alloy. Then, water-quench the alloy at 450 °C and get the alloy stick. The rectangular specimen with a size of 8 rum x 6 rum x 4 mm was sliced from the as-prepared alloy stick. The experiment surface of a specimen was mechanically grinded with 400, 600, 800, 1200, 1500, 2000, and 4000 grit SiC papers, followed with ultrasonic cleaning by ethanol to remove grease on surface. 0.5 M NaCI solution was prepared with analytical grade NaCl chemicals and distilled water. Zn-30Sn alloy specimens were immersed in 0.5 M NaCI solution for 20 h, 30 h, 60 h and 100 h at room temperature (25 ± 0.5 0c). After immersion, rinsing of the sample was performed with distilled water for 1 minutes to remove the deposited NaCI. Then high pressure cold air was used to dry the samples. The surface morphology of Zn-30Sn after immersion was observed with Field Emission-Scanning Electron Microscope (FE-SEM, Hitachi SU8020, Japan), and the corrosion products were further characterized with Energy Dispersive X-ray Analysis (EDXA), X-Ray diffraction (XRD, Rigaku SmartLab, Japan)
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and X-ray photoelectron spectroscopy (XPS, JPS-9010, JEOL, Japan). The angular range of XRD was from 10° to 90° (29), and Jade 5.0 software was used to analyze the XRD patterns. XPS experiment was performed using monochromatized Mg Ka photons (1253.6 eV) with a resolution of 0.05 eV and input power of 100 W (10 kV, 10 rnA). III. A.
RESULTS AND DISCUSSION
Surface morphology ofSEM
Fig.l shows the evolution of the corrosion products with different immersion time in neutral 0.5 M NaCI solution. The corresponding chemical composition by EDXA of the marked points in Fig.l was illustrated in Fig.2. After 20 h immersion in 0.5 M NaCI solution (Fig.l (a)), the surface of Zn-30Sn was covered with tiny and dense particle structure. Additionally, a small amount of spindle-like structure with a length of about 5 J.tm was observed. The chemical composition of these two kinds of structure by EDXA was depicted in Fig.2 (1) and (2), respectively. Obviously, these structures are mainly composed of Zn element and 0 element, while the content of Sn and CI is very low. Specifically, the atomic ratio of Zn and 0 is about 1:1 for the particle structure on the surface, indicating that these particles are likely to be ZnO. As for the spindle-like structure, though 0 content shows a slight growth, the atomic ratio of Zn and 0 is still close to 1:1, which indicates that these spindle-like structure may be the same substance with the particle structure. The similar chemical composition of these two structure indicates that spindle-like structure may be formed from particle structure.
was covered with porous structure. The chemical composition of these porous structure was depicted in Fig.2 (3) with atomic ratio of Zn and 0 at about 2:3. Compared to the particle structure, the porous structure presents higher 0 content. After 100 h immersion, the morphology of corrosion products changed to platelet-like structure. Fig.2 (4) listed the chemical composition of platelet-like structure. Compared to the other structure, the Cl-content of these platelet-like structure is much higher, reaching at 1l.52 wt.%, revealing that CI is involved in the formation of corrosion products. The atom ratio of it indicates that the platelet-like structure can be Zn5(OH)sCh'H20 [16,17]. For all the corrosion products observed on the surface, the content of Sn is very low, namely almost no Sn compound was detected, indicating that Zn phase is corroded preferentially.
Fig.2 corresponding EDXA spectrum of points marked in Fig.!
XRD was further used to confirm the phase composition of the corrosion products in Fig. 1(d). After 100 h immersion, as shown in Fig. 3, a large amount of �-Sn phases were observed, while no a-Zn phase was detected, Clearly, a-Zn phase was preferentially corroded during the corrosion process, corresponding to the results of EDXA. According to the XRD pattern, the main corrosion products were indentified as Zn5(OH)sCb·H20. Moreover, some ZnO and Zn(OH)2 were detected. Therefore, dominant corrosion products can be characterized as a mixture of ZnO, Zn(OHh and Zn5(OH)sCh'H20.
Fig.! Corrosion products morphology ofZn-30Sn with different immersion time in neutral 0.5 M NaCI solution. (a) 20 h; (b) 30 h; (c)60 h; (d)! 00 h
With immersion time extended to 30 h, a significant microstructure evolution was observed, as shown in Fig.l (b). Due to the corrosion conditions for different part of alloy is not completely consistent, an obvious evolution direction of the corrosion product microstructure was observed. Along the evolution direction, the particle structure evolves into the porous structure. With the immersion time further increased to 60 h immersion in 0.5 M NaCI solution, the surface of Zn-30Sn
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Fig.3 XRD pattern ofZn-30Sn alloy after! 00 h immersion in 0.5 M NaCI solution
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B. XPS results
For further investigation of the corrosion process of Zn30Sn, XPS method with etching was conducted and the XPS spectra of 01s and Zn2p3/2 recorded at the surface Zn-30Sn after 60 h immersion in 0.5 M NaCI solution was depicted in Fig. 4. During the whole experiment, though the peak intensity of Zn2p3/2 presents a slight change, the binding energy remains within a pretty small range at about lO2l.5 eV. When focusing on the peak of 01s, a dramatic evolution was observed. In the beginning of the experiment (etching time 0 minute), only one peak with a binding energy of 531.5 eV was observed, indicating the presence of only one chemical type of O. When etching time increased to 1 min, a completely new peak with a binding energy of 530 eV was monitored, which means the appearance of a second chemical type of O. Specifically, with the increasing of etching time, the intensity of new peak presents an increasing trend, while the original one presents a decreasing trend. These two peaks of Ols could be used to characterize ZnO and Zn(OH)2, respectively, namely, 01s at 53l.5 eV to Zn(OH)2 and 01s at 530 eV to ZnO [18,19]. The evolution of 01s peaks corresponds to the change in the content of Zn(OH)z and ZnO. Therefore, it can be concluded that the Zn(OH)2 is rich at the surface, while ZnO is rich at the inner layer.
Fig. 5 four elements depth profile ofZn-30Sn alloy after60 h immersion in 0.5 M NaCI solution
C.
Corrosion process discussion
Based on the evolution of morphology and chemical composition with different immersion time, the corrosion process of Zn-30Sn can be inferred. In the initial corrosion stage, it is the active dissolution of Zn to form particle ZnO. With the growth of immersion time, part of the ZnO would convert into Zn(OH)2, forming the mixture of ZnO and Zn(OH)2 with porous structure. And further increasing the immersion time, those porous structure would react with CI- to form complex Zns(OH)sCb·H20. IV.
Fig. 4 XPS spectra of 01s andZn2p3/2 recorded at the surfaceZn-30Sn after 60 h immersion in 0.5 M NaCI solution
Fig. 5 shows the depth profile of four elements Sn, Zn, 0 and CI after 60 h immersion. Clearly, the main chemical composition on the surface consists of 0 element and Zn element, while the concentration of Sn and CI is very low, in fact their content is close to 0, consistent with the EDXA result illustrated in Fig. 2 (3). At the surface of sample, there shows enrichment phenomenon of 01s (Zn(OH)z) concentration, and along the depth of sample, the 01s concentration presents a significant drop, at last remain stable. However, concentration of Ols (ZnO) shows an opposite trend. These two 01s peaks indicate that the corrosion products of Zn-30Sn alloy after 60 h immersion consist of the mixture of Zn(OH)2 and ZnO. Moreover, the evolution of them reveals that the formation of surface Zn(OH)2 may be related to inner ZnO.
2017 18th International Conference on Electronic Packaging Technology
CONCLUSION
The corrosion process of Zn-30Sn high-temperature alloy was investigated in neutral 0.5 M NaCI solution by SEM, EDXA,XRD and XPS. The results indicate that Zn phase is corroded preferentially, and the dominant corrosion product consists of ZnO, Zn(OHh and Zn5(OH)8Cb·H20. The possible corrosion process was proposed, hopefully providing information for corrosion resistance improvement of Zn-Sn alloy system based on the understanding of its corrosion process. V.
ACKNOWLEDGMENT
The present research was partially supported by Japan Science and Technology Agency (JST) Advanced Low Carbon Technology Research and Development Program (ALCA) project (Grant No. Jl65lO1047). Zheng-Hong Wang would like to express gratitude to the support from the Chinese Scholars Council (File No.201606210397). REFERENCES [1 ]
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