Formation Of Cu6Sn5 Research Articles

Mechanism and control of preferred Cu6Sn5 growth on single crystal (001)Cu

Preferred orientation Cu6Sn5 prisms are usually formed on (001)Cu single crystal, making use of which can highly improve the packaging reliability of electronic devices. A practical method for morphology controlling of preferred orientation Cu6Sn5 was put forward preliminarily in this article based on temperature variation. The roof-type Cu6Sn5 prisms actually behaved as scallops of highly preferred orientation relationship with (001)Cu substrate in isothermal heating of reflow process and began to transform into prismatic roofs without grain number and orientation changing following it cooled down to solidification. This new phenomenon undiscovered before can pave the way for understanding the precise mechanism of roof-type Cu6Sn5 formation and provide a scientific viewpoint that orientation, size, and outline control of oriented Cu6Sn5 should be realized in different stages of reflow process. This is quite significant for manufacture of morphology-controllable solder interconnections in electronic industry.

Effect of the fabrication route on the phase and volume changes during the reaction heat treatment of Nb3Sn superconducting wires

Accelerator magnets that can reach magnetic fields well beyond the Nb-Ti performance limits are presently being built and developed, using Nb3Sn superconductors. This technology requires reaction heat treatment (RHT) of the magnet coils, during which Nb3Sn is formed from its ductile precursor materials (a “wind and react” approach). The Nb3Sn microstructure and microchemistry are strongly influenced by the conductor fabrication route, and by the phase changes during RHT. By combining in situ differential scanning calorimetry, high energy synchrotron x-ray diffraction, and micro-tomography experiments, we have acquired a unique data set that describes in great detail the phase and microstructure changes that take place during the processing of restacked rod process (RRP), powder-in-tube (PIT), and internal tin (IT) Nb3Sn wires. At temperatures below 450 °C the phase evolutions in the three wire types are similar, with respectively solid state interdiffusion of Cu and Sn, Cu6Sn5 formation, and Cu6Sn5 peritectic transformation. Distinct differences in phase evolutions in the wires are found when temperatures exceed 450 °C. The volume changes of the conductor during RHT are a difficulty in the production of Nb3Sn accelerator magnets. We compare the wire diameter changes measured in situ by dilatometry with the phase and void volume evolution of the three types of Nb3Sn wire. Unlike the Nb3Sn wire length changes, the wire diameter evolution is characteristic for each Nb3Sn wire type. The strongest volume increase, of about 5%, is observed in the RRP wire, where the main diameter increase occurs above 600 °C upon Nb3Sn formation.

The effect of Aluminium addition on the microstructure of lead-free solder alloys: A short review

The ban on using lead in electronic packaging has been taken effect since the year 2006. The alternative to replacing lead (Pb) based solder has been given highly concerns by the electronic industry. Both Sn- Ag-Cu and Sn-Cu solder alloys were considered to be the most acceptable candidates to replace SnPb solder alloy. From the findings, it showed that β-Sn appears as island-liked shape and eutectic phases appear as a darker shade. The grain size is refined significantly after incorporated with a suitable amount of Al. When exorbitant amount of Al addition will cause Al-rich particles, which has high potential to acts as the heterogeneous nucleation site and result in a lower degree of undercooling. The most desired amount of Al addition is between 0.025 - 0.05 wt%. The primary intermetallic compounds found in Sn-based solder alloy are Cu6Sn5 and Ag3Sn. New intermetallic compouds were found after addition of Al, which are Cu-Al and Ag-Al intermetallic. The formation of new intermetallic compounds causes the formation of Cu6Sn5 and Ag3Sn to be suppressed.

Effect of Zn addition on Cu3Sn formation in Sn-10Cu alloys

Sn-Cu alloys have generated interest as potential candidates for high-temperature soldering applications, as they are economically feasible and environmentally safe. Sn-Cu alloys with copper contents beyond 7.6wt.% solidify with a primary Cu3Sn phase, followed by peritectic and eutectic reactions. Typically, phases which form as elongated needles are undesirable, as they tend to be more brittle and in turn reduce the strength and toughness of the alloy. The effect of Zn on the peritectic reaction and phase formation in a Sn-10wt.%Cu alloy is investigated through cooling curves and microstructural analysis using optical and electron microscopy. Zn was alloyed in the range of 0.1-1wt.% to the base Sn-10Cu alloy system. The results indicate that the amount of the primary Cu3Sn phase that forms is inversely proportional to the concentration of Zn. Although complete suppression of Cu3Sn was not observed, there was a clear effect of Zn reducing the amount of the primary Cu3Sn phase, with the greatest reduction of Cu3Sn occurring with a 1wt.% Zn addition. The reduction of the elongated needle-shaped primary Cu3Sn phase may find application in the development of more reliable bulk solder microstructures.

Scanning 3DXRD Measurement of Grain Growth, Stress, and Formation of Cu6Sn5 around a Tin Whisker during Heat Treatment.

The 3D microstructure around a tin whisker, and its evolution during heat treatment were studied using scanning 3DXRD. The shape of each grain in the sample was reconstructed using a filtered-back-projection algorithm. The local lattice parameters and grain orientations could then be refined, using forward modelling of the diffraction data, with a spatial resolution of 250 . It was found that the tin coating had a texture where grains were oriented such that their c-axes were predominantly parallel to the sample surface. Grains with other orientations were consumed by grain growth during the heat treatment. Most of the grain boundaries were found to have misorientations larger than 15, and many coincidence site lattice (CSL) or other types of low-energy grain boundaries were identified. None of the grains with CSL grain boundaries were consumed by grain growth. During the heat treatment, growth of preexisting Cu6Sn5 occurred; these grains were indexed as a hexagonal phase, which is usually documented to be stable only at temperatures exceeding 186 . This indicates that the phase can exist in a metastable state for long periods. The tin coating was found to be under compressive hydrostatic stress, with a negative gradient in hydrostatic stress extending outwards from the root of the whisker. Negative stress gradients are generally believed to play an essential role in providing the driving force for diffusion of material to the whisker root.

Characterisation of lithium-ion battery anodes fabricated via in-situ Cu6Sn5 growth on a copper current collector

Li-ion batteries (LIBs) are favoured in many applications due to their high energy density and good cyclic performance. However, some safety concerns remain with respect to the risk of fires and explosions with LIBs. Carbon-based anode materials in LIBs operate close to the Li metal reduction potential, therefore Li dendrites can grow during voltage fluctuations, causing short circuits which may lead to fires. The Cu6Sn5 anodes has the lithiation potential at about 0.4 V vs Li/Li+ and therefore less prone to Li metal plating. A new manufacturing method involving direct in-situ formation of Cu6Sn5 on a Cu current collector via melt-solid contact is proposed. This method combines the active material production and the anode fabrication into a single process. Lithiation and delithiation mechanisms of the anode produced are studied by in-situ synchrotron X-ray powder diffraction (XRPD) and ex-situ high voltage transmission electron microscope (HV-TEM). The in-situ XRPD study shows a reversible two-step reaction during cycling, and also reveals the differences in the reaction mechanisms at higher charge/discharge rates than those in published data.

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.

Microstructure, IMCs layer and reliability of Sn-58Bi solder joint reinforced by Mo nanoparticles during thermal cycling

The microstructure, IMCs layer and reliability of Sn-58Bi and Sn-58Bi-0.25Mo solder joints during thermal cycling were investigated. The results showed that the microstructure of Sn-58Bi and Sn-58Bi-0.25Mo solder joints is coarsened with increasing thermal cycling times and it is refined by adding Mo nanoparticles. The IMCs morphologies of Sn-58Bi and Sn-58Bi-0.25Mo solder joints are both changed to be flat and thick with increasing thermal cycling times and fewer voids are found in the IMCs layer of Sn-58Bi-0.25Mo solder joint than that of Sn-58Bi solder joint. The IMCs layer thickness of Sn-58Bi and Sn-58Bi-0.25Mo solder joints is increased with increasing thermal cycling times due to the formation of Cu6Sn5 and Cu3Sn compounds, and slow IMCs layer growth is obtained by Mo-reinforced solder joint during thermal cycling. The shear and tensile strength of Sn-58Bi-xMo (x = 0, 0.25) solder joints are both decreased with increasing thermal cycling times and the mechanical properties of Sn-58Bi solder joint are improved by adding Mo nanoparticles. The fracture mode of Mo contained composite solder joint is mixed with ductile and brittle fracture with lots of dimples, and it is changed to brittle fracture after 1200 thermal cycles with the reduction in dimples.

Evolution of the Microstructure of Sn58Bi Solder Paste with Sn-3.0Ag-0.5Cu Addition During Isothermal Aging

Eutectic Sn58Bi (SnBi) solder paste mixed with 0 wt.%, 3 wt.%, 5 wt.%, 8 wt.% and 15 wt.% of Sn-3.0Ag-0.5Cu (SAC) paste was prepared by mechanical mixing. The effects of SAC paste additions on the microstructure evolution of SnBi-SAC/Cu composite solder joints during isothermal aging were investigated. The results indicated that the number of large Bi-rich phases decreased and the relative areas of β-Sn increased with increasing SAC content. Moreover, 1-μm Bi-rich particles were found near the Bi-rich phases. During the isothermal aging process, the diameter of the 1-μm Bi-rich particles in the solder bulk increased by about 50% with aging time by Ostwald ripening. The thickness of the interfacial intermetallic compound in all the solder joints increased slightly during the aging process. The formation of Cu6Sn5 was suppressed by the Bi-rich phases above the Cu6Sn5 layer with the aging time increasing. In addition, the solder bulk showed many cracks along the β-Sn grain boundaries after isothermal aging when the content of SAC paste was 5 wt.%. With 8 wt.% or 15 wt.% SAC, fractures were more obvious near the interface than away from the interface.

The fast formation of full Cu3Sn solder joints in Cu/Sn/Cu system by thermal gradient bonding

To form full intermetallic compounds (IMCs) solder joints becomes widely available for the die bonding of the third generation semiconductor power devices. The fast formation of Cu3Sn in Cu/Sn(10 µm)/Cu solder joints were investigated by thermal compression bonding in a few seconds and under a low pressure of 0.1 MPa at ambient temperature. The results show that the temperature gradient produced by thermal compression bonding contributes to enhancing the interfacial reaction at the liquid Sn/solid Cu metallization interface. The scallop-liked Cu6Sn5 is formed while planar-liked Cu3Sn is formed between Cu and Cu6Sn5 at initial bonding stage. After that, the growth rate of scallop-liked Cu6Sn5 layer from cold end is faster than that from hot end and the thin planar-liked Cu3Sn layer becomes thick. This abnormal growth behavior of Cu6Sn5 is due to the fact that Cu atoms migrate from hot end to cold end by temperature gradient. The middle Sn layer is completely consumed with increasing bonding time. Cu6Sn5 and Cu3Sn are consisted of the solder joints and the planar-liked Cu3Sn grows with a transition to scallop-liked morphology until the full Cu3Sn solder joint is eventually formed, which indicates that the formation time of full IMCs solder joints can be narrowed by temperature gradient. This bonding process provides a new solution for the rapid acquisition of interconnect material for the third generation semiconductor power devices.

Effect of Ni addition into the Cu substrate on the interfacial IMC growth during the liquid-state reaction with Sn–58Bi solder

The interfacial reactions between Sn–58Bi solder and Cu–xNi (x = 0, 0.5, 1.5, 5 and 10 wt.%) substrates at 200 °C with different liquid-state reaction durations were investigated to reveal the effect of the Ni addition into the Cu substrate on the growth of intermetallic compound (IMC), and grains evolution in this study. The results of this research indicated that addition of Ni significantly changed the interfacial microstructure of solder joints. Moreover, the formation of Cu3Sn was suppressed with the addition of Ni element, the Cu3Sn was only observed on Sn–58Bi/Cu–0.5Ni and Sn–58Bi/Cu–1.5Ni interfaces after liquid-state reaction for 150 min while Cu3Sn was completely eliminated when 5 or 10 wt.% Ni was added to Cu substrate regardless of the reaction time, and one new single phase (Cu,Ni)6Sn5 was formed between solder and Cu–Ni substrate. The total thickness increased as the liquid-state reaction time increased and the line relationship existed between total thickness and reaction time. The total thickness and growth rate of IMC gradually increased with the mass percentage of Ni increasing from 0 to 5 wt.% for the same liquid-state reaction time. In contrast, when the Ni content increased to 10%, the thickness and growth rate decreased slightly. The grain size became fine and the prismatic grain perpendicular to the interface tended to be dominant as the Ni element was added.

Low-temperature-solderable intermetallic nanoparticles for 3D printable flexible electronics

Functional materials for flexible and wearable smart devices have attracted much attention in recent years. This paper describes structure and properties of uniquely prepared, functional interconnectable nanoparticles (NPs) of Cu6Sn5 intermetallic compound that can allow 3D flexible packaging and nano-circuits. In situ TEM analysis confirms that size-controllable Cu6Sn5 NPs as small as ∼6.40 nm can be made sinterable at the start temperature as low as ∼130 °C, which is much lower than its bulk melting point (MP) of 415 °C. After sintering, its high MP provides mechanical and thermal stability. Based on the in situ TEM observation and calculation, particle size and distribution affects the sintering process. More interestingly, the relative orientations of adjacent particles also play an important role. A new orientation related sintering mechanism noted as orientation unification (OU) is revealed as two adjacent particles exhibit orientation change to slowly match their orientation with each other during the heating process. The interesting interaction between nano-Cu6Sn5 and micro-Cu substrate during in situ TEM heating gives first hand atomic level proof of the formation of Cu3Sn. The nano-Cu6Sn5 joints possess high enough bonding strength and great high temperature working capability. This intermetallic nano-soldering approach can pioneer a novel strategy of circuit connection, by providing high working temperature interconnection materials for 3D flexible packaging and ultra-high-density micro/nano interconnections.

Copper effects in mechanical, thermal and electrical properties of rapidly solidified eutectic Sn–Ag alloy

The Sn–3.5 wt%Ag alloy considered as a good alternative to Pb–Sn alloys. This study aims to investigate the effects of Cu or Sb additions by 3 or 5 wt% to melt-spun Sn–3.5%Ag alloy. Ternary melt-spun Sn–Ag–Cu and Sn–Ag–Sb alloys investigated using X-ray diffractions (XRD), Scanning electron microscope (SEM), Dynamic resonance technique (DRT), Instron machine, Vickers hardness tester and Differential scanning calorimetry (DSC). The results revealed that the microstructures of the β-Sn phase, Ag3Sn and Cu3Sn intermetallic compounds (IMCs) in the solder matrices were refined due to the effect of Cu additions and melt-spun process. Moreover, increasing Cu content promotes Ag3Sn intermetallic compound (IMC) formation. Consequently, the addition of “3 wt%” of Cu reduced the creep rate ɛ from (3.79 × 10−3) to (1.65 × 10−3) and delayed the fracture point. The tensile results showed an improvement in Young’s modulus by 47% (30.3 GPa), ultimate tensile strength (UST) by 11.6% (23.9 MPa), and in toughness by 20.5% (952.32 J/m3) compared to the eutectic Sn–Ag alloy. Vickers hardness has improved by 3.3% (136.71 MPa) and thermal activation energy by 54% (90.40 KJ/mol) when compared with that of eutectic Sn–Ag alloy. Those improvements are related to the lack of lattice strain from 7.56 × 10−4 without “3 wt%” of Cu to 5.26 × 10−4 with “3 wt%” of Cu. Its melting temperature (Tm) increased by 3 °C due to Ag3Sn IMC increased and Cu3Sn formation, but the pasty rang (mushy zone) decreased by 4 °C with “3 wt%” of Cu. The small lattice strains resulted with “3 wt%” of Cu made the electrical resistivity of this alloy more stable at elevated temperatures. The mechanical, thermal and electrical improvements of Sn93.5–Ag3.5–Cu3 alloy provide good physical performance for soldering process and electronic assembly.

The fast formation of Cu-Sn intermetallic compound in Cu/Sn/Cu system by induction heating process

The fast formation of Cu3Sn in a Cu/Sn(10 μm)/Cu solder joints were investigated by induction heating process in a few minutes and under a low pressure of 0.01 MPa at ambient temperature. The introducing of induction heating process showed benefit effects on enhancing the interfacial reaction at the liquid Sn/solid Cu metallization interface. From the three-dimensional microstructural observation of interfacial IMCs at different bonding times, scallop-liked Cu6Sn5 was formed while planar-liked Cu3Sn was formed between Cu and Cu6Sn5 at initial soldering stage. After that, the scallop-liked Cu6Sn5 grew with a transition to dendritic-liked morphology while planar-liked Cu3Sn grew with a transition to columnar-liked morphology until the full Cu3Sn solder joint was eventually formed. Accompanying with the movement of Cu atoms in the molten Sn, the morphology of columnar-liked Cu3Sn compounds surrounded by dendritic-liked Cu6Sn5 compounds was formed due to constitutional super-cooling. The rapid microstructure evolution of the interfacial IMCs was caused by the joule heating effect as well as the electromagnetic stirring effect.

Electromigration and Thermomechanical Fatigue Behavior of Sn0.3Ag0.7Cu Solder Joints

The anisotropy of Sn crystal structures greatly affects the electromigration (EM) and thermomechanical fatigue (TMF) of solder joints. The size of solder joint shrinkage in electronic systems further makes EM and TMF an inseparably coupled issue. To obtain a better understanding of failure under combined moderately high (2000 A/cm2) current density and 10–150°C/1 h thermal cycling, analysis of separate, sequential, and concurrent EM and thermal cycling (TC) was imposed on single shear lap joints, and the microstructure and crystal orientations were incrementally characterized using electron backscatter diffraction (EBSD) mapping. First, it was determined that EM did not significantly change the crystal orientation, but the formation of Cu6Sn5 depended on the crystal orientation, and this degraded subsequent TMF behavior. Secondly, TC causes changes in crystal orientation. Concurrent EM and TC led to significant changes in crystal orientation by discontinuous recrystallization, which is facilitated by Cu6Sn5 particle formation. The newly formed Cu6Sn5 often showed its c-axis close to the direction of electron flow.

A comparison between complete and incomplete cellular precipitations

Classical cellular precipitation is an incomplete reaction, meaning not all the supersaturated solutes can be precipitated out. Consequently, the kinetic parameters such as lamellar spacing and growth velocity cannot be defined. Recently, in 3-Dimension Integrated Circuits (3D IC) microbump technology, the porous Cu3Sn formation is a complete cellular precipitation; the kinetic parameters can be defined. This is rare and unique in phase transformations. In this letter, kinetic models of both precipitations are analyzed for a deeper understanding.

Interfacial Reaction and Mechanical Properties of Sn-Bi Solder joints

Sn-Bi solder with different Bi content can realize a low-to-medium-to-high soldering process. To obtain the effect of Bi content in Sn-Bi solder on the microstructure of solder, interfacial behaviors in solder joints with Cu and the joints strength, five Sn-Bi solders including Sn-5Bi and Sn-15Bi solid solution, Sn-30Bi and Sn-45Bi hypoeutectic and Sn-58Bi eutectic were selected in this work. The microstructure, interfacial reaction under soldering and subsequent aging and the shear properties of Sn-Bi solder joints were studied. Bi content in Sn-Bi solder had an obvious effect on the microstructure and the distribution of Bi phases. Solid solution Sn-Bi solder was composed of the β-Sn phases embedded with fine Bi particles, while hypoeutectic Sn-Bi solder was composed of the primary β-Sn phases and Sn-Bi eutectic structure from networked Sn and Bi phases, and eutectic Sn-Bi solder was mainly composed of a eutectic structure from short striped Sn and Bi phases. During soldering with Cu, the increase on Bi content in Sn-Bi solder slightly increased the interfacial Cu6Sn5 intermetallic compound (IMC)thickness, gradually flattened the IMC morphology, and promoted the accumulation of more Bi atoms to interfacial Cu6Sn5 IMC. During the subsequent aging, the growth rate of the IMC layer at the interface of Sn-Bi solder/Cu rapidly increased from solid solution Sn-Bi solder to hypoeutectic Sn-Bi solder, and then slightly decreased for Sn-58Bi solder joints. The accumulation of Bi atoms at the interface promoted the rapid growth of interfacial Cu6Sn5 IMC layer in hypoeutectic or eutectic Sn-Bi solder through blocking the formation of Cu6Sn5 in solder matrix and the transition from Cu6Sn5 to Cu3Sn. Ball shear tests on Sn-Bi as-soldered joints showed that the increase of Bi content in Sn-Bi deteriorated the shear strength of solder joints. The addition of Bi into Sn solder was also inclined to produce brittle morphology with interfacial fracture, which suggests that the addition of Bi increased the shear resistance strength of Sn-Bi solder.

Coupling effect of thermomigration and cross-interaction on evolution of intermetallic compounds in Cu/Sn/Ni ultrafine interconnects undergoing TLP bonding

Abstract

Suppression of Cu3Sn layer and formation of multi-orientation IMCs during thermal aging in Cu/Sn–3.5Ag/Cu–15Zn transient liquid-phase bonding in novel 3D-IC Technologies

The evolution of grain structures and microstructures in Cu/Sn–3.5Ag/Cu–xZn (x=0 or 15wt%) TLP bonding before and after aging were investigated. Cu–Sn intermetallic compounds (IMCs) with a strong [001] orientation rapidly form from the Cu pads and cause impingement in TLP bond, allowing cracks to easily propagate. During aging, the Cu6Sn5 becomes one layer with a homogeneous structure, thick Cu3Sn layers form, and Kirkendall voids occur at bonding interfaces, causing degradation of bonding strength. This study demonstrates that doping Zn into one of the Cu substrates efficiently retains the multi-orientation structure of Cu–Sn IMCs and suppresses the formation of Cu3Sn during lone-time aging. The Cu/Sn–3.5Ag/Cu–15Zn TLP bonding is potentially useful in strengthening the interconnections for novel 3D-IC technologies.

Comparative study on the wettability and interfacial structure in Sn–xZn/Cu and Sn/Cu–xZn system

Zn is the common used alloying element in Pb-free solder to depress the growth of interfacial intermetallic compound (IMC) in solder joints, but is also an oxidation preferred element to decrease the wettability of solder. This paper provides a potential replacement on the Zn-contained solder with small amount of Zn alloyed Cu substrate to avoid the deterioration on wettability but keep the depressing effect on growth of IMC through the comparative study on Sn–xZn/Cu and Sn/Cu–xZn. To systematically study the effect of Zn alloying, 0.2–0.8 wt% content of Zn was added into pure Sn solder and 2.29 and 4.89 wt% content of Zn was alloyed into Cu substrate. The wettability, interfacial reaction during soldering and interfacial IMC growth of Sn–xZn/Cu and Sn/Cu–xZn during isothermal aging were studied and compared with Sn/Cu system. Zn alloyed into Cu substrate exhibited a far better wettability than Zn alloyed into solder. Although there was no significant change on the composition, morphology and thickness of interfacial of IMC in Zn-contained system during soldering, Zn alloyed into Cu substrate or Sn solder obviously depressed the formation of Cu3Sn and the growth of IMC layers during isothermal aging compared with Sn/Cu system. The depressing effect and calculated activation energy on the growth of IMC increased with the content of Zn in Cu or in Sn, and can be attributed to the CuZn solid solution existed at the interface because it decreased the driving force on formation of Cu3Sn and the diffusion rate of Cu atoms. Compared Sn/Cu–xZn with Sn–xZn/Cu, Cu–4.89Zn substrate provided a similar depressing effect on IMC growth but an exceeding wettability compared with Sn–Zn solder, and can be offered as an alternative under bump metallurgy (UBM) in solder joints.