Sn Diffusion Research Articles

Interfacial reaction of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates

The interfacial reactions of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates after soldering and subsequent aging have been investigated in this study. Ni–W alloy layers with tungsten content of 3.0 and 10.0 at.% were electrodeposited on copper substrate. The interfacial micrographs of solder joints prepared at 250 °C for 15 s and aged at 150 °C for 24, 96 and 216 h are shown. Double-layer IMC composed of Cu5Zn8 and Ag3Sn was observed at the interface of Sn–2Ag–2.5Zn and Cu couple, which was compact and acted as a barrier layer to confine the further growth of Cu–Sn IMC. On Ni–W barrier layer, a thin Ni3Sn4 film appeared between the solder and Ni–W layer, whose thickness decreases with the increase of W content. During the aging process, a thin layer of the Ni–W substrate transforms into an amorphous bright layer, and the thickness of amorphous layer increased as aging time extended. Referring to the elemental line-distribution and the thickness of different layers at the interface, the formation of the bright layer is caused by the fast diffusion of Sn into Ni–W layer.

Evaluation of the bond strength of a low-fusing porcelain to cast Ti–24Nb–4Zr–7.9Sn alloy

The purpose of this study was to compare the bonding characteristics of titanium porcelain Duceratin bonded to Ti–24Nb–4Zr–7.9Sn (TNZS) alloy and commercial pure titanium (cp Ti). The bond strengths between porcelain and TNZS were tested by a three-point flexural device. The same tests for the cp Ti were used as for the control. Coefficient of thermal expansion (CTE) of TNZS was evaluated with a push-rod dilatometer. Interfacial characterization was carried out by X-ray energy-dispersive spectrometry (EDS) analysis operating in line scan mode. Additionally, microstructure characterizations of TNZS and cp Ti after debonding fracture were analyzed by scanning electron microscope (SEM) and EDS. The porcelain bond strength of TNZS alloy was 31.51MPa, showing a significant increase relatively to that of cp Ti (23.89MPa) (P<0.05). Mean CTE values of TNZS alloy was 9.51×10−6/°C exceeding the porcelain by 0.81×10−6/°C, attesting to a better mechanical performance. Interfacial characterization showed the mutual diffusion of Ti, Si, O and Sn along the TNZS–ceramic interface. Both SEM and EDS results revealed that fracture modes of TNZS specimens exhibited a mixed mode of cohesive and adhesive failures. The results demonstrated that TNZS could be a good alternative for the metal–ceramic restoration in the future.

Development and Evaluation of Direct Deposition of Au/Pd(P) Bilayers over Cu Pads in Soldering Applications

The thermal reliability of Sn-3Ag-0.5Cu/Au/Pd(P)/Cu solder joints was evaluated in this study. After reflow and subsequent solid-state aging (180°C), the reaction product species at the interface included Cu6Sn5 [or (Cu,Pd)6Sn5] and Cu3Sn, and their growth was strongly dependent on the Pd(P) thickness, δPd(P). As δPd(P) increased, the growth of Cu6Sn5 was significantly enhanced, while that of Cu3Sn was suppressed. Computer coupling of phase diagrams and thermochemistry (CALPHAD) analysis showed that minor incorporation of Pd (~2 at.%) into the Cu6Sn5 phase decreased the Gibbs free energy of Cu6Sn5 from −7339 J/mol to −9191 J/mol. This effect might enhance Sn diffusion in Cu6Sn5 but diminish Cu diffusion in Cu3Sn, thereby facilitating the growth of Cu6Sn5 but retarding that of Cu3Sn. High-speed ball shear (HSBS) test results showed that the mechanical properties of the solder joints were slightly enhanced by an increase in δPd(P). These findings suggest that direct deposition of Au/Pd(P) bilayers over the Cu pads can effectively modify the mechanical reliability of solder joints.

