Microstructure and chemical characterization of the intermetallic phases in Cu/(Sn,Ni) diffusion couples with various Ni additions

The paper presents new results concerning the influence of nickel addition (1 and 5 at.%) into tin on the development of the Cu/(Sn,Ni) interface area in diffusion couple experiment. The morphology and chemical composition of the intermetallic phases growing in the Cu/(Sn,Ni) diffusion couples were examined by means of the scanning (SEM) and transmission (TEM) electron microscopy after annealing at 215 °C in vacuum for different period time.It was shown that even 1 at.% of nickel addition into tin resulted in formation of intermetallics of complex microstructure. The presence of (Cu_1−xNi_x)₆Sn₅ in two morphological and compositional variants was noted. The discontinuous layer consisting up to 7.2 at.% of Ni closer to copper end-member coexisted with needle-like and faceted precipitates with even 22.3 at.% of Ni, which intensively detached from the interface. At the Cu/(Cu_1−xNi_x)₆Sn₅ interface the formation of Cu₃Sn wavy layer compound was observed in all examined diffusion couples which became thicker with time. The porosity within the both formed intermetallic phases existed irrespective of the amount of added nickel.     

Ultrafast formation of unidirectional and reliable Cu3Sn-based intermetallic joints assisted by electric current

High-temperature-stable Cu₃Sn-based joints were selectively fabricated using electric current-assisted bonding process within an extremely short time (∼200 ms) and under a low pressure of 0.08 MPa in a Cu/Sn/Cu interconnection system at ambient temperature. The experimental results showed that the imposed electric current density (∼10⁴ A/cm²) resulted in sharply increased local temperature as well as accelerated growth of Cu₃Sn intermetallic compounds (IMCs). Under the effects of electron wind force-induced electromigration and joule heat-induced temperature, the transient formation of Cu₃Sn-based joints can thus be obtained across the interfaces. Furthermore, highly unidirectional 〈1 0 0〉 growth of Cu₃Sn IMCs was achieved along the direction of electron flow. By calculating the planar atomic densities of projected images on different planes, the particular growth direction was confirmed to represent the low-scattering path for the traveling electron flow. The oriented Cu₃Sn-based joints exhibited more reliable shear properties than the Sn-based joints.     

Development and characterization of cube-textured Ni–Cu–Co substrates for YBCO-coated conductors

Ni–Cu–Co alloys were studied for the development of textured substrates for YBCO-coated conductor application. Three compositions were obtained by adding a fixed amount of 3 at.% Co to the binary Ni_xCu_100?x, where x = 40, 50 and 60. Cube texture was induced by conventional cold rolling followed by high-temperature annealing. The structural, microstructural, morphological, electrical, magnetic, mechanical and oxidation properties were evaluated and compared with those exhibited by the binary Ni–Cu alloy, as well as by Ni–W and pure Ni. A low Ni content is detrimental for the development of the cube texture with respect to higher concentrations. Nevertheless, the use of high annealing temperatures enabled an area fraction of cube orientation as high as 95% to be obtained for x = 40, and >97.5% in the case of Ni-richer alloys. Compared with Ni and Ni–W, Ni–Cu–Co alloys oxidize more easily and exhibit higher electrical resistance. In addition, the presence of copper enables the Curie temperature to be reduced to 60 K for x = 40 and to 155 K for x = 50. Furthermore, the introduction of cobalt reduces the oxidation rate at temperatures normally used for the deposition of ceramic buffer layers, thus allowing the successful development of a CeO₂/YSZ/CeO₂ architecture on ternary Ni–Cu–Co alloy. YBCO/buffer multilayer architecture deposited by pulsed laser deposition on a selected alloy tape exhibits a critical current density exceeding 1 MA cm^?2 at 77 K in self-field, indicating that this alloy substrate is suitable for YBCO-coated conductor application.     

