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Materials and Corrosion 1/2013

2013, Materials and Corrosion

This paper presents an investigation on the corrosion performance of Ni/Au, NiP/ Au, Ni/NiP/Au and Ni/NiPd/Au electrical connector coating systems. The coating systems were exposed to 10 days Class III mixed flowing gas and were subsequently examined by X-ray diffraction, focused ion beam microscopy, scanning electron microscopy and energy dispersive spectroscopy to evaluate the performance of each coating system. The Ni/Au coating system showed the worst performance followed by NiP/Au and Ni/NiP/Au. The Ni/NiPd/Au coated connector materials exhibited the least surface corrosion and this was attributed to a number of factors including a thicker coating system, more compact gold layer with fewer defects and a reduction in the electrochemical potential difference between layers reducing the local cell effect.

Materials and Corrosion 2013, 64, No. 1 DOI: 10.1002/maco.201106340 Corrosion under mixed flowing gas conditions of various connector coatings M. N. Collins*, M. Reid, E. Dalton, K. Shannon and L. F. Garfias-Mesias This paper presents an investigation on the corrosion performance of Ni/Au, NiP/ Au, Ni/NiP/Au and Ni/NiPd/Au electrical connector coating systems. The coating systems were exposed to 10 days Class III mixed flowing gas and were subsequently examined by X-ray diffraction, focused ion beam microscopy, scanning electron microscopy and energy dispersive spectroscopy to evaluate the performance of each coating system. The Ni/Au coating system showed the worst performance followed by NiP/Au and Ni/NiP/Au. The Ni/NiPd/Au coated connector materials exhibited the least surface corrosion and this was attributed to a number of factors including a thicker coating system, more compact gold layer with fewer defects and a reduction in the electrochemical potential difference between layers reducing the local cell effect. 1 Introduction Reliability of an electronic material or device is defined as the probability that the package will perform its intended function for a specified period of time under a given operating condition without failure. In particular, for most electronic devices, the reliability of the electronic package and the metallic connectors are the most important parts of the device, when it comes to corrosion susceptibility. Most electronic devices and systems use metallic electrical connections that can react when exposed to an aggressive environment. Therefore, reliable connectors are essential to manufacture reliable electronic devices and systems [1]. Corrosion of the connectors of most electronic systems can manifest itself in a wide range of consequences ranging from intermittent electrical faults to complete functional breakdown [2–5]. The majority of connectors comprise of a copper base alloy substrate used because of its low cost and high electrical conductivity. Unfortunately, copper is a very reactive metal to the atmospheric environment; therefore, a protective coating is required to prevent corrosion products developing on the surface M. N. Collins, M. Reid, E. Dalton CTVR, Stokes Institute, University of Limerick, Limerick (Ireland) E-mail: maurice.collins@ul.ie K. Shannon Southwest Research Institute, San Antonio, TX (USA) L. F. Garfias-Mesias Materials & Corrosion Technology Center, Det Norske Veritas (U.S.A.), Inc., 5777 Frantz Road, Dublin, OH 43017-1386 (USA) www.matcorr.com wileyonlinelibrary.com which can cause intermittent failures. Typically the industry uses metallic coatings, which consist of a decorative gold top layer with excellent electrical properties and underneath a barrier layer (in contact with the copper) that has the main objective of impeding copper diffusion and improve wear resistance [1–3, 6]. In recent years, the economics of using a thick gold top layer of approximately 0.8 mm has come under closer scrutiny and a gold flash (a thin layer less that 0.25 mm of gold) is now being used for electrical connections [7, 8]. With the introduction of the gold flash, interest has turned towards the effectiveness of the barrier layer(s), in order to improve the reliability of connectors with a gold flash and other barrier layers. The interest in barrier layer(s) is due to the overriding evidence indicating that using a gold flash can be detrimental to reliability [7]. Since typical gold flash coatings are very thin, they tend to have a high level of defects, namely surface porosity; as a result, pore corrosion is responsible for most corrosion failures of gold