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Article

The Role of High-Temperature-Formed Surface Oxide Film in Corrosion Protection of SAC305 Solder

by
Taoyu Zhou
1,
Guanglin Zhu
1,
Cean Guo
1,* and
Xiahe Liu
2
1
School of Equipment Engineering, Shenyang Ligong University, Shenyang 110159, China
2
School of Metallurgy, Northeastern University, Shenyang 110819, China
*
Author to whom correspondence should be addressed.
Metals 2026, 16(6), 563; https://doi.org/10.3390/met16060563
Submission received: 2 April 2026 / Revised: 30 April 2026 / Accepted: 6 May 2026 / Published: 22 May 2026
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Abstract

The structural stability of high-temperature-formed oxide films (HTOFs) on SAC305 solder plays a critical role in determining corrosion reliability during long-term thermal exposure, yet the coupled effects of oxide evolution and substrate microstructure changes remain unclear. In this work, SAC305 solder was thermally aged at 150 °C for 10–60 days, and the evolution of the oxide film structure and substrate microstructure was systematically investigated using SEM, XRD, XPS, and electrochemical techniques. The results reveal that HTOF mainly consists of a SnO/SnO2-layered structure with thickness increasing slightly from approximately 16.5 nm to 18 nm, while increasing micro-cracks and Ag3Sn coarsening induced by the Kirkendall effect lead to significant reductions in impedance parameters and corrosion resistance. These findings demonstrate that the degradation of HTOF is governed by the coupled effects of oxide defect accumulation and intermetallic phase coarsening, providing a mechanistic insight into the corrosion failure of SAC305 solder under long-term thermal aging conditions.

1. Introduction

Solder joints based on Sn-Ag-Cu (SAC) alloys are widely used in modern electronic packaging due to their good mechanical properties, environmental compatibility, and reliable electrical performance [1,2]. Among them, SAC305 solder has become one of the most commonly applied lead-free solder materials in microelectronic devices [1,2]. The long-term stability of SAC solder joints plays a critical role in ensuring the reliability and service lifetime of electronic systems, especially under harsh working environments involving elevated temperatures and corrosive conditions [3,4]. Therefore, understanding the degradation behavior of SAC solder under thermal exposure is of great importance for improving its corrosion reliability.
Under high-temperature conditions, oxidation reactions inevitably occur on the surface of SAC solder, leading to the formation of oxide films [5,6]. These high-temperature-formed oxide films (HTOFs) are generally considered to provide certain barrier effects against corrosive media [7,8,9,10]. However, prolonged thermal exposure may significantly alter the structure and stability of oxide films, resulting in changes in their protective performance, and the influence of high-temperature exposure on SAC solder is highly complex and involves the coupled evolution of oxide film structure and substrate microstructure [11,12,13,14,15]. On the one hand, the oxide film may undergo structural changes such as thickening, defect formation, and cracking during thermal aging [14,15]. These structural defects can significantly reduce the compactness of the oxide film and provide diffusion channels for corrosive species [14,15]. On the other hand, thermal diffusion of alloying elements, especially Ag atoms, may lead to the coarsening of Ag3Sn intermetallic compounds within the β-Sn matrix [6,16,17]. The formation of interfacial defects caused by the Kirkendall effect and the growth of intermetallic phases may further enhance local microstructural heterogeneity [3]. The combined influence of oxide film degradation and intermetallic phase evolution is expected to play an important role in determining the long-term corrosion behavior of SAC solder.
Although many studies have investigated the oxidation behavior and corrosion resistance of SAC solder under different environmental conditions [18,19,20,21], most existing works mainly focus on either oxide formation or electrochemical response individually [8,9,10,21,22,23]. There are still limited studies that systematically clarify the coupled effects of high-temperature oxide film evolution and substrate microstructure changes on the corrosion degradation behavior of SAC305 solder. In particular, the structural stability of high-temperature-formed oxide films and their interaction with evolving intermetallic compounds remain insufficiently understood. Therefore, a detailed investigation of the dynamic evolution of HTOF and its associated corrosion mechanism is still necessary.
In this work, the structural evolution and the corrosion mechanism of HTOF on SAC305 solder during thermal aging were systematically investigated. SAC305 solder samples were subjected to thermal aging at 150 °C [9,21] for different durations in order to simulate long-term thermal exposure conditions. The evolution of oxide film structure and substrate microstructure was characterized using SEM, XRD, and XPS techniques, while the corresponding corrosion behavior was evaluated through electrochemical impedance spectroscopy and polarization measurements. Special attention was given to the coarsening behavior of Ag3Sn intermetallic compounds and the formation of interfacial defects associated with thermal diffusion processes. The objective of this work is to clarify the coupled evolution behavior of oxide film and substrate microstructure and to provide a mechanistic understanding of the corrosion degradation of SAC305 solder under long-term thermal exposure conditions.

