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.     

A new phase in stoichiometric Cu6Sn5

The intermetallic compound Cu₆Sn₅ is a significant microstructural feature of many electronic devices where it is present at the solder–substrate interfaces. The time- and temperature-dependent thermomechanical properties of Cu₆Sn₅ are dependent on the nature and stability of its crystal structure, which has been shown to exist in at least four variants (η, η′, η⁶ and η⁸). This research details an additional newly identified monoclinic-based structure in directly alloyed stoichiometric Cu₆Sn₅ using variable-temperature synchrotron X-ray diffraction (XRD) and transmission electron microscopy. The phase is associated with a departure from the equilibrium temperature of the polymorphic monoclinic–hexagonal transformation temperature. The new monoclinic phase can be treated as a modulation of four η⁸-Cu₅Sn₄ unit cells plus one η′-Cu₆Sn₅ unit cell. It has been labeled as η⁴⁺¹ and has cell parameters of a = 92.241 Å, b = 7.311 Å, c = 9.880 Å and β = 118.95° determined from electron diffraction patterns. The XRD results could be fitted well to a Rietveld refinement using the new crystal parameters.     

Microstructural and mechanical analysis on Cu–Sn intermetallic micro-joints under isothermal condition

This study focuses on the mechanism of phase transformation from Cu₆Sn₅ into Cu₃Sn and the homogenization process in full intermetallics (IMCs) micro-joints, which were prepared by soldering the initial Cu/Sn/Cu structure through high temperature storage in vacuum environment as the Transient Liquid Phase (TLP) process. From the microstructural observation by electron backscatter diffraction (EBSD), a mixture of IMCs phases (Cu₆Sn₅ and Cu₃Sn) has been found to constitute the sandwich-structured Cu/IMCs/Cu joints. With the dwell time increasing at 533 K, there were two layers of Cu₃Sn emerging from both sides of copper substrates with the depletion of Cu₆Sn₅ layer, toward merging each other in the IMCs interlayer. Then the Cu₃Sn grains with various sizes became more homogenous columnar crystallites. Meanwhile, some equiaxial ultra-fine grains accompanied with the Kirkendall voids, were found only in adjacent to the electroplated copper. In addition, a specific type of micropillar with the size ∼5 μm × 5 μm × 12 μm fabricated by focus ion beam (FIB) was used to carry out the mechanical testing by Nano-indentation, which confirmed that this type of joint is mechanically robust, regardless of its porous Cu₃Sn IMC interconnection.     

Morphologies,orientation relationships and evolution of Cu6Sn5 grains formed between molten Sn and Cu single crystals

The morphologies and orientation relationships of Cu₆Sn₅ grains formed between Sn and (0 0 1), (0 1 1), (1 1 1) and (1 23) Cu single crystals under liquid- and solid-state aging conditions were systematically investigated. The regular prism-type Cu₆Sn₅ grains formed on (0 0 1) and (1 1 1) Cu single crystals are elongated either along two perpendicular directions or along three preferential directions with an angle of 60° between each pair of directions. The orientation relationships between Cu and Cu₆Sn₅ lattice structures were determined by electron backscatter diffraction and were explained in terms of their minimum misfit. However, on (0 1 1) and (1 2 3) Cu single crystal surfaces, the Cu₆Sn₅ grains were mainly scallop-type, with only a few regular prism-type grains. Furthermore, the regular prism-type Cu₆Sn₅ grains will change into scallop-type after long reflow or aging times. Meanwhile it is considered that the growth of the scallop-type grains is supplied by two fluxes: the flux of the interfacial reaction and the flux of ripening. However, the growth of the prism-type grains is only supplied by the flux of the interfacial reaction. The kinetics of IMCs growth between Sn and Cu single crystals was also investigated.     

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.     