Texture evolution and its effects on growth of intermetallic compounds formed at eutectic Sn37Pb/Cu interface during solid-state aging

The growth orientation of Cu6Sn5 intermetallic compounds (IMCs) formed at the eutectic Sn37Pb/polycrystalline Cu interface during solid-state aging was investigated. The results indicate that the interfacial Cu6Sn5 grains exhibit textured growth under solid-state conditions, and their preferred orientations are affected by the initial joint preparation conditions. Cu6Sn5 grains in the [0001] direction normal to the interface are stable in solid and molten Sn37Pb solder at 200°C, but are rapidly consumed at 280°C. This effect leads to the formation of different textures in the Cu6Sn5 layer during the solid-state aging treatment of joints formed at 200 and 280°C. In addition, the influence of texture evolution on the growth of interfacial IMCs was evaluated. The results indicate that Sn diffusion is faster along the [0001] direction of the Cu6Sn5 crystal than along an angle of 25–45° to the [0001] direction; therefore, more IMCs are generated at the interface of the joints formed at 200°C than at those formed at 280°C under the same solid-state reaction conditions.

Diffusion and growth mechanism of phases in the Pd-Sn system

Growth mechanism of phases and atomic mechanism of diffusion are discussed in the Pd-Sn system. The Kirkendall marker plane location indicates that the PdSn4 phase grows because of diffusion of Sn. Atomic arrangement in the crystal indicates that Sn can diffuse through its own sublattice but Pd cannot diffuse unless antisites are present. The negligible diffusion of Pd indicates the absence of Pd antisites. The activation energy value indicates that the contribution from grain boundary diffusion cannot be neglected although experiments were conducted in the homologous temperature range of 0.7–0.79.

Reliability evaluation on a submicron Ni(P) thin film for lead-free soldering

The solderability between a Sn–Ag–Cu alloy and a submicron Ni(P) film (0.2μm) was examined using a focused ion beam and field-emission transmission electron microscope. The Ni(P) was electrolessly deposited between the Au and Cu layers, which possessed a low P content (less than 5wt.%) and a nanocrystalline structure. After one typical reflow, the Ni(P) film was mostly eliminated from the interface, where Cu6Sn5 with a significant Ni content [(Cu0.6Ni0.4)6Sn5] nucleated. The subsequent diffusion of Sn to the underlying Cu through molten solder channels among the (Cu0.6Ni0.4)6Sn5 grains yielded a second Cu6Sn5 layer at the (Cu0.6Ni0.4)6Sn5/Cu interface. The removal of Ni from the Ni(P) during soldering reaction allowed P to nucleate as nanocrystalline Ni3(Sn,P) between the two Cu6Sn5 layers, which subsequently translated into a chain of amorphous P–Sn–O pores. The propagation of the porous P–Sn–O destroyed the stability of (Cu0.6Ni0.4)6Sn5 and drove the compound layer to separate from the second Cu6Sn5 after the third reflow. These observations suggest that the exhaustion of the Ni(P) induced spallation of the compound layer, thereby degrading the reliability of the joining interface.

Effect of Cr doping on the photoelectrochemical performance of hematite nanorod photoanodes

Hematite (α-Fe2O3) nanorod arrays modified by surface doping of chromium (III) ions (Cr3+) for photoelectrochemical (PEC) water splitting are fabricated using a general process involving a combination of aqueous chemical growth and spin coating. The PEC activity of Cr-doped α-Fe2O3 nanorod films first increases then decreases with increasing dopant content. At the optimal content of Cr, Cr-doped α-Fe2O3 nanorod films exhibit about 3.5 and 6 times higher PEC activity than the undoped material under solar light (AM 1.5, 100mWcm−2) and visible light (λ>430nm) irradiation, respectively. A comprehensive characterization of the chemical, morphological, PEC, structural, and optical properties of the doped α-Fe2O3 films are presented to assess the mechanisms by which the dopants influence photoelectrode performance. The relationship between dopant content, photoluminescence intensity, and PEC performance suggests Cr doping alters charge transfer in the films under irradiation. At low Cr doping contents, Cr dopants act as electron (or hole) traps and retard photoinduced charge recombination, leading to enhanced PEC activity. Whereas, at high Cr doping contents, Cr dopants act as charge recombination sites and lower the charge separation efficiency, leading to decreased PEC activity. High temperature annealing proves to be effective for further improvement of the PEC activities of doped and undoped hematite films, by Sn diffusion.