2. Experimental

2.1. Sample Preparation

SAC305 solder alloy was used as the experimental material in this study. The nominal composition of SAC305 solder is Sn-3.0Ag-0.5Cu (wt.%). In order to obtain a relatively stable microstructure before thermal aging, the SAC305 solder specimens with dimensions of 10 mm × 10 mm × 5 mm were subjected to heat preservation at 400 °C for 0.5 h in a thermostatic drying oven, followed by furnace cooling to room temperature. Thermal aging treatment was then carried out at 150 °C [9,21] in a thermostatic drying oven under ambient air atmosphere for different durations of 10, 30, and 60 days. These samples were labeled as 10 d, 30 d, and 60 d, respectively, while the sample without thermal aging was marked as 0 d. Before subsequent tests, all samples were ground sequentially using SiC abrasive papers up to 2000 grit, and then polished using 0.5 μm diamond paste to obtain a smooth surface. After polishing, the samples were ultrasonically cleaned in ethanol and dried in air.

2.2. Morphology Characterization

The surface morphology of oxide films formed on SAC305 solder after thermal aging was observed using scanning electron microscopy (SEM; XL30 FEG, Philips Electron Optics, Eindhoven, The Netherlands). The SEM observation was conducted under an accelerating voltage of 20 kV. The elemental composition and distribution of different regions were analyzed using energy-dispersive spectroscopy (EDS) attached to the SEM system. Several typical positions were selected for elemental analysis to obtain representative compositional information of oxide films and substrates.

2.3. Composition and Thickness Characterization

The phase composition of oxide films formed on SAC305 solder after thermal aging was analyzed using X-ray diffraction (XRD; D/max 2000, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å). The diffraction patterns were collected in the 2θ range from 10° to 85°, with a scanning speed of 5°/min. The chemical composition and thickness of oxide films were further investigated using X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) equipped with a monochromatic Al Kα X-ray source. The photoelectrons were collected using a hemispherical electron energy analyzer. The survey spectra were acquired with a pass energy of 100 eV, while the high-resolution spectra of Sn 3d and O 1s were recorded with a pass energy of 30 eV. A β-Sn-rich region on the sample surface was selected for depth profile analysis. Before measurement, Ar+ ion sputtering was applied to remove surface contamination. The sputtering process was conducted with a sputtering rate of approximately 0.2 nm/s. The thickness of oxide film was estimated according to the sputtering depth corresponding to the transition from oxide signals to metallic substrate signals during the XPS depth profiling. The binding energy values were calibrated using the C 1s peak at 284.8 eV.

2.4. Electrochemical Measurement

Electrochemical measurements were carried out to evaluate the corrosion behavior of SAC305 solder after thermal aging. All electrochemical tests were performed using an electrochemical workstation (Gamry Reference 600, Gamry Instruments, Warminster, PA, USA) in a conventional three-electrode system. In the electrochemical cell, the SAC305 solder sample was used as the working electrode, a saturated calomel electrode (SCE) was used as the reference electrode, and a platinum sheet was used as the counter electrode. The exposed area of the working electrode was approximately 1.0 cm2, and the remaining surface was sealed using insulating material. The electrolyte used in this study was a 3.5 wt.% NaCl aqueous solution, which was prepared using analytical grade sodium chloride and deionized water. All electrochemical measurements were conducted at room temperature (about 25 °C). Before electrochemical measurements, the working electrode was immersed in the electrolyte and allowed to stabilize at open circuit potential (OCP) for 7200 s in order to obtain relatively stable electrochemical condition. The variation of OCP with immersion time was recorded during this period. Electrochemical impedance spectroscopy (EIS) measurements were then performed at the stabilized OCP. The frequency range was from 100 kHz to 0.01 Hz, with an AC signal amplitude of 10 mV. The impedance spectra were recorded and analyzed using the corresponding electrochemical software. Polarization measurements were conducted after the EIS test. The polarization curves were recorded from −0.15 V to 0.5 V relative to the OCP, with a scanning rate of 0.167 mV/s. The corrosion behaviors of samples with different thermal aging times were evaluated according to the polarization curves.
In addition, in order to further evaluate the influence of substrate microstructural evolution on corrosion resistance, the oxide films on selected samples were carefully removed by mechanical polishing before electrochemical testing. This method was used because it can remove the oxide film from the whole exposed area of the working electrode, which is necessary for electrochemical measurements. This comparison was designed because high-temperature aging may affect not only the structure of the surface oxide film, but also the microstructure of the underlying SAC305 substrate. Therefore, the corrosion behavior of the exposed substrate was further investigated to clarify the contribution of substrate microstructural evolution to the overall corrosion resistance. For the detail of the experiments, the OCP stabilization time for these samples was set to 300 s, and the corresponding electrochemical measurements were carried out under the same experimental conditions as described above. The microstructure of SAC305 solder substrate without oxide film covering after thermal aging was studied by optical microscopy (OM; Carl Zeiss AG, Oberkochen, Germany). The size of Ag3Sn IMCs of SAC305 solder after thermal aging was analyzed by statistical software Image J (version 1.53).

