Fabrication of Cu6Sn5 single-crystal layer for under-bump metallization in flip-chip packaging

To inhibit the rapid consumption of the copper substrate at the intergranular regions (grain boundaries or solder channels), a Cu₆Sn₅ single-crystal layer was fabricated via 1 min reflow at 250 °C. The orientation maps showed that the host-controlled growth behavior of the Cu₆Sn₅ phase existed during this single-crystal-forming procedure. By combining surface morphologies and kinetic analyses, the physical mechanism behind this behavior was identified as grain boundary migration rather than Ostwald ripening. This study provided a strong foundation for the fabrication of Cu₆Sn₅ under-bump metallization, as well as other similar intermetallic diffusion barriers.     

Effects of Sn thickness on morphology and evolution of Ni3Sn4 grains formed between molten Sn and Ni substrate

The morphology and growth behavior of Ni₃Sn₄ grains at the Sn/Ni interface during soldering were systematically investigated. Sn/Ni reaction samples were examined in three electroplated Sn thicknesses, 5 μm, 10 μm and 100 μm. In soldering at 280 °C, the formed Ni₃Sn₄ grains exhibit two kinds of morphologies, needle-like grains and larger chunk-type grains. Experimental observations reveal that the amount of the chunk-type grains increased as the Sn layer thickness was decreased, which is attributed to the Ni content in molten Sn. Moreover, needle-like grains display a hexagonal columnar structure with (100) and (110) facets. With the reaction proceeding, the needle-like grains coarsen with cube root of reaction time. The faceted chunk-type grains are transformed from the boomerang-shaped grains, consisting of two inverted triangular plates sharing an edge. The crystal plane orientations of the chunk-type Ni₃Sn₄ grains were also analyzed based on its crystalline structure. Additionally, the Sn/Ni reactions at 250 °C indicate that the morphology of the chunk-type grains is also affected by the reaction temperature.     

Inhibiting CoSn3 growth at the Sn/Co system by minor Zn addition

CoSn₃ is the major reaction product in Sn/Co interfacial reactions. Compared with other substrate materials, e.g., Cu and Ni, the CoSn₃ growth is much faster during reflow soldering and thermal aging. However, excessive formation of intermetallic compound (IMC) layer might deteriorate the reliability of solder joints. This study demonstrated that the CoSn₃ growth was effectively slowed down by adding minor amounts of Zn in Sn solder. The solder reactions between Co and Sn with addition of various Zn contents (0.05 wt% ∼ 2 wt%) were systematically examined in liquid-state and solid-state reactions. For liquid-state reactions at 240 °C–260 °C, the CoSn₃ growth rate was decreased by 50% with 0.1 wt%Zn addition, and further decreased by 75% with 0.5 wt%Zn. Moreover, the growth kinetics of CoSn₃ was investigated with various Zn additions. The CoSn₃ growth was found to be proportional with reaction time, and their apparent activation energies were very close (between 105 kJ/mol ∼ 110 kJ/mol). For solid-state reactions, the effect of Zn addition on the inhibition of CoSn₃ growth was more significant. The growth rate was greatly reduced by 75% with only 0.05 wt%Zn addition. The possible inhibition mechanism was discussed.     

Minor Ga addition to effectively inhibit PdSn4 growth between Sn solder and Pd substrate

PdSn₄ is the major reaction phase in the Sn-based or Sn–Ag–Cu solder joints with Pd substrate and it exhibits an extremely high growth rate. Ga is considered as a candidate alloying element in the Sn–Ag–Cu solders. This paper investigates the effects of Ga addition on the interfacial reactions between Pd and Sn–Ga (0.1wt.%–1wt.%Ga) solders by solid-state aging at 160, 180, and 200 °C and liquid-state aging at 250 °C. The most important finding is that minor Ga addition can effectively inhibit the fast PdSn₄ growth. In the solid-state reaction, with only 0.1wt.%Ga addition, the PdSn₄ growth was suppressed by ∼50%, compared with the pure Sn/Pd reaction. When the Ga content increased to 0.5wt.%, the PdSn₄ growth was further reduced by over 90%. In addition, a thin PdGa phase layer was formed at the interface between PdSn₄ and Pd, which was the main cause for the inhibition of PdSn₄ growth. The growth kinetics was systematically explored. The PdSn₄ growth had a higher activation energy for the higher Ga addition (>0.5wt.%). Furthermore, a similar reduction in the PdSn₄ growth was observed in the liquid-state reaction, but it was not as strong as in the solid-state reactions. The PdGa phase was not formed in the liquid-state reaction and the growth inhibition could be attributed to the Ga doping in the PdSn₄ lattice to retard the Sn diffusion.     

