Effect of Cu addition on whisker formation in tin-rich solder alloys under thermal shock stress

This article focuses on the influence of thermal shocks and Cu addition on tin whiskers growth on the surface of tin-rich materials and alloys. The tests were carried out on real samples manufactured with classical PCB technology. Four Pb-free materials i.e. pure Sn, Sn99Cu1, Sn98Cu2 and Sn97Cu3 were tested from the point of view of susceptibility to whisker formation after thermal shocks. Results show that all tested materials were prone for whisker formation. Copper addition in coexistence with thermal shocks did not promote the growth of filament-like whiskers.     

Growth of Si whiskers by MBE: Mechanism and peculiarities

We analyzed the stress-driven mechanism of MBE Si whisker growth. It is shown that the driving force for MBE whisker growth is determined by the relaxation of elastic energy stored in the overgrown layer L_s due to gold intrusion. In this case the supersaturation is determined by the interplay between elastic stresses and surface energy. The latter is considerably decreased due to decoration of the Si surface by gold resulting in formation of thin liquid Si/Au eutectic layer. This suggests that in our case the Si supersaturation is not an independent growth parameter as it is in the chemical vapor deposition growth method. Instead it is determined by stress in the overgrown Si layer. This approach allows us to explain quite well the growth kinetic and the relationship between the radius and the length of the whiskers. The whisker growth in our case can be considered as a stress relaxation mechanism, where the stress relaxation occurs due to transition from the two-dimensional system to the three-dimensional one.     

磁场对滤纸上Zn电解沉积物形貌的影响

研究了外加磁场对滤纸上Zn分枝状电解沉积物生长形貌的影响.用金相显微镜和原子力显微镜观察Zn沉积物的显微结构.发现在非饱和浓度下生长的沉积物生长形貌受电解液浓度和滤纸纤维影响，外加磁场对其形貌的影响不明显.在饱和电解液浓度下获得的沉积物分枝分两层，并且贴近滤纸表面的薄层比其上面离滤纸较远的那一层生长快.在饱和浓度电解液情形下，0.40T外加磁场使得沉积物分枝发生明显的螺旋状偏转.在实验基础上结合相关理论，认为外加磁场通过霍尔效应影响滤纸上Zn分枝状电解沉积物形貌. 关键词： 电解沉积 磁场 霍尔效应 生长形态     

Electrodeposition of Pb-free Sn alloys in pulsed current

A pulsed electrodeposition method is applied to the preparation of Pb-free Sn alloys solder bumps for flip-chip bonding with the aid of a photolithography. Sn-Ag alloy films with near eutectic compositions (Sn-3.5% Ag) were obtained using a pyrophosphate-iodide plating baths regardless under direct or pulsed current. The composition and the morphology of electrodeposits were examinated by SEM and X-ray photoelectron spectroscopy (XPS). The main results revealed that the organic additives affect the electrochemical reduction of tin-silver and the direct consequence on making Sn-Ag alloy is a decreased deposition rate. However, the addition of additives in the plating bath suppressed the dendritic tin-silver growth by adsorption on the deposited surface. Pulsed electrodeposition is shown to be an interesting approach to elaborate bumps with smooth and homogeneous surfaces.     

Spontaneous alloying of tin atoms into nanometer-sized gold clusters and phase stability in the resultant alloy clusters

Alloying behavior and phase stability has been studied in situ by transmission electron microscopy using clusters in the Au-Sn system. When tin atoms are vapor-deposited onto nm-sized gold clusters, rapid dissolution of tin atoms into gold clusters takes place and as a result Au-rich solid solution, amorphous-like Au-Sn alloy and AuSn compound clusters are formed depending upon the concentration of tin. The remarkable enhancement of solubility has been observed in Au-rich solid solution and AuSn compound. It becomes more difficult to form two phases in the interior of individual clusters even if the composition of alloy clusters falls in the two-phase region in the phase diagram for the bulk alloy and as a result amorphous-like phase is stabilized in nm-sized Au-Sn alloy clusters. Received 2 August 1999 and Received in final form 8 November 1999     

Giant magnetoresistance (GMR) and ferromagnetic properties of DC and pulse electrodeposited Cu–Co alloys

Cu–Co ferromagnetic alloys occurring as granular films and exhibiting giant magnetoresistance (GMR) property have been synthesized using both DC and pulse electrodeposition techniques. The growth process of these electrodeposits comprising multiple granules of disparate morphology, magnetic features exhibits critical dependence on electrodeposition conditions. Using ferromagnetic resonance and magnetic hysteresis data, we have attempted a correlation between evolution of these electrodeposits and their ferromagnetic properties with special emphasis on GMR property.     