3. Results and Discussion

3.1. Surface Morphology and Microstructural Evolution of HTOF

The surface morphology of SAC305 solder after thermal aging for different durations is shown in Figure 1. As shown in Figure 1a, the surface of the sample without thermal aging (0 d) appears relatively smooth, and the typical microstructure of SAC305 solder can be clearly observed. The matrix is mainly composed of β-Sn, with dispersed Ag3Sn intermetallic compounds distributed in the Sn-rich region [24,25]. After thermal aging for 10 days, the surface morphology changes obviously, as shown in Figure 1b. Some micro-cracks begin to appear on the surface, indicating the formation and growth of oxide film during thermal aging process. With further increasing aging time to 30 days, more obvious micro-cracks can be observed on the surface, as shown in Figure 1c. The cracks become longer and more interconnected compared with those observed in the 10 d sample, suggesting that the oxide film becomes more heterogeneous with increasing aging time. When the thermal aging time reaches 60 days, as shown in Figure 1d, the surface morphology becomes much rougher and a larger number of micro-cracks are observed. The crack width and density increase significantly compared with those in shorter aging conditions. These micro-cracks may provide possible diffusion paths for electrolyte penetration during subsequent corrosion process [11,13,15]. In order to further analyze the elemental composition of different regions, EDS analysis was carried out at the selected positions shown in Figure 1b. The corresponding EDS spectra of Position 1 and Position 2 are presented in Figure 1e and Figure 1f, respectively. As shown in Figure 1e, the detected elements at Position 1 mainly include Ag, Sn, and O, indicating that oxidation occurs in the region containing intermetallic compounds. For Position 2, as shown in Figure 1f, the detected elements mainly include Sn and O, suggesting that oxidation also takes place in Sn-rich regions. Overall, the SEM and EDS results demonstrate that thermal aging at 150 °C promotes the formation of oxide film on the surface of SAC305 solder [9,11,12,13,21]. With increasing aging time, the oxide film gradually develops more micro-cracks and becomes structurally less compact, which may reduce its protective ability during corrosion exposure.

3.2. Composition and Structure of Oxide Film

The phase composition of oxide films formed on SAC305 solder after thermal aging for different durations was analyzed using XRD, and the results are shown in Figure 2. As shown in Figure 2, the 0 d sample mainly exhibits diffraction peaks corresponding to β-Sn, Ag3Sn, and Cu6Sn5, while no obvious diffraction peaks of SnO or SnO2 are detected. After thermal aging, several oxide-related diffraction peaks appear and can be assigned to SnO and SnO2 phases, confirming the formation of high-temperature oxide films during thermal aging at 150 °C. For the 10 d sample, weak diffraction peaks corresponding to SnO and SnO2 can be observed, suggesting the initial formation of the oxide film. With increasing thermal aging time to 30 d, the intensity of oxide-related diffraction peaks becomes more obvious, indicating further growth of the oxide film. When the aging time reaches 60 d, the diffraction peaks of SnO and SnO2 become more pronounced, suggesting that the oxide film continues to develop during prolonged thermal exposure. It should be noted that the diffraction peaks corresponding to Ag3Sn and Cu6Sn5 phases remain detectable in all samples after thermal aging. This indicates that these intermetallic compounds still exist in the substrate after aging treatment. Meanwhile, no obvious diffraction peaks corresponding to Ag or Cu oxides are detected in the XRD patterns, suggesting that the dominant oxide products formed during thermal aging are mainly Sn-based oxides.
In order to further investigate the chemical states and thickness evolution of oxide films formed on SAC305 solder during thermal aging, XPS analysis was performed, and the results are shown in Figure 3 and Figure 4. The high-resolution XPS spectra of Sn 3d for samples aged for 10 d, 30 d, and 60 d are shown in Figure 3a. The deconvolution data of the corresponding XPS peaks are gathered in Table 1 [9,10,21,22,26,27]. At the surface region, corresponding to the sputtering depth of 0 nm, the dominant peak corresponding to Sn4+ can be clearly observed for all samples, indicating the presence of SnO2 on the outer surface of the oxide film [11,21]. After sputtering to a depth of 6 nm, additional peaks corresponding to Sn2+ and Sn0 begin to appear, suggesting that the inner region of the oxide film mainly consists of SnO, and gradually transitions to metallic Sn substrate [14,15]. It should be noted that the binding energies of Sn0 and Sn2+ are close to each other, and partial overlap between these components may occur in the Sn 3d5/2 spectra. In the present fitting results, the binding energy difference between Sn0 and Sn2+ is about 0.5 eV, which is consistent with the commonly reported chemical shift between metallic Sn and SnO [9,10,21,22,26,27]. Similar binding energy positions of Sn0 and Sn2+ have also been reported in previous studies on tin oxides [28,29]. In addition, the thickness of the oxide film is only about 16–18 nm. After sputtering to a depth of 6 nm, the remaining oxide layer is relatively thin, and the detected Sn 3d signal may contain contributions from both the inner oxide region and the underlying metallic Sn substrate. Therefore, the simultaneous appearance of Sn2+ and Sn0 at this sputtering depth is reasonable. The corresponding high-resolution spectra of O 1s are presented in Figure 3b. The O 1s peaks for all samples are mainly located around 530 eV, which can be attributed to O2− species in metal oxides [12,30]. The presence of O-related peaks further confirms the formation of Sn-based oxide films during thermal aging. With increasing sputtering depth, the O 1s peak intensity decreases slightly, indicating the gradual transition from oxide layer to metallic substrate.
The depth distribution of elemental composition for different aging conditions is shown in Figure 4. As shown in Figure 4a–c, the atomic percent of O decreases gradually with increasing sputtering depth, while the atomic percent of Sn increases correspondingly. The dashed lines in the figures mark the transition point between the oxide layer and the substrate, which was determined according to the criterion that the oxygen content decreased below 10 at.% and was used to estimate the oxide film thickness. For the 10 d sample, the oxide film thickness is approximately 16.5 nm, as shown in Figure 4a. A similar thickness of about 16.5 nm is also observed for the 30 d sample, as shown in Figure 4b. When the aging time increases to 60 d, the oxide film thickness slightly increases to approximately 18 nm, as shown in Figure 4c. According to previous studies on Sn-based solders, the thickness of the native oxide film on the 0 d sample is usually only about 3–4 nm [9,22]. Therefore, the much larger oxide film thickness observed in the thermally aged samples confirms that thermal aging at 150 °C significantly promotes the formation and growth of high-temperature oxide films on SAC305 solder. Furthermore, it should be noted that the surface oxygen concentration of the 60 d sample is slightly lower than those of the 10 d and 30 d samples. This does not necessarily indicate a lower overall oxidation degree. After prolonged thermal aging, more micro-cracks and defects are formed in the oxide film, as shown in Figure 1. These defects may expose more underlying metallic Sn or sub-oxide regions within the XPS detection depth, resulting in a relatively lower apparent oxygen atomic percentage at the outermost surface. Meanwhile, the oxide film thickness of the 60 d sample slightly increases to about 18 nm, indicating that the overall oxidation degree still increases after long-term thermal aging.
In general, the XRD and XPS results demonstrate that the oxide film formed on SAC305 solder during thermal aging exhibits a layered structure, consisting of SnO2 on the outer surface and SnO in the inner region. With prolonged thermal aging, the oxide film becomes slightly thicker and maintains a relatively stable layered structure.