Ni segregation in the interfacial (Cu,Ni)6Sn5 intermetallic layer of Sn-0.7Cu-0.05Ni/Cu ball grid array (BGA) joints

Ni segregation in the interfacial (Cu,Ni)₆Sn₅ intermetallic layer of Sn-0.7Cu-0.05Ni/Cu BGA solder joints was investigated by using synchrotron micro X-ray fluorescence (XRF) analysis and synchrotron X-ray diffraction (XRD). Compared to Sn-0.7Cu/Cu BGA joints, Ni containing solder show suppressed Cu₃Sn growth in both reflow and annealed conditions. In as-reflowed Sn-0.7Cu-0.05Ni/Cu BGA joints, Ni was relatively homogenously distributed within interfacial (Cu,Ni)₆Sn₅. During subsequent annealing, the diffusion of Ni in Cu₆Sn₅ was limited and it remained concentrated adjacent the Cu substrate where it contributes to the suppression of Cu₃Sn formation at the interface between the Cu substrate and Cu₆Sn₅ intermetallics.     

Microstructure and shape memory behavior of Ti55.5Ni44.5−xCux (x = 11.8–23.5) thin films

The microstructure and shape memory behavior of Ti_55.5Ni_45.5−xCu_x (x = 11.8–23.5) thin films annealed at 773, 873, and 973 K for 1 h were investigated. None of the films except the Ti_55.4Ni_32.8Cu_11.8 film annealed at 773 K for 1 h had any precipitates in the B2 grain interiors and their grain sizes were small (less than 1 μm). Increasing the annealing temperature caused grain growth and thus a decrease in the critical stress for slip and an increase in the martensitic transformation start temperature (Ms). The grain size was also controlled by the growth of a second phase. In the three-phase equilibrium region of Ti₂Ni, Ti₂Cu and TiNi, Ti₂Cu grains grew faster than Ti₂Ni grains, leading to a decrease in the critical stress for slip and an increase in the Ms temperature with increasing Cu content.     

Vacuum brazing of TiAl-based intermetallics with Ti–Zr–Cu–Ni–Co amorphous alloy as filler metal

An amorphous Ti41.7–Zr26.7–Cu14.7–Ni13.8–Co3.1 (wt%) ribbon fabricated by melt spinning was used as filler to vacuum braze Ti–48Al–2Nb–2Cr (at%) intermetallics. The influences of brazing temperature and time on the microstructure and strength of the joints were investigated. It is found that intermetallic phases of Ti₃Al and γ-Ti₂Cu/Ti₂Ni form in the brazed joints. The tensile strength of the joint first increases and then decreases with the increase of the brazing temperature in the range of 900–1050 °C and the brazing time varying from 3 to 15 min. The maximum tensile strength at room temperature is 316 MPa when the joint is brazed at 950 °C for 5 min. Cleavage facets are widely observed on all of the fracture surfaces of the brazed joints. The fracture path varies with the brazing condition and cracks prefer to initiate at locations with relatively high content of γ-Ti₂Cu/Ti₂Ni phases and propagate through them.     

Employment of a bi-layer of Ni(P)/Cu as a diffusion barrier in a Cu/Sn/Cu bonding structure for three-dimensional interconnects

This study explored the possibility of employing a bi-layer barrier of electroless-plated Ni(P)/thin Cu layers in a Cu/Sn/Cu bonding structure for three-dimensional interconnects. Our materials analysis revealed that the bi-layer barrier served effectively as a diffusion barrier and prevented full-scale materials interaction for temperatures higher than 300 °C. Such suppression of an intermetallic compound reaction and limiting Cu diffusion led to the formation of a rod-shaped Cu₆Sn₅ compound, rendering a unique microstructure of ductile Sn embedded with strong Cu₆Sn₅ rods. Our mechanical characterization using lap-shear testing and fracture analysis revealed that the sample with such a microstructure displayed a high bonding strength with some ductility, a desirable combination for high mechanical reliability.     

