Mechanisms for the intermetallic formation during the Sn-20In-2.8Ag/Ni soldering reactions

The morphology and growth kinetics of intermetallic compounds formed during the interfacial reactions between liquid Sn-20In-2.8Ag solder and Ni substrates are investigated. Energy-dispersive x-ray (EDX) analysis identifies the composition of the interfacial intermetallics as Ni₃(In_0.99In_0.01)₄. The soldering reactions at lower temperatures (225–275°C) result in the predominant formation of a homogeneous intermetallic layer whose growth is diffusion controlled. At higher soldering temperatures (300–350°C), the interfacial intermetallics appear to be long needlelike crystals, and the grooves in between the intermetallics provide fast-diffusion paths for Ni atoms to react with Sn atoms at the intermetallic front, which leads to interface-controlled growth kinetics. The intermetallic needles turned out to be flat slablike after selective etching of the unreacted solder. Kinetics analysis showed that they not only lengthened in the longitudinal direction, but also coarsened transversely by the Ostwald ripening mechanism.     

Mechanisms for interfacial reactions between liquid Sn-3.5Ag solders and cu substrates

Intermetallic compounds formed during the soldering reactions between Sn-3.5Ag and Cu at temperatures ranging from 250°C to 375°C are investigated. The results indicate that scallop-shaped η-Cu₆(Sn_0.933 Ag_0.007)₅ intermetallics grow from the Sn-3.5Ag/Cu interface toward the solder matrix accompanied by Cu dissolution. Following prolonged or higher temperature reactions, ɛ-Cu₃ (Sn_0.996 Ag_0.004) intermetallic layers appear behind the Cu₆(Sn_0.933 Ag_0.007)₅ scallops. The growth of these interfacial intermetallics is governed by a kinetic relation: ΔX=tⁿ, where the n values for η and ɛ intermetallics are 0.75 and 0.96, respectively. The mechanisms for such nonparabolic growth of interfacial intermetallics during the liquid/solid reactions between Sn-3.5Ag solders and Cu substrates are probed.     

Interfacial reactions between liquid indium and nickel substrate

The morphologies and growth kinetics of intermetallic compounds for the interfacial reaction between liquid In and solid Ni substrate in the temperature range from 225 to 500°C are examined in this study. Experimental results showed that the thickness of intermetallic compounds formed during the Ni_(s)/In₍₁₎ interfacial reaction increased with the reaction temperature and the square root of reaction time. The x-ray diffraction pattern revealed the formation of intermetallic compounds Ni₁₀In₂₇ (T<300°C) and Ni₂In₃ (T>300°C). Moreover, the activation energies for the interdiffusion of Ni and In atoms in the Ni₁₀In₂₇ and Ni₂In₃ are 94.74 and 33.51 kJ/mol, respectively. Using the Ta thin film as a diffusion mark, the formation mechanism of intermetallic compounds during interfacial reaction was clarified.     

Reflow and burn-in of a Sn-20In-0.8Cu ball grid array package with a Au/Ni/Cu pad

The intermetallic compounds formed after reflow and burn-in testing of a Sn-20In-0.8Cu solder ball grid array (BGA) package are investigated. Along with the formation of the Cu₆(Sn_0.78In_0.22)₅ precipitates (IM1) in the solder matrix, scallop-shaped intermetallic compounds (IM2) with a compositional mixture of Cu₆(Sn_0.87In_0.13)₅ and Ni₃(Sn_0.87In_0.13)₄ appear at the interfaces between the solder balls and Au/Ni/Cu pads. A significant number of intermetallic particles (IM3), with a composition of (Au_0.80Cu_0.20)(In_0.33Sn_0.67)₂, can also be found in the solder matrix. After aging at 115°C for 750 h, an additional intermetallic compound layer (IM4) with a composition of (Ni_0.91Cu_0.09)₃(Sn_0.77In_0.23)₂ is formed at the interface between IM2 and the Ni layer. The ball shear strength of the Sn-20In-0.8Cu BGA solder after reflow is 4.5 N and will rise to maximum values after aging at 75°C and 115°C for 100 h. With a further increase of the aging time at both temperatures, the joint strengths exhibit a tendency to decline linearly at about 1.7×10⁻³ N/h.     

