Kinetics of reactive diffusion between Au and Sn during annealing at solid-state temperatures

The reactive diffusion between Au and Sn was experimentally studied at solid-state temperatures using Sn/Au/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were annealed at temperatures of T = 393 and 473 K for various times in an oil bath with silicone oil. After annealing, compound layers composed of AuSn₄, AuSn₂ and AuSn were recognized to form at the Au/Sn interface. The thickness of the AuSn₄ layer is about six and four times greater than those of the AuSn₂ and AuSn layers at T = 393 and 473 K, respectively. The ratio of the thicknesses of the compound layers is kept constant independently of the annealing time. The total thickness l of the compound layers is described as a function of the annealing time t by the equation l = k(t/t₀)ⁿ, where t₀ is unit time, 1 s. The exponent n is nearly equal to 1/2 at T = 393 K but takes a value between 1/4 and 1/2 at T = 473 K. Such an intermediate value of n at T = 473 K indicates that the grain boundary diffusion contributes to the reactive diffusion and the grain growth occurs at certain rates. As the annealing temperature decreases, the contribution of the grain boundary diffusion should become more remarkable, but the grain growth will slow down. Consequently, n becomes close to 1/2 at T = 393 K. According to the constancy of the ratio of the thicknesses, it is concluded that the same rate-controlling process works in the AuSn₄, AuSn₂ and AuSn layers at a constant annealing temperature.     

Growth behavior of compounds due to solid-state reactive diffusion between Cu and Al

To examine experimentally the kinetics of the reactive diffusion between solid-Cu and solid-Al, sandwich Al/Cu/Al diffusion couples were prepared by a diffusion-bonding technique and then isothermally annealed in the temperature range of T = 693–753 K for various times up to 336 h. Owing to annealing, compound layers of the γ ₁, δ, ζ ₂, η ₂, and θ phases are formed between the Cu and Al specimens. The γ ₁, δ, ζ ₂, η ₂, and θ phases are the only stable compounds at T = 693–753 K in the binary Cu–Al system. At each annealing time, the thickness of the θ phase is much greater than those of the δ, ζ ₂, and η ₂ phases but smaller than that of the γ ₁ phase. Hence, the overall growth of the compound layers is governed by the γ ₁ and θ phases. The mean thickness of each compound layer is proportional to a power function of the annealing time. For the γ ₁ phase, the exponent m of the power function is 0.5 at T = 753 K. Such a relationship is called a parabolic relationship. As the annealing temperature T decreases, however, m gradually increases and then reaches to 0.66 at T = 693 K. On the other hand, for the θ phase, m is close to 0.5 at T = 723–753 K and becomes 0.42 at T = 693 K. In the γ ₁ and θ phases, grain growth occurs at T = 693–753 K. Thus, the layer growth of the θ phase is controlled by volume diffusion at T = 723–753 K but partially by boundary diffusion at T = 693 K. On the other hand, for the γ ₁ phase, volume diffusion is the rate-controlling process of the layer growth at T = 753 K, but interface reaction contributes to the rate-controlling process at T = 693–723 K. Consequently, the rate-controlling process varies depending on the annealing temperature in a different manner for each compound.     

Effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu and SnPb solders with Au/Ni/Cu substrate during aging

The effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu (SAC) and SnPb solders with Au/Ni/Cu substrate during aging was investigated in this study. AuSn₄ needle-like intermetallic compound (IMC) formed at the interfaces of both SAC and SnPb solder joints after laser reflow. In SnPb solder joints, a layer-type ternary IMC, (Au,Ni)Sn₄, formed over a thin layer quaternary IMC (Au,Ni,Cu)₃Sn₄ after aging at 150 °C for 4 days, in which Cu came from the Cu pad below the Ni layer through diffusion. This (Au,Ni,Cu)₃Sn₄ IMC evolved into a new kind of quaternary IMC (Au,Ni,Cu)₆Sn₅ with further aging at 150 °C for 8 days because more Cu diffused through Ni and was involved into the IMC formation. For SAC solder joints, needle-like binary AuSn₄ IMC in as-soldered state changed into a layer-type quaternary IMC (Au,Ni,Cu)Sn₄. The quaternary IMC (Au,Ni,Cu)₆Sn₅, which has the same composition as those found in SnPb solder joints, was developed below the (Au,Ni,Cu)Sn₄ layer after aging at 150 °C for 8 days. It was found that Cu diffusion played a key role in the interfacial reaction and compound formation.     

