Effect of direct current and pulse plating parameters on tin whisker growth from tin electrodeposits on copper and brass substrates

Abstract

Electroplated tin finishes are widely utilised in the electronics industry due to their advantageous properties such as excellent solderability, electrical conductivity and corrosion resistance. However, the spontaneous growth of tin whiskers during service can be highly deleterious, resulting in localised electrical shorting or other harmful effects. The formation of tin whiskers, widely accepted as resulting from the formation of compressive stresses within the electrodeposit, has been responsible for a wide range of equipment failures in consumer products, safety critical industrial and aerospace based applications. The numbers of failures associated with tin whiskers is likely to increase in the future following legislation banning the use of lead in electronics, the latter when alloyed with tin, being an acknowledged tin whisker mitigator. Using a bright tin electroplating bath, the effect of process parameters on the characteristic structure of the deposit has been evaluated for deposition onto both brass and copper substrates. The effect on whisker growth rate of process variables, such as current density and deposit thickness, has been evaluated. In addition, the effect of pulse plating on subsequent whisker growth rates has also been investigated, particularly by varying duty cycle and pulse frequency. Whisker growth has been investigated under both ambient conditions and also using elevated temperature and humidity to accelerate the growth of whiskers. Studies have shown that whisker formation is strongly influenced by pulse plating parameters. Furthermore, increasing both current density and thickness of the deposit reduce whisker growth rates. It is also observed that whisker formation is greatly accelerated on brass substrates compared with copper. The basis for this observation is explained.     

The effect of different post-electroplating surface modification treatments on tin whisker growth

There are very few studies that have investigated directly the effect of an oxide film on tin whisker growth, since the ‘cracked oxide theory’ was proposed by Tu in 1994. The current study has investigated the effect of both a molybdate conversion coating and a tungstate conversion coating on tin whisker growth from Sn–Cu electrodeposits on Cu, and compared it with that from an electrochemically formed oxide produced from a potassium bicarbonate-potassium carbonate electrolyte. X-ray photoelectron spectroscopy (XPS) has been used to investigate the effect of both immersion time and applied potential on the thickness and composition of the oxide film. The XPS studies show that the oxide film formed using either of the conversion coating baths is significantly thicker than that produced from the potassium bicarbonate-potassium carbonate bath. Initial observations suggest that both the tungstate-based conversion coatings and the molybdate-based conversion coatings significantly reduce whisker growth by over 80% for all conversion coating systems compared with a native air-formed oxide and provide improved mitigation compared with the electrochemically formed oxides previously investigated.     

添加剂对甲基磺酸盐镀锡电沉积过程的影响

为了探讨添加剂对锡电沉积的作用机理,运用循环伏安法、计时安培法和阴极极化曲线法研究了添加剂2-巯基苯并噻唑对锡电沉积过程的影响.结果表明锡的电沉积过程经历了晶核形成过程,其电结晶按照三维瞬时成核方式进行,加入添加剂没有改变锡的电结晶机理,对锡的电沉积过程起阻化作用,但有利于晶核的形成,从而可以得到结晶细致的镀层.     

Porous TiO2 film prepared by micro-arc oxidation and its electrochemical behaviors in Hank's solution

Porous titanium oxide film was prepared by micro-arc oxidation (MAO) method on the surface of titanium alloy in electrolyte containing Ca and P. Surface characterizations of the film were carried out using X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) before and after immersion in Hank's solution. Electrochemical behaviors and corrosion resistance were studied by electrochemical techniques. The film was mainly composed of titania, α-tricalcium phosphate (α-TCP) and amorphous Ca-P compounds. α-TCP and amorphous compounds could transformed into hydroxyapatite (HA) when immersed in Hank's solution. MAO film showed higher corrosion potential and lower corrosion current than the titanium alloy and its chemical stability was slightly changed after formation of HA. Fitted electrochemical impedance spectroscopy (EIS) data indicated that after immersion for 2 weeks the MAO film kept good corrosion resistance. Porous TiO₂ film on titanium alloy by MAO method showed good chemical stability in Hank's solution and the transformation of Ca-P compounds into HA indicated that MAO was an effective method for preparing titanium alloys as bioactive artificial bone substitute even when Ca and P in the tissue environment were not abundant.     