Effect of Ga content in Cu(Ga) on the growth of V3Ga following bronze technique

Diffusion–controlled growth rate of V₃Ga in the Cu(Ga)/V system changes dramatically because of a small change in Ga content in Cu(Ga). One atomic percent increase from 15 to 16 leads to more than double the product phase layer thickness and a decrease in activation energy from 255 to 142 kJ/mol. Kirkendall marker experiment indicates that V₃Ga grows because of diffusion of Ga. Role of different factors influencing the diffusion rate of Ga and high growth rate of V₃Ga are discussed.     

Microstructure development during pack aluminization of nickel and nickel–chromium wires

Pack aluminization – a chemical vapor deposition process widely used to form protective coatings on Ni-based superalloy components – was used to form shells of Ni₂Al₃, NiAl and/or γ′-Ni₃Al on the surface of γ-Ni wires with diameters of 127 μm. The growth kinetics of these Al-rich intermetallic shells are studied as a function of aluminization time and pack activity at 1000 °C. Similar kinetics but additional phases (Cr/Ni two-phase shell, Cr silicide particles and Al-rich particles distributed in Ni₂Al₃) are found in the shells of pack-aluminized Ni–20 wt.% Cr wires with similar diameters. Fully homogenous Ni–Al and Ni–Cr–Al wires are achieved by interdiffusion at 1200 °C between the deposited Al-rich intermetallic shells and the Ni-rich core of both types of wires. Upon subsequent aging at 900 °C, wires with γ/γ′ structure and high hardness indicative of precipitation strengthening are obtained.     

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.     

Creep corrosion cracking of Sn–3.0Ag and Sn–0.5Cu solder alloys in NaCl solution

The surface crack nucleation of Sn–3.0Ag and Sn–0.5Cu solder alloys has been examined by performing sustained tensile-loading tests in 0.9 mass% NaCl solution at room temperature. For Sn–3.0Ag alloy, many cracks nucleate and propagate on the side surface of the specimen, similarly to Sn–3.0Ag–0.5Cu alloy reported previously. For Sn–0.5Cu alloy, such cracks are not observed, and ordinary creep deformation occurs in the solution. The effect of sustained applied stress, i.e., creep, on the dissolution of ions is smaller for Sn–0.5Cu alloy than for Sn–3.0Ag alloy. The present results suggest that there are differences in the susceptibility to cracking under applied stress in a solution, i.e., creep corrosion cracking, among lead-free solder alloys.     

Morphology and formation of Fe–Al intermetallic layers on iron hot-dipped in Al–Mg–Si alloy melt

We have examined the morphology and the growth of Fe–Al intermetallic layers of η-Fe₂Al₅ and θ-FeAl₃ phases formed on pure Fe sheets dipped in an Al-8.2Mg-4.8Si (wt.%) alloy melt and pure Al melt at 750 °C. The η phase layer grows one order of magnitude slower in the Al–Mg–Si alloy melt than in the pure Al melt. The change in thickness of Fe sheets with dipping time is less pronounced in the Al–Mg–Si alloy melt than in a pure Al melt. Microstructure observations suggest that the retarded interfacial reaction between solid Fe and liquid Al–Mg–Si alloy is associated with a continuous θ phase layer formed in the Al–Mg–Si alloy melt, which acts as the diffusion barrier.     

