Corrosion Resistance of Multilayered Sn/Ag3Sn Films Electroplated on Cu Alloys for Highly Reliable Automotive Connectors

Sn ﬁlms showed low and stable electrical contact resistance equivalent toas-plated samples even after aging at 473 K for 3000 h, whereas the conventional Ag and reﬂowed Sn ﬁlms delivered unstable andever-increasing contact resistances at 500 h. The excellent corrosion resistance of the multilayered Sn/Ag

In the past, automotive connectors mounted in various electric control units (ECU) were used at temperatures up to 473 K, or Class II -III referring to the standards of United States Council for Automotive Research (USCAR). [1][2][3] Conventional coating materials on Cu alloy base materials, e.g., various tin-based films like reflowed Sn (electrodeposited Sn that has been melted and resolidified), electroplated Sn and hot-dip Sn films, and the noble-metal-based coatings like Ag, Au-Co, Au-Ni, and Au/Pd/Ni electroplating films, have been used in automobile applications to meet the demands at temperatures below 473 K. 1 Recently, with the development of automatic electronic systems equipped on automobiles, the use of electronic and/or electric devices is continuously growing to meet various needs for security and convenience. Accordingly, more sensors and junction boxes connected to various electronic devices are increasingly mounted in engine compartments close to combustion chambers or near the exhaust systems. This requires that the materials for connectors and sensors withstand higher temperature of at least 453-473 K (USCAR-Class V) and a more corrosive environment than usual throughout the lifetime of vehicles. However, the conventional coating materials discussed above cannot meet the ever-increasing requirements for the harsher environment. Therefore, exploring new coatings with stable contact performance for automotive connectors is essential for higher functionality and higher reliability of electric control units.
Various Sn-based coating materials are mostly used for regular connectors because of the low cost, good soldering ability, and excellent ductility, yet the applicable temperature for stable electric contact resistance is usually up to 393 K. It is known that the failure of Snbased coating materials as automotive connectors at high temperature, interpreting by the phenomenon of ever-increasing contact resistance with aging, can be attributed to the formation of insulating Cu oxide films on the surface by the fast Cu diffusion from the base materials. 4 To solve this problem, a Ni underlayer electroplated beneath the Sn film has been adopted as a barrier layer to prevent the diffusion of Cu onto the surface. 2 The Ni barrier layer, however, can only maintain stable contact resistance up to 423 K, above which Ni diffusion also occurred toward the surface and a more insulating Cu-Sn-Ni oxide film would form at high temperature. Recently, several works have been reported on applications of a multilayered Ni (1-3 µm)/Pd (0.3-1 µm)/Au (50-100 nm) coating on Cu for high reliable automotive connectors at high temperature. [5][6][7] Despite of high cost, the high temperature automotive reliability of the noble metal coatings as special connectors was assessed to be equivalent to 423 K for 5000 h 6 or 448 K for 500 h. 8 Electrodeposited pure Ag films on Cu alloys are widely used in various electronic devices due to their excellent electric conductivity and good fretting performance. However, silver sulfide corrosion has been a big concern in various industrial applications, not only in the automotive industry, but also in vehicle exhaust fumes (exit/entrance ramps), rubber manufacturing, sewage/waste-water treatment plants, petroleum refineries, coal-generation power plants, large-scale farms, etc. 9,10 In addition, other disadvantages of pure Ag films included electro-migration and softening because of recrystallization of Ag at temperature above 373 K. 11 The present study reports a new solution to use a multilayered Sn/Ag 3 Sn film on a Cu alloy base material, which can be produced at lower cost than conventional pure Ag coatings. The multilayered Sn/Ag 3 Sn films demonstrated a high reliability of stable electric contact resistance at different aging temperatures, good fretting performance, and excellent resistance to the growth of Sn whiskers, thus being thought as a promising coating material for various electric connectors and terminals. [12][13][14][15] This paper focuses on evaluation of the corrosion resistance, i.e., sulfidizing resistance and oxidation resistance of the multilayered Sn/Ag 3 Sn films, compared to a commercial Ag film with 2 µm thickness and a conventional reflowed Sn film with 1 µm thickness. The surface states and microstructures of the multilayered Sn/Ag 3 Sn films before and after accelerated corrosion tests were investigated by various analysis methods and the mechanism of improved corrosion resistance was discussed.