Synthesis and growth mechanism of carbonated hydroxyapatite whisker from natural collophanite via a template-directed homogeneous precipitation method

We reported a method to prepare carbonated hydroxyapatite (CHAP) whisker by a template-directed homogeneous precipitation process. In this process, natural collophanite was well purified and used as raw materials to promote the utilization of natural phosphorite resource. The results derived from XRD and FT-IR revealed that the as-prepared CHAP whisker had no impurities and crystallized completely. Its morphology observed by SEM and AFM showed a distinct whisker shape and a uniform distribution with the diameter in sub-micron scale and the length/diameter ratio of about 120–140, respectively. In light of the experimental results and the interface diffusion theory, a “step growth mechanism” was proposed to explain the formation of CHAP whisker in this process.     

High-temperature electrochemical synthesis and properties of intermetallic compounds of the Ni---Sc system Part 3. Synthesis of intermetallic compounds of the Ni---Sc system in molten salts by currentless transfer Part 4. Electrochemical synthesis of the intermetallic compounds of the Ni---Sc system

The formation mechanism of the Ni---Sc intermetallic compounds (lMCs) in molten salts by currentless transfer is discussed. It is shown that this mechanism is typical for the alloy-formation process involving electronegative metals having no cations of low oxidation state. The influence of several parameters of the currentless transfer (temperature, process duration, size of scandium metal surface in contact with the electrolyte) on the kinetics of the alloy-formation and the phase composition of the IMC layers has been studied. An electrochemical synthesis of Ni---Sc intermetallic compounds was performed. Some details of this process are discussed. The parameters of the solid state interdiffusion are estimated using chronopotentiometry data.     

Annealing and oxidation study of Ta-Ru hard coatings

Refractory metal alloy coatings have been widely used as protective coatings on glass molding dies. The formation of intermetallic compounds in the coatings inhibits grain growth at high-temperature environment in the mass production of optical components. The current work presents Ta-Ru coatings with a Cr interlayer on cemented carbide substrates and silicon wafers deposited by direct current magnetron co-sputtering at 400 °C. The as-deposited Ta-Ru coatings possessed a hardness of 13-14 GPa and a surface roughness of 1.3-4.0 nm. The annealing treatments were carried out at 600 °C under two vacuum levels of 3 × 10^− 3 and 3 Pa, respectively. After annealing in vacuum at 3 × 10^− 3 Pa, the Ta-Ru coatings showed grain size, hardness, surface roughness and phase stability comparable to those of the as-deposited coatings. While annealing in vacuum at 3 Pa, preferential oxidation of Ta in the Ta-Ru coatings was verified by X-ray photoelectron spectroscopy, a variation of the chemical composition in depth was analyzed by Auger electron spectroscopy and the internal oxidation zone consisting of a laminated structure was observed by transmission electron microscopy.     

Microstructural characteristics of immersion tin coatings on copper circuitries in circuit boards

Immersion tin coating, which is used as a lead-free surface finish, is deposited on the surface of copper circuitries on circuit boards by a replacement reaction. The characteristics of immersion tin coatings and the formation of tin whiskers and the intermetallic compound (IMC) are described. A Cu₆Sn₅ phase forms at the beginning of the immersion-plating process and expands until all of the tin has been transformed into a copper-tin alloy. The IMC layer becomes thicker and the volume of the pure tin layer decreases during storage. The tin involved in the formation of whiskers must originate from the tin layer of the immersion tin coating. Therefore, the formation of tin whiskers stops when all of the tin has been expended. Moreover, relatively larger whiskers grow on thicker coating layers, which contain more tin.     

Formation and evolution of phosphorus-containing species during high-temperature oxidation of TiAl dipped in a low-concentrated phosphoric acid solution

TiAl samples were dipped in a low-concentrated phosphoric acid solution, air dried, then heated to 800 °C under argon gas before oxidation under reconstituted air at this temperature. At the end of the heating ramp, sample surfaces were covered by a titanium pyrophosphate (TiP₂O₇) layer, well adhered to the substrate. This pyrophosphate was shown to be detectable from 400 °C. Chemical reactions have been proposed for the formation of this compound. TiP₂O₇ coating strongly increased the oxidation resistance of TiAl at 800 °C for 100 h. After 100 h, a transition period occurred leading to the change of TiP₂O₇ into TiO₂, which was achieved after 230 h, total mass gain always remaining below that of raw substrate. This evolution was demonstrated not resulting from a thermal decomposition of pyrophosphate compound.     