3.3. Electrochemical Corrosion Behavior of SAC305 Samples After High-Temperature Aging

3.3.1. Corrosion Behavior of SAC305 Samples with HTOF

The electrochemical corrosion behavior of SAC305 solder with HTOF covering after different thermal aging time was investigated by EIS and a potentiodynamic polarization test. The Bode plots of SAC305 solder with HTOF covering are shown in Figure 5. As shown in Figure 5a, the impedance modulus of the 0 d sample is the highest in the low-frequency region, and the corresponding impedance modulus value at 0.01 Hz (∣Z∣0.01Hz) reaches about 105 Ω·cm2, indicating that the surface oxide film provides relatively good corrosion protection before thermal aging [31,32,33]. After thermal aging for 10 days, the low-frequency impedance modulus decreases obviously, suggesting that the corrosion resistance of the sample decreases after thermal aging [34,35]. With further increasing aging time to 30 d and 60 d, the ∣Z∣0.01Hz continues to decrease, and the 60 d sample shows the lowest ∣Z∣0.01Hz among all aged samples, indicating the weakest corrosion resistance. The corresponding phase angle plots are shown in Figure 5b. It can be observed that the 0 d sample exhibits a relatively wide phase angle plateau with a high phase angle value, suggesting that the surface oxide film is relatively compact and shows obvious capacitive behavior [31,36]. After thermal aging, the phase angle curves of the aged samples shift to a lower frequency region, and the phase angle plateau becomes broader but less ideal [23,37]. This result indicates that the electrochemical response of the oxide film changes obviously after thermal aging, and the oxide film becomes less compact due to the formation of micro-cracks and structural heterogeneity. Nyquist plots were also plotted to further evaluate the impedance response and verify the applicability of the equivalent electrical circuit. As shown in Figure 5c, the Nyquist plots exhibit two time constant characteristics, one in the high-frequency region and the other in the low-frequency region, corresponding to the oxide film response and the electrochemical corrosion process, respectively. With increasing thermal aging time, the capacitive loop diameter decreases, indicating the gradual degradation of corrosion resistance for the SAC305 solder with the HTOF covering.
In order to further analyze the corrosion process of SAC305 solder with the HTOF covering, EIS data were fitted using the equivalent electrical circuit shown in Figure 6, and the fitting results are listed in Table 2 [25,31,32]. In this equivalent circuit, Rs represents the solution resistance, Ros and CPEos are related to the resistance and capacitance behavior of the oxide film, and Rct and CPEdl correspond to the charge transfer process and double-layer capacitance at the substrate/electrolyte interface. The fitting data are drawn in Figure 5, and the χ2 in Table 2 are in the magnitude of 10−3, indicating that the fitting results are reliable. As shown in Table 2, the fitted Ros value decreases significantly from 9.502 × 104 Ω·cm2 for the 0 d sample to 0.799 × 104 Ω·cm2 for the 60 d samples. At the same time, the Rct value also decreases obviously from 5.512 × 104 Ω·cm2 for the 0 d sample to 0.517 × 104 Ω·cm4. These results indicate that both the protective ability of the oxide film and the corrosion resistance of the substrate decrease with an increasing thermal aging time. Meanwhile, the values of Qos and Qdl increase after thermal aging, especially for the 30 d and 60 d samples. This result suggests that the oxide film becomes less dense and the electrolyte penetration into the oxide film/substrate interface becomes easier. Combined with the SEM results in Figure 1, it can be considered that the formation of micro-cracks and the increase in structural heterogeneity in HTOF are responsible for the decrease in corrosion resistance.
Table 2. Fitting parameters from EIS results of the high-temperature-aged SAC305 solder (with HTOF covering) measured in 3.5 wt.% NaCl electrolyte using the equivalent electrical circuits in Figure 5.
Table 2. Fitting parameters from EIS results of the high-temperature-aged SAC305 solder (with HTOF covering) measured in 3.5 wt.% NaCl electrolyte using the equivalent electrical circuits in Figure 5.
Aging Time
(d)		Rs
(Ω·cm2)		Ros × 104
(Ω·cm2)		Qos × 10−5
(F·cm−2·sn−1)		nosRct × 104
(Ω·cm2)		Qdl × 10−5
(F·cm−2·sn−1)		ndlχ2 × 10−3
03.859.5021.3480.94475.5120.6220.99633.04
104.122.00928.930.93771.9381.5770.85966.18
302.890.97948.550.93380.56723.310.85554.26
603.540.79938.940.89290.51725.830.81554.76
The potentiodynamic polarization curves of SAC305 solder with the HTOF covering are shown in Figure 7. As shown in the figure, the polarization behavior changes obviously after thermal aging. Compared with the 0 d sample, the aged samples exhibit more positive corrosion potential, while the anodic and cathodic branches also show visible changes. Although the difference in the anodic branch is not very large, the overall polarization behavior indicates that the electrochemical protective effect of HTOF decreases after long-term thermal aging. In particular, the aged samples show higher current responses in the polarization process, suggesting a reduction in corrosion resistance after thermal exposure. Overall, the electrochemical results demonstrate that the HTOF formed on SAC305 solder before thermal aging can provide certain corrosion protection in a 3.5 wt.% NaCl solution. However, with increasing thermal aging time, the protective ability of HTOF decreases gradually. Table 3 summarizes the corrosion potential (Ecorr) and corrosion current density (icorr) obtained by Tafel fitting, where the decreased icorr is observed. Furthermore, the obtained Ecorr is similar to the OCP value obtained before the polarization measurement (Table 3). In this case, the decreased icorr is mainly related to the structural evolution of the oxide film during thermal aging [11,25,36], especially the formation of micro-cracks and the decrease in film compactness, which facilitate the penetration of electrolytes and accelerate the corrosion process of SAC305 solder.
Table 3. The Ecorr and icorr obtained by Tafel fitting and the OCP data obtained before polarization test.
Table 3. The Ecorr and icorr obtained by Tafel fitting and the OCP data obtained before polarization test.
Aging Time (d)βaβcEcorr (V vs. SCE)icorr (×10−8 A·cm−2)OCP (V vs. SCE)
0 d0.0500.057−0.5163.23−0.492
10 d0.0260.23−0.43210.05−0.453
30 d0.0110.13−0.44722.32−0.478
60 d0.01720.16−0.43098.40−0.449

3.3.2. Corrosion Behavior of SAC305 Samples with HTOF

In practical corrosion environments, the protective oxide film formed during thermal aging may gradually degrade or be locally damaged, leading to the direct exposure of substrate to corrosive electrolytes [11,36]. Therefore, it is necessary to investigate the corrosion behavior of SAC305 substrate without an oxide film covering in order to clarify the effect of substrate microstructure evolution on corrosion resistance. In order to further investigate the effect of substrate microstructure evolution on corrosion behavior, the oxide film was removed before electrochemical testing, and the corresponding microstructures of substrate are shown in Figure 8 and Figure 9. As shown in Figure 8a, the microstructure of the 0 d sample mainly consists of fine Ag3Sn intermetallic compounds distributed in the β-Sn matrix. The Ag3Sn phases exhibit a relatively small size and uniform distribution. After thermal aging for 10 days, as shown in Figure 8b, the Ag3Sn phases begin to grow and become slightly coarser compared with those in the 0 d sample. With further increasing aging time to 30 days, as shown in Figure 8c, the Ag3Sn phases become more obviously coarsened, and the spacing between adjacent intermetallic phases increases. When the aging time reaches 60 days, as shown in Figure 8d, the Ag3Sn phases show significant coarsening and aggregation behavior, and the intermetallic phases become much larger compared with those in shorter aging conditions. These results indicate that prolonged thermal aging promotes the coarsening of Ag3Sn intermetallic compounds in SAC305 solder [6,36]. In order to quantitatively evaluate the size evolution of Ag3Sn phases, the statistical results of minor axis size distribution are presented in Figure 9. As shown in the figure, the 0 d sample mainly contains Ag3Sn phases with sizes in the range of 1–2 μm, accounting for the highest percentage. After thermal aging for 10 days, the proportion of Ag3Sn phases with sizes of 2–3 μm increases obviously. With further increasing aging time to 30 days and 60 days, the fraction of larger Ag3Sn phases in the range of 3–4 μm and 4–5 μm increases gradually, while the fraction of smaller-sized Ag3Sn phases decreases correspondingly. This result further confirms that thermal aging leads to the continuous coarsening of Ag3Sn intermetallic compounds.
The electrochemical impedance spectra of samples without HTOF coverings are shown in Figure 10a,b. Compared with the samples with the oxide film covering, the impedance modulus of samples without the oxide film is generally lower, indicating a reduced corrosion resistance after removing the protective oxide film. As shown in Figure 10a, the ∣Z∣0.01Hz of the 0 d sample is slightly higher than that of the aged samples, suggesting relatively better corrosion resistance for the unaged substrate. With increasing aging time, the ∣Z∣0.01Hz gradually decreases, and the 60 d sample exhibits the lowest impedance value among all samples. The corresponding phase angle plots shown in Figure 10b indicate that the phase angle plateau becomes slightly narrower with increasing aging time, suggesting that the electrochemical response becomes less capacitive and more resistive after prolonged thermal aging. Furthermore, as shown in Figure 10c, the Nyquist plot displays a large capacitive arc, indicating that the sample after the removal of the HTOF exhibits the characteristic of being covered by a native oxide film. With the increase in aging time, the extension of this capacitive arc decreases, indicating the decreased corrosion resistance. Therefore, the EEC as shown in Figure 10d is employed to fit the EIS data, and the fitted parameters are summarized in Table 4. The fitted Rct value decreases gradually from 17.23 × 104 Ω·cm2 for the 0 d sample to 15.91 × 104 Ω·cm2 for the 60 d sample. Meanwhile, the Ros value also shows a decreasing trend with increasing aging time. These results indicate that the corrosion resistance of the substrate decreases gradually after thermal aging. Combined with the microstructural observations shown in Figure 8 and Figure 9, it can be considered that the coarsening of Ag3Sn intermetallic compounds during thermal aging plays an important role in the deterioration of corrosion resistance. The increase in the Ag3Sn phase size may lead to microstructural inhomogeneity and facilitate localized corrosion behavior in SAC305 solder substrate.