The 260 °C phase equilibria of the Sn–Sb–Cu ternary system and interfacial reactions at the Sn–Sb/Cu joints

The isothermal section of the Sn–Sb–Cu ternary system at 260 °C has been determined in this study by experimental examination. Experimental results show no existence of ternary compounds in the Sn–Sb–Cu system. An extensive region of mutual solubility existing between the two binary isomorphous phases, Cu₃Sn and Cu₄Sb, was determined and labeled as δ. Intermetallic compounds (IMCs) Cu₂Sb, SbSn, and Cu₆Sn₅ are in equilibrium with the δ solid solution. Up to about 6.5 at.%Sb can dissolve in the Cu₆Sn₅ phase, and the solubility of Sn in the Cu₂Sb is approximately 6.2 at.%. Each of the Sb and SbSn phases has a limited solubility of Cu. Only one stoichiometric compound, Sb₂Sn₃, exists. Besides phase equilibria determination, the interfacial reactions between the Sn–Sb alloys and Cu substrates were investigated at 260 °C. Sb was observed to be present in the Cu₆Sn₅ and δ phases, and Sb did not form Sn–Sb IMCs in the interfacial reactions. Moreover, the addition of up to 7 wt% of Sb into Sn does not significantly affect the total thickness of IMC layers. It was found that the phase formations in the Sn–Sb/Cu couples are very similar to those in the Sn/Cu couples.     

Dissolution and precipitation kinetics of Cu6Sn5 intermetallics in Cu/Sn/Cu micro interconnects under temperature gradient

Multiple reflows are often required in 3D packaging. To elucidate the effect of temperature gradient during subsequent reflow on existing intermetallic compounds (IMCs), Cu₆Sn₅ IMC layers were initially formed in Cu/Sn/Cu micro interconnects. Upon subsequent reflow, synchrotron radiation real-time imaging technology was used to in situ study the dissolution and precipitation behavior of the pre-formed Cu₆Sn₅ under different temperature gradients. The pre-formed Cu₆Sn₅ IMC at the cold end continued to grow linearly with increasing aspect ratio, whereas that at the hot end dissolved linearly and then maintained a critical thin layer. The thick pre-formed Cu₆Sn₅ IMC at the hot end significantly hindered the dissolution of the neighboring Cu substrate until a dynamic equilibrium between chemical potential gradient and temperature gradient was satisfied. The thermomigration of Cu atoms from the hot end towards the cold end was responsible for the asymmetrical evolution of the interfacial Cu₆Sn₅ between the cold and hot ends. A theoretical model was proposed based on Cu diffusion flux to calculate the IMC thickness at the both ends as a function of reflow time and the equilibrium IMC thickness at the hot end under temperature gradient.     

Reliability studies of Sn–9Zn/Cu solder joints with aging treatment

Sn–9Zn (in wt.%) solder ball was bonded to Cu pad, and the effect of aging on shear reliability was investigated. After reflow, the intermetallic compound (IMC) phase formed at the interface was Cu₅Zn₈, and the as-reflowed Sn–9Zn/Cu joint had sufficient shear strength. In the isothermal aging test, only Cu₅Zn₈ IMC was observed in the samples aged at temperatures between 70 and 120 °C. On the other hand, after aging at 150 °C for 250 h, Cu₆Sn₅ phase was observed at the interface between the interfacial Cu₅Zn₈ IMC layer and the Cu substrate. And, the layer-type Cu₅Zn₈ IMC layer was disrupted locally at the interface. In the ball shear test conducted after aging treatment, the shear strength significantly decreased after aging at all temperatures for initial 100 h, and then remained constant by further prolonged aging. The fracture mainly occurred at the interface between the solder and Cu₅Zn₈ IMC layer. The aged Sn–9Zn/Cu solder joint had an inferior joint reliability.     