The effect of growth rate on microstructure and microindentation hardness in the In–Bi–Sn ternary alloy at low melting point

In–21.5 at.% Bi–17.8 at.% Sn ternary alloy which has 333 K melting point was directionally solidified upward with a constant temperature gradient (G = 0.91 K/mm) in a wide range of growth rates (3.2–157.1 μm/s) with a Bridgman type directional solidification furnace. The lamellar spacings (λ) and microhardness values (H_V) were measured from both transverse and longitudinal sections of the samples. The dependence of lamellar spacings (λ) and microhardness (H_V) on the growth rate (V) was determined by using linear regression analysis. According to these results, it has been found that the value of λ decreases with the increasing value of V and whereas, the value of H_V increases for a constant G. The values of λ²V were determined by using the measured values of λ and V. The results obtained in this work have been compared with the previous similar experimental results obtained for binary or ternary alloys.     

Infrared brazing of Ti50Ni50 shape memory alloy using pure Cu and Ti–15Cu–15Ni foils

Infrared brazing of Ti₅₀Ni₅₀ using two brazing filler metals was investigated in the study. Three phases, including Cu-rich, CuNiTi (Δ) and Ti(Ni,Cu), were observed in the Ti₅₀Ni₅₀/Cu/Ti₅₀Ni₅₀ joint after brazing at 1150 °C. The Cu-rich phase was rapidly consumed in the first 10 s of brazing, and the eutectic mixture of CuNiTi and Ti(Ni,Cu) phases were subsequently observed in the joint. Samples brazed for longer time resulted in less CuNiTi and more Ti(Ni,Cu) phases in the joint. The existence of CuNiTi phase deteriorated the shape memory effect of the joint, but Ti(Ni,Cu) could still preserve shape memory behavior even alloyed with a large number of Cu. Therefore, higher shape recovery ratio was observed for specimens brazed for a longer time period. Extensive presence of Ti₂(Ni,Cu) phase was observed in Ti₅₀Ni₅₀/Ticuni^®/Ti₅₀Ni₅₀ joint upon brazing the specimens up to 1150 °C. The bending test could not be performed due to the inherent brittleness of Ti₂(Ni,Cu) matrix. Moreover, the stable Ti₂(Ni,Cu) phase was difficult to be removed completely by increasing either brazing time and/or temperature.     

The effect of strain on dynamic recrystallization of PM Ti–45Al–10Nb intermetallics during isothermal forging

The strain in different regions of the pancake was different during isothermal forging process. Different strain could greatly influenced the forged microstructure. In the present work, the relationships between strain, dynamic recrystallization (DRX) and microstructure in different regions of powder metallurgy Ti–45Al–10Nb alloy during isothermal forging were investigated. The results show that the microstructure from the center to edge of the forged pancake was inhomogeneous due to the degree of DRX. And DRX was related with the degree of strain, large strain can promote DRX.     

In-situ alloying of Sn–3.5Ag solder during reflow through Zn nanoparticle addition and its effects on interfacial intermetallic layers

Driven by the necessity to improve the reliability of lead free electronic products and by the trend towards miniaturization, researchers are putting intense efforts to improve the properties of Sn based solders. The present work investigates the effects of Zn nanoparticle addition to Sn-3.5Ag (SA) alloy through paste mixing on the interfacial structure between solder and copper substrate during reflow. Results show that the addition of Zn nanoparticles does not alter the morphology of the interfacial intermetallic compounds although they substantially suppress their growth. Zn nanoparticles are seen to be most efficient compared with Co and Ni nanoparticles in suppressing the growth of Cu₃Sn layers. It is suggested that Zn nanoparticles exert their influence through an in-situ dissolution and alloying effect.     

Single-crystal growth of the complex metallic alloy phase β-Al–Mg

The development of a single-crystal growth route for the complex metallic alloy phase β-Al₃Mg₂ is presented. After initial probing of the phase diagram in the vicinity of the existence range of the β-phase, we performed single-crystal growth experiments employing various techniques. The Czochralski and self-flux growth turned out to be the most suitable for this phase, and with both we reproducibly achieved single crystals of several cubic centimeters in volume. While the Czochralski technique allows for the production of deliberately oriented single crystals, the self-flux technique is capable of producing very large but unoriented single grains.     