Intermetallic compounds formed during the reflow and aging of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder ball grid array packages

After reflow of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder balls on Au/Ni surface finishes in ball grid array (BGA) packages, scallop-shaped intermetallic compounds (Cu_0.70Ni_0.28Au_0.02)₆Sn₅ (IM1a) and (Cu_0.76Ni_0.24)₆(Sn_0.86In_0.14)₅ (IM1b), respectively, appear at the interfaces. Aging at 100°C and 150°C for Sn-3.8Ag-0.7Cu results in the formation of a new intermetallic phase (Cu_0.70Ni_0.14Au_0.16)₆Sn₅ (IM2a) ahead of the former IM1a intermetallics. The growth of the newly appeared intermetallic compound, IM2a, is governed by a parabolic relation with an increase in aging time, with a slight diminution of the former IM1a intermetallics. After prolonged aging at 150°C, the IM2a intermetallics partially spall off and float into the solder matrix. Throughout the aging of Sn-20In-2Ag-.5Cu solder joints at 75°C and 115°C, partial spalling of the IM1b interfacial intermetallics induces a very slow increase in thickness. During aging at 115°C for 700 h through 1,000 h, the spalled IM1b intermetallics in the solder matrix migrate back to the interfaces and join with the IM1b interfacial intermetallics to react with the Ni layers of the Au/Ni surface finishes, resulting in the formation and rapid growth of a new (Ni_0.85Cu_0.15)(Sn_0.71In_0.29)₂ intermetallic layer (IM2b). From ball shear tests, the strengths of the Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder joints after reflow are ascertained to be 10.4 N and 5.4 N, respectively, which drop to lower values after aging. An erratum to this article is available at .     

Study of interfacial reactions between Sn3.5Ag0.5Cu composite alloys and Cu substrate

For development of a lead-free composite solder for advance electrical components, lead-free Sn3.5Ag0.5Cu solder was produced by mechanically mixing 0.5 wt.% TiO₂ nanopowder with Sn3.5Ag0.5Cu solder. The morphology and growth kinetics of the intermetallic compounds that formed during the soldering reactions between Sn3.5Ag0.5Cu solder with intermixed TiO₂ nanopowder and Cu substrates at various temperatures ranging from 250 to 325 °C were investigated. A scanning electron microscope (SEM) was used to quantify the interfacial microstructure at each processing condition. The thickness of interfacial intermetallic layers was quantitatively evaluated from SEM micrographs using imaging software. Experimental results show that a discontinuous layer of scallop-shaped Cu-Sn intermetallic compounds formed during the soldering. Kinetics analysis shows that the growth of such interfacial Cu-Sn intermetallic compounds is diffusion controlled with an activation energy of 67.72 kJ/mol.     

Intermetallic compounds formed during the interfacial reactions between liquid In-49Sn solder and Ni substrates

The interfacial reactions between liquid In-49Sn solder and Ni substrates at temperatures ranging from 150°C to 450°C for 15 min to 240 min have been investigated. The intermetallic compounds formed at the In-49Sn/Ni interfaces are identified to be a ternary Ni₃₃In₂₀Sn₄₇ phase using electron-probe microanalysis (EPMA) and x-ray diffraction (XRD) analyses. These interfacial intermetallics grow with increasing reaction time by a diffusion-controlled mechanism. The activation energy calculated from the Arrhenius plot of reaction constants is 56.57 kJ/mol.     

Thermo‐electrochemical Activation of an In–Cu Intermetallic Electrode for the Anode in Lithium Secondary Batteries

Lithium ion batteries (LIBs) have been emerging as a major power source for portable electronic devices and hybrid electric vehicles (HEV) with their superior performance to other competitors. The performance aspects of energy density and rate capability of LIBs should, however, be further improved for their new applications. Towards this end, many Li‐alloy materials, metal oxides, and phosphides have been tested, some of which have, however, been discarded because of poor activity at ambient temperature. Here, it is shown that the In? Cu binary intermetallic compound (Cu₇In₃), which shows no activity at room temperature as a result of activation energy required for In? Cu bond cleavage, can be made active by discharge–charge cycling at elevated temperatures. Upon lithiation at elevated temperatures (55–120 °C), the Cu₇In₃ phase is converted into nanograins of metallic Cu and a lithiated In phase (Li₁₃In₃). The underlying activation mechanism is the formation of new In‐rich phase (CuIn). The de‐lithiation temperature turns out to be the most important variable that controlling the nature of the In‐rich compounds.     

