Cu6Sn5 Phase Research Articles

The monoclinic lattice distortion of η′-Cu6Sn5

In commonly used Sn-based solder systems the η′-Cu6Sn5 phase, which develops during field operation, is of huge interest concerning solder joint reliability in electronic components. For all mentioned results in this paper, the η′-Cu6Sn5 was produced via solid state interdiffusion by employing an industrial like set up. The specimens were characterized via SEM, EDS and XRD measurements. The main focus was set on getting really precise lattice parameters, which gave rise to the problem of partially poor refinements during XRD evaluations. Strongly overlapping fundamental reflections of different occurring phases lead to the danger of getting trapped into false, local minima during refinement. For that reason an unusual refinement method was applied, which only considers the superstructure reflections of the η′-Cu6Sn5 phase. Thereby, the obtained precise lattice parameters could be used for calculating the spontaneous strain tensors of the η′-Cu6Sn5. The concept of the spontaneous strain initially introduced for ferroelastics was applied to quantify the metric distortion of the η′-Cu6Sn5 phase. The calculated eigenvectors of the corresponding strain tensors reveal the direction of the most prominent distortion, which concludes referred to the NiAs/Ni2In structure in a contraction in the [110]h and an expansion in a corresponding, perpendicular [11¯0]h direction. Therefore, the corresponding eigenvalues (−0.00088 and 0.00080) of η′-Cu6Sn5 are 5–6 times smaller than the ones from the comparable structure Ni3Sn2. Overall, a refinement strategy for evaluating lattice parameters based on XRD pattern of η′-Cu6Sn5 was worked out, which is also applicable for other problematic superstructures. Hence, the obtained really precise lattice parameters could be used to characterize for the first time the exact monoclinic metric and therefore the monoclinic distortion of the η′-Cu6Sn5 phase.

Structure-induced metastable phase transformation in Cu6Sn5 intermetallics

The retarded η → η′ phase transformation in Cu6Sn5 has become a potential risk to the service reliability of micro-solder joints. In this paper, both Cu6Sn5 crystal and powder samples were fabricated to explore the phase transformation behaviour of Cu6Sn5. Because of the difference in stress accumulation, η′ ↔ ηc ↔ η and η′ ↔ η phase transformations occurred in the Cu6Sn5 core layers and surface layers, respectively. Moreover, formation of the metastable intermediate ηc phase was confirmed to be both stress induced and structure dependent. In addition, by decreasing the crystal size or using a proper isothermal treatment, formation of the metastable phase in Cu6Sn5 can be significantly reduced. Our study can provide an understanding of the phase transformation behaviour of Cu6Sn5 and may provide visual data for solving the risk of retarded Cu6Sn5 phase transformation.

Effects of Zn and Sb additions on micro-structure, creep behavior and thermal properties of binary eutectic Sn-0.7%Cu lead-free solder

The effects of Zn and Sb additions on microstructure, creep behavior and thermal properties of binary eutectic Sn-0.7%Cu lead-free solder were investigated. Results show that Zn-addition reduced the melting temperature of Sn-0.7Cu and enhanced the creep resistance due to the formation of Cu5Zn8 intermetallic compound (IMC) and reduction of Cu6Sn5 phase in β-Sn matrix. Conversely, the Sb-addition showed negative effect on both the creep resistance and melting temperature compared to the plain Sn-0.7Cu and Sn-0.7Cu-2.0Zn alloys due to the formation of SnSb IMC particles and coarse Cu6Sn5 phase through the β-Sn rich phase. The average stress exponents of 4.9 - 6.0 and activation energies of 50-60 KJ/mol were obtained for three solders; suggesting that the dislocation climb controlled by lattice diffusion is the dominant creep mechanism. X-ray diffraction (XRD) analysis revealed that the lattice parameters (a), (c), the axial rate (c/a) and the peak height intensities (hkl) of some crystallographic planes are changed with alloying additions and the crystallite size is decreased.