Formation of intermetallic compound layers in Sn/Au/Sn diffusion couple during annealing at 433 K

The kinetics of the reactive diffusion between Au and Sn was experimentally studied in a metallographical manner. Sn/Au/Sn diffusion couples were prepared by a solid-state diffusion bonding technique and then annealed at 433 K for various times between 1 and 127 h (3.6 × 10³ and 4.57 × 10⁵ s) in an oil bath with silicone oil. Due to annealing, intermetallic compound layers consisting of AuSn₄, AuSn₂ and AuSn are produced at the Au/Sn interface in the diffusion couple. The thickness of the AuSn₄ layer is four times greater than those of the AuSn₂ and AuSn layers. The ratio of the thicknesses of the compound layers remains constant during annealing. The total thickness l of the compound layers increases with annealing time t. Such a relationship is mathematically described by the equation l = k(t/t ₀)ⁿ, where t ₀ is unity time, 1 s. From the experimental results, the proportionality coefficient and the exponent are determined to be k = 2.7 × 10^–7 m and n = 0.42, respectively, by a least-squares method. The exponent n is slightly smaller than 0.5. This implies that the grain boundary diffusion partially contributes to the rate-controlling process for the growth of the compound layers.     

Study on important diffusion parameters of binary Ni3Sn4 phase

Important diffusion parameters, such as—parabolic growth constant, integrated diffusivity, ratio of intrinsic diffusivities of species Ni and Sn, Kirkendall marker velocity and the activation energy for diffusion kinetics of binary Ni₃Sn₄ phase have been investigated with the help of incremental diffusion couple technique (Sn/Ni_0.57Sn_0.43) in the temperature range 200–150 °C. Low activation energy extracted from Arrhenius plot indicates grain boundary controlled diffusion process. The species Sn is three times faster than Ni at 200 °C. Further, the activation energy of Sn tracer diffusivity is greater than that of Ni.     

Substantial diffusion of Zr during solid-state amorphization in Ni/Zr multilayers

To examine the proposed model that amorphization is achieved by the motion of one species only, Ni/Zr multilayers have been prepared by sputter deposition at a pressure of 1 Pa. The as-deposited specimen shows the planar and sharp interface between Ni and Zr layers. The amorphous phase is clearly observed at the residual Ni layers and around the void edge in the Zr layer from the cross-sectional TEM images of the samples annealed at 300 C for 1 h. Composition analysis verifies that considerable Zr diffusion occurs through the grain boundary in the Ni layer during the annealing. The activation energy for amorphization is determined to be 1.17 eV per atom through a Kissinger-type analysis. This paper clearly shows the formation of amorphous NiZr alloys when Zr diffusion is comparable to Ni diffusion.     

Growth kinetics of monofilamentary Nb3Sn and V3Ga synthesized by solid-state diffusion

Studies of growth kinetics of Nb₃Sn and V₃Ga formation have been carried for mono-filamentary composites of niobium and vandium filaments embedded in bronze wires containing varying concentrations of tin and gallium, respectively. The samples are diffusion reacted at different temperatures and for different lengths of time and the thickness and the microstructure of the resulting A-15 layer are investigated using optical and scanning electron microscopy techniques. The results are discussed in the light of the analytical model previously proposed by the present authors and it is shown that while the rate controlling step for the formation of Nb₃Sn is diffusion of tin through the bronze matrix, for V₃Ga it is the diffusion of gallium through the grain boundaries of the compound layer. The data are used to calculate the activation energies for Nb₃Sn and V₃Ga formation.     

Carburation of an Ni70Cr30 substrate by laser solid-state diffusion

Laser surface treatments provide interesting solutions to the problem of accelerated wear of materials. Some results are presented for chromium carbide-rich surface alloys after laser-beam solid-state treatment of an Ni₇₀Cr₃₀ carbon-precoated substrate. The wear results are satisfactory with respect to pure Ni₇₀Cr₃₀ substrate, but are below those obtained by superficial melting of a graphitized Ni₇₀Cr₃₀ substrate.     

Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density

Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate current density of 6.0 × 10² A/cm² at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement, microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating, which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening, volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing.     

Interfacial reaction between Au and Sn films electroplated for LED bumps

Au-20 wt% Sn eutectic solder is used as bumps in flip chip package of power LED (Light Emitting Diode) due to its excellent properties. The Au/Sn dual-layer films were fabricated on Si wafer by pulse electroplating of Au and Sn sequentially, and the solid?Csolid interfacial reaction during aging and the eutectic reaction during reflow soldering were investigated in the present work. After storage at room temperature for 1 week, three phases of AuSn, AuSn₂ and AuSn₄ were sequentially formed at the Au/Sn (10 ??m/10 ??m) interface, and the thickness of this reaction region was about 5 ??m. Firstly, AuSn₄ was formed at the Au/Sn interface, and then AuSn and AuSn₂ were formed at the Au/AuSn₄ interface. After aging at 150 °C for 5 and 10 h, a similar layered structure of AuSn/AuSn₂/AuSn₄ was also observed. Due to the faster diffusion of Au to Sn layer, all the Sn elements were consumed after aging at 150 °C for 15 h and AuSn₄ layer gradually transformed into AuSn and AuSn₂ layers. For the specimen of Au/Sn (9 ??m/6 ??m) films on Si chip, a bamboo-shoot-like microstructure of Au₅Sn/AuSn/AuSn₂ was formed in the reaction region after reflowed at 280 °C for 10 s; while a typical two-phase (Au₅Sn and AuSn) eutectic microstructure was formed after reflowed at 280 °C for 60 s.     