coated connectors [1–10]. Nickel and some Ni alloys with other noble materials are typically used as a barrier layer. Although the nickel layer does impede the copper diffusion from the substrate, it does not prevent it. Pinnel and Bennett [11, 12] revealed that the diffusion of copper to the gold surface can take place at relatively low temperatures and that the nickel layer only slow the diffusion of copper to the gold surface but do not totally prevent the diffusion process. The copper diffuses out along the short circuit diffusion paths, i.e. grain boundaries, dislocations, pores, etc. Alloying the nickel layer with phosphorus modifies the microstructure of the nickel layer. Coatings containing 10–13 wt% phosphorus are mostly amorphous. By removing the grain boundaries, it is possible to reduce the diffusion paths and anodic sites for corrosion to initiate [13]. An additional layer of a nickel– ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 7 8 Collins, Reid, Dalton, Shannon and Garfias-Mesias Materials and Corrosion 2013, 64, No. 1 palladium (NiPd) is used in certain applications, where wear and corrosion related failures have previously been reported. The NiPd coatings increase the hardness in those connectors, and as a consequence it improves the wear resistance and at the same time decreases the diffusion of copper to the surface [14]. The aim of this paper is to investigate the corrosion performance of Ni, NiP, Ni/NiP and Ni/NiPd connector coating systems with a gold flash finish. A Class III mixed flowing gas (MFG) test was used to evaluate the corrosion resistance of the different coating systems. This test enables years of service to be mimicked by days of testing in which the temperature, relative humidity and concentration of gaseous pollutants are carefully defined, monitored and controlled [1–3, 6, 15–17]. The Class III environment represents a particular harsh corrosive environment in the North America Region and Europe, in which the corrosion film of pure copper in one year would be between 80 and 400 nm [15–17]. 2 Experimental 2.1 Sample preparation Pre-stamped strips of copper-zinc alloy with dimensions 25 mm  42 mm  0.50 mm were coated with a specific thickness of different layers of nickel, nickel–phosphorous, NiPd and gold. The materials and plating conditions are detailed in Table 1. 2.2 Mixed flowing gas exposure system Mixed flowing gas exposure was performed at a constant temperature and relative humidity of 30 8C and 70% RH, respectively. The corrosive gases used in the chamber were 100 ppb of H2S, 200 ppb of SO2, 200 ppb of NO2 and 20 ppb of Cl2. Air was used as a carrier gas. The experimental set-up and settings are detailed elsewhere [14]. Duplicate samples of the coupons of the different connector coating systems were exposed to the above MFG conditions, and removed after 10 days exposure for further detailed analysis. 2.3 Characterization The samples were characterized before and after the MFG exposure using optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), focused ion beam (FIB) milling and energy dispersive X-ray spectroscopy (EDS). The EDS spectra were collected at 20 kV, a beam current of 0.26 nA and a working distance of 39 mm, which was Table 1. Plating conditions Bath characteristics Temperature pH-value Current density Plating speed Chloride content Boric acid content Ni/Au NiP/Au Ni/NiP/Au Ni/NiPd/Au 45 8C 60 8C 60 8C 4 2.6 4 3 A/dm2 2 A/dm2 4 A/dm2 5–10 cm/s 5–10 cm/s 5–10 cm/s – – 8 g/L Cl – – 45 g/L H3BO3 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 45 8C 6 4 A/dm2 5–10 cm/s – – reproducible to a high degree of accuracy. The spectra were processed using PGT software. For XRD a filtered Cu Ka radiation was used and diffractograms were obtained over a scanning range 20–908 2u with a scan speed of 0.0088 2u/s. To observe metallographic cross-sections of the coating, the coatings were protected by epoxy and then cut with a diamond saw and metallographically prepared to a 1 mm finish. 3 Results and discussion 3.1 As-received contacts Figure 1a–d shows typical FIB cross-sections of the different connector coating systems – Ni/Au, NiP/Au, Ni/NiP/Au and Ni/ NiPd/Au, respectively. The Ni/Au connector coating demonstrated a two layer structure comprising approximately 0.1 mm thick gold outer layer and a nickel under layer. Close examination of the nickel under layer demonstrates a uniform fine grain size. The NiP/Au connector coating also demonstrated a two layer structure comprised of approximately 0.1 mm thick gold outer layer and a 1.2 mm thick nickel under layer. The nickel under layer illustrated an amorphous microstructure owing to the P content in the Ni layer [13]. The Ni/NiP/Au connector coating system comprised approximately of 0.1 mm thick gold outer layer, a 1 mm thick NiP intermediate layer and a 1.2 mm Ni under layer. The Ni/NiPd/Au connector coating system showed a three layer structure, comprising approximately of 0.1 mm thick gold outer layer, a 4 mm thick NiPd intermediate layer and a 1.2 mm nickel under layer. Figure 2 a–d shows X-ray patterns of the different connector coating systems. The XRD pattern of the Ni/Au coating system (Fig. 2a) displays cubic Cu6Zn4 reflections together with the peaks for the gold layer. The NiP/Au connector coating system displayed identical reflections to Ni/NiP/Au (Fig. 2b and c). These XRD patterns demonstrate strong Cu6Zn4 reflections and minor Au reflections. The XRD pattern of Ni/NiPd/Au connector coating system (Fig. 2d) showed the cubic Cu6Zn4 reflections together with peaks of Ni, NiPd and Au. The overall intensity of the Cu6Zn4 reflections decreased in comparison to the intensity observed in the other coating systems. This is most likely due to the presence of the thicker NiPd layer observed in Fig. 1d. 3.2 Analyses of corrosion products and degradation mechanism Figure 3a–d shows photographs of the different connector coating systems – Ni/Au, NiP/Au, Ni/NiPd/Au and Ni/NiP/Au, respectively, after 10 days exposure in Class III MFG environment. Owing to the sometimes localized nature of the corrosion processes, the measurements of the corroded areas were statistically studied using quantitative analysis with digital image processing aid: ‘‘The Image Processing ToolKit’ 4.000 . The image analysis was based on reflective light microscopy (using a Leica Zoom 2000 stereozoom microscope) imaging acquisition. After image acquisition the corrosion sites were highlighted and its www.matcorr.com Materials and Corrosion 2013, 64, No. 1 Mixed flowing gas (MFG) corrosion Figure 1. FIB cross-section of the as-received connector coating systems: (a) Ni/Au, (b) NiP/Au, (c) Ni/NiP/Au and (d) Ni/NiPd/Au threshold was calculated to provide a measure of the coverage of corrosion products. From the images in Fig. 3a–d it is clear that a significant spread of corrosion product occurred on Ni/Au, NiP/Au and Ni/ NiP/Au coating systems, demonstrating their propensity to corrode. The Ni/NiPd/Au, however, did not show the same degree of corrosion over the entire surface – developing in isolated areas but not spreading over the surface to the same extent as observed on Ni/Au, NiP/Au and Ni/NiP/Au coating systems. The corrosion on Ni/NiPd/Au appears to have originated at two different sites over the surface. Firstly, as ring defects periodically spread over the surface and secondly line defects spreading inwards from the edge of the sample. Table 2 summaries the average measured area of corrosion products that developed on the different connector coating systems. The images depicted in Fig. 4a–d show high magnification representative backscattered electron images of corrosion features on the coating systems Ni/Au, NiP/Au, Ni/NiP/Au and, Ni/NiPd/Au, respectively, after 10 days exposure in Class III MFG environment. The original surface of the Ni/Au coating system is only slightly visible as bright areas in Fig. 4a. The extent of surface corrosion on the Ni/Au coating system makes it impossible to determine the location of sites where the initial corrosion took place. The corrosion products that developed on the surface of the coating system were analyzed by EDS and comprised mainly of copper, zinc and sulphur. www.matcorr.com On the NiP/Au and Ni/NiP/Au coating systems the corrosion developed as sharply defined corrosion mounds. The backscattered electron images of NiP/Au and Ni/NiP/Au in Fig. 4b and d show the development of such corrosion mounds. The mound consisted of a well know halo and bloom effect [9]. The bloom at the centre which has grown to a considerable height above the surface is surrounded by a halo which encloses the bloom at the centre. The Ni/NiPd/Au coating system in Fig. 4d shows a backscattered electron image of the tip of a line defect spreading inwards from the edge of the sample. Creep of the corrosion product over the coating surface was observed from the initial defect. Creep corrosion is generally understood as mass transport process in which solid corrosion products migrate over a surface [18, 19]. Although the combination of visual, SEM and EDS of the surface provide valuable results of extent of corrosion on the different coating systems – metallographic cross-sections were conducted on each sample in order to get an overall picture of failure mechanisms and corrosion layer development. Figure 5a– d shows backscattered electron cross-section images of the different connector coating systems – Ni/Au, NiP/Au, Ni/NiP/Au and Ni/NiPd/Au after 10 days exposure in Class III MFG environment. Each of the cross-sections show the development of corrosion products on the surface over a corrosion defect. EDS analysis (not shown) confirmed the corrosion products that developed on the surface were mostly compounds containing ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 9 10 Collins, Reid, Dalton, Shannon and Garfias-Mesias Materials and Corrosion 2013, 64, No. 1 Figure 2. X-ray diffractometer traces of as-received connector coating systems: (a) Ni/Au, (b) NiP/Au, (c) Ni/NiP/Au and (d) Ni/NiPd/Au sulphur. Over the defect the corrosion product was mainly comprised of copper, zinc, nickel and sulphur, whereas the area spreading away from the defect comprised mainly of copper and sulphur. These results are in good agreement with previous reports of Cu exposed to MFG III for several days (up to 20 days) in which the main corrosion products found are sulphur rich [16]. Two sites on the Ni/Au coating system from which corrosion initiated and corrosion products subsequently migrated across the surface are shown in Fig. 5a. Since the gold layer is very thin (approximately 0.1 mm) for these types of metallic coatings, it is expected that a certain degree of porosity will be present in such thin layers [8, 19]. The mechanism by which the corrosion occurs is as follows: the area at the bottom of a defect (within the gold layer), is oxygen depleted; and the less thermodynamically stable nickel area becomes anodic with respect to the surrounding Aucoated layer. The gold surface at the top, surrounding the pit, has access to the oxygen in the air and becomes the cathodic site. Under the MFG conditions, an invisibly thin but finite, water layer forms on the gold surface due to the high relative humidity [20–23]. The combination of corrosive gas dissociates to different ionic species in the thin water layer [24]. This concentrated surface solution provides a low conductivity electrolytic path between the gold and nickel through the surface defect. Corrosion proceeds because of a local cell effect resulting from a potential difference between the cathodic gold and the anodic Ni area exposed. This mechanism is enhanced due to the cathodic portion of the gold plate having a much larger area than the minute nickel point at the bottom of the defect (small anode and ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim large cathode effect). Once the nickel is corroded, it exposes the copper substrate and allows easy transport of the corrosive gases and water. Corrosion of the substrate occurs at a very rapid rate, with the corrosion products migrating in all directions over the gold surface. The extent of creep corrosion observed can be explained by the chemical species of the corrosion products, in this case copper and sulphur. Copper sulphide when formed has significantly higher mobility than copper oxides [19]. In the complex corrosive environment in this study copper-sulphur corrosion product is continuously generated from the defects with extensive migration across the gold surface. The continued growth of the electrical insulating corrosion products that creeps over the otherwise uncorroded surface is likely to cause signal intermittence on connectors and result in device failure. Underneath the nickel layer, undercutting corrosion of the copper substrate has occurred, where attack of the copper substrate under the nickel is observed. The severity of the undercutting varied. This leads to de-alloying of the zinc, by which the more active constituent of the Cu–Zn alloy is initially selectively removed from the alloy, leaving behind a weak deposit of more noble copper. This effect is notable in Fig. 5b that shows the NiP/Au coating system, where the Zn is preferentially corroded. With the subsequent corrosion of the remaining copper the area under the pit is significantly weakened structurally. This allows further gases and humidity to penetrate the outer layer and causes further corrosion. The difference between the two phosphorous coating systems could possibly be attributed to the nickel layer. The grain boundaries in the Ni layer may act as www.matcorr.com Materials and Corrosion 2013, 64, No. 1 Mixed flowing gas (MFG) corrosion Figure 3. Photographs of connector coating systems: (a) Ni/Au, (b) NiP/Au, (c) Ni/NiP/Au and (d) Ni/NiPd/Au after 10 days exposure to exposed to Class III MFG Table 2. The percentage of area covered with corrosion product on connector coating systems after 10 days exposure in Class III mixed flowing gas environment % Average surface area covered by corrosion product Ni/Au (%) NiP/Au (%) Ni/NiP/Au (%) Ni/NiPd/Au (%) 81 84 16 99 sites of structural discontinuity (both microstructurally and chemically). These discontinuities and differences may lead to preferential corrosion of specific constituents of the nickel layer, as some grains may be more noble or active than others [10]. The Ni/NiPd/Au demonstrated significantly less pore corrosion and creep corrosion which may be attributed to a thicker NiPd layer which will significantly reduce pinholes than www.matcorr.com the other three thinner coating systems used in the study. Additionally, the structure of the flash gold and its properties are significantly influenced by the surface upon which it is plated. Typically, a deposited metal will try to mimic the structure of the surface on which it is plated. The fine microstructure of the NiPd layer in comparison to the slightly coarser nickel–phosphorous layer (shown in the FIB cross-sections in Fig. 1b and d, respectively) may facilitate the nucleation of a more compact gold layer with fewer defects. Despite the Ni/NiPd/Au coating system significantly reducing the pore corrosion, cracking of the NiPd layer near the edge of the coupon allowed penetration to the substrate and subsequent corrosion. The cracking of the NiPd layer near the edges may have been a result of stress near the edge of the coupon causing fracture of the more brittle NiPd layer. Application of a thinner layer may prove more protective reducing the stress in the layer and eliminating the stress cracks. ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 11 12 Collins, Reid, Dalton, Shannon and Garfias-Mesias Materials and Corrosion 2013, 64, No. 1 Figure 4. Backscattered electron images of corrosion products on (a) Ni/Au, (b) NiP/Au, (c) Ni/NiP/Au and (d) Ni/NiPd/Au after 10 days exposure to exposed to Class III MFG Figure 5. Backscattered electron cross-section images of (a) Ni/Au, (b) NiP/Au, (c) Ni/NiP/Au and (d) Ni/NiPd/Au after 10 days exposure to exposed to Class III MFG ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com Materials and Corrosion 2013, 64, No. 1 4 Conclusions In this study, four different coating systems (Ni/Au, NiP, Ni/NiP and Ni/NiPd) with flash gold finish were assessed under MFG stimulus. It is strongly evident that the Ni/NiPd/Au connector is superior to the Ni/Au, NiP/Au and Ni/NiP/Au, coating systems in terms of surface coverage of corrosion. Moreover, the Ni/Au coating system is inferior to the NiP/Au and Ni/NiP/Au, when exposed to a MFG environment for 10 days. The superior performance of Ni/NiPd/Au coating systems was attributed to a thicker coating system, more compact gold layer with fewer defects and a reduction in the electrochemical potential difference between layers reducing the local cell effect. These results would be of interest to end users concerned with high reliability of electronic devices. Acknowledgements: The financial support for this work is through Science Foundation Ireland under grant number 03/ CE3/I405. The authors are indebted to Franz Gassner at Umicore Galvanotechnik GmbH, Schwaebisch Gmuend, Germany. 5 References [1] R. S. Mroczkowski, Presented at the 44th IEEE Holm Conference on Electrical Contacts, 1998, pp. 57. [2] R. Martens, M. G. Pecht, IEEE Trans. Adv. Pack. 2000, 23, 561. [3] C. Maul, J. W. McBride, J. Swingler, IEEE Trans. Compon. Pack. Technol. 2001, 24, 370. [4] M. Sun, M. Pecht, R. Martens, Scr. Mater. 2000, 42, 1. [5] R. Martens, M. Pecht, J. Mater. Sci.: Mater. Elecron. 2000, 11, 209. Mixed flowing gas (MFG) corrosion [6] M. Sun, M. Pecht, M. A. E. Natishan, Microelectron. J. 1999, 30, 211. [7] M. Peel, Connector Specifier 1999, 15, 9. [8] J. Xie, M. Sun, M. Pecht, D. Barbe, J. Electron. Pack. 2004, 126, 37. [9] M. Reid, J. Punch, G. Grace, L. F. Garfias, S. Belochapkine, J. Elec. Soc. 2006, 153, B513. [10] S. J. Krumbein, IEEE Trans. Parts Mater. Pack. 1969, 5, 89. [11] M. R. Pinnel, J. E. Bennett, Metall. Trans. A 1972, 3, 1989. [12] M. R. Pinnel, J. E. 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Graedel, Atmospheric Corrosion, Wiley, New York 2000. (Received: August 28, 2011) (Accepted: October 5, 2011) www.matcorr.com W6340 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 13