Tin amalgam mirrors: investigation by XRF, SEM-EDS, XRD and EPMA-WDS mapping

Ancient mirrors were constituted by a tin-mercury amalgam layer superimposed to a glass sheet. This was the only one method used until the nineteenth century, when the wet silvering process was invented. The tin amalgam is a binary alloy of tin and mercury constituted by two different phases: a mercury-rich liquid phase and a tin-rich solid phase. The amalgam alteration produces mercury loss and a general growth of the solid crystalline phase. In addition, tin dioxide and monoxide are formed with a consequent decrease of the amalgam adhesion to the glass. These degradation phenomena led to reduction or disappearance of the mirror reflective power. The aim of this study was the characterization of the amalgam layers of eight mirror samples dating during the seventeenth and nineteenth centuries. The samples were analyzed by X-ray diffraction and by a Scanning Electron Microscope with an Energy Dispersive Spectrometer (SEM-EDS), and for the first time on this type of alloy by X-ray Fluorescence and EPMA-WDS (Electron Probe Micro Analysis with Wavelength Dispersive Spectrometry) elemental mapping. The contents of tin, mercury, and some trace elements in the amalgam layers have been determined. The investigation of the superficial patterns of the amalgam by SEM, EPMA-WDS mapping, and SEM-EDS allowed a first understanding of some morphologies and processes of the degradation of the amalgam layer.     

Tin/Indium nanobundle formation from aggregation or growth of nanoparticles

Shape and size controlled gram level synthesis of tin/indium (SnIn) alloy nanoparticles and nanobundles is reported. Poly(N-vinylpyrrolidone) (PVP) was employed as a capping agent, which could control the growth and structure of the alloy particles under varying conditions. Transmission electron microscopy showed that unique SnIn alloy nanobundles could be synthesized from the bulk materials above a certain concentration of PVP and below this concentration, discrete spherical nanoparticles of variable size were evolved. The morphology and the composition of the as-synthesized SnIn alloy nanobundles were investigated by high-resolution transmission electron microscopy (TEM). The possible mechanisms on the formation of these structures were discussed.     

Phonon emission spectra of superconducting tunnel junctions

Resonant scattering of phonons by the stress split acceptor ground state in Si:B has been used to investigate the phonon emission spectra of superconducting tunnel junctions. Symmetric Sn, Pb and PbBi alloy junctions have been studied in the single particle tunneling regime. The spectra show many features as expected from relaxation and recombination of quasiparticles via one-phonon emission. The results are compared with the steady state solutions of the kinetic equations for quasiparticles and phonons. Spatially homogeneous distribution functions are assumed. This approach describes the spectra of tin junctions fairly well. However, there are marked deviations for Pb and PbBi junctions which are attributed to a decay of transverse phonons within the junction.Dedicated to K. Dransfeld on the occasion of his 60th birthday     

New insight into the nature and properties of pale green surfaces of outdoor bronze monuments

The present study concerns the chemical–physical and electrochemical characterisations of the pale green surfaces formed on outdoor bronzes exposed in urban conditions. In the first part, results from investigations performed on the equestrian statue of the French king Louis XIV exposed in the Palace of Versailles (France) are given. Analyses by energy-dispersive spectrometry and Raman spectroscopy, coupled with scanning electron microscopy, show that the external layer is characterised by a marked selective dissolution of copper and zinc of the alloy leading to an important relative enrichment in tin compounds. The same phenomenon with the same magnitude, determined from dissolution factors f_Cu and f_Zn, has also been evidenced on other bronze monuments used for comparison. Proportionality between the amount of dissolved copper and zinc cations to their respective initial content in the alloy is evidenced independently of the tin content. The pale green patina appears to be a complex mixture of copper and tin compounds whose structure still needs to be more precisely characterised. In the second part, the electrochemical reactivity of tin compound enriched patina was investigated by cyclic voltammetry from a Cu10Sn electrode in sulfate solution at pH=2 and 5.6. This patina is stable at pH 5.6 but reactive at pH 2 in relation to the modification of properties of tin species in the patina. Mott–Schottky application in pH 5.6 solution revealed that the bronze patina exhibits two types of semiconducting properties according to the potential domains, similar to what has been observed for pure tin in aqueous solutions. The destabilisation of bronze patinas in outdoor conditions and consequently the cyclic erosion due to rainfall have been attributed to the modification of tin species properties rather than to the transformation of copper compounds. PACS  07.78.+s; 81.05.-t; 81.15.-z; 81.65.Kn     

Effects of platinum nano electrodeposits on corrosion of carbon substrate

The electrodes for the experiments were prepared by electrodepositing Pt on a carbon substrate. In the cyclic voltammograms of the carbon electrodes with and without Pt nano electrodeposits, the total anodic current including the currents from the oxygen evolution reaction and carbon corrosion increased abruptly above a critical potential. The oxidation overpotential of the carbon electrodes with Pt nano electrodeposits was lower than that of the bare carbon electrode. This phenomenon was more prominent at 75 °C than at 25 °C. In the potentiostatic experiments, the current transients and corresponding power spectral density (PSD) increased with increasing applied potential to the electrodes. Furthermore, the current transients for the carbon electrodes with Pt nano electrodeposits were much higher than those for the bare carbon electrode. This suggests that the corrosion of the carbon substrate can be accelerated by the Pt nano electrodeposits.     