Phase Formation, Composition and ${\rm T}_{\rm c}$ Distribution of Binary and Ta-Alloyed ${\rm Nb}_{3}{\rm Sn}$ Wires Produced by Various Techniques

Low temperature calorimetry was used to determine the distribution of the superconducting transition temperature in binary and Ta-doped wires fabricated by the Bronze route and by the Powder-In-Tube (PIT) method. From this analysis we were able to discern the effects of Sn and Ta compositions on the distribution of the superconducting parameters Tc and Bc2 in the different samples. The influence of the heat treatment conditions on the superconducting properties was investigated for the PIT wires. In particular we determined the field dependence of the distribution for a commonly used reaction schedule (675 /84 h) and for an optimized heat treatment (625 /320 h). For the first time, we show that the wire reacted at 625/320 h exhibits two separated contributions in the distribution directly related to the grain morphology of the A15 layer: a narrow peak determined by the large grains, with a lower Bc2, and a broad peak due to the fine grains, with a higher Bc2. The kinetics of the Sn diffusion in Nb and the growth rate of the A15 layer were experimentally studied. The influence of Ta doping on the A15 phase formation was analysed by electron microscopy, the growth rate and the grain morphology in binary and Ta-alloyed Bronze route wires with the same filament layout being compared at different stages of the heat treatment. At the end of reaction, the well known microstructure comprising equiaxed and columnar regions was observed in the filaments of both the binary and the Ta-alloyed wires. Based on these observations and from the growth rate analysis we conclude that Ta does not affect the Sn diffusion rate.

Cu5Zn8 Growth Reversion in Cu/Sn-8Zn-3Bi/Cu During Discontinuous Electromigration

A Cu/Sn-8Zn-3Bi/Cu structure was used to investigate the intermetallic compound (IMC) growth behavior during discontinuous electromigration under current density of 104 A/cm2 at 70°C. Cu5Zn8 IMC formed at both the anode and the cathode interfaces, and the thickness increased with the stressing time. With prolonging the current stressing time, a bulged Cu5Zn8 layer was squeezed out between the former Cu5Zn8 layer and Cu substrate in the samples to relax the excess compressive stress. Additionally, due to the back stress gradient built up by the Sn diffusion, the Zn atomic flux reacted with Cu to form Cu5Zn8 at the cathode side when the power was turned off. Finally, the total IMC thickness of the anode and the cathode under discontinuous current stressing showed a “reversion” in the 69 h and 310 h samples.

Defect levels in SnS thin films prepared using chemical spray pyrolysis

AbstractThe origin of various defect levels in the SnS thin films deposited using chemical spray pyrolysis (CSP) technique has been explored in this manuscript, by employing low‐temperature photoluminescence (PL) technique. Concentration of Sn in the samples was varied purposefully by ex situ diffusion in order to alter the defect levels. The acceptor level obtained at 0.22 eV from the Arrhenius plot, has been assigned as the defect level caused by the Sn vacancies present in the lattice. Two shallow donor levels are conclusively identified and their activation energies have been estimated. The present study could also unearth a trap level in the forbidden energy gap which was due to the oxygen contaminant occupied by the vacancy of Sn. This trap level could be removed by annealing the sample in vacuum or through the ex situ diffusion of Sn. Employing Kelvin probe force microscopy (KPFM), the work‐function of SnS was obtained as 4.925 eV, from which the position of the Fermi level could be assigned. Based on the present work, an energy level scheme for SnS thin films is proposed outlying origin of various defect levels.

Influence of current density on microstructure of pulse electrodeposited tin coatings

Abstract Pulse electrodeposited tin coatings on copper substrate have been synthesized from an aqueous solution containing sodium stannate (Na 2 SnO 3 .3H 2 O) and sodium hydroxide (NaOH). The effect of current density on surface morphology of the deposits has been investigated. As deposited coatings are characterized by X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, and line profile analysis. The X-ray diffraction analysis shows that the deposits consist of tetragonal (β-Sn) structure with microcrystalline grains. The deposits plated at lower current density exhibit (110) texture which decreases with increasing current densities. The effects of current density on Cu–Sn diffusion and whisker growth of the electrodeposited tin coatings are also reported here.