Interfacial reaction and IMCs growth behavior of Sn3Ag0.5Cu/Ni solder bump during aging at various temperatures

The interfacial reaction, morphology, and growth behavior of interfacial intermetallic compound (IMC) between the Sn–3Ag–0.5Cu (in wt%) solder and Ni substrate during reflow at 523 K for 300 s and aging at different temperatures for up to 360 h were investigated, and the growth kinetics of the interfacial Ni–Cu–Sn ternary IMC layers were monitored during the isothermal aging. The experimental results showed that a bi-layer of IMCs including (Cu, Ni)₆Sn₅/Ni and (Ni, Cu)₃Sn₄ was detected at the Sn3Ag0.5Cu/Ni interface. A (Ni, Cu)₃Sn₄ layer formed at the (Cu, Ni)₆Sn₅/Ni interface and once the (Ni, Cu)₃Sn₄ layer had formed, it grew at an exceptionally rapid rate by consuming the (Cu, Ni)₆Sn₅ layer. The (Cu, Ni)₆Sn₅ layer was much thicker than the (Ni, Cu)₃Sn₄ layer after reflowing, but the (Ni, Cu)₃Sn₄ layer thickened rapidly and would to be thicker than the (Cu, Ni)₆Sn₅ layer after prolonged aging. The (Ni, Cu)₃Sn₄ grains revealed an elongated, rod-like shape while the (Cu, Ni)₆Sn₅ IMC was Polyhedral shape, which was on the top of (Ni, Cu)₃Sn₄ phase. The thicknesses of total IMC and (Ni, Cu)₃Sn₄ increased linearly with square root of aging time, while the thickness of (Cu, Ni)₆Sn₅ did not increase significantly. The activation energies for the diffusion constants of the total interfacial IMC and (Ni, Cu)₃Sn₄ IMC layers were obtained by plotting the diffusion constants (D) as a function of the aging temperature (1/T), and were 91.43 and 89 KJmol^?1, respectively.     

Growth rate of Nb<Subscript>3</Subscript>Sn for reactive diffusion between Nb and Cu–9.3Sn–0.3Ti alloy

In order to examine experimentally the growth behavior of Nb₃Sn during reactive diffusion between Nb and a bronze with the α + β two-phase microstructure, a sandwich (Cu–Sn–Ti)/Nb/(Cu–Sn–Ti) diffusion couple was prepared from pure Nb and a ternary Cu–Sn–Ti alloy with concentrations of 9.3 at.% Sn and 0.3 at.% Ti by a diffusion bonding technique. Here, α is the primary solid-solution phase of Cu with the face-centered cubic structure, and β is the intermediate phase with the body-centered cubic structure. The diffusion couple was isothermally annealed at temperatures between T = 923 and 1,053 K for various times up to 843 h. Owing to annealing, the Nb₃Sn layer is formed along each (Cu–Sn–Ti)/Nb interface in the diffusion couple, and grows mainly into Nb. Hence, the migration of the Nb₃Sn/Nb interface governs the growth of the Nb₃Sn layer. The mean thickness of the Nb₃Sn layer is proportional to a power function of the annealing time. The exponent of the power function is close to unity at T = 923 K, but takes values of 0.8–0.7 at T = 973–1,053 K. Consequently, the interface reaction at the migrating Nb₃Sn/Nb interface is the rate-controlling process for the growth of the Nb₃Sn layer at T = 923 K, and the interdiffusion across the Nb₃Sn layer as well as the interface reaction contributes to the rate-controlling process at T = 973–1,053 K. Except the effect of Ti, the growth rate of the Nb₃Sn layer is predominantly determined by the activity of Sn in the bronze and thus the concentration of Sn in the α phase. As a result, the growth rate is hardly affected by the volume fraction of the β phase, though the final amount of the Nb₃Sn layer may depend on the volume fraction.     

