Sn Coatings Research Articles

Electrodeposition of Sn and NiSn alloys coatings using choline chloride based ionic liquids—Evaluation of corrosion behavior

Abstract The present paper presents several experimental results regarding the electrodeposition and corrosion behavior of Sn and NiSn alloy coatings from some choline chloride based ionic liquids, applied onto various metallic substrates including Cu and steel. There are discussed electrolytes containing Sn and Ni salts in choline chloride–ethylene glycol and choline chloride–malonic acid eutectic mixtures. The obtained Sn and Ni–Sn alloy deposits are adherent and uniform onto metallic substrates The Ni–Sn alloys contained about 62–72 wt% Sn according to EDX analysis, with a very slight variation against the applied current density domain. According to XRD investigations both electrochemical coatings showed a nanocrystalline structure, with average sizes of the crystallites between 50 and 80 nm for Sn and around 11–14.5 nm for NiSn alloy. To evaluate corrosion resistance of Sn and Ni–Sn alloy coatings, potentiodynamic polarization curves and electrochemical impedance spectra (at open circuit potential) in 0.5 M NaCl solution have been recorded after different immersion times of coated specimens. Different corrosion performances are discussed taking into account the used electrodeposition procedures.

Electrolytic deposition of Sn-coated mesocarbon microbeads as anode material for lithium ion battery

Deposited of crystalline tin (Sn) coatings on mesocarbon microbead (MCMB) powder as anodes of lithium ion (Li-ion) battery was conducted in the SnSO4 solution by a cathodic electrochemical synthesis. The Sn-coated MCMB specimens were characterized by X-ray diffraction, scanning electron microscopy, and charge/discharge tests. The synthesis condition of Sn-coated MCMB was optimized by considering the agglomeration, size, and adhesion of the samples to the current collectors in the battery. The Sn-coated MCMB electrodes exhibit increased reversible capacity without sacrificing its cycling behavior, compared with bare MCMB electrodes. It is concluded that electrolysis-deposited Sn-coated MCMB electrodes may emerge as a practical and promising anode material for secondary Li-ion batteries.

Altering the Mechanical Properties of Sn Films by Alloying with Bi: Mimicking the Effect of Pb to Suppress Whiskers

Stress is believed to be the main driving force for whisker formation in Sn coatings on Cu. This suggests that whiskering can be suppressed by enhancing stress relaxation in the Sn layer, which is believed to be the reason why Sn-Pb alloys do not form whiskers. However, Pb is no longer acceptable for use in electronics manufacturing. As an alternative, we used pulsed plating to create Sn-Bi coatings with an equiaxed microstructure similar to that of Sn-Pb alloys. An optical wafer curvature technique was used to measure stress relaxation kinetics in Sn, Sn-Pb and Sn-Bi alloy thin films during thermal cycles. The results show that Sn-Bi films have significantly enhanced stress relaxation relative to pure Sn films. Comparison between Sn-Bi samples with equiaxed and columnar microstructures shows that both microstructure and alloy composition play a role in enhancing the stress relaxation.

Correlating whisker growth and grain structure on Sn-Cu samples by real-time scanning electron microscopy and backscattering diffraction characterization

Whiskers/hillocks grow out of Pb-free Sn coatings used in electronics manufacturing. To determine which grains form whiskers/hillocks, we use scanning electron microscopy and backscattering diffraction to simultaneously monitor the surface morphology and grain structure. To reduce surface roughness, we developed a “peel-off” method to prepare ultra-flat samples that were measured repeatedly while whiskers/hillocks formed. We find grains that form into whiskers/hillocks are present in the as-deposited film (i.e., not re-nucleated) and many have horizontal grain boundaries beneath them. Grain rotation during whisker/hillock formation means that measurements performed after the features grow do not indicate their initial grain orientations.