Calculating the diffusivity of Cu and Sn in Cu3Sn intermetallic by molecular dynamics simulations

In this paper, molecular dynamics (MD) simulations are performed to calculate the diffusivity of Cu and Sn atoms in the Cu3Sn intermetallic compound. Our MD results show that Cu atoms are the dominant diffusion species in Cu3Sn. The diffusion coefficient of Cu atoms in Cu3Sn is about 17 times higher at room temperature (300K), and 7 times higher at the annealing temperature (423K) than that of Sn. This large difference in diffusivity between Cu and Sn is believed to be the main cause of Kirkendall void formation within the Cu3Sn layer in lead-free solder joints.

Kinetics of Sn(II) reduction in acid sulphate solutions containing gluconic acid

Voltammetry, electrochemical quartz crystal microgravimetry (EQCM) and electrochemical impedance spectroscopy (EIS) were applied to study kinetics of cathodic processes occurring in acidic Sn(II) gluconate solutions containing an excess of sulphate. Simulations based on the material balance equations showed a rather complicated composition of the solutions involving gluconate, sulphate and hydroxide complexes of Sn(II). Equilibrium potentials following from the Nernst equation satisfactorily coincide with experimental open-circuit potentials. According to EQCM data, Sn(II) reduction is the main process occurring at sufficiently low cathodic polarizations (ΔE). An onset of hydrogen evolution was observed at ΔE≈−0.3V (pH 4). The rotating disc electrode (RDE) and linear potential sweep (LPS) voltammetric data were analyzed using equations based on the charge transfer and diffusive mass transport regularities. The observed considerable decrease in the rate of Sn(II) reduction with solution pH is embodied in the respective diminution of the effective Sn(II) diffusion coefficient. Its values obtained from the limiting (RDE) and peak (LPS) current densities vary from ∼6×10−6 to 6×10−7cm2s−1 at 2<pH<5. Nyquist plots (EIS data) contain two semicircles that become clearly pronounced on subtraction of the double layer impedance. It is assumed that the electroreduction of Sn(II) in gluconate solutions is accompanied by adsorption phenomena whose inhibitive character enhances with solution pH.

Application of AuSn Adjusting Layer for a Novel Ohmic Back Metal for n-Type Si Devices

In this paper, we present a novel ohmic back metal for n-type Si devices. Using an AuSn adjusting layer, a simple and low-cost process is provided, as well as assuring strong adhesion between the metal and the Si substrate. The thickness of the Au layer under the AuSn layer for adjusting the Sn concentration at the interface between the metal and the Si substrate, and the deposition temperature are optimized. A novel back metal is deployed as a good substitute for the conventional AuSb back metal. The eutectic melting ratio (Au 80 wt %, Sn 20 wt %) of the AuSn alloy is confirmed by focused ion beam (FIB) and energy dispersive X-ray spectrometry (EDX). A good ohmic characteristic is obtained upon Sn diffusion to the Au layer. This method is useful for the metallization of various devices owing to its simple and low cost process and its high performance.

Thermal diffusivity of Sn–Ag–Cu-based, Pb-free, micro- and nano-sized solder balls

In order to determine the effects of ball size and porosity on the thermophysical properties of solder materials, several Sn–3.0Ag–0.5Cu solder balls with average ball diameters of 170nm, 10μm, 29μm, and 140μm were prepared, and disk-type samples were formed under compaction pressures of 100, 200, and 300psi. The thermal diffusivity of each sample was then measured using a laser flash apparatus over a temperature range of room temperature to 150°C. The results showed that the thermal diffusivity increased as both the diameter of the solder ball and the compaction pressure increased. On the other hand, the thermal diffusivity decreased by as much as 28% for the same ball sizes and pressures at higher temperatures. Overall, the sample with a ball diameter of 140μm prepared under a compaction pressure of 300psi exhibited the highest thermal diffusivity (about 30×10−6m2/s). Thus, it was found that the thermal diffusivity of a sample composed of solder balls is strongly dependent on the ball size, porosity, and preparation temperature.

High-lead flip chip bump cracking on the thin organic substrate in a module package

Flip chip bump cracking was observed after Si die attach reflow on the organic substrate of a module package. High-lead bump and eutectic SnPb cladding were used on Si die and the substrate sides, respectively. The reflow peak temperature was 260 °C for compatibility with passive components attach using lead-free solder. Flip chip bump cracking occurred at high-lead solder close to the die side. The cracking was eliminated by lowering the reflow peak temperature down to 225 °C. Main cause of the cracking at 260 °C reflow was attributed to the extensive Sn diffusion into high lead bump. This decreased the melting point of the high-lead solder around the die side, which in turn worsened the adhesion between solder and die due to the coexistence of solid and liquid. Diffusion length estimation showed both of the liquid- and solid-state diffusions of Sn. Crack gap in the solder bump was consistent with thermal expansion mismatch between Si die and organic substrate. The bump cracking was mitigated by use of 225 °C reflow, limiting Sn diffusion and providing a good integrity of high lead bumps on die side.

Evolution of the Sn/Ni–8.0 at.%V interfacial reaction paths

Abstract

Fast Concurrent Growth of Ni3Sn4 and Voids During Solid-State Reaction Between Sn-Rich Solder and Ni Substrates

To simulate the growth of Ni3Sn4 phase layers in Sn-based solder joints with Ni substrates during solid-state aging, Sn/(Cu1−x Ni x )6Sn5/Ni and Sn/Ni diffusion couples were aged isothermally at 180°C and 200°C, and the growth kinetics of the (Ni,Cu)3Sn4 and Ni3Sn4 layers in the respective couples were monitored during the isothermal aging. Once the (Ni,Cu)3Sn4 layer was formed at the (Cu,Ni)6Sn5/Ni interface, it grew unexpectedly fast with concurrent growth of voids formed in the Sn layer during prolonged aging at both temperatures. The results obtained from the various types of diffusion couples revealed that the voids formed in the Sn layer were Kirkendall voids, due to the (Ni,Cu)3Sn4 layer growing predominantly at the (Ni,Cu)3Sn4/Ni interface by fast diffusion of Sn across the (Ni,Cu)3Sn4 layer. It is proposed that the accelerated growth of the (Ni,Cu)3Sn4 and Ni3Sn4 layers after the formation of voids in the Sn layer is due to the relaxation of vacancy oversaturation and the enhanced annihilation rate of incoming vacancies in the presence of the voids in the Sn layer.

Mechanism of tin diffusion in ZnTe single crystals

The diffusion coefficient of Sn in single-crystal zinc telluride has been measured. In the temperature range 1070–1270 K, Sn diffusion follows a vacancy mechanism, producing for the most part SnZn substitutional atoms, which act as donors with an ionization energy of ∼0.26 eV.

Growth Behavior of Meta-Stable ${\rm NiSn}_{3}$ Intermetallic Compound and Its Potential Influence on the Reliability of Electronic Components

The morphology, texture evolution and growth behavior of the meta-stable ${\rm NiSn}_{3}$ intermetallic compound (IMC) in Sn electrodeposits over Ni were investigated. It was found that at lower storage temperatures $({ , the plate-like ${\rm NiSn}_{3}$ is the dominant IMC phase in the Sn layers. Upon raising the temperature, a competition between the initiations of ${\rm NiSn}_{3}$ and ${\rm Ni}_{3}{\rm Sn}_{4}$ phases was observed as identified by energy dispersive X-ray. The crystal structure of ${\rm NiSn}_{3}$ was determined by the powder diffraction pattern. The investigations revealed that the growth rate of ${\rm NiSn}_{3}$ in terms of the penetration depth from the Sn/Ni interface to the Sn surface follows the square-root power law, implying a diffusion controlled phase growth. The diffusion coefficients of the ${\rm NiSn}_{3}$ phase growth were determined at different temperatures (25 $^{\circ}{\rm C}$ , 50 $^{\circ}{\rm C}$ , and 75 $^{\circ}{\rm C}$ ), which are significantly higher than that reported for ${\rm Ni}_{3}{\rm Sn}_{4}$ under the same temperatures. The activation energy for the growth of ${\rm NiSn}_{3}$ obtained is 39.90 kJ/mol. Hardness and Young's modulus of ${\rm NiSn}_{3}$ were determined by means of nanoindentation. This paper discusses the potential influences of the ${\rm NiSn}_{3}$ phase growth on the reliability of electronic components containing Ni–Sn diffusion couple.