基于微弧氧化和电化学沉积在纯Ti基体表面制备FHAP/MAO复合涂层的腐蚀行为和机械性能研究

本论文采用微弧氧化和电化学沉积（ED）技术在工业纯钛（CPTi）表面沉积氟掺杂羟基磷灰石（FHAP）/微弧氧化（MAO）复合涂层。并在Hsnk"s溶液中对未涂覆的CPTi基材和涂覆的样品进行电化学耐腐蚀性测试。本文研究了MAO界面层对涂层微观结构，力学性能和电化学性能的影响。结果表明，HAP / Ti，FHAP / Ti和FHAP / MAO / Ti复合涂层样品在模拟Hank"s溶液中显着提高了CPTi基体的耐腐蚀性能。然而，力学性能测试表明，与具有MAO界面层的FHAP / Ti涂层的结合强度（18.1MPa）相比，FHAP / Ti涂层的结合强度较差（10.7MPa）。此外，FHAP / MAO / Ti涂层与去离子水的接触角约为35.8°，这更有利于促进细胞附着和增殖。     

Recovery of copper and tin from stripping tin solution by electrodeposition

In this paper, the method of recycling copper and tin by constant-current and constant-voltage electrolysis from the stripping tin solution of tinned copper wastes was studied. The experimental results show that the elements could be deposited on the cathode in turn by different deposition potentials, therefore, the copper and tin were separated by constant-voltage electrolysis but not constant-current electrolysis. In this study, the influence of anode materials was also investigated. Graphite anode is stable without impurities dissolved into the stripping tin solution, while 316 stainless steel anode is dissolved into Fe2？and Fe3？as anodic corrosion, which could decrease the deposition efficiency of tin. The copper and tin in the stripping tin solution are separated orderly by electrodeposition at different voltages using the graphite anode. The recovery rate of copper is up to 100 % at 2.00 V, while that of tin is 80 % at 3.00 V.     

Mechanistic interpretation of electrochemical behaviour of galvannealing coating in saline environment

A comparative study on galvannealed (GA) and galvanised plain-skin passed (GP-SP) coatings was carried out to evaluate coating microstructures, the corrosion resistance and electrochemical behaviour in 3.5% NaCl solution. The corrosion resistance behaviour of GA coating was found to be superior compared to GP-SP coating. The Fe-Zn intermetallic phases formed in GA coating provided galvanic protection i.e., a cell reaction between Fe and Zn within a single phase. This gives rise to protective potential plateau and are believed to be responsible for the electrochemical polarisation resulting in sluggish corrosion reaction kinetics, thereby reducing the corrosion rate significantly after prolonged exposure in saline environment. It was also observed from the electrochemical impedance spectroscopy (EIS) study that the coating capacitance (C_c) decreased and polarisation resistance (R_p) increased with exposure time indicating a continuous charge transfer reaction across the coating and electrolyte interface. Although the corrosion potential of both coatings increased towards more noble direction with exposure time, it was observed that the potential for GA coatings was always nobler than GP-SP coating as E_corr shifted towards more positive potential corresponding to a flat potential band of −850 ± 20 mV.     

低锡青铜的吹氧精炼

对低锡青铜进行了吹氧精炼试验。试验结果表明，在０．０３ＭＰａ压力下，吹氧５～６ｓ，即可去除５００ｋｇ铜水中所含氢气。     

Synthesis and electrochemical properties of Li4Ti5O12

The spinel compound Li₄Ti₅O₁₂ was synthesized by a solid state method. In this synthesizing process, anatase TiO₂ and Li₂CO₃ were used as reactants. The influences of reaction temperature and calcination time on the properties of products were studied. When calcination temperature was 750 °C and calcination temperature was 24 h, the products exhibited good electrochemical properties. Its discharge capacity reached 160 mAh g⁻¹ and its capacity retention was 97% at the 50th cycle when the current rate was 1 C. When current rate increased to 10 C, its first discharge capacity could reach 136 mAh g⁻¹, and its capacity retention was 85% at the 50th cycle.     

Study of iron oxidation in sulfur dioxide atmospheres by means of the35S radioisotope

Results of investigations of the mechanism of iron oxidation in atmosphere containing sulfur dioxide are presented. Experiments were carried out both by the platinum marker method and by means of the two -stage oxidation method using sulfur dioxide labeled with the³⁵S radioisotope. SO₂ partial pressures applied were 0.03 and 1 atm at 800°C. The reaction occurred by outward diffusion of iron ions at both sulfur pressures. In addition to cation diffusion there was also some inward sulfur penetration which occurred, however, not by volume diffusion but by short-circuit paths. At the lower SO₂ partial pressure these paths are probably discontinuities in the scale (microfissures), whereas, forp = 1 atm, the inward penetration paths are probably large cavities produced by dissociation at the edges of the specimen when the reaction time is long enough.     