Investigation of diffusion and electromigration parameters for Cu–Sn intermetallic compounds in Pb-free solders using simulated annealing

An efficient numerical method was developed to extract the diffusion and electromigration parameters for multi-phase intermetallic compounds (IMC) formed as a result of material reactions between under bump metallization (UBM) and solder joints. This method was based on the simulated annealing (SA) method and applied to the growth of Cu–Sn IMC during thermal aging and under current stressing in Pb-free solder joints with Cu-UBM. At 150 °C, the diffusion coefficients of Cu were found to be 3.67 × 10¹⁷ m² s⁻¹ for Cu₃Sn and 7.04 × 10¹⁶ m² s⁻¹ for Cu₆Sn₅, while the diffusion coefficients of Sn were found to be 2.35 × 10¹⁶ m² s⁻¹ for Cu₃Sn and 6.49 × 10¹⁶ m² s⁻¹ for Cu₆Sn₅. The effective charges of Cu were found to be 26.5 for Cu₃Sn and 26.0 for Cu₆Sn₅, and for Sn, the effective charges were found to be 23.6 for Cu₃Sn and 36.0 for Cu₆Sn₅. The SA approach provided substantially superior efficiency and accuracy over the conventional grid heuristics and is particularly suitable for analyzing many-parameter, multi-phase intermetallic formation for solder systems where quantitative deduction for such parameters has seldom been reported.     

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.     

Significant tensile ductility induced by cold rolling in Cu47.5Zr47.5Al5 bulk metallic glass

Significant tensile plasticity up to 0.7 ± 0.1% together with work-hardening and larger fracture strength was obtained in Cu_47.5Zr_47.5Al₅ bulk metallic glass (BMG) upon cold rolling with only 2.9 ± 0.3% thickness reduction. The good deformability could be attributed to the multiple pre-existing shear bands and structural inhomogeneity induced by rolling. The distributions of shear bands upon rolling can be predicted by a simplified rolling model. The underlying mechanism for the tensile plasticity was further discussed in the frame of potential energy landscape theory (PEL).     

Solid state interracial reactions in electrodeposited Cu/Sn couples

Cu/Sn couples, prepared by sequentially electroplating Cu and Sn layers on metallized Si wafers, were employed to study the microstructures, phases and the growth kinetics of Cu-Sn intermediate phases, when electroplated Cu/Sn couples were aged at room temperature or annealed at temperatures from 373 K to 498 K for various time. Only Cu₆Sn₅ formed in aged couples or couples annealed at temperature below 398 K. The Cu₆Sn₅ layer was continuous, but not uniform, with protrusions extending into the Sn matrix. When Cu/Sn couples were annealed at temperatures from 423 K to 498 K, two continuous and uniform Cn₆Sn₅/Cu₃Sn layers formed within the reaction region between Sn and Cu. There were many voids near the Cu₃Sn/Cu interface and within the Cu₃Sn layer. Cu₆Sn₅ and Cu₃Sn formations both follow parabolic growth kinetics with activation energies of 41.4 kJ/mol for Cu₆Sn₅ and 90.4 kJ/mol for Cu₃Sn, respectively.     

Effects of Co nanoparticle addition to Sn–3.8Ag–0.7Cu solder on interfacial structure after reflow and ageing

Effects of Co nanoparticle additions to Sn–3.8Ag–0.7Cu on the structure of solder/copper interface have been studied after reflow and high temperature ageing (150 °C, up to 1008 h). Results show that the Co nanoparticles substantially suppress the growth of Cu₃Sn but enhance Cu₆Sn₅ growth. Cobalt nanoparticles reduce interdiffusion coefficient in Cu₃Sn. It is suggested that the Co nanoparticles undergo surface dissolution during reflow and exert their influence, at least partially, through alloying effect.     

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.     

Interface reaction systematics in the Cu/In–48Sn/Cu system bonded by diffusion soldering

An alternative lead-free solder alloy In–48 at%Sn with a melting point of 120 °C and its implementation to bond Cu substrates in a diffusion soldering joining method are presented. According to the EPMA, TEM/EDX and electron diffraction analyses, two different behaviors were observed in the interconnection zone depending on the temperature range: (i) a single layer consisting of η phase below 200 °C; (ii) a Cu-poor region consisting of η phase and a Cu-rich layer formed by a mixture of thin alternate regions of ζ-Cu₁₀Sn₃ and δ-Cu₇In₃ phases perpendicular to the interconnection plane above 200 °C. The η layer shows two morphologies: large grains and fine grains at the η/In–48Sn (liquid) and at the η/Cu-rich interfaces, respectively. Additionally, the η region shows a gradual change in composition, suggesting a change from the Cu₆Sn₅ to the Cu₂In structures. Thermal stability tests indicate that the thermal resistance of the bonds is about 750 °C.     