Effects of Co and Ni addition on reactive diffusion between Sn–3.5Ag solder and Cu during soldering and annealing

The reactive diffusion between Sn–Ag solders and Cu was experimentally examined during soldering and isothermal annealing. Three sorts of solders with compositions of Sn–3.5Ag, Sn–3.5Ag–0.1Ni and Sn–3.5Ag–0.1Co were used for the experiment. Each solder was soldered on a Cu plate at 523 K (250 °C) for 1–60 s in a pure nitrogen gas, and then the solder/Cu diffusion couple was isothermally annealed at 423 K (150 °C) for 168–1008 h. Due to soldering, only Cu₆Sn₅ is formed at the interface in each diffusion couple. On the other hand, Cu₃Sn is produced between Cu₆Sn₅ and Cu owing to the isothermal annealing. The composition of Cu₆Sn₅ is (Cu_0.8Ni_0.2)₆Sn₅ and (Cu_0.93Ni_0.07)₆Sn₅ on the solder and Cu₃Sn sides, respectively, in the (Sn–3.5Ag–0.1Ni)/Cu diffusion couple, and it is (Cu_0.9Co_0.1)₆Sn₅ and (Cu_0.99Co_0.01)₆Sn₅ on the solder and Cu₃Sn sides, respectively, in the (Sn–3.5Ag–0.1Co)/Cu diffusion couple. Different rate-controlling processes were suggested for the (Sn–3.5Ag)/Cu, (Sn–3.5Ag–0.1Ni)/Cu and (Sn–3.5Ag–0.1Co)/Cu diffusion couples. Finally, thermodynamic models were herein adopted to explore influences of the additives on the thermodynamic interaction of the component elements and the driving force for the growth of intermetallics.     

Effects of diffusion and primary creep on intergranular cavitation at high temperatures

International Journal of Fracture - Next-generation reactors are expected to play a crucial role in power production in the foreseeable future. Due to the extreme anticipated operating temperatures...     

Metallic Contact between MoS2 and Ni via Au Nanoglue

A critical factor for electronics based on inorganic layered crystals stems from the electrical contact mode between the semiconducting crystals and the metal counterparts in the electric circuit. Here, a materials tailoring strategy via nanocomposite decoration is carried out to reach metallic contact between MoS₂ matrix and transition metal nanoparticles. Nickel nanoparticles (NiNPs) are successfully joined to the sides of a layered MoS₂ crystal through gold nanobuffers, forming semiconducting and magnetic NiNPs@MoS₂ complexes. The intrinsic semiconducting property of MoS₂ remains unchanged, and it can be lowered to only few layers. Chemical bonding of the Ni to the MoS₂ host is verified by synchrotron radiation based photoemission electron microscopy, and further proved by first‐principles calculations. Following the system's band alignment, new electron migration channels between metal and the semiconducting side contribute to the metallic contact mechanism, while semiconductor–metal heterojunctions enhance the photocatalytic ability.     

Q-factor of acoustic modes of a solid-state resonator at low temperatures

The acoustic characteristics of a cylindrical sample made of an aluminum alloy (model of gravity detector) in the course of cooling to temperatures on the order of the boiling point of liquid helium are studied. A specialized cryostat with working chamber having a large volume is presented. The technique and results of measurements of the Q-factor of the longitudinal modes of a model weighing 8 kg are described. The results are extrapolated to a cryogenic variant of the OGRAN large-scale (2.3 tons) gravity detector.     

Influence of Thermal Cycling on the Microstructure and Shear Strength of Sn3.5Ag0.75Cu and Sn63Pb37 Solder Joints on Au/Ni Metallization

The influence of thermal cycling on the microstructure and joint strength of Sn3.5Ag0.75Cu （SAC） and Sn63Pb37 （SnPb） solder joints was investigated. SAC and SnPb solder balls were soldered on 0.1 and 0.9 μm Au finished metallization, respectively. After 1000 thermal cycles between -40℃ and 125℃, a very thin intermetallic compound （IMC） layer containing Au, Sn, Ni, and Cu formed at the interface between SAC solder joints and underneath metallization with 0.1 μm Au finish, and （Au, Ni, Cu）Sn4 and a very thin AuSn-Ni-Cu IMC layer formed between SAC solder joints and underneath metallization with 0.9 μm Au finish. For SnPb solder joints with 0.1 μm Au finish, a thin （Ni, Cu, Au）3Sn4 IMC layer and a Pb-rich layer formed below and above the （Au, Ni）Sn4 IMC, respectively. Cu diffused through Ni layer and was involved into the IMC formation process. Similar interfacial microstructure was also found for SnPb solder joints with 0.9μm Au finish. The results of shear test show that the shear strength of SAC solder joints is consistently higher than that of SnPb eutectic solder joints during thermal cycling.     

Grain boundary self-diffusion in evaporated Au films at low temperatures

Direct self-diffusion measurements in vapor-deposited polycrystalline Au films have been made using ¹⁹⁵Au radioactive tracer and an r.f. back-sputtering technique for serial sectioning. A temperature range of 117°–177°C was investigated. It has been demonstrated that self-diffusion in thin Au films at these low temperatures takes place by rapid transport of the tracer atoms along the grain boundaries. The grain boundary self-diffusion parameters are Q_b=1.0±0.1 eV and δD_b⁰ = 9 × 10^?10 cm³/sec, which compare well with those in bulk polycrystalline Au.