Mössbauer Investigation of Electrodeposited Sn–Zn,Sn–Cr,Sn–Cr–Zn and Fe–Ni–Cr Coatings

⁵⁷Fe and ¹¹⁹Sn CEMS, XRD and electrochemical measurements were used to investigate the effect of the preparation parameters and the components on the structure and phase composition of electrodeposited Fe-Ni-Cr alloys in connection with their corrosion behavior. XRD of the electrodeposits reflect an amorphous-like character. ⁵⁷Fe CEM spectra of Fe-Ni-Cr electrodeposited samples, prepared in a continuous flow plating plastic circulation cell with variation of current density, electrolyte velocity and temperature, can be evaluated as a doublet associated with a highly disordered paramagnetic solid solution phase. This phase was identified earlier in Fe-Ni-Cr electrodeposits that were prepared by another plating method and contained both ferromagnetic and paramagnetic metastable phases [1]. This is the first time that we have succeeded to prepare Fe-Ni-Cr alloys containing only the metastable paramagnetic phase. The effect of the plating parameters on the structure is also analysed by the quadrupole splitting distribution method. ¹¹⁹Sn CEM spectra of all Sn-containing plated alloys show a broad line envelop which can be decomposed at least into two components. One can be associated with β-tin. The other one can be assigned to an alloy phase. The structure and distribution of microenvironments of these phases depends on the plating parameters especially on the parameters of the reverse pulse applied. This revised version was published online in August 2006 with corrections to the Cover Date.     

电感耦合等离子体发射光谱法测定铝合金中痕量锡

本文报道了电感耦合等离子体发射光谱法测定铝合金中痕量锡，通过标准样品分析，加标回收实验以及对铝合金实际样品的不同分析测试手段锡的测定结果的相互对照，分析结果令人满意。该方法简便，快速，灵敏度高，适合于铝合金中低含量锡的分析测定。     

Direct evidence for reversible diffusional phase change in nanometer-sized alloy particles

Solid solubility in isolated nanometer-sized particles has been studied by in situ transmission electron microscopy using alloy particles in the Pb-Sn binary system. In approximately 17-nm-sized particles of a lead-56 at. % Sn solid solution, a phase change from a single phase of the lead solid solution to two phases, a lead solid solution and a tin-solid solution, takes place when the temperature is reduced from 110 degrees C to room temperature (RT). Furthermore, it is confirmed that this phase change could occur rather reversibly when the temperature is cyclically changed between 110 degrees C and RT. This observation provides direct evidence for reversible diffusional phase change in nanometer-sized alloy particles. It seems safe to conclude that the solubility limit of tin in lead is higher than 56 at. % at 110 degrees C, which is almost 5 times higher than the solubility limit of tin in bulk lead (i.e., 10 at. %).     

Mutual influence of voltage steps in the V–I characteristics of current-carrying superconducting tin whiskers

The mutual influence of the voltage steps in the V–I characteristics of tin whiskers near the superconducting/normal transition temperature was investigated by means of several potential probes along one sample. It is shown that the steps generated at one part of a whisker may enhance the critical currents related to the appearance of steps at other parts of the whiskers. Especially the width between the steps may become larger by the presence of other voltage steps.     

In situ XPS investigation of Pt(Sn)/Mg(Al)O catalysts during ethane dehydrogenation experiments

Calcined hydrotalcite with or without added metal (Mg(Al)O, Pt/Mg(Al)O and Pt,Sn/Mg(Al)O) have been investigated with in situ X-ray photoelectron spectroscopy (XPS) during ethane dehydrogenation experiments. The temperature in the analysis chamber was 450 °C and the gas pressure was in the range 0.3-1 mbar. Depth profiling of calcined hydrotalcite and platinum catalysts under reaction, oxidation and in hydrogen-water mixture was performed by varying the photon energy, covering an analysis depth of 10-21 Å. It was observed that the Mg/Al ratio in the Mg(Al)O crystallites does not vary significantly in the analysis depth range studied. This result indicates that Mg and Al are homogeneously distributed in the Mg(Al)O crystallites. Catalytic tests have shown that the initial activity of a Pt,Sn/Mg(Al)O catalyst increases during an activation period consisting of several cycles of reduction-dehydrogenation-oxidation. The Sn/Mg ratio in a Pt,Sn/Mg(Al)O catalyst was followed during several such cycles, and was found to increase during the activation period, probably due to a process where tin spreads over the carrier material and covers an increasing fraction of the Mg(Al)O surface. The results further indicate that spreading of tin occurs under reduction conditions.A PtSn₂ alloy was studied separately. The surface of the alloy was enriched in Sn during reduction and reaction conditions at 450 °C. Binding energies were determined and indicated that Sn on the particle surface is predominantly in an oxidised state under reaction conditions, while Pt and a fraction of Sn is present as a reduced Pt-Sn alloy.     