Experimental
Specimens.-The multilayered Sn/Ag 3 Sn specimens were produced by consecutive electrodeposition of Sn and Ag films on Cu alloys. Briefly, a Sn film with 1 µm thickness was first electrodeposited on Cu alloys using a sulfuric bath, which mainly contained 0.25 M SnSO 4 and 1.0 M H 2 SO 4 , followed by a heat-treatment at around 593 K to get a reflowed Sn film (denoted as Sn-RF later). Here, the Sn-RF film means that the electroplated Sn films was heated at a temperature above the melting point of metallic Sn, 505 K, to achieve a bright Sn film with smooth surface, better soldering ability, and more importantly, to inhibit the growth of Sn whiskers. After the C442 Journal of The Electrochemical Society, 161 (10) C441-C449 (2014) reflowed treatment, silver electroplating was performed on the Sn-RF specimens to achieve Ag films with 30-80 nm thickness using a cyanide bath, which mainly contained 0.1 M AgCN and 1.0 M KCN, followed by an alloying treatment below 373 K, to achieve a Sn-Ag alloy film on the Sn-RF film. Moreover, a commercial Ag film with 2 µm thickness and the Sn-RF film without Ag electroplating were used in various corrosion tests as reference samples.
Corrosion evaluation methods.-The corrosion resistance against sulfidization of the coating materials mentioned above was evaluated by immersing the specimens in 0.2 or 2 mL/L (NH 4 ) 2 S x solution (Japan Industrial Standard JIS-H 8621) at 298 K for 30 min -120 h. The high temperature oxidization resistance or high temperature reliability for automotive connectors was assessed by aging in air at 473 K up to 3000 h, according to the Class-V standard of The United States Council for Automotive Research (USCAR).
The electrical contact resistance of the specimens before and after corrosion tests was measured by an electric contact simulator (Yamazaki, CRS-113-AU), in a 4 terminal method vs. Au wire, using a sliding mode with a distance of 0.5 mm, at low load from 0 to 0.49 N. A 'dimple on flat' sample style was adopted for contact pairs, which is similar to many real connectors with 'male' and 'female' contacts. Here, a sliding mode instead of a static mode was chosen to simulate the real movements of automotive connectors or terminals due to the vibration during driving and idling. A lower load (or small clapping force) of 0.49 N/pin compared to normal load of 5 N/pin was selected as a critical assessment for promising applications as mini-terminals and multi-pin type connectors or terminals. 13,16 Characterizations.-The morphologies of the surface and the cross-sections of the multilayered Sn/Ag 3 Sn films and reference specimens before and after various corrosion tests were observed by field emission scanning electron microscopy (FE-SEM: JEOL-JSM-7001F) with X-ray energy-dispersive analysis (EDS) at accelerated voltages of 10-15 kV. The specimens for cross-sectional FE-SEM observation were first polished by a diamond paste solution (0.1 µm), and then cleaned in ultrasonic bath for 10 min in acetone, and finally polished using Ag + ion beam sputtering in form of flat-milling method. The crystalline structures of the specimens were determined by a grazing incidence X-ray diffractometer (XRD: Rigaku-2000, 35 kV/ 30 mA, Cu Kα). Moreover, the texture and crystal structures for the multilayered Sn/Ag 3 Sn film after aging at 473 K for 500 h was observed by a transmission electron microscope (TEM: JEOL-JEM-2100) equipped with X-ray energy-dispersive detector at 200 kV. The specimen for the TEM observation was prepared by focused ion beam (FIB) after evaporation of carbon layer on the film surface. Furthermore, the chemical states before and after corrosion tests were analyzed by an X-ray photoelectron spectroscope (XPS: ULVAC-PHI-5600CIM, Al Kα), which was performed using a five channeltron