Novel insights in growth of intermetallic compounds between Sn–3.0Ag–0.5Cu solder and flexible PCB substrates under strain

The study investigated the effect of tensile and compressive strain on the interfacial reaction of SAC305/Cu system. Experimental results indicated that the thickness of Cu6Sn5 layer on non-strained samples slightly decreased after aging for 120 h and increased with prolonged aging time. The applied tensile strain had an enhanced effect on the growth of Cu6Sn5 phase, while the applied compressive strain inhibited the growth of Cu6Sn5 phase. However, the thickness of Cu3Sn layer at SAC305/Cu solder joints increased gradually with increasing aging time, regardless of whether strain was or not applied on the solder joints. Besides, the mean diameter of Cu6Sn5 grain increased in three systems as the aging time was prolonged to 480 h. The morphology of Cu6Sn5 grain on non-strained samples gradually changed from round-shape into polyhedron-shape with increased aging time. The morphology of Cu6Sn5 grain under tensile or compressive strain was polyhedron-shape. The Kirkendall voids appeared within Cu3Sn layer in three systems after aging for 120 h and the amount of voids increased gradually with prolonged aging time. The applied tensile or compressive strain had an enhanced effect on the formation of voids within Cu3Sn layer and the enhanced effect of applied tensile strain was much remarkable than that of other systems.

Investigation of the interfacial reactions between Sn-3.0 wt%Ag-0.5 wt%Cu solder and Cu[sbnd]Ti alloy (C1990HP)

The interfacial reactions between Sn-3.0 wt%Ag-0.5 wt%Cu (SAC) solder and CuTi alloy (C1990HP) using the solid/liquid reaction couple technique reflowed at 240, 255, and 270 °C for 10 min to 20 h was first investigated. The results revealed that the Cu6Sn5 phase and the precipitated Ti2Sn3 and Ag3Sn phases with a small grain size were formed and no Cu3Sn phase was observed in the SAC/C1990HP couple when the reflowing time was longer than 30 min. In addition, the Ti2Sn3 phase was observed among the upper the Cu6Sn5 phase. Comparing the SAC/Cu reaction couple, the thicker intermetallic compound (IMC) layers were observed in the SAC/C1990HP reaction couple. It was also shown that the massive spalling of the Cu6Sn5 phase in the SAC/C1990HP reaction couple for the prolonged reflowing time. Meanwhile, the Sn-rich layer was observed between the upper and lower Cu6Sn5 phases. When the reaction temperatures and times were increased, the small grain of the Cu6Sn5 phase was merged and ripened each other, and then spread over to the SAC solder. The hexagonal prism-shaped Cu6Sn5 phase was found when the Cu6Sn5 layer detached at the SAC/C1990HP interface. The IMC growth mechanism was diffusion controlled.

Research on soldering AlN ceramics with Cu substrate using Sn-Ag-Ti solder

PurposeThis study aims to solder AlN ceramics with a Cu substrate using an active type Sn-Ag-Ti solder. Soldering was performed with power ultrasound. The Sn3.5Ag2Ti alloy was first studied.Design/methodology/approachIt was found to contain a Sn matrix, where both Ag phase – ɛ-Ag3Sn – and Ti phases ɛ-Ti6Sn5 and Ti2Sn3 – were identified. Ti contained in these phases is distributed to the interface with ceramic material. A reaction layer was thus formed. This layer varies in thickness from 0.5 to 3.5 µm and ensures the wettability of an active solder on the surfaces of ceramic materials.FindingsX-ray diffraction analysis proved the presence of new NTi and AlTi2 phases on the fractured surface. Sn plays the main role in bond formation when soldering the Cu substrate with Sn-Ag-Ti solder. The Cu3Sn and Cu6Sn5 phases, which grow in direction from the phase interface to solder matrix, were found in all cases within the solder/Cu substrate interface. The combination of AlN ceramics/Cu joint maintained a shear strength of 29.5 MPa, whereas the Cu/Cu joint showed a somewhat higher shear strength of 39.5 MPa.Originality/valueThe present study was oriented towards soldering of AlN ceramics with a Cu substrate by the aid of ultrasound, and the fluxless soldering method was applied. Soldering alloy type Sn-Ag-Ti was analysed, and the interactions between the solder and ceramic and/or Cu substrate were studied. The shear strength of fabricated soldered joints was measured.