Intrinsic and Interdiffusion in Cu-Sn System

Solid-solid diffusion couples assembled with disks of copper, tin and intermetallics (Cu₃Sn and Cu₆Sn₅) were employed to investigate the Kirkendall effect in the copper-tin system at the temperature of 200 °C. In the Cu(99.9%)/Sn diffusion couple, inert alumina particles used as markers were identified in the Cu₆Sn₅ phase, while microvoids were observed at the Cu/Cu₃Sn interface. The Cu(99.9%)/Sn and Cu(99.9%)/Cu₆Sn₅ diffusion couples annealed at 200 °C for 10 days were analyzed for intrinsic diffusion coefficients of Cu and Sn in the Cu₆Sn₅ and Cu₃Sn phases, respectively with due consideration of changes in molar volume. Interdiffusion, integrated and effective interdiffusion coefficients were also calculated for the intermetallic phases. Diffusion couples annealed at 125-400 °C for various times were analyzed for the kinetic parameters such as growth rate constants and activation energies for the formation of Cu₃Sn and Cu₆Sn₅ phases. Uncertainties in the calculated intrinsic diffusivities of Cu and Sn arise mainly from the non-planar morphologies of the interfaces and the non-planar distribution of the markers. Intrinsic diffusion coefficients based on average locations of the marker plane indicate that Cu is the faster diffusing component than Sn in both the Cu₃Sn and Cu₆Sn₅ phases.     

Tribological behaviour and residual stress of electrodeposited Ni/Cu multilayer films on stainless steel substrate

In the present study, [Ni (4.5 nm)/Cu (t_Cu = 2, 4 and 8 nm)] multilayers were pulse electrodeposited on stainless steel (AISI SS 304) substrate from sulphate based single bath technique. X-ray diffraction (XRD) was used to investigate the structure and stress of the Ni/Cu multilayer. The results from XRD analysis indicated that the deposited multilayers had a preferred crystal orientation of [111] and presence of satellite reflection suggested the formation of superlattice. The stress level within the deposited multilayers was found to be sensitive to the sublayer thickness. Sliding wear behaviour of electrodeposited Ni/Cu multilayer films has been investigated against a tungsten carbide (WC) ball as the counter body and compared with that of the constituents, Cu and Ni coatings. The wear tests were carried out by using a reciprocating ball-on-flat geometry at translation frequencies of 5 and 10 Hz, slip amplitude of 1 mm and at five different loads of 3, 5, 7, 9 and 11 N. Friction force was recorded on-line during the tests. At the end of the tests, the wear scars were examined by laser surface profilometry and scanning electron microscopy (SEM). Friction coefficient was found to be dependent on load and Cu layer thickness (t_Cu) and the values for multilayers were border between Ni and Cu. Among multilayers, sample with minimum t_Cu has shown the lowest friction coefficient and wear rate. With increasing t_Cu, the wear mechanism changes from pure abrasive wear at t_Cu = 2 nm, to particle entrapment at t_Cu = 4 nm to particle embedding at t_Cu = 8 nm. Detailed investigation of the wear scar morphology as well as wear rate measurement revealed that at low loads, (H/E) ratio and residual stress governed the wear rate and the principle wear mode was abrasive cutting. At intermediate loads, the role of residual stress became insignificant while wear was governed by (H/E) ratio and plastic deformation. However, at higher loads, plastic deformation played the major role.     

Reactive oxide-dispersed Ni3Al intermetallic coatings by sediment co-deposition

Ni–Al-reactive oxide (REO) ternary composite coatings were successfully deposited from a Watt's nickel bath containing Al particles and REO particles via the sediment co-deposition (SCD) technique. Three different composite systems, Ni–Al-nano CeO₂, Ni–Al-5 μm CeO₂, and Ni–Al–Y₂O₃ (<1 μm), were studied. The volume fraction of the Al particles in the composite coatings was significantly decreased with an increase of the REO bath loading, while the REO particle content positively increased. The REO particles in the plating bath evidently interfered with the deposition of Al particles. The development of intermetallic phases in the annealed Ni–Al-REO composite coatings mainly depended on the Al content in the coatings. REO-dispersed Ni₃Al intermetallic coatings could be formed as long as the REO particle loading in the bath was controlled below a critical level. Transformation of CeO₂ phase to CeAlO₃ was found in the Ni–Al-nano CeO₂ composite coatings during the annealing treatment at 800 °C.     