3D tin anodes prepared by electrodeposition on a virus scaffold

A patterned core–shell tin (Sn) nanorod anode is fabricated by pulse electrodeposition of Sn onto a self-assembled Tobacco Mosaic Virus (TMV) structured nickel current collector. Pulse electrodeposition onto the virus assembled 3D electrode surfaces produces homogenous Sn coatings with significant void space to accommodate the large volume change associated with Sn lithiation. The TMV enabled 3D Sn anodes shows high capacity retention of 560 mAh g−1 after 100 cycles with the average capacity fading rate of 0.4% per cycle. The high electronic conductivity of Sn, short diffusion length for Li-ions, and large interface between Sn nano-rods and electrolyte greatly enhance the rate performance of the TMV enabled Sn anodes.

Fabrication of Sn Coatings on Alumina Balls by Mechanical Coating Technique and Relevant Process Analysis

Sn coatings were fabricated by mechanical coating technique for the first time. The coatings were characterized by XRD and SEM, among others. The SEM showed that the coatings had an irregular and uneven morphology. The influence of the rotation speed of planetary ball mill on the evolution and formation of the coatings was also investigated. The results indicated that continuous Sn coatings can be formed under a moderate rotation speed. In other words, the coatings cannot be formed when rotation speed was too high or too low. The evolution of the coatings was examined and discussed. The results showed that it followed the universal evolution law of metal coatings which included four stages. However, the exfoliation of the coatings was not seen even the milling time reached 30 h.

Tin-coated graphite electrodes as composite anodes for Li-ion batteries. Effects of tin coatings thickness toward intercalation behavior

The electrochemical behavior of graphite anodes, coated by 50–500 Å-thick Sn layers, is discussed in the present paper. Morphology and structure of the modified electrode surfaces are described, and the charge/discharge behavior is evaluated by galvanostatic cycles at temperatures down to −30 °C. The enhanced kinetics of the intercalation/deintercalation process is studied by cyclic voltammetry and electrochemical impedance spectroscopy, focusing on the role played by the Sn coatings in the intercalation/deintercalation mechanism. The results show that the metal layers modify and stabilize the electrode/electrolyte interphase and that the intercalation process is mediated by reversible Li–Sn alloys formation. Moreover, all the Sn coatings are effective in modifying the energy barriers related both to the Li + desolvation step and to the migration of the desolvated Li + ion through the modified surface layers. As a consequence, the overall polarization for the charge-transfer process is reduced, and enhanced low-temperature intercalation performances are obtained.

Stress Relaxation Mechanisms of Sn and SnPb Coatings Electrodeposited on Cu: Avoidance of Whiskering

The interrelations of microstructural evolution, phase formation, residual stress development, and whiskering behavior were investigated for the systems of Sn coating on Cu and SnPb coating on Cu during aging at room temperature. It was shown that the whisker-preventing effect of Pb addition to pure Sn can be attributed to a Pb-induced change of the stress relaxation mechanism in the coating: Pure Sn coatings, with a columnar grain morphology, relax mechanical stress via localized, unidirectional grain growth from the surface of the coating (i.e., whisker formation occurs), whereas SnPb coatings, with an equiaxed grain morphology, relax mechanical stress via uniform grain coarsening without whisker formation. It can thus be suggested that tuning of the Sn grain morphology (i.e., establishing an equiaxed grain morphology) is a straightforward method of microstructural control to suppress whisker formation at room temperature. Experimental results obtained in this project validate this conclusion.

Whisker Growth Behavior of Sn and Sn Alloy Lead-Free Finishes

Sn whisker growth behavior, over periods of time up to 10,080 h at room temperature, was examined for Sn and Sn-Cu, Sn-Ag, Sn-Bi, and Sn-Pb coatings electroplated on copper in 2 μm and 5 μm thicknesses to understand the effects of the alloying elements on whisker formation. Sn-Ag and Sn-Bi coatings were found to significantly suppress Sn whisker formation compared with the pure Sn coatings, whereas whisker growth was enhanced by Sn-Cu coatings. In addition, annealed Sn and Sn-Pb coatings were found to suppress Sn whisker formation, as is well known. Compared with the 2-μm-thick coatings, the 5-μm-thick coatings had high whisker resistance, except for the Sn-Cu coating. Whisker growth was correlated with coating crystal texture and its stability during storage, crystal grain microstructure, and the formation of intermetallic compounds at Sn grain boundaries and substrate–coating interfaces.