Three-dimensional characterisation and modelling of small fatigue corner cracks in high strength Al-alloys

The growth of fatigue cracks at small length scales is known to be influenced by a variety of factors, including local microstructure, varying stress states and crack shape. High resolution computed tomography allows for sub-micron resolution imaging of failure processes in small test coupons undergoing in situ cyclic loading, providing detailed three-dimensional (3D) assessment of propagation processes across the entire crack front (surface and depth). In this work fatigue crack growth has been examined in an advanced Direct Chill (DC) cast aluminium alloy, along with a fine grained powder-metallurgy alloy. The latter is identified as a model material, offering considerably simpler microscopic crack paths than the DC cast alloy, and hence a means of separating bulk mechanical effects (such as stress state variations across a crack front and plasticity induced closure) from microstructural effects (such as crystallographic deflection and roughness induced crack closure). Crack growth has been studied in both materials under both constant amplitude (CA) and single peak overload (OL) conditions. Experimental results are presented in the present paper, particularly in relation to micromechanical understanding of failure. A modelling approach based on those results, and some typical results, is also presented.     

微波射频场在微波管中产生爆炸电子发射特性初步分析

 对微波射频场在微波管内引起的场致发射和爆炸电子发射及等离子体的产生进行了分析,推导了等离子体产生强度与微波振幅、材料的电阻率、热传导系数、质量密度和比热容之间的关系,得到了晶须温度分布的表达式,通过数值解析的方式总结出在远大于微波周期的时间尺度上晶须温度提高随时间线性上升。在模型所述材料特性下,温度的上升率达到了3.22×10¹⁰ ℃/s,在100 ns量级就可以使晶须发生气化形成等离子体。     

Size, position and direction control on GaAs and InAs nanowhisker growth

The self-organized, position-controlled and parallel growth of GaAs and InAs nanowhiskers is successfully demonstrated by using a metal–organic chemical vapour deposition method. The growth takes place preferentially along the 111 As direction with the aid of the catalytic effect of Au nanodroplets, and not along 111 Ga or In directions. The diameter and length of the whisker can be controlled artificially down to 10 nm and to over 1 μm, respectively. Doping and composition control of p- or n-type such as GaAs–InAs heterostructure formation are possible along the length direction of the whisker by changing the source gases. In order to control the growth position of the whisker, positioning of a Au nanodroplet is essential and realized by a lithographic method. By choosing the [111]B direction to the substrate surface and normal to the patterned side edges, and by positioning the Au nanodroplet on the side wall, the positioned planar nanowhisker growth and bridging are successfully demonstrated. The growth mechanism of the nanowhiskers is revealed by the scanning and transmission electron microscope observations. Nanometer-size Au-alloy droplets play an important role in the growth of the whiskers. The whisker growth process is governed by the vapor–liquid–solid growth mechanism.     

弱磁场对Zn分枝状电解沉积物形貌的影响

用金相显微镜和原子力显微镜分析了存在外加磁场情况下的Zn分枝状电解沉积物的微观形貌 ，并且根据实验结果提出了磁场使沉积物分枝呈现螺旋状偏转的机理.1)在沉积物晶粒的早 期生长阶段，当晶粒已经形成了择优生长方向但又非常小,仍然悬浮于电解液中时，磁流体 动力学对流作用于它上面的不平衡力使得晶粒连同它的择优生长方向向着对流下方偏转，直 到晶粒足够大，和它的前一个单晶枝晶连接在一起为止.2)当晶粒和它的前一个单晶枝晶连 接在一起以后，磁流体动力学对流的作用力已不足使它偏转.此时，如果电解液的浓度比较 高，单晶枝晶的一级分枝和二级分枝之间不存在对电解质的竞争关系，则此单晶枝晶将完全 按照初期形成的晶体择优生长方向直线生长，直到新的晶粒形核为止;如果电解液浓度较低 ，造成第一级分枝与第二级分枝的竞争关系，二级分枝总在第一级分枝的上游侧生长，使得 一级分枝向下游方向偏转.通过上述两个步骤，无论是枝晶沉积物还是分形沉积物都产生螺 旋状偏转. 关键词： 电解沉积 磁流体动力学对流 晶体择优生长方向 生长形态