Novel multilayer Mg–Al intermetallic coating for corrosion protection of magnesium alloy by molten salts treatment

A novel multilayer Mg–Al intermetallic coating on the magnesium alloy was obtained by AlCl₃–NaCl molten salt bath treatment. The molten salt was treated at 400 °C, which is lower than the treatment temperature of solid diffusion Al powder. The thick Al₁₂Mg₁₇, Al_0.58Mg_0.42 and Al₃Mg₂ multilayer Mg–Al intermetallic coating forms on the magnesium alloy. The corrosion resistance of AZ91D alloy with and without coating by multilayer of Mg-Al intermetallic compound was evaluated by electrochemical impedance spectroscopy measurements in 3.5% (mass fraction) NaCl solution. The polarization resistance value of the multilayer coating on the magnesium alloy by molten salt bath treatment is greater than that of the uncoated one, which is attributed to the homogenously distributed intermetallic phases.     

Cu content and annealing temperature dependence of martensitic transformation of Ti36Ni49−xHf15Cux melt spun ribbons

The martensitic transformation in Ti₃₆Ni_49−xHf₁₅Cu_x (x = 1, 3, 5, 8) ribbons has been investigated. Only B2 to B19′ transformation was detected in all the present ribbons. The martensitic transformation temperatures do not change obviously with increase in the Cu content except that they decrease when the Cu content is 3 at.%. The lattice parameters of B19′ martensite, a and c increase, b almost remains constant, while the monoclinic angle β decreases with increase in the Cu content. For the ribbons with Cu content of 1 and 3 at.%, the martensitic transformation temperatures change slightly when the annealing temperature increases. For the ribbons with Cu content of 5 and 8 at.%, with increase in the annealing temperature, the martensitic transformation temperatures almost do not change and then decrease rapidly when the annealing temperature is higher than 873 K. TEM observation shows that the microstructure of the ribbons with Cu content of 1 and 3 at.% contains the martensite matrix and the (Ti,Hf)₂Ni particles with the size of about 150 nm, which does not change obviously when the annealing temperature increases. This results in that the martensitic transformation temperatures are not sensitive to the annealing temperature in the ribbons with 1 and 3 at.% Cu content. However, nano-scale (Ti,Hf)₂Ni particles precipitate in the ribbons with Cu content of 5 and 8 at.% when the annealing temperature is 773 and 873 K, and then the (Ti,Hf)₂Ni particles grow and coarsen rapidly with further increase in the annealing temperature. The coarsening of the (Ti,Hf)₂Ni particles should be responsible for the dramatic decrease of the martensitic transformation when the annealing temperature is higher than 873 K. For all the present ribbons, the substructure of B19′ martensite is (001) compound twins, and the inter-variant relationship is mainly (011) type I twinning.     

A first quantitative XPS study of the surface films formed, by exposure to water, on Mg and on the Mg-Al intermetallics: Al3Mg2 and Mg17Al12

An XPS investigation was carried out of the surface films, formed by exposure to ultrapure water, on mechanically ground Mg and the two Mg-Al intermetallic compounds: Al₃Mg₂ and Mg₁₇Al₁₂. The mechanically ground Mg surface had a film of MgO at the Mg metal surface covered by a Mg(OH)₂ layer, formed by the reaction of the MgO with water vapour in the air. Upon immersion in ultrapure water, this film converted to a duplex film with an inner MgO layer next to the Mg metal and an external porous hydroxide layer. For both intermetallics, the XPS data is consistent with (i) preferential dissolution of Mg and (ii) a 10 nm thick film on the surface after immersion in ultrapure water; the film composition on Al₃Mg₂ was AlMg_1.4O_0.2(OH)_5.4 whilst on Mg₁₇Al₁₂ the composition was AlMg_2.5(OH)₈.     

On the influence of sol-gel derived CeO2 coatings on high-temperature oxidation of Co, Ni and Cu

The effects of CeO₂ coatings on high-temperature oxidation of Co, Ni and Cu have been investigated as a function of temperature at oxygen pressures from 1×10⁻⁴ to 1 atm. The oxidation mechanisms for Co and Cu are essentially unaffected by CeO₂ coatings, whereas the oxidation rate of Ni decreases by approximately one order of magnitude. The oxygen pressure dependence does not change markedly with CeO₂ coatings for any of the metals studied. For oxidation of Ni plus CeO₂ coatings, the temperature dependence is less marked at lower temperatures, whereas essentially the same behavior is observed for Co and Cu with and without the coating. Differences in the effects of CeO₂ coatings for the three metal systems have been attributed to the relative influence of grain boundary transport on the overall rates of oxidation.     