Breakthroughs in understanding elastic grain interaction and whisker formation made possible by advances in X-ray powder diffraction

Recent work on laboratory parallel-beam diffraction is summarized. Special attention is paid to instrumental aberrations and their corrections. Two types of primary beam optics are considered: X-ray lenses and X-ray multilayer mirrors. A high photon flux, high brilliance and minimal divergence of the incident beam make a recently pioneered setup, based on a combination of a microfocus rotating anode X-ray source with a single-reflection mirror, particularly suitable for grazing-incidence diffraction, and thus for the investigation of very thin films (yielding low diffracted intensities) and of stress and the anisotropy of the microstructure of materials (such as the crystallographic texture, requiring the acquisition of large measured data sets, corresponding to the variation of the orientation of the diffraction vector with respect to the specimen frame of reference). A comparative discussion of primary optics which can be used to realize parallel-beam geometry demonstrates the range of possible applications of parallel-beam diffractometers. Such advances in laboratory parallel-beam diffraction instrumentation enable highly accurate diffraction investigations of thin films and, in particular, diffraction measurement at constant, tuneable penetration depth. These novel possibilities thereby provide a pathway for fundamental investigations of thin films and nanocrystalline materials. In particular, breakthroughs have been realized in the fields of elastic grain interaction in ultrathin films and the understanding of whisker formation on Sn coatings.

Introduction of an exponential formulation to quantify the electrical endurance of micro-contacts enduring fretting wear: Application to Sn, Ag and Au coatings

Abstract The massive use of electronic control in automotive, plane and energy systems requires the extensive application of in board low current electrical connectors. Subjected to vibrations, the electrical contacts endure severe fretting wear damage (i.e. wear of contacts induced by oscillating small amplitude displacements). One consequence of this fretting wear damage is that oxide debris are formed and maintained within the interface, decaying the electrical transmission of information. To prevent this critical damage, various soft and thin electrolytic coatings like Sn, Ag and Au layers are usually applied. In order to better understand the damage processes and provide a quantitative strategy to select the coating palliatives, an original approach coupling representative experiments and analytical models is introduced. Using a representative micro-fretting system, the electrical resistance evolution is shown to be mainly controlled by the applied displacement and sliding condition. It is confirmed that below a threshold displacement amplitude, related to a stabilised partial slip condition, the electrical endurance is infinite. Above this threshold, when full gross slip conditions are running, a finite endurance behaviour is observed. The electrical endurance is then controlled by the interfacial oxide debris layer formation and the nature of the coating. For non-noble coatings, like Sn, the electrical failure comes about as soon as an oxide debris layer, the so-called “third body” is trapped within the interface, while the endurance of noble and semi-noble materials (i.e. Au, Ag), which is significantly greater, is controlled by the progressive decrease of noble element concentrations in the third body layer and the related increase of its current resistivity. Finally, simple and basic exponential formulations are introduced to rationalise the global electrical endurance of “connector like” micro-contacts subjected to fretting wear. Using this formulation, an endurance ratio is developed to compare the endurance performance of the different coating palliatives.

Mechanism of Whisker Growth on Pure Sn Coating of Cu Leads in the High Temperature/Humidity Storage Tests

Sn whiskers can grow spontaneously on the surface in Sn alloy coatings. This paper focused on the mechanism of whisker growth on the pure Sn coating to Cu leads in electronic components. After the rack plating process the pure Sn coating was annealed at 150°C for one hour and measured about 10mm in thickness. Then samples endured the high temperature/humidity storage tests (60°C and 87%RH) from 1 month to 6 months. The experimental results revealed that Sn whiskers were prone to grow from the surface with local corrosion in the Sn coating. This was the high temperature/humidity storage provided much moisture to cause an electrochemistry corrosion between Sn and H2O. The excess formation of SnO2 had a large molar volume comparing to Sn and this generated a high inner stress in the corroded areas to shear the coating surface producing the defects. The compressive stress deriving from the formation of Cu-Sn intermetallic compounds drove Sn whiskers initially grew from the corroded areas.