Stabilisation of Cu6Sn5 by Ni in Sn-0.7Cu-0.05Ni lead-free solder alloys

Cu₆Sn₅ exists at least in two crystal structures with an allotropic transformation from monoclinic η'-Cu₆Sn₅ at temperatures lower than 186 °C to hexagonal η-Cu₆Sn₅. We recently discovered that the hexagonal structure of Cu₆Sn₅ in lead-free solder alloys with trace Ni additions is stable down to room temperature using high resolution TEM/ED/EDS. This report further confirm the phase stabilising effect of Ni by analysing samples of Cu₆Sn₅ extracted from a Sn-0.7wt%Cu-0.05wt%Ni lead-free solder alloy. Techniques used include X-ray diffraction, transmission electron microscopy and differential scanning calorimetry.     

Preparation of bulk β-FeSi2 by synthetical control of melt cyclical superheating,solidification and cooling process

A new method of melt cyclical superheating combined with the control of solidification and cooling process as well as appropriate heat treatment was proposed to prepare bulk β-FeSi₂. FeSi₂ precursor of φ18 × 17 mm in size was obtained under the conditions of melt superheating temperature 1550 °C, superheating time 10 min, recycling times 3, solidification rate 30 °C/s, cooling rate 12 °C/min from solidification temperature to 700 °C and cooling naturally from 700 °C to room temperature. The precursor had homogeneous and complete α + ε eutectic structure, with the rod-like ε phase of 1–2 μm in diameter. After the precursor with complete α + ε eutectic structure were annealed at 900 °C for 150 h, both the α and ε phases totally disappeared and were transformed into β-FeSi₂ except few residual Si-rich phase.     

Quantification of growth kinetics and adherence of oxide scales formed on Ni-based superalloys at high temperature

Cyclic and isothermal oxidation behaviors of first and fourth-generation superalloys AM1 and MCNG were investigated to evaluate the ability of the scratch test to quantify the adhesion of multi-layered oxide scales. Effects of sulfur content and of scale thickness were studied independently. Available models lead to large discrepancies in the calculated work of adhesion values with the evaluation of the residual stress being the largest source of error. Nevertheless, models can assess the effect of sulfur content and the scratch test can be used to correlate the long-term cyclic oxidation behavior and the adhesion of oxide scales.     

The difference in the types of intermetallic compound formed between the cathode and anode of an Sn–Ag–Cu solder joint under current stressing

An investigation of microstructural evolution with various current densities in a lead-free Cu/SnAgCu/Au/Cu solder system was conducted in this study. Current stressing induced migration of Cu toward the anode and resulted in the formation of Cu₆Sn₅ at the interface. The consumption rates of Cu were calculated to be 2.24 × 10⁻⁷ μm/s and 5.17 × 10⁻⁷ μm/s at 1.0 × 10³ A/cm² and 2.0 × 10³ A/cm², respectively, while the growth rates of Cu₆Sn₅ were 6.33 × 10⁻⁷ μm/s and 7.72 × 10⁻⁷ μm/s. The atomic fluxes of Cu were found to be 2.50 × 10¹² atom/cm² s and 5.88 × 10¹² atom/cm² s at the above-mentioned current densities. The diffusivities of Cu in Cu₆Sn₅ were 2.02 × 10⁻¹¹ cm²/s and 2.38 × 10⁻¹¹ cm²/s under 1.0 × 10³ A/cm² and 2.0 × 10³ A/cm² of current stressing. Current stressing effectively enhances the migration of Cu in Cu₆Sn₅ and results in a 1000-fold increase of magnitude in diffusivity compared to thermal aging. (Cu_1−x,Au_x)₆Sn₅ compound was formed near the anode after a long period of current stressing.