Intermetallic Compounds Formed in In-3Ag Solder BGA Packages with ENIG and ImAg Surface Finishes

During the reflow process of In-3Ag solder ball grid array (BGA) packages with electroless nickel immersion gold (ENIG) and immersion silver (ImAg) surface finishes, continuous (Au_0.9Ni_0.1)In₂ and scallop-shaped (Ag_0.9Cu_0.1)In₂ intermetallic layers form at the interfaces of In-3Ag solder with Au/Ni/Cu and Ag/Cu pads, respectively. The (Au_0.9Ni_0.1)In₂ layer breaks into clusters with increases in the aging time and temperature. Aging at 115 °C results in the formation of an additional continuous Ni₁₀In₂₇ layer on the Ni/Cu pads and the migration of (Au_0.9Ni_0.1)In₂ intermetallic clusters into the solder matrix. In contrast, the (Ag_0.9Cu_0.1)In₂ scallops grow into a continuous layer after aging treatment. Accompanying the interfacial reactions, AgIn₂ precipitates in the interior of In-3Ag solder balls and coarsens during aging, causing the ball shear strengths of reflown ENIG (1.18 N) and ImAg (1.11 N)-surface-finished solder joints to decrease gradually. However, the migration of (Au_0.9Ni_0.1)In₂ clusters into the solder matrix of ENIG-surface-finished In-3Ag packages leads to an increase in their ball shear strengths after aging at 115 °C over 300 h. Both the ENIG- and ImAg-surface-finished In-3Ag solder joints, after ball shear tests, have fractured across the solder balls with ductile characteristics.     

Changes in mass and stress during anodic oxidation and cathodic reduction of the Cu/Cu2O multilayer film

The anodic oxidation and cathodic reduction processes of the Cu/Cu₂O multilayer film and pure Cu film in pH 8.4 borate buffer solution were analyzed by electrochemical quartz crystal microbalance (EQCM) for gravimetry and bending beam method (BBM) for stress measurement. The mass loss of the multilayer film during anodic oxidation at 0.8 V (SHE) in the passive region was less than that of the pure Cu film. The comparison between current transients and mass changes during anodic oxidation has succeeded in separating the anodic current density into two partial current densities of oxide film growth, i_O^2-, and of Cu²⁺ dissolution through the passive film, i_Cu²⁺. As a result, in the case of the pure Cu film, the anodic current density was mainly due to i_Cu²⁺, while in the case of the multilayer film, i_Cu²⁺ was almost equal to i_O^2-. The compressive stress for the multilayer film was generated during anodic oxidation, while the tensile stress for the pure Cu film was generated.The mass loss of the multilayer film during cathodic reduction at a constant current density (i_c = −20 μA cm⁻²) was significantly less than that estimated from coulometry, suggesting that H₂O produced by cathodic reduction remained in the multilayer film. The compressive stress was generated during cathodic reduction of the multilayer film, which was ascribed to H₂O remained in the multilayer film.     

Nanocrystalline matrix Al3Ni2–Al–Al3Ni composites produced by reactive hot-pressing of milled powders

Mechanically alloyed nanocrystalline Al₆₃Ni₃₇ powder with a metastable structure of NiAl intermetallic phase was mixed with 30 vol.% of Al powder. This powder mixture was consolidated under the pressure of 7.7 GPa at 600, 700, 800 and 1000 °C for 15 and 180 s. During the consolidation, in all cases, the metastable NiAl phase transformed into the equilibrium Al₃Ni₂ intermetallic. Moreover, a solid-state reaction between the intermetallic matrix and Al occurred, yielding an Al₃Ni phase. Progress of this reaction depended on the consolidation temperature and temperature exposure time, thus Al₃Ni₂–Al, Al₃Ni₂–Al–Al₃Ni or Al₃Ni₂–Al₃Ni composites were produced by hot-pressing with various parameters. The mean crystallite size of the Al₃Ni₂ intermetallic matrix in the composites is 39–67 nm, depending on consolidation parameters. The composites hardness is in the range of 6.02–7.51 GPa.     

Effect of intermetallic compound thickness on shear strength of 25 μm diameter Cu-pillars

The chip-to-chip bonding technique using Cu-pillars bump is widely applied in 3D chip stacking technology. The excessive growth of intermetallic compounds (IMC) is expected to increase the propensity to brittle failure. Typically, the thickness of the IMC layer is used to indicate the risk of failure of solder joints. This study investigates the effects of Cu₆Sn₅ and Cu₃Sn compounds on the single-joint shear strength of Cu-pillar bumps 25 μm in diameter joined with Sn–3.5 wt%Ag–0.7 wt%Cu (SAC 357) alloy. The influence of heat treatment on the shear strength of the Cu-pillar structures is studied by applying two types of heat treatment: (i) a standard conveyor furnace process and (ii) isothermal holding at 240 °C for holding times up to 30 min. The change in mechanical strength was then established as a function of total IMC thickness through shear test experiments. Shear strength was measured with different displacement rates of 70, 130 and 500 μm s⁻¹. The shear height, from the tip of the shear tool to Cu pad substrate, varied from 11 μm to 16 μm in 1 μm steps. The variation in shear force values through the interfacial system, from pure Cu-pillar to solder ball, are discussed in relation to the failure shape observations. For low shearing heights (11–12 μm), mainly Cu material is probed and high shear force values are measured. For high shearing heights (15–16 μm), the probed materials are Cu₆Sn₅ and Sn and low shear force values are measured. For intermediate shearing heights (13–14 μm) the probed materials are Cu₃Sn and Cu₆Sn₅ and the Cu/Cu₃Sn interface seems to be as strong as the Cu₃Sn/Cu₆Sn₅ interface, despite the fact that almost all the Kirkendall voids are located in Cu/Cu₃Sn interface.     

Growth behavior of Cu6Sn5 in Sn–6.5 Cu solders under DC considering trace Al: In situ observation

High resolution time-resolved X-ray imaging with synchrotron radiation has been used to in situ observe the growth behavior of Cu₆Sn₅ intermetallic compounds (IMCs) during solidification in Sn–6.5 Cu and Sn–6.5 Cu–0.2 Al (wt. %) solders under applied direct current (DC) field. The morphological evolution of Cu₆Sn₅ with I-like, X-like, Y-like and bird-like shapes is directly observed. It is shown that trace levels of Al have a marked effect on the solder microstructures and refining the size of the primary Cu₆Sn₅. The solidification pathway leading to the refinement is observed in real time using synchrotron microradiography. After adding the trace Al, I-like shapes bifurcate into X-like shapes. Furthermore, when DC field with 10 A/cm² is applied, both the growth rate and the mean size of Cu₆Sn₅ are increased but decreased when 100 A/cm² is applied. Meanwhile, the effect of thermodynamic potential barrier caused by DC field on the growth behavior of Cu₆Sn₅ is discussed.     

Strong coupling effects during Cu/In/Ni interfacial reactions at 280 °C

The substantial heat generated in three-dimensional integrated circuits and high-power electronics has made thermal management a critical challenge for reliability in the electronics industry. Pure indium solder has been used as a thermal interface material to minimize the contact thermal resistance between a chip and its heat sink. Indium and indium-based alloys are potential lead-free solder for low-temperature applications. Heat sinks in the heat dissipation system as well as substrates of electronic joints are usually made of copper, with nickel being the most commonly used diffusion barrier on the chip side. Therefore, the Cu/In/Ni sandwich structure would be encountered in electronic devices. The soldering process for forming the Cu/In/Ni structure crucially determines the reliability of devices. In this study, Cu/In/Ni interfacial reactions at 280 °C were investigated. Intermetallic compounds were identified and the microstructural evolution was observed. A strong coupling effect between Cu and Ni was found, which caused several peculiar phenomena: (1) the formation of a Cu–In compound (the Cu₁₁In₉ phase) at the In/Ni interface; (2) the formation of two sub-layers of the Cu₁₁In₉ phase at the Cu/In interface; (3) the formation of faceted rod-like Cu₁₁In₉ grains; and (4) the formation of a half-Cu₁₁In₉, half-Ni₃In₇ microstructure after prolonged reactions. The mechanism of phase transformations is elucidated based on the calculated Cu–In–Ni ternary phase diagram using CALPHAD thermodynamic modeling.