Evaluation of Alloy 146, 279, 900, and 926 sensitization to intergranular corrosion by means of electrochemical methods and image analysis

Properties of stainless steels are affected by improper heat treatment. When that happens, the material can become sensitive to intergranular corrosion. This loss of properties is due to a microstructural changes. In this paper, electrochemical potentiokinetic reactivation tests (single and double loop) were used to evaluate steel sensitization to intergranular corrosion. Four alloys, Alloy 146 (UNS 1.4331) and 279 (UNS 1.4429) (austenitic stainless steels), Alloy 900 (UNS 1.4462) (duplex stainless steel), and Alloy 926 (UNS N08926) (high alloyed stainless steel), were employed as working electrodes. The specimens were sensitized at different temperatures (600–825 °C) for periods of 1 or 2 h in argon atmosphere. Electrochemical tests were conducted using the electro-optical devices P-200002525 and P-200002526. These devices permit to obtain images of the electrode surface and electrochemical data simultaneously. In addition, these results were compared to standardized etching with oxalic acid (ASTM A-262), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Two alloys showed sensitization to intergranular corrosion, Alloy 900 and Alloy 926 at 725 and 825 °C. Additionally, the single and double loop methods showed different passivation results. In the double loop test, the formation of a layer over the entire electrode surface was visible during the process of activation. The film remained until the end of experiment. However, in the single loop method no significant variations were observed on the electrode surface during passivation process, but if material is sensitized a film was formed during reactivation branch.     

Amphiphilic and photocatalytic behaviors of TiO2 nanotube arrays on Ti prepared via electrochemical oxidation

Amphiphilic TiO₂ nanotube arrays (TiO₂ NTs) were fabricated through electrochemical oxidation of Ti in solution containing H₃PO₄ and NaF. Scanning electron microscopic analysis shows that the as-prepared TiO₂ NTs have an average pore diameter of 100 nm and a wall thickness of 15 nm. The electrochemical oxidation of Ti can be divided into four stages. In the first stage, when the potential is very low, oxygen formation and Ti dissolution are the major reactions. The second stage corresponds to a slightly higher potential, but less than 2.5 V. In this stage, the formation of TiO₂ film occurs. When the potential is increased to the even higher range from 2.5 V to 6 V, the TiO₂ film dissolves and nanoporous surface structure is generated. This is the third stage. Further increase of the potential enters stage four. The high potentials cause the self-organization of the nanostructure and allow the formation of well-aligned TiO₂ NTs. We also found that the change in surface condition of Ti by annealing heat treatment affects the film dissolution kinetics. As compared with TiO₂ thin film, the TiO2 NTs show higher photocatalytic activity on decomposing Rhodamine B. The surface of the TiO₂ NTs can be wetted by both water and oil. Such an amphiphilic property comes from the capillary effect of the nanochannel structure of the TiO₂ NTs. Because of the amphiphilic property and the photocatalytic activity, we conclude that the TiO₂ NTs have the capability of self-cleaning.     

Electrochemical properties of microarc oxidation films on a magnesium alloy modified by high-intensity pulsed ion beam

Microarc oxidation (MAO) films on AZ31 magnesium alloy were treated by high-intensity pulsed ion beam (HIPIB) irradiation with ion energy of 300 keV at ion current density of 200 A/cm². A remelted layer of a few micrometers was produced on irradiated MAO films. The corrosion resistance of MAO films was characterized by potentiodynamic electrochemical test in 3.5% NaCl solution. The anodic polarization behavior of MAO samples exhibited a transition from the active anodic dissolution for the original one to the passivation-pitting breakdown for the modified films. The passivation-pitting breakdown voltage of modified films increased with multi-shot irradiation, i.e. from a value of − 1420 mV(SCE) at 1 shot to − 800 mV(SCE) at 5 shots, and the corresponding passivation current density decreased by two orders of magnitude. The irradiated MAO films have a higher corrosion potential than the original one, reaching a maximal value of − 1350 mV(SCE) at 5 shots from the original − 1580 mV(SCE). The electrochemical impedance spectrum (EIS) of modified MAO films was measured with varying the immersion time in 3.5% NaCl solution. The Nyquist impedance plots from modified films may show only a capacitive loop at the immersion time of 5 h, and the inductive loop occur at longer immersion time, whereas original films presenting both capacitive and inductive loops at all the immersion time. Bode plots of MAO films before and after irradiation were obviously distinguished in shape at low frequency range, especially at short immersion time. The evolution of Nyquist and Bode plots with immersion time was discussed in association with the processes of electrolyte penetration into the MAO structure and resultant reaction at the film-substrate interface. It is concluded that the improvement in the continuity and compaction of MAO films accounts for the enhanced corrosion resistance of the films irradiated by HIPIB.