Nanosized samarium oxide (Sm2O3) particles suppressed the IMC phases and enhanced the shear strength of environmental-friendly Sn–Ag–Cu material

Environmental-friendly Sn–Ag–Cu-based composite material was prepared through evenly mixing of similar density nonreacting samarium oxide (Sm2O3) nanoparticles into Sn–3Ag–0.5Cu (wt%) powders. Further, the interfacial structural morphology and strength on various surface-finished e.g., Cu, Au/Ni-plated and Ag-plated Cu pads were studied on aging period with different temperatures. At low-temperature aging, a layer-type Cu6Sn5 intermetallic compound (IMC) phase was detected on Cu and Ag-plated Cu pad surface. Further, a thin film-type Cu3Sn IMC phase was grown during the aging treatment. Moreover, in Au/Ni-plated Cu substrate, a (Cu, Ni)6Sn5 IMC phase was grown at the pad surface. However, these IMC layers become thicker with the aging temperature and time. Additionally, in the solder matrix, a spherical-shape Cu6Sn5 and needle-shape Ag3Sn IMC phases were evenly dispersed in β-Sn matrix. However, it was worthy noted that these IMC particles and layers growth nature of interconnection made by composite material was slower than that of reference solder systems. This phenomenon was occurred due to set the surface-active Sm2O3 nanoparticles on IMC phase. Moreover, nanoparticles doped composite material ensured the higher shear strength due to reduce the growth rate of IMC thickness, control the IMC particles and evenly distributed of Sm2O3 nanoparticles in the solder matrix.

Directional Solidification of Sn-Cu6Sn5 In Situ Composites

The Sn-Cu system presents an important interest from academic and technological point of view because it is part of the family of alloys proposed as lead-free solder alloys for electronic components and also due to the mechanisms involved during the growth of the different phases. Sn-Cu system has two intermetallic phases, i.e., ε-Cu3Sn and η-Cu6Sn5, and η can be used as the negative (anode) electrode in Li-ion batteries, alone or as part of (Co,Ni)xCu6−xSn5-type composites. Obtaining this η phase from liquid with the appropriate chemical composition is a very difficult task because it has a formation temperature lower than liquidus for such a composition. In this way, the η phase appears as a consequence of a solid-solid transformation from the ε phase However, it is possible to find the η phase as the primary or secondary phase after a eutectic reaction for lower concentrations of Cu. On the other side, the Cu6Sn5 phase shows a hexagonal to monoclinic solid-solid transformation around 187°C, which could affect the mechanical system stability when it is used as solder. In this work, directional solidification at different growth velocities of hypereutectic Sn-Cu samples was performed. The resultant microstructure varies with the growth velocity, but it is formed for a fibber-like primary phase Cu6Sn5 which is projected towards the liquid phase. Behind this region, these fibbers are rounded by a two-phase Sn-Cu6Sn5 structure. In this way, three zones could to be defined in the sample during the directional growth: (i) an entirely solid two-phase region, formed by η rounded by β(Sn) + η eutectic-like structure, (ii) a two-phase solid (η) + liquid, and (iii) the remnant liquid in front of the interface.

Phase separation during the sedimentation process in Cu–Sn alloy

In the present paper the sedimentation process in Cu–Sn alloy is discussed in two ways: by experimental and numerical approach. The sedimentation process was executed at the following conditions — the rotation speed 35,000 RPM, the radii was between 88 and 93 mm, temperature 180 °C and time 2 h. During the experiment the separation of the Cu3Sn and Cu6Sn5 phase was observed. The sedimentation process clearly influenced the morphology of the alloy. Moreover, the model based on the bi-velocity method is presented. The model allow for determination of the phase fraction when centrifugal force acting on the sample.

Size-Dependent Phase Transformation during Gas Atomization Process of Cu⁻Sn Alloy Powders.