analyzer (VG Scientific ESCALAB 200-X) in an operating vacuum better than 1 × 10 −9 torr (1 torr ∼ = 133 Pa). The binding energy of the spectra was corrected by the C 1s signal at 284.5 eV, with an accuracy of within 0.1 eV. The element distributions along the film thickness of the various specimens were investigated by a glow discharge optical emission spectrometry (GDOES: HORIBA-JY-5000RF, φ4 mm, 13.56 MHz, 35 W, Ar-600 Pa). Moreover, cyclic voltammetry (CV) measurements of multilayered Sn/Ag 3 Sn and Sn-RF films before and after aging were carried out using an Electrochemical Measurement System (Hokuto Denko -HZ-5000) at a scan rate of 10 mV s −1 in a buffer solution composing of 0.5 M NH 4 Cl and 0.5 M NH 4 OH at room temperature, with a pure Pt plate as the counter electrode and an Ag/AgCl (3M NaCl) as the reference electrode. 17,18

Results and Discussion
Microstructures of multilayered Sn/Ag 3 Sn films on Cu alloys.- Figure 1 exhibits the as-deposited multilayered Sn/Ag 3 Sn, Sn-RF, and Ag films on Cu alloys substrates before corrosion testing. The Sn-RF specimen constitutes a smooth Sn film with uneven thickness and a unique Cu-Sn intermetallic compound layer in form of Cu 6 Sn 5 . The Cu 6 Sn 5 was formed on Cu substrates due to the reflow treatment at high temperature above the Sn melting point (Fig. 1a, 1d). The multilayered Sn/Ag 3 Sn specimen consists of an Ag-Sn alloy layer with thickness of 50 nm, and a Sn layer and a Cu 6 Sn 5 layer similar to the Sn-RF specimen. It should be noted that the multilayered Sn/Ag 3 Sn film is different from conventional Sn-Ag alloy electroplating [19][20][21][22][23][24] and commercial Sn-Ag hot-dip coatings, 1,25 in which the Ag component (the former) or the Ag 3 Sn particles (the latter) are distributed the whole thickness of coatings. The multilayered Sn/Ag 3 Sn film in the present study was designed to utilize effectively the highly conductive Ag alloy film on the top layer and to minimize Ag usage by 1/40. Moreover, the Ag-Sn alloy layer was composed of nano-grains 50-100 nm across (Fig. 1b), which are much smaller than the Ag grains observed in a commercial Ag film that are 200 nm -2 µm across ( Fig. 1c, 1f). From the cross-sectional image (Fig. 1e), the Ag-Sn alloy layer in nano-grains on the Sn film agglomerated into  nano-flakes around 200 nm, which can be ascribed to the sampling process for FE-SEM observation. Figure 2 gives XRD patterns for a multilayered Sn/Ag-80 nm film and a Sn-RF film before Ag electroplating. Except for the Sn peaks corresponding to the Sn layer, all of the peaks that relate to Ag component for the multilayered Sn/Ag specimen are indexed to be ε−Ag 3 Sn intermetallic compound. This indicates that the Ag electrodeposit was completely alloyed with Sn layer by the alloying treatment after the Ag electroplating. The Ag 3 Sn intermetallic compound has a hexagonal close-packed structure with lattice constants of a = 2.995 Å, c = 4.780 Å, c/a = 1.596, and presents a preferential <111> orientation. It is known that Ag readily reacts with sulfur and forms silver sulfide. Hence, it is essential to transform Ag electrodeposits to Ag 3 Sn alloy completely for high corrosion resistance in various circumstances. Figure 3 illustrates the influence of immersion time in a 0.2 mL/L (NH 4 ) 2 S x solution on the electric contact resistance of the multilayered Sn/Ag 3 Sn films, compared to a pure Ag film. The contact resistance of the pure Ag film increased greatly with immersion time, and eventually failed after 120 h, beyond the measurable range of the equipment in 2 or permissible contact resistance in practical applications. The multilayered Sn/Ag 3 Sn films, on the other hand, exhibited stable contact resistance around 1 m until 95-h immersion, and changed to around 2 m even after being immersed for 120 h, indicating an excellent sulfidizing resistance. Moreover, it was also observed from the appearance of specimens before and after sulfidizing tests for different times that the multilayered Sn/Ag 3 Sn film kept its mirror-like brightness and exhibited little change even after being immersed for 95 h, while the Ag film lost its metallic luster within 30 min, changing its color from silvery to brown and dark blue (see inserts in the Fig. 3). Figure 4 gives the GDOES spectra showing the element distribution across the film thickness for the multilayered Sn/Ag 3 Sn and Ag films before and after a sulfidizing test. As the specimen was immersed in 2 mL/L (NH 4 ) 2 S x solution for 60 min, a small amount of sulfur was detected on the surface of multilayered Sn/Ag 3 Sn film (Fig. 4d). In contrast, a strong sulfur peak was detected on the pure Ag film (Fig. 4e). From the relative intensity and the corresponding sputtering time of sulfur profiles in regions shown in dashed line, it can be deduced that only a thin Ag 2 S film formed on the top of the Ag 3 Sn layer, but a thick Ag 2 S film formed on the pure Ag film. This phenomenon can be explained by the chemical reactions between sulfur and silver in the (NH 4 ) 2 S x solution as discussed later. From the viewpoint of chemical kinetics, intermetallic compounds (i.e., Ag 3 Sn) are generally more stable than pure metals (i.e., Ag). Therefore, the alloying treatment after Ag electroplating played an important role in enhancing the sulfidizing resistance of the multilayered Sn/Ag 3 Sn film. For comparison, GDOES spectra for Sn-RF films before and after the sulfidizing test are also given in Fig. 4. A strong sulfur peak was detected for the specimens after 60-min immersion, which can be ascribed to the formation of the CuS from the element distribution (Fig. 4f). This indicated that, although a thin Sn oxide film existed on the surface of initial Sn-RF specimen, the sulfidizing corrosion progressed through the Sn top layer into the underlying Cu 6 Sn 5 alloy layer (Fig. 4c).

Sulfidizing resistance of multilayered Sn/Ag 3 Sn films.-
In order to investigate the mechanism of sulfidizing resistance of the multilayered Sn/Ag 3 Sn film, high-resolution XPS analysis was used to further determine the chemical states of the surface components for specimens before and after immersing in 0.2 mL/L (NH 4 ) 2 S x solution for 7 h and 95 h, as shown in Fig. 5. In good accordance with the GDOES analysis results, a strong S peak corresponding to Ag 2 S was detected on the Ag film after immersion for 7 h, while a weak S peak with less than 1/5 intensity was measured on the multilayered Sn/Ag 3 Sn film even after a prolonged immersion for 95 h. This confirms that the sulfidizing resistance of the multilayered Sn/Ag 3 Sn film is apparently advantageous over the conventional Ag film. Moreover, it is noticeable that strong Sn and O peaks were present on both the initial multilayered Sn/Ag 3 Sn specimen and the ones after sulfidizing tests. The binding energy values of the centers of Sn 3d 5/2 peaks are 486.8, 487.1, and 487.3 eV for the specimens before immersion and after immersion for 7 and 95 h, respectively, which close to those of SnO (E b = 486.0-486.8 eV) and SnO 2 (E b = 486.4-486.9 eV). 26 This indicates that tin oxide films, probably in forms of SnO and/or SnO 2 , existed on the surface of multilayered Sn/Ag 3 Sn specimens irrespective of the sulfidizing test. The tin oxide films on multilayered Sn/Ag 3 Sn specimens can be attributed to the oxidization of Ag 3 Sn layer during the alloying treatment after Ag electroplating. It appears reasonable to hypothesize that it is the tin oxide films that protected the Ag 3 Sn layer by isolating the Ag in Ag 3 Sn layer from the corrosive S 2− ions in the test solution.
Moreover, with increasing immersion time from 7 to 95 h, the peak centers for Sn 3d 5/2 shifted from 486.8 to 487.3 eV. This infers an increase in the SnO 2 component of the tin oxide film, possibly through the following reactions: The SnO 2 film, in turn, also protected the Ag 3 Sn, thus hindered the sulfidizing reaction. In contrast, strong sulfur peaks were detected on the pure Ag film after being immersed for 7 h, indicating the formation of a large amount of Ag 2 S simply by a chemical reaction between Ag and S 2− ions as below: It should be noted here that a Sn oxide film also existed on the Sn-RF specimen due to the solidifying process of electrodeposited Sn, yet it did not provide enough protection from the corrosion of chemically active Sn and the underlying Cu 6 Sn 5 in ammonia sulfide solution (Fig. 4f). Since it is difficult to differentiate SnO and SnO 2 by XPS analysis and peak fitting, cyclic voltammetry measurements were utilized to determine the chemical states of the tin oxide films on  multilayered Sn/Ag 3 Sn and Sn-RF specimens. Figure 6 illustrates the linear sweep voltammentric (LSV) curves for the as-plated Sn/Ag 3 Sn and Sn-RF specimens. For comparison, LSV curves for specimens after aging at 433 K for 24 h were also given in Fig. 6 as a reference of stable SnO 2 films. It can be seen from Fig. 6a that the as-plated multilayered Sn/Ag 3 Sn film presents one reduction peak at around -1.17 V (vs. Ag/AgCl), whereas the specimen after aging at 433 K gives a relative negative reduction peak at -1.40 V (vs. Ag/AgCl), which can be indexed to the reduction peaks for anhydrous SnO and SnO 2 , respectively. 18 This confirms that the tin oxide film on the asplated Sn/Ag 3 Sn film exists in form of anhydrous SnO, consistent with the XPS analysis (Fig. 5). In contrast, the as-plated Sn-RF film gives a main reduction peak at -1.02 V (vs. Ag/AgCl) and a weak peak at -1.35 V (vs. Ag/AgCl), which can be ascribed to hydrated Sn 3 O 2 (OH) 2 and SnO 2 · nH 2 O, respectively, according to Nakayama et al. 27 Since tin hydroxides and hydrates are generally less stable than the corresponding oxides, it follows that the tin oxide films on Sn-RF specimens provided less protection in the sulfidizing tests than that on Sn/Ag 3 Sn (Fig. 4). This proved that the chemical stability of intermetallic Ag 3 Sn phase plays a critical role in the advantageous performance of corrosion resistance for the multilayered Sn/Ag 3 Sn specimens over the Sn-RF and pure Ag films. In addition to the corrosion enhancement of Ag 3 Sn alloy due to the inclusion of noble metal Ag component, the chemically stable anhydrous SnO film formed on the Ag 3 Sn layer physically isolated the Ag from the corrosive circumstance. Therefore, the excellent sulfidizing resistance of the Sn/Ag 3 Sn film can be mainly attributed to both the chemically stable Ag 3 Sn alloy layer and the protective SnO film on it, which effectively hinders the chemical reaction between the Ag and the S and enhances the sulfidizing resistance of the Sn/Ag 3 Sn film.
Oxidization resistance of multilayered Sn/Ag 3 Sn films at high temperature.- Figure 7 illustrates the profiles of electric contact resistance versus applied load for the specimens before and after aging at 473 K (USCAR-Class V) for 500 and 2000 h, measured at 3-5 locations on the samples for reproducibility. The multilayered Sn/Ag 3 Sn films showed excellent stable contact resistance at all load ranges even up to 2000 h, thus indicating the excellent high temperature reliability for automotive connectors. As for conventional Ag and Sn-RF films, vigorous oscillations in contact resistance at the lower load side occurred after aging for 500 h, implying that insulating oxide films formed in relatively short period at the high temperature. Figure 8 summarizes the average values of contact resistance at load of 0.49 N for various specimens with the aging time at 473 K from    7. The initial contact resistance of the multilayered Sn/Ag 3 Sn film was 1.36 m , which was slightly higher than that of Sn-RF film in 0.95 m and Ag film in 0.34 m . This may be attributed to the existence of the insulating SnO film on hard Ag 3 Sn alloy layer as described earlier in XPS analysis (Fig. 5) and LSV measurement (Fig. 6). It should be emphasized, however, that the multilayered Sn/Ag 3 Sn films exhibited low and stable electrical contact resistances of around 1 m even after aging for a prolonged period of 3000 h, equivalent to those of as-plated samples, which indicates an excellent electric conductive stability. In contrast, both the Sn-RF film and the conventional Ag film showed ever-increasing contact resistance with the progressing of aging. In particular, the contact resistance of the Ag film exceeded the measuring range of 20 m after aging at 473 K for 2000 h, which indicates an insufficient stability at high temperature environment. Figure 9 demonstrates the surface morphologies and the cross sections for Sn-RF, multilayered Sn/Ag 3 Sn, and Ag films after aging at 473 K for 2000 h. The selected area EDX analysis results were also labeled in the Fig. 9. Please note that thick Cu oxide films formed on both Sn-RF and Ag films, whereas no apparent oxides were observed on the multilayered Sn/Ag 3 Sn film, though the Ag 3 Sn nano-grains agglomerated into micro-particles of 0.5-1 µm at prolonged high temperature. Through quantitative EDX analysis, the oxide films were determined to be CuO on the Sn-RF film and Cu 2 O · CuO on the Ag film, respectively. It was found in a previous study 14 that the crystalline structure of Ag 3 Sn grains was unchanged even after aging at 473 K for 3000 h in air, indicating the excellent chemical stability under the high temperature. In addition, it was also confirmed that the underlying Cu-Sn alloy layers of Sn-RF and Sn/Ag 3 Sn specimens had transformed completely from η-Cu 6 Sn 5 to ε-Cu 3 Sn after aging at 473 K for 240 h because of the outward Cu diffusion from the base materials. This means that both Ag 3 Sn and Cu 3 Sn are stable crystalline phases at high temperature.
Moreover, it can be seen from the cross-sectional images that some conspicuous Kirkendall voids formed at the film/Cu substrate interfaces for both the Sn-RF and Ag films. This can be attributed to the reactive diffusion between Cu base materials and the Sn or Ag films at high temperature, of which the outward Cu diffusion occurred preferentially to the inward Sn or Ag diffusion, thus leading to the formation of thick Cu oxide films. [28][29][30] In contrast, only small Kirkendall voids can be observed at the Cu 3 Sn/Cu interface for the multilayered Sn/Ag 3 Sn specimen, which may be ascribed to the volume reduction of phase transformation from η-Cu 6 Sn 5 (ρ = 8.31 g cm −3 ) to ε-Cu 3 Sn (ρ = 11.3 g cm −3 ) during aging. 31 Noticeably, the Ag 3 Sn grains were retained on the top of the coating film, irrespectively of the disappearance of Sn layer and the progress of Sn-Cu alloying with the outward Cu diffusion at prolonged high temperature. More importantly, some tiny particles, containing Ag component from EDX analysis, were observed at the Cu 3 Sn/Cu interface. This reveals that the Ag component also diffused inward across the Cu-Sn alloy layer and ultimately accumulated at the film/substrate interface with aging. 14,15 The Ag component that accumulated at the interface could also hinder the outward Cu diffusion locally, thus decreasing the formation of Kirkendall voids at the Cu 3 Sn/Cu interface and improving the adhesion between the film and the base materials and the stability of electric contact resistance as well.  and Ag during aging at high temperature circumstance, various specimens after aging at 473 K for 120, 500, and 2000 h were investigated by GDOES analysis, as shown in Fig. 10. After aging for 120 h, a small peak of oxygen corresponding to Sn oxide film was detected for the Sn/Ag 3 Sn specimen (Fig. 10a), whereas a broad oxygen peak corresponding to Sn and Cu oxides was measured on the surface of Sn-RF specimen (Fig. 10g). This indicates that the oxide film formed on the multilayered Sn/Ag 3 Sn specimen is thinner than that on the Sn-RF specimen, or in other words, the former exhibited higher oxidization resistance than the latter. As for the conventional Ag film, Cu and O elements were detected within the Ag film region (Fig. 10d). This revealed that outward Cu diffusion from the base material and inward O diffusion occurred concurrently with aging at high temperature, thus leading to the intrinsic oxidation of Cu inside the Ag film. With the progress of aging from 500 h to 2000 h (Fig. 10b-10c), the oxide film on the surface of Sn/Ag 3 Sn specimen became thicker, yet only Sn rather than Cu oxide formed even after aging for prolonged 2000 h, consistent to the FE-SEM observation as shown in Fig. 9. In contrast, both the conventional Ag and Sn-RF specimens exhibited apparently Cu diffusion toward the film surface with aging ( Fig. 10e-10f and Fig. 10h-10i), and finally producing thick Cu oxide films on the surface. According to the sputtering times and the profiles of oxygen element, it is calculated that the oxide films on Ag and Sn-RF specimens after aging at 473 K for 2000 h are approximately 50 and 10 times thicker than that on Sn/Ag 3 Sn one, respectively. This clearly elucidates the advantageous oxidation resistance of the Sn/Ag 3 Sn film over the conventional Ag and Sn-RF films. Moreover, it should be noticed that a small Ag peak appeared at the Sn-Cu/Cu substrate interface and its intensity increased gradually with aging time (Fig. 10b-10c). This indicates that Ag diffusion inward occurred simultaneously with aging at high temperature, which agrees with the result of the FE-SEM observation (Fig. 9e). It is known that the specific resistivity of Cu oxides, 10 6 -10 7 · m, is significantly higher than that of Sn oxides, 4 × 10 −4 · m. Therefore, the excellent stability of electric resistance for the multilayered Sn/Ag 3 Sn films can be predominantly attributed to the effective inhibition of the formation of insulating Cu oxides on the film surface, which in turn, can be ascribed to the inward Ag diffusion which countered the outward Cu diffusion from the Cu base materials. Figure 11 shows a TEM image for the cross section of the Sn/Ag 3 Sn film after aging at 473 K for 500 h. The Ag 3 Sn grains that agglomerated during aging existed as single crystals from the selected area X-ray diffraction pattern. Noticeably, the Cu 3 Sn grains in vicinity to the Ag 3 Sn gains on the surface were much larger than those inside the film, and exhibited a different texture compared to the normal Cu 3 Sn grains inside the film, inferring a different crystalline structure with different chemical components. Table I lists the chemical compositions at different areas corresponding to the numbers in Fig. 11, which were measured by EDX analysis. A small amount of Ag was detected in the Cu 3 Sn grain close to the Ag 3 Sn grain (No.3). This reveals that Ag included in the Cu 3 Sn  phase is in the form of solid solution, which can be caused by interdiffusion between Ag 3 Sn and Cu 3 Sn phases at high temperature. This is similar to the inhibiting effects on the growth of Cu 3 Sn and Kirkendall voids in the Cu/(Sn-Ag-Cu) system produced by minor Pd alloying additions. 30 The inclusion of the Ag component in the Cu 3 Sn phase may also improve the corrosion resistance and the chemical stability of Cu-Sn alloy matrix by effectively suppressing the formation of Cu oxides on the surface of Sn/Ag 3 Sn specimen, thus leading to a stable electric contact resistance at high temperature. Moreover, the oxygen component on Ag 3 Sn grain (No.1) is less than that on Cu 3 Sn (No.4), indicating that the oxidation resistance of the Ag 3 Sn with noble metal component is higher than that of the latter Cu 3 Sn. Figure 12 illustrates the scheme of the element diffusion models of multilayered Sn/Ag 3 Sn film during aging at high temperature, compared to the conventional Ag and the Sn-RF on Cu alloys. It is known that Cu from the substrate materials diffuses easily via electroplating films toward the film surface due to the highly diffusive nature at high temperature. [28][29][30] In case of multilayered Sn/Ag 3 Sn film, elemental Ag diffuses simultaneously inward toward the film/substrate interface because of its strong migration nature with aging time. 1,9 The inward Ag diffusion may be predominant to the outward Cu diffusion, thus leading to the accumulation of Ag nanoparticles at the film/substrate interface (see Fig. 9e and Fig. 10b-10c). This, in turn, constituted a discontinuous barrier layer and hindered the outward Cu diffusion at prolonged aging time to some extent (Fig. 10). More importantly, Ag component also included in the intermetallic Cu 3 Sn phase in form of solid solution with aging ( Fig. 11 and Table I), thus enhanced the chemical stability or oxidation resistance of the Cu 3 Sn phase at high temperature. Generally, it is known that, for Sn-based coatings on Cu substrates, the Cu component that diffused from the substrates during aging was trapped by Sn to form Cu-Sn alloys in forms of intermetallic Cu 6 Sn 5 and Cu 3 Sn phases, 28 thus retarding the formation of Cu oxides on the film surface compared to the Ag films on Cu materials (Fig. 10). However, as the Sn layer disappeared and totally transformed into the Cu 3 Sn phase with aging at high temperature, insulating Cu oxide films then formed rapidly on the Cu 3 Sn layer, assisted by the continuous supply of Cu that diffused from the base materials. 2 Therefore, the stable electric conductivity of multilayered Sn/Ag film at high temperature can be mainly attributed to the chemically stabile Ag 3 Sn particles and the Ag-included Cu 6 Sn 5 /Cu 3 Sn phase on the film surface (Fig. 9b,  Fig. 12a). The Ag 3 Sn particles provided electric conductive sites irrespective of the oxidation of the surrounding Cu-Sn alloy regions (Fig. 11), and the inclusion of Ag component in Cu 6 Sn/Cu 3 Sn phases enhanced the oxidizing resistance of the Cu-Sn alloy matrix, thus effectively inhibiting the formation of insulating Cu oxides on the surface and leading to the excellent stability of contact resistance at prolonged high temperature (Fig. 8).

Mechanism of oxidization resistance of multilayered Sn/Ag 3 Sn films under high temperature.-
In contrast, in case of Ag and Sn-RF films, highly insulating Cu oxide films grew continuously on the film surface with aging. This can be ascribed to the faster diffusion behavior of Cu component with highly intrinsic diffusion coefficient via the Ag and Cu-Sn alloy layers (see Fig. 9, 10, and 12b-12c), thus leading to the ever-increasing electric contact resistances at high temperature (see Fig. 8).

Conclusions
The corrosion resistance of multilayered Sn/Ag 3 Sn films with 50 nm Ag thickness on Cu alloy materials was investigated through an accelerated sulfidizing test in a (NH 4 ) 2 S x solution according to a JIS standard and an accelerated aging test at 473 K up to 3000 h referring to a USCAR standard, compared to a commercial pure Ag film and a conventional Sn-RF film on Cu alloy materials.
The multilayered Sn/Ag 3 Sn films went through the accelerated sulfidizing test successfully, with unchanged appearance and stable electrical contact resistance, even after being immersed in a 0.2 mL/L (NH 4 ) 2 S x solution for 120 h. In contrast, the conventional Ag film failed the sulfidizing test, losing its metallic gloss turning a dark blue and delivering an ever-increasing contact resistance with prolonged immersing period. The excellent sulfidizing resistance and durability of the multilayered Sn/Ag 3 Sn film can be mainly attributed to the chemically stable Ag 3 Sn alloy and the overlying anhydrous SnO film. The Ag 3 Sn alloy provides better corrosion resistance against S 2ions than pure Ag, and the chemically stable anhydrous SnO film protects the underlying Ag 3 Sn layer by isolating the Ag from the corrosive S 2− ions. This hindered the chemical reaction between the Ag and the S and eventually improved the sulfidizing resistance of multilayered Sn/Ag 3 Sn film.
Moreover, the multilayered Sn/Ag 3 Sn films exhibited low and stable electrical contact resistances of around 1 m even after aging at 473 K for 3000 h, equivalent to those of as-plated samples, indicating excellent oxidation resistance and reliably conductive stability at high temperature. In contrast, both the conventional Ag and Sn-RF films showed ever-increasing electrical contact resistance at 500 h, which can be ascribed to the formation of highly insulating Cu oxide films on the surface and of the generation of conspicuous Kirkendall voids at the film/Cu substrate interfaces due to the intrinsic outward Cu diffusion at high temperature. The excellent oxidizing resistance of the multilayered Sn/Ag 3 Sn film at high temperature can be mainly attributed the chemically stable Ag 3 Sn grains and the Ag-included Cu 3 Sn phase on the film surface. The Ag 3 Sn grains provided electric conductive paths irrespective of the oxidization of the surrounding Cu-Sn alloy regions, and Ag inclusion in Cu-Sn alloy enhanced the oxidization resistance of the Cu-Sn alloy matrix. Consequently, this effectively inhibited the formation of highly insulating Cu oxides on the surface at high temperature, thus leading to the excellent stability of contact resistance for highly reliable electric connectors at various harsh environments.