A Hermetic Seal Using Composite Thin-Film In/Sn Solder as an Intermediate Layer and Its Interdiffusion Reaction with Cu

A bonding joint between Cu metallization and evaporated In/Sn composite solder is produced at a temperature lower than 200°C in air. The effects of bonding temperature and duration on the interfacial bonding strength are studied herein. Cross sections of bonding joints processed at different bonding conditions were examined by scanning electron microscopy (SEM). The optimal condition, i.e., bonding temperature of 180°C for 20 min, was chosen because it gave rise to the highest average bonding strength of 6.5 MPa, and a uniform bonding interface with minimum voids or cracks. Good bond formation was also evidenced by scanning acoustic imaging. For bonding couples of patterned dies, a helium leak rate of 5.8 × 10⁻⁹ atm cc/s was measured, indicating a hermetic seal. The interfacial reaction between Cu and In/Sn was also studied. Intermetallic compounds (IMCs) such as AuIn₂, Cu₆Sn₅, and Cu₁₁In₉ were detected by means of x-ray diffraction analysis (XRD), and transmission electron microscopy (TEM) accompanied by energy-dispersive x-ray (EDX) spectroscopy. Chemical composition analysis also revealed that solder interlayers, Sn, and In were completely converted into IMCs by reaction with Cu. All the IMCs formed in the joints have remelting temperatures above 300°C according to the Cu-In, Cu-Sn, and Au-In phase diagrams. Therefore, the joint is able to sustain high service temperatures due to the presence of these IMCs.     

Phase identification and growth kinetics of the intermetallic compounds formed during in-49Sn/Cu soldering reactions

For the application of In-49Sn solder in bonding recycled-sputtering targets to Cu back plates, the intermetallic compounds formed at the In-49Sn/Cu interface are investigated. Scanning electron microscopy (SEM) observations show that the interfacial intermetallics consist of a planar layer preceded by an elongated scalloped structure. Electron-probe microanalyzer analyses indicate that the chemical compositions of the planar layer and the scalloped structure are Cu_74.8In_12.2Sn_13.0 and Cu_56.2In_20.1Sn_23.7, respectively, which correspond to the ε-Cu₃(In,Sn) and η-Cu₆(In,Sn)₅ phases. Kinetics analyses show that the growth of both intermetallic compounds is diffusion controlled. The activation energies for the growth of η- and ε-intermetallics are calculated to be 28.9 kJ/mol and 186.1 kJ/mol. Furthermore, the formation mechanism of intermetallic compounds during the In-49Sn/Cu soldering reaction is clarified by marking the original interface with a Ta-thin film. Wetting tests are also performed, which reveal that the contact angles of liquid In-49Sn drops on Cu substrates decline to an equilibrium value of 25°C.     

Kinetics of Intermetallic Compound Formation at the Interface Between Sn-3.0Ag-0.5Cu Solder and Cu-Zn Alloy Substrates

The growth kinetics of an intermetallic compound (IMC) layer formed between Sn-3.5Ag-0.5Cu (SAC) solders and Cu-Zn alloy substrates was investigated for samples aged at different temperatures. Scallop-shaped Cu₆Sn₅ formed after soldering by dipping Cu or Cu-10 wt.%Zn wires into the molten solder at 260°C. Isothermal aging was performed at 120°C, 150°C, and 180°C for up to 2000 h. During the aging process, the morphology of Cu₆Sn₅ changed to a planar type in both specimens. Typical bilayer of Cu₆Sn₅ and Cu₃Sn and numerous microvoids were formed at the SAC/Cu interfaces after aging, while Cu₃Sn and microvoids were not observed at the SAC/Cu-Zn interfaces. IMC growth on the Cu substrate was controlled by volume diffusion in all conditions. In contrast, IMC growth on Cu-Zn specimens was controlled by interfacial reaction for a short aging time and volume diffusion kinetics for a long aging time. The growth rate of IMCs on Cu-Zn substrates was much slower due to the larger activation energy and the lower layer growth coefficient for the growth of Cu-Sn IMCs. This effect was more prominent at higher aging temperatures.     