3.4. Properties’ Evolution and Mechanism of HTOF on SAC305

Based on the experimental results presented above, the structural evolution and failure mechanism of the high-temperature-formed oxide film (HTOF) on SAC305 solder during thermal aging can be further discussed (Figure 11). High temperature plays a critical role in the structural evolution of HTOF, and the influence of thermal exposure at 150 °C significantly accelerates the dynamic evolution of both the oxide film and the substrate microstructure as a function of aging time [6,36]. Before thermal aging, a relatively intact native oxide film exists on the solder surface, presenting good compactness and providing initial corrosion protection [23,38]. At this stage, the Ag3Sn phase is randomly distributed within the β-Sn matrix in rod-like and plate-like morphologies, and the interface between Ag3Sn and β-Sn remains structurally intact without observable defects. After thermal aging for 10 d, the fresh SAC305 solder surface further reacts with oxygen to form a high-temperature oxide film composed mainly of SnO and SnO2, as confirmed in Figure 2, Figure 3 and Figure 4. The oxidation process proceeds through the sequential transformation of Sn into SnO and subsequently into SnO2, resulting in the formation of a bilayer-type oxide structure [11,12,15]. The thickness of this oxide film increases to approximately 16.5 nm, as indicated in Figure 4, suggesting the formation of a relatively stable HTOF during the early stage of thermal aging. Meanwhile, significant microstructural changes occur within the substrate [6,36]. As shown in Figure 8, defects gradually appear at the Ag3Sn/β-Sn interface, which can be attributed to the Kirkendall effect arising from unequal diffusion rates between Ag and Sn atoms. In addition, the thermal diffusion of Ag atoms within β-Sn promotes the coarsening of Ag3Sn intermetallic compounds, as confirmed by the statistical results in Figure 9, where the minor axis of Ag3Sn increases noticeably after thermal aging [6,36]. The growth of Ag3Sn phases introduces local stress concentration and structural heterogeneity, which weakens the uniformity and compactness of the overlying oxide film. The electrochemical results shown in Figure 5, Figure 6 and Figure 7 and Table 2 further confirm that the coarsening of Ag3Sn is unfavorable to corrosion resistance, leading to reduced impedance values compared with the 0 d sample. With further thermal aging to 30 d and 60 d, the thickness of HTOF shows little increase, as indicated in Figure 4, suggesting that oxide growth gradually reaches a quasi-steady state. However, the structural stability of the oxide film continues to deteriorate. As shown in Figure 1, increasing numbers of micro-cracks and defects develop within the oxide film, indicating the accumulation of internal stress during prolonged thermal exposure. The difference in the Pilling–Bedworth ratios between SnO and SnO2 should also be considered. The formation of SnO and SnO2 is accompanied by different degrees of volume expansion relative to metallic Sn, which may generate internal growth stress within the oxide film. Since the HTOF consists of a SnO/SnO2 layered structure, the mismatch in volume expansion between the inner and outer oxide layers may further weaken the adherence of the oxide film. During prolonged thermal aging, the accumulation of internal stress promotes the development of micro-cracks and defects, leading to the degradation of oxide film compactness and corrosion protection [11,12,15]. Furthermore, the failure of HTOF in the NaCl solution can be reasonably explained by the point defect model (PDM) proposed by Macdonald [39]. In chloride-containing environments, structural defects within the degraded HTOF provide diffusion pathways for Cl− ions. Chloride ions are preferentially adsorbed at oxygen vacancies ( V O · · ) within the oxide lattice and promote the generation of cation vacancies through defect reactions. These vacancies migrate through the oxide film toward the film/substrate interface and accumulate at localized regions, eventually leading to the breakdown of the oxide structure. In this process, Cl− ions function as catalytic species that accelerate defect generation and transport, thereby promoting the progressive degradation of the oxide film. Furthermore, the continuous coarsening of Ag3Sn intermetallic compounds enhances microstructural heterogeneity within the substrate, which increases the tendency for micro-galvanic corrosion during electrochemical exposure [37]. This effect is confirmed by the electrochemical results of the substrate shown in Figure 10 and Table 4, where the corrosion resistance decreases progressively with increasing aging time. In general, the degradation of HTOF during thermal aging is governed by a coupled mechanism involving oxide film growth stabilization, defect accumulation, interfacial degradation induced by the Kirkendall effect, and Ag3Sn coarsening-driven microstructural instability. These synergistic effects lead to the gradual loss of structural integrity of HTOF and ultimately result in the deterioration of corrosion resistance of SAC305 solder with increasing thermal aging time.

4. Conclusions

The high temperature oxide film forms on the SAC305 solder surface during thermal aging in 150 °C dry atmosphere for 10 d, 30 d and 60 d. The aging process affects the structure evolution dynamic of the high temperature oxide film thereby making an impact on the corrosion behavior of the film. By characterizing the above two aspects, the following conclusions are obtained:
  • The high temperature oxide film forming on the SAC305 solder surface is composed of SnO2 and SnO. The outer layer is mainly composed of SnO2 and the inner layer is mainly composed of SnO. As a function of thermal aging time, the change of thickness of the film is much tinier, locating in a dynamic stable state.
  • High temperature provides the relative energy for the diffusion of Sn and Ag atoms, resulting in the emergence of defects at the interface of Ag3Sn/β-Sn and the coarsening of Ag3Sn. Moreover, defects are increasingly damaging with the extension in aging time. And the coarsening of Ag3Sn increases as a function of aging time, which may prompt the development of defects and the galvanic corrosion behavior of the solder substrates.
  • The structure of the high-temperature oxide film seriously deteriorates after thermal aging. The corrosion resistance in the NaCl solution of the oxide film forming on SAC305 solder after aging is lower than that without thermal aging, and the corrosion resistance decreases as a function of aging time. The results indicate that the oxide film after aging for a long time is not benefit to the corrosion protective of the SAC305 solder substrate.