For binary element atomization, it is essential to investigate the phase transformation from liquid to solid as a functions of the droplet sizes, as well as the reaction competitiveness, during gas atomizing solidification of their nuclei. In the present work, a series of phase transformations of undercooled Cu (60.9 wt.%)/Sn droplets were analyzed when atomized by pressure gas. The results indicated that the microstructures of the obtained powders and their morphologies were highly relevant to the droplet size. According to the phase characteristics analyzed by the microstructural observations in combination with the transient nucleation theory, powders with sizes from 10 to 100 μm were divided into three categories, exhibiting lotus-leaf, island, and stripe morphologies. The competitive formation of Cu6Sn5 or Cu3Sn was also controlled by the droplet sizes, and a diameter of approximately 45 μm was identified as the threshold size. After heat treatment at 300 °C for 4 h, the powders consisted of a single η’ Cu6Sn5 phase. The obtained Cu6Sn5 phase powders can be used in the field of high-temperature applications as intermetallic balls for integrated chip interconnects.

Evolution of Transient Liquid-Phase Sintered Cu–Sn Skeleton Microstructure During Thermal Aging

The evolution of the transient liquid-phase sintered (TLPS) Cu–Sn skeleton microstructure during thermal aging was evaluated to clarify the thermal reliability for die-attach applications. The Cu–Sn skeleton microstructure, which consists of Cu particles connected with Cu–Sn intermetallic compounds partially filled with polyimide resin, was obtained by the pressure-less TLP sintering process at 250 °C for 1 min using a novel Cu-solder-resin composite as a bonding material in a nitrogen atmosphere. Experimental results indicate that the TLPS joints were mainly composed of Cu, Cu6Sn5, and Cu3Sn in the as-bonded state, where submicron voids were observed at the interface between Cu3Sn and Cu particles. After thermal aging at 150, 175, and 200 °C for 1000 h, the Cu6Sn5 phase fully transformed into Cu3Sn except at the chip-side interface, where the number of the submicron voids appeared to increase. The averaged shear strengths were found to be 22.1 (reference), 22.8 (+3%), 24.0 (+9%), and 19.0 MPa (−14%) for the as-bonded state and specimens aged at 150, 175, and 200 °C for 1000 h, respectively. The TLPS joints maintained a shear strength over 19 MPa after thermal aging at 200 °C for 1000 h because of both the positive and negative impacts of the thermal aging, which include the transformation of Cu6Sn5 into Cu3Sn and the formation of submicron voids at the interface, respectively. These results indicate an excellent thermal reliability of the TLPS Cu–Sn skeleton microstructure.

Ni Barrier Symmetry Effect on Electromigration Failure Mechanism of Cu/Sn–Ag Microbump

Ni barrier symmetry effect on the electromigration (EM) failure mechanism of Cu/Sn–Ag microbump were systematically investigated by studying the intermetallic compound (IMC) growth characteristics at 150 °C with a current density of 1.5 × 105 A/cm2. In the symmetric Ni barrier structure, Cu diffusion to Sn–Ag solder was restricted by the Ni barrier at both interfaces and the Ni3Sn4 phase formed by the inter-diffusion between Ni and Sn atoms just after bonding, which was gradually transformed to (Ni,Cu)3Sn4 phase and later to (Cu,Ni)6Sn5 during current stressing with relatively slow resistance increase with time. By the way, in the asymmetric structure, extensive Cu6Sn5 phase grew by the inter-diffusion between Cu and Sn atoms due to there is no Ni barrier at the upper interface, which was rapidly transformed into only Cu6Sn5 and Cu3Sn IMCs during electron downward flow, with relatively fast resistance increase with time. Therefore, the symmetric Ni barrier structure is very effective in restricting extensive IMC reactions during EM of Cu-solder microbump structure.

Interfacial Behaviors in Cu/Molten Sn–58Bi/Cu Solder Joints Under Coupling with Thermal and Current Stressing

The interfacial behaviors of Cu/molten Sn–58Bi/Cu solder joints under the coupling effect of a temperature gradient and the current stressing have been investigated. The most obvious change of the interfacial behaviors under the individual electromigration (EM) and thermomigration (TM) in molten solder was the asymmetrical growth of interfacial Cu–Sn intermetallic compounds (IMCs), which grew rapidly as the stressing time prolonged. The growth rates of the interfacial IMCs induced by TM under a temperature gradient of 40 °C/cm were slightly faster than EM under the current density of 0.5 × 104 A/cm2. However, the microstructure evolution and interfacial behaviors changed obviously when the thermal was distributed unevenly across the entire solder stripe under current stressing. It was found that there was a Bi-rich layer adhered to the anode side and a distributed Cu6Sn5 phase existed in the solder matrix when the anode and the cold end were at the same end. Additionally, a large number of Bi-blocks and Cu–Sn IMCs were dramatically observed in the solder matrix when the anode and the hot end were on the same side. The main reason for this result may be attributed to the significant change of the diffusion of Bi atoms under the thermo-electric coupling conditions. In the initial melting stage of solder, the Bi atoms in molten Sn–58Bi solder rapidly diffused to the anode and then began to be reversely dissolved into the solder matrix. The experimental results proved the additional temperature gradient played a positive or negative role on the reverse dissolution of Bi atoms in the heat preservation process.

Enhanced corrosion resistance by SnCu-graphene oxide composite coatings

SnCu-graphene oxide composite coatings with different amount of graphene oxide (GO) content were electrodeposited over a mild steel substrate. Morphological, microstructural evolution and electrochemical properties of coatings were investigated as a function of GO content. All the coatings exhibited compact and crack free morphology. Structural characterization revealed the presence of Sn and Cu6Sn5 phase in all the coatings. Addition of GO considerably reduced the grain size of Sn phase while the Cu6Sn5 phase remained unaltered. GO altered the coating texture facilitating growth along the low index planes. Corrosion behaviour of the coatings was examined through potentiodynamic polarization and electrochemical impedance spectroscopy methods. An aspect of “optimum” GO amount for best corrosion resistance for the SnCu-GO composite coatings was witnessed. With the addition of GO, the corrosion rate initially decreased to a minimum value and then, upon further addition of GO, it increased to values much higher than the pristine coating. In the case of lower GO amounts, the textured growth coupled with inertness of GO increased the corrosion resistance whereas, at higher GO concentrations, the galvanic coupling between cathodic GO and anodic SnCu dominated, leading to lowering of the corrosion resistance.

The effects of TiO2 nanoparticles addition on the thermal shock resistance, shear strength and IMC layer growth of SAC305 alloy

The present study explores the effect of nano-sized TiO2 ceramic particles on interfacial structure, creep and shear strength of eutectic Sn-3Ag-0.5Cu (SAC305 in wt.%) interconnect material under isothermal aging and thermal shock cycles. The study revealed that the non-reacting and non-coarsening TiO2 nanoparticles improved the creep, shear strength and thermal shock resistance through refined the structure. An interfacial microstructural analysis confirmed that a layer types Cu6Sn5 and Cu3Sn phases are grown at the interface of SAC305/Cu system. When SAC305 was in contact with the Au/Ni-plated Cu pad ball grid array (BGA) substrate, a ternary (Cu, Ni)-Sn phase was detected. The thickness of IMC phase is increased with the aging time. However, the growth of IMC layer as well as the IMC phases inside the matrix e.g., Ag3Sn, Cu6Sn5, and AuSn4 is slower in the composite interconnection systems than that of reference interconnection systems. Furthermore, during the thermal shock cycles some micro-cracks are found at the interface and matrix regions in the reference interconnection systems. The measured shear strength of composite interconnections exhibited higher values as compare to the reference interconnection systems. However, the thermal shock cycles degrades the shear strength of interconnection faster than the situation in isothermal aging.

Crystal Structure of the Intermetallic Thin-Film Cu–Sn Condensate

The synthesis of intermetallic compounds in multilayer Cu–Sn films is confined within reaction islands, which can attain 100 nm in size. X-ray diffraction analysis reveals the predominance of one intermetallic Cu6Sn5 phase, while electron microscopy evidences a high crystallographic orientation of its islands. They have the same orientation to the film surface, but are rotated with respect to each other at different angles and randomly oriented to the axis, which is perpendicular to the film plane and coincide with the crystallographic axis [100] of individual single crystals.

Sn–3.0Ag–0.5Cu solder with minor added Ge and Pd–Cu reaction

To improve solder-joint reliability, the Sn–3.0Ag–0.5Cu (SAC) solder had minor Ge and Pd additions to form composite solders. The results indicated that the Cu6Sn5 and Cu3Sn phases formed at the SAC–[Formula: see text]Ge/Cu interface ([Formula: see text], 0.5, 1.0, 2.0 wt.%) and the Ag3Sn phase precipitated in the solder. Cu6Sn5 and Cu3Sn phases formed at the interface and PdSn4 phase in the solder when the SAC–1.0Pd and SAC–2.0Pd reacted with the Cu. The SAC–1.0 Pd solder had the lowest liquidus temperature, and Ge addition improved the wettability.

The effect of platinum contact metallization on Cu/Sn bonding

In this work, formation and evolution of microstructures in CuSn/Pt bonding were investigated after 320 °C reflow process as well as after high temperature storage test at 150 °C. Sputtered thin film platinum on silicon wafer and high purity platinum sheet were applied as contact metallizations for electroplated copper-tin based bonding metallurgy. As bonded microstructure showed PtSn4 intermetallic compound growth at the Pt/Sn interface, and both Cu6Sn5 and Cu3Sn phases formed at the Cu/Sn fiinterface. Both hexagonal and monoclinic Cu6Sn5 were found to coexist after 1000 h high temperature storage test. Platinum was discovered to dissolve into the Cu6Sn5 phase during soldering process and form (Cu, Pt)6Sn5 intermetallic compound exhibiting hexagonal allotropy. Meanwhile, under annealing, monoclinic Cu6Sn5 phase layer without platinum was observed to form between (Cu, Pt)6Sn5 grains and tin. Thermodynamic analysis was performed in order to reason the effects of Pt on the phase equilibria and phase stabilities. Results show that platinum has a significant impact on the stability of hexagonal Cu6Sn5.

Effect of Ni layer on the microstructure evolution and mechanical property of Cu-based bearing shell with aging treatment

Cu-based bearing shell containing Ni layer was prepared by centrifugal casting. The effect of Ni layer on the microstructure evolution and mechanical property of Cu-based bearing shell with aging treatment was studied. The microstructure of the Sn-Sb-Cu alloy (ZChSnSbCu8-4) without aging is consisted of Sn-based solid solution and Cu6Sn5 phases which distributed uniformly in the matrix. After aging treatment, the grain boundary segregation is slowly disappeared, and the matrix transforms into a more uniform Sn-based solid solution. The Ni layer hinders the diffusion of Cu and Sn atoms at the interface, and suppresses the volume fraction increment of Cu6Sn5 phase during aging. The Cu6Sn5 phases are gradually coarsened during aging. The average bonding strength of Cu-based bearing shell with Ni layer is 70.3 MPa. After aging at 175 °C with 500 h, the bond strength is still higher than 50 MPa, which is greater than the specimen without Ni layer. The hardness of Cu matrix, Ni layer and Sn-Sb-Cu alloy decreases with increasing aging time. While the hardness of the region near the Ni/Sn-Sb-Cu interface increases from 47.5Hv to 88.4Hv during aging. The fracture surface of the samples after aging treatment is changed from intergranular/transgranular fracture mode to intergranular fracture mode. The change of the fracture mode is related to the coarsening and growth of Cu6Sn5 phases near the interface between Ni layer and Sn-Sb-Cu matrix.

Investigation on the Interfacial IMCs Layer of Sn-58Bi/Cu Solder Joints

The interfacial intermetallic compounds (IMCs) layer of Sn-58Bi lead-free solder reacting with Cu substrate were explored at aging stage after soldering for 1day, 3days, 6days, 8days and 10days, respectively. Results show that Bi doesn’t react with Cu atoms or Sn atoms. The IMCs layer is composed of Cu6Sn5 phase and Cu3Sn phase. The IMCs layer evolves from scalloped shape to layer shape with addition of aging time, meanwhile, Bi phase in matrix grows bigger. The growth kinetic equation of IMCs layer is obtained and described as X=1.06+0.69t1/2, where 0.69μm/d1/2 is the growth coefficient. Compared with Sn-3Ag-0.5Cu lead-free solder, the growth coefficient of Sn-58Bi solder is much smaller. Accordingly, the growth of IMCs layer of Sn-58Bi solder is more slowly than that of Sn-3Ag-0.5 Cu solder.