The Effects of Solid-State Aging on the Intermetallic Compounds of Sn-Ag-Bi-In Solders on Cu Substrates

The growth behavior of the intermetallic compounds that formed at the interfaces between Sn-Ag-Bi-In solders and Cu substrates during solid-state aging is investigated. The compositions of the intermetallic compounds are Cu₃(Sn,In) near the Cu substrates and Cu₆(Sn,In)₅ near the solders; very little Bi or Ag was dissolved in the compounds. The aging temperatures were 120°C, 150°C, and 180°C for 5 days, 10 days, 20 days, and 40 days. The change in the morphology of Cu₆(Sn,In)₅ from scallop type to layer type was prominent at the aging temperature of 180°C. The thickness of the compound layers did not vary much at the lower aging temperatures but followed the diffusion- controlled mechanism at 180°C. Massive Kirkendall voids were observed in Cu₃(Sn,In) layers at the aging temperature of 180°C.     

Intermetallic reactions in reflowed and aged Sn-9Zn solder ball grid array packages with Au/Ni/Cu and Ag/Cu pads

During the reflowing of Sn-9Zn solder ball grid array (BGA) packages with Au/Ni/Cu and Ag/Cu pads, the surface-finished Au and Ag film dissolved rapidly and reacted with the Sn-9Zn solder to form a γ₃-AuZn₄/γ-Au₇Zn₁₈ intermetallic double layer and ε-AgZn₆ intermetallic scallops, respectively. The growth of γ₃-AuZn₄ is prompted by further aging at 100°C through the reaction of γ-Au₇Zn₁₈ with the Zn atoms dissolved from the Zn-rich precipitates embedded in the β-Sn matrix of Sn-9Zn solder BGA with Au/Ni/Cu pads. No intermetallic compounds can be observed at the solder/pad interface of the Sn-9Zn BGA specimens aged at 100°C. However, after aging at 150°C, a Ni₄Zn₂₁ intermetallic layer is formed at the interface between Sn-9Zn solder and Ni/Cu pads. Aging the immersion Ag packages at 100°C and 150°C caused a γ-Cu₅Zn₈ intermetallic layer to appear between ε-AgZn₆ intermetallics and the Cu pad. The scallop-shaped ε-AgZn₆ intermetallics were found to detach from the γ-Cu₅Zn₈ layer and float into the solder ball. Accompanied with the intermetallic reactions during the aging process of reflowed Sn-9Zn solder BGA packages with Au/Ni/Cu and Ag/Cu pads, their ball shear strengths degrade from 8.6 N and 4.8 N to about 7.2 N and 2.9 N, respectively.     

Effects of Minor Additions of Zn on Interfacial Reactions of Sn-Ag-Cu and Sn-Cu Solders with Various Cu Substrates during Thermal Aging

The effects of Zn additions to Sn-0.7Cu and Sn-3.8Ag-0.7Cu (all in wt.% unless specified otherwise) Pb-free solders on the interfacial reactions with Cu substrates were investigated. The study was focused on the intermetallic compound (IMC) growth, Cu consumption and void formation as a function of thermal aging and solder composition. Four different kinds of Cu substrates (high-purity Cu, oxygen-free Cu, vacuum-sputtered Cu, and electroplated Cu) were prepared to compare their interfacial reaction behaviors with Zn-added solders. Thermal aging was performed at 150°C for up to 1000 h to accelerate the interfacial reactions between solders and Cu substrates. Growth of IMCs (Cu₆Sn₅ and Cu₃Sn) in Zn-added solders was slower than those without Zn additions. The growth of the Cu₃Sn phase, in particular, was drastically reduced in the Zn-added solders on all four Cu substrates. On an electroplated Cu substrate, numerous voids were observed in the Cu₃Sn phase for Sn-Cu and Sn-Ag-Cu solders aged at 150°C for 1000 h. However, these voids were largely eliminated in the Zn-added solders. On the other three Cu substrates, the conditions which produce a high density of voids were not found after aging both solders with and without Zn. The Cu consumption with Zn-added solders was also significantly lower. The beneficial effects of Zn additions on interfacial reaction behaviors are reported, and the corresponding mechanisms in suppressing void formation and Cu consumption due to Zn additions will be discussed.     