Investigation of Sn–Zn electrodeposition from acidic bath on EQCM

Tin–zinc (Sn–Zn) alloy with low tin content was deposited on gold electrode and steel substrate with use of chronoamperometric technique from an acidic bath. In order to evaluate coating efficiency of Sn–Zn alloy in 0.5 M NaCl solution, open circuit potential–time curve ( E OCP– t), polarization curves, mass change of the electrode (Δ m– t) using quartz crystal microbalance (QCM) were compared to those of pure Sn and Zn coatings. Anodic stripping measurements were carried out simultaneously with the mass loss of the deposit. Scanning electron microscopy (SEM) and energy dispersive X-ray spectra (EDS) analysis were performed to characterize the surface morphology. Anodic stripping experiment and EDS analysis indicated that Sn, Zn, and SnO 2 formed on the electrode surface when Sn–Zn was coated from acidic bath. Furthermore, local mapping demonstrated homogeneous distribution of Sn and Zn atoms throughout the surface.

Estimation of Whiskers Growth on Sn Coating of Cu-Leads in Different Accelerated Tests

With the ban of Sn-Pb alloys coated on Cu leads in electronic devices, the electronic packaging industry uses Pb-free Sn alloys as the coating materials. This replacement meets a widely concern that Pb-free Sn alloys have a stronger propensity to grow whiskers on the surface of the coating. The experimental samples have been plated a pure Sn coating on Cu leads by an industrial plating process before the accelerated tests. The study executed three different environment conditions to estimate whiskers growth on the Sn coating of Cu leads. They were Room Temperature/Humidity Storage (RS), High Temperature/Humidity Storage (TH) and Temperature Cycling (TC). No whisker had been found in the samples during the RS tests since this environment condition had not destroyed the surface continuity to promote whiskers grow out of the Sn coating. In the TH tests, the moisture degraded the coating surface to produce the electrochemical corrosion on some local areas. Sn whiskers were easy prone to grow from the corroded areas on the coating surface with a high rate. In the TC tests, Sn whiskers grew from the cracks on the coating surface with a high density based on the mechanism of the thermal fatigue. The alternating stress in the coating surface induced by the temperature fluctuation tore the Sn oxide grains to form cracks and provide a growth path for Sn whiskers on the coating surface.

Effect of Substrate Composition on Sn Whisker Growth in Pure Sn Films

Pure Sn films deposited on Cu and Cu alloys are prone to spontaneous whisker formation. One way of preventing whisker formation is to alloy Pb into Sn coatings. However, restriction on the use of Pb demands the development of alternative methods for preventing whisker growth. The present work reports the effect of substrate composition on whisker formation and morphology. Despite employing identical plating conditions, long filament-like whiskers grew only on Sn-plated Cu samples and not on brass. The presence or lack of Sn whiskers has been explained via the thermodynamic stability of various intermetallic compounds at the Sn/substrate interface.

Codeposition of copper and tin from acid sulphate solutions containing gluconic acid

RDE voltammetry, XPS and XRD techniques were applied to investigate the codeposition of copper and tin from acid gluconate solutions. Voltammograms contain two plateaus of limiting current of diffusive nature which obeys Levich equation. Underpotential deposition of tin occurs at potentials 0.3 V higher than the equilibrium potential of Sn|Sn(II) electrode. Cu–Sn deposits contain carbon and oxygen that prevail at the surface and are found in the deeper layers of coatings. Chemosorption of gluconic acid with its partial destruction is supposed to take place. Bright Cu–Sn coatings are obtained when the quantity of inclusions is sufficiently high. Cu–Sn coatings obtained in the UPD area consist of α phase or solid solution of tin in copper and a small amount of pure copper. The β-CuSn and δ-CuSn phases as well as a small amount of η phase were detected at −0.31 V. At a more negative potential (−0.4 V) Cu–Sn coatings contain the pure Sn phase, η phase and a small amount of β-CuSn.

Corrosion resistance of Pb–Sn composite coatings reinforced by carbon nanotubes

Carbon nanotubes/Pb–Sn composite coatings were prepared by electrodeposition technology. The polarization curves and electrochemical impedance of the Pb–Sn coatings and carbon nanotube/Pb–Sn composite coatings were studied in 3.0 wt% HCl, 10 wt% NaOH, and 3.5 wt% NaCl electrolyte solutions, respectively. The results show that the corrosion potential of carbon nanotubes/Pb–Sn composite coatings were improved in the three kinds of corrosive medium, especially in 3.5 wt% NaCl electrolyte solution, where it increased significantly from −0.592 V (vs SCE) to −0.535 V (vs SCE). In addition, composite coatings have higher electrochemical impedance. Carbon nanotubes can improve the corrosion resistance of lead–tin electroplated coatings.

Thin film deposition using a plasma source with a hot refractory anode vacuum arc

Vacuum arc generated plasma was used to deposit metallic Al, Zn, and Sn coatings on glass substrates. An arc mode with a refractory anode and an expendable cathode (the “hot refractory anode vacuum arc”), overcomes macroparticle (MP) contamination experienced in other arc modes. I = 100–225 A arcs were sustained between a water-cooled coating source cathode and an anode, which was heated by the arc, separated from each other by a 10-mm gap, for times up to 150 s. The distance from the arc axis to the substrate (L) was 80–165 mm. Film thickness was measured with a profilometer. It was found that the deposition rate increased with time to a peak, and then decreased to a steady-state value. The peak occurred earlier when using short anode (9 mm long), e.g., with the Al cathode, L = 110 mm, and I = 200 A, the peak was at t p = 15 s after arc ignition while with the long anode t p = 45 s. t p decreased with I, from 45 s with I = 100 A, to 10 s with I = 225 A with the short anode. The peak is believed to appear due to initial condensation of cathode material (including MPs) on the cold anode, and its subsequent evaporation as the anode heated. In the later HRAVA steady state, a balance between condensation and evaporation on the anode is established. The deposition rate peak was significant with low melting temperature Al and Zn cathodes, which produce many MPs, and negligible with Cu and Ti cathodes.

Improvement of Electrochemical Migration Resistance by Sn Coating on Cu Electrode in Printed Circuit Board

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Dynamic Recrystallization (DRX) as the Mechanism for Sn Whisker Development. Part II: Experimental Study

In Part I of this study, a dynamic recrystallization (DRX) model was proposed to describe the development of metal whiskers. A diffusion-assisted, dislocation-based mechanism would support the DRX steps of grain initiation (refinement) and grain growth. This, Part II, describes experiments investigating the time-dependent deformation (creep) of Sn under temperature conditions (0°C, 25°C, 50°C, 75°C, and 100°C) and stresses (1 MPa, 2 MPa, 5 MPa, and 10 MPa) that are commensurate with Sn whisker development, in order to parameterize the DRX process. The samples, which had columnar grains oriented perpendicular to the stress axis similar to their morphology in Sn coatings but of larger size, were tested in the as-fabricated condition as well as after 24 h annealing treatments at 150°C or 200°C. The steady-state creep behavior fell into two categories: low ( 10−7 s−1). The apparent activation energy (ΔH) at low strain rates was 8 ± 9 kJ/mol for the as-fabricated condition, indicating that an anomalously or ultrafast diffusion mass transport mechanism assisted deformation. Under the high strain rates, the ΔH was 65 ± 6 kJ/mol (as-fabricated). The rate kinetics were not altered significantly by the annealing treatments. The critical strain (e c) and Zener–Hollomon parameter (Z) confirmed that these stresses and temperatures were nearly capable of causing cyclic DRX in the Sn creep samples, but would certainly do so in Sn coatings with the smaller grain size. The effects of the annealing treatments, coupled with the DRX model, indicate the need to maximize the creep strain rate during stress relaxation so as to avoid conditions that would favor whisker growth. This study provides a quantitative methodology for predicting the likelihood of whisker growth based upon the coating stress, grain size, temperature, and the similarity assumption of creep strain.