Author Contributions

Conceptualization, T.Z., G.Z. and C.G.; Methodology, T.Z. and G.Z.; Software, X.L.; Formal analysis, T.Z., G.Z. and X.L.; Investigation, T.Z. and G.Z.; Data curation, T.Z. and X.L.; Writing—original draft, T.Z. and C.G.; Writing—review & editing, C.G. and X.L.; Visualization, G.Z.; Supervision, C.G.; Project administration, C.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Advanced Materials-National Science and Technology Major Project (2024ZD0601100), Advanced Materials-National Science and Technology Major of the Joint Science and Technology Program of Liaoning Province (2025JH2/101800434), Research Project of the Education Department of Liaoning Province of China (1030040000675), Research support funding for attracting high-level talents in 2025 (6030109777), 2026 Liaoning Province Doctoral Research Launch Project, Shenyang Municipal Special Project for Scientific and Technological Talents (RC23095), Special Fund of Basic Scientific Research Operating Expense of Undergraduate Universities in Liaoning Province (LJ212410144077).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Surface morphology and the corresponding composition evolution of SAC305 solder after thermal aging for different durations: (a) 0 d, (b) 10 d, (c) 30 d, (d) 60 d, (e,f) the EDS results.
Figure 1. Surface morphology and the corresponding composition evolution of SAC305 solder after thermal aging for different durations: (a) 0 d, (b) 10 d, (c) 30 d, (d) 60 d, (e,f) the EDS results.
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Figure 2. The XRD pattern of the high-temperature-aged SAC305 samples with HTOFs.
Figure 2. The XRD pattern of the high-temperature-aged SAC305 samples with HTOFs.
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Figure 3. The composition of HTOF of the SAC305 solder detected by XPS: high-resolution spectra of Sn 3d5/2 (a) and O 1s (b) at a sputtering depth of 0 nm and 6 nm in HTOF formed on SAC305 solder.
Figure 3. The composition of HTOF of the SAC305 solder detected by XPS: high-resolution spectra of Sn 3d5/2 (a) and O 1s (b) at a sputtering depth of 0 nm and 6 nm in HTOF formed on SAC305 solder.
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Figure 4. Atomic percent evolution of Sn and O elements as a function of sputtering depth in the HTOF obtained on SAC305 solder substrate surface as the aging time evolves in 150 °C dry atmosphere, where (a) the 10 d sample, (b) the 30 d sample, (c) the 60 d sample. Note the dash line in the figure represents the oxide film/substrate interface.
Figure 4. Atomic percent evolution of Sn and O elements as a function of sputtering depth in the HTOF obtained on SAC305 solder substrate surface as the aging time evolves in 150 °C dry atmosphere, where (a) the 10 d sample, (b) the 30 d sample, (c) the 60 d sample. Note the dash line in the figure represents the oxide film/substrate interface.
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Figure 5. The EIS results of SAC305 solder with HTOF covering after thermal aging for a variety of times, where (a) the Bode-phase angle plot, (b) the Bode-modulus plot, (c) the Nyquist plot.
Figure 5. The EIS results of SAC305 solder with HTOF covering after thermal aging for a variety of times, where (a) the Bode-phase angle plot, (b) the Bode-modulus plot, (c) the Nyquist plot.
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Figure 6. Equivalent electrical circuit for fitting EIS data.
Figure 6. Equivalent electrical circuit for fitting EIS data.
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Figure 7. Potentiodynamic polarization curves of SAC305 solders with HTOF covering after thermal aging for a variety of times.
Figure 7. Potentiodynamic polarization curves of SAC305 solders with HTOF covering after thermal aging for a variety of times.
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Figure 8. The geometry of Ag3Sn in SAC305 solder as a function of thermal aging time in 150 °C dry atmosphere, where (a) the 0 d sample, (b) the 10 d sample, (c) the 30 d sample, (d) the 60 d sample.
Figure 8. The geometry of Ag3Sn in SAC305 solder as a function of thermal aging time in 150 °C dry atmosphere, where (a) the 0 d sample, (b) the 10 d sample, (c) the 30 d sample, (d) the 60 d sample.
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Figure 9. The percentage of the variety rank of Ag3Sn minor axis after thermal aging for different times.
Figure 9. The percentage of the variety rank of Ag3Sn minor axis after thermal aging for different times.
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Figure 10. The electrochemical test results of SAC305 solder surface without HTOF covering after thermal aging for a variety of times: (a) impedance plots; (b) phase angle plot; (c) Nyquist plot; (d) equivalent electrical circuit for fitting EIS data.
Figure 10. The electrochemical test results of SAC305 solder surface without HTOF covering after thermal aging for a variety of times: (a) impedance plots; (b) phase angle plot; (c) Nyquist plot; (d) equivalent electrical circuit for fitting EIS data.
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Figure 11. The mechanism regarding the failure of SAC305 solder under high-temperature environment.
Figure 11. The mechanism regarding the failure of SAC305 solder under high-temperature environment.
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Table 1. Binding energy (Eb) values and full width at the half-line maximum (FWHM) values used for analyzing XPS spectra of the HTOF on SAC305 solder, expressed as mean ± standard deviation, adapted from Refs. [9,10,21,22,26,27].
Table 1. Binding energy (Eb) values and full width at the half-line maximum (FWHM) values used for analyzing XPS spectra of the HTOF on SAC305 solder, expressed as mean ± standard deviation, adapted from Refs. [9,10,21,22,26,27].
SpeciesChemical StateEb (eV)FWHM (eV)
Sn4+Sn 3d5/2486.3 ± 0.11.63 ± 0.32
Sn2+Sn 3d5/2485.1 ± 0.21.35 ± 0.08
Sn0Sn 3d5/2484.6 ± 0.21.13 ± 0. 3
O2−O 1s530.4 ± 0.22.02 ± 0.52
Table 4. Fitting parameters from EIS results of the high-temperature-aged SAC305 solder (without HTOF covering) measured in 3.5 wt.% NaCl electrolyte using the equivalent electrical circuits in Figure 10.
Table 4. Fitting parameters from EIS results of the high-temperature-aged SAC305 solder (without HTOF covering) measured in 3.5 wt.% NaCl electrolyte using the equivalent electrical circuits in Figure 10.
Aging Time
(d)		Rs
(Ω·cm2)		Ros × 104
(Ω·cm2)		Qos × 10−5
(F·cm−2·sn−1)		nosχ2 × 10−3
01.822.1581.8620.90851.16
102.492.0951.9530.90561.27
304.691.9532.0120.90461.36
603.461.7522.1590.90240.81
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Zhou, T.; Zhu, G.; Guo, C.; Liu, X. The Role of High-Temperature-Formed Surface Oxide Film in Corrosion Protection of SAC305 Solder. Metals 2026, 16, 563. https://doi.org/10.3390/met16060563

AMA Style

Zhou T, Zhu G, Guo C, Liu X. The Role of High-Temperature-Formed Surface Oxide Film in Corrosion Protection of SAC305 Solder. Metals. 2026; 16(6):563. https://doi.org/10.3390/met16060563

Chicago/Turabian Style

Zhou, Taoyu, Guanglin Zhu, Cean Guo, and Xiahe Liu. 2026. "The Role of High-Temperature-Formed Surface Oxide Film in Corrosion Protection of SAC305 Solder" Metals 16, no. 6: 563. https://doi.org/10.3390/met16060563

APA Style

Zhou, T., Zhu, G., Guo, C., & Liu, X. (2026). The Role of High-Temperature-Formed Surface Oxide Film in Corrosion Protection of SAC305 Solder. Metals, 16(6), 563. https://doi.org/10.3390/met16060563

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