Ultrafine eutectic Ti x Sn y /TiNi composites with high damping capacity

Three kinds of bulk-type ultrafine Ti _x Sn _y /TiNi (Ti _x Sn _y represents Ti₃Sn, Ti₂Sn, and Ti₅Sn₃ or Ti₆Sn₅) composites with homogeneous eutectic microstructure were prepared by arc melting. The composites exhibit high damping capacity (tanδ greater than 1 × 10^?2) and enhanced mechanical strength (the highest fracture strength is 2.15 GPa). The damping capacity originates from TiNi and Ti₃Sn, while the eutectic contributes to the mechanical strength.     

Microstructural evolution of intermetallic compounds in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn and 1.5In)/Cu solder joints during liquid aging

In this paper, the microstructural evolution of IMCs in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn, 1.5In)/Cu solder joints and their growth mechanisms during liquid aging were investigated by microstructural observations and phase analysis. The results show that two-phase (Ni₃Sn₄ and Cu₆Sn) IMC layers formed in Sn–3.5Ag–0.75Ni/Cu solder joints during their initial liquid aging stage (in the first 8 min). While after a long period of liquid aging, due to the phase transformation of the IMC layer (from Ni₃Sn₄ and Cu₆Sn phases to a (Cu, Ni)₆Sn₅ phase), the rate of growth of the IMC layer in Sn–3.5Ag–0.75Ni/Cu solder joints decreased. The two Cu₆Sn₅ and Cu₅Zn₈ phases formed in Sn–3.5Ag–1.0Zn/Cu solder joints during the initial liquid aging stage and the rate of growth of the IMC layers is close to that of the IMC layer in Sn–3.5Ag/Cu solder joints. However, the phase transformation of the two phases into a Cu–Zn–Sn phase speeded up the growth of the IMC layer. The addition of In to Sn–3.5Ag solder alloy resulted in Cu₆(Sn_x,In_1?x)₅ phase which speeded up the growth of the IMC layer in Sn–3.5Ag–1.5In/Cu solder joint.     

A synchrotron radiation x-ray microdiffraction study on orientation relationships between a Cu6 Sn5 and Cu substrate in solder joints

The orientation distribution of Cu₆Sn₅ scallops and its relationship with the orientation of copper substrate was studied using synchrotron-radiation-based micro-x-ray diffraction. Laue spots were obtained both from the Cu₆Sn₅ and copper at the same time. From the Laue patterns, orientation distribution maps of the Cu₆Sn₅ and copper were obtained. The orientation of the Cu₆Sn₅ scallops had a strong dependence on that of copper. The [001] direction of Cu₆Sn₅ is always parallel to the [110] of copper, and either the (110) or the (010) plane of Cu₆Sn₅ is parallel to the (001) plane of copper. It was also found that the scallops of Cu₆Sn₅ gradually gain texture in the early stage of reflow, but lose the texture after a long reflow.     

The 473 K isothermal section of the Cu–Ti–Sn ternary system

The phase relationships of the Cu–Ti–Sn ternary system at 473 K have been investigated mainly by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), optical microscopy (OM) and differential thermal analysis (DTA). The isothermal section consists of 17 single-phase regions, 33 two-phase regions and 17 three-phase regions. The existence of 12 binary compounds and 2 ternary compounds, namely Cu₄Ti, Cu₃Ti₂, Cu₄Ti₃, CuTi, CuTi₂, Cu₃Sn, Cu₆Sn₅, Ti₃Sn, Ti₂Sn, Ti₅Sn₃, Ti₆Sn₅, Ti₂Sn₃, CuTi₅Sn₃ and CuTiSn, are confirmed in the Cu–Ti–Sn ternary system at 473 K. No new ternary compound is found. The maximum solid solubility of Cu in Ti₆Sn₅ was approximately 10 at.% Cu.