Effect of Bi Segregation on the Asymmetrical Growth of Cu-Sn Intermetallic Compounds in Cu/Sn-58Bi/Cu Sandwich Solder Joints During Isothermal Aging

An asymmetrical interfacial microstructure was observed at both top and bottom interfaces of Cu/Sn-58Bi/Cu solder joints after isothermal aging at 120°C for different times. The asymmetrical interfacial microstructure resulted from asymmetrical Bi segregation, which was attributed to the density difference between Bi and Sn atoms. Bi atoms were driven to the bottom solder/Cu interface by gravity during the liquid soldering procedure since Bi atoms are more massive than Sn atoms. With increasing aging time, Bi accumulated at the bottom Cu₃Sn/Cu interface and the Bi segregation enhanced Cu₆Sn₅ intermetallic compound growth, blocked Sn transport to the Cu₃Sn intermetallic compound, and facilitated growth of the Cu₆Sn₅, based on the measured thicknesses of intermetallic compounds (including Cu₆Sn₅ and Cu₃Sn) at both bottom and top interfaces for Cu/Sn-58Bi/Cu sandwich joints under the same aging process.     

The Melting Characteristics and Interfacial Reactions of Sn-ball/Sn-3.0Ag-0.5Cu-paste/Cu Joints During Reflow Soldering

In this work, the melting characteristics and interfacial reactions of Sn-ball/Sn-3.0Ag-0.5Cu-paste/Cu (Sn/SAC305-paste/Cu) structure joints were studied using differential scanning calorimetry, in order to gain a deeper and broader understanding of the interfacial behavior and metallurgical combination among the substrate (under-bump metallization), solder ball and solder paste in a board-level ball grid array (BGA) assembly process, which is often seen as a mixed assembly using solder balls and solder pastes. Results show that at the SAC305 melting temperature of 217°C, neither the SAC305-paste nor the Sn-ball coalesce, while an interfacial reaction occurs between the SAC305-paste and Cu. A slight increase in reflow temperature (from 217°C to 218°C) results in the coalescence of the SAC305-paste with the Sn-ball. The Sn-ball exhibits premelting behavior at reflow temperatures below its melting temperature, and the premelting direction is from the bottom to the top of the Sn-ball. Remarkably, at 227°C, which is nearly 5°C lower than the melting point of pure Sn, the Sn-ball melts completely, resulting from two eutectic reactions, i.e., the reaction between Sn and Cu and that between Sn and Ag. Furthermore, a large amount of bulk Cu₆Sn₅ phase forms in the solder due to the quick dissolution of Cu substrate when the reflow temperature is increased to 245°C. In addition, the growth of the interfacial Cu₆Sn₅ layer at the SAC305-paste/Cu interface is controlled mainly by grain boundary diffusion, while the growth of the interfacial Cu₃Sn layer is controlled mainly by bulk diffusion.     

Intermetallic compound layer formation between copper and hot-dipped 100In, 50In-50Sn, 100Sn,and 63Sn-37Pb coatings

The growth kinetics of intermetallic compound layers formed between four hot-dipped solder coatings and copper by solid state, thermal aging were examined. The solders were l00Sn, 50In-50Sn, 100In, and 63Sn-37Pb (wt.%); the substrate material was oxygen-free, high conductivity Cu. The total intermetallic layer of the 100Sn/Cu system exhibited a combination of parabolic growth at lower aging temperatures and t^0.42 growth at the higher temperatures. The combined apparent activation energy was 66 kJ/mol. These results are compared to the total layer growth observed with the 63Sn-37Pb/Cu system which showed parabolic kinetics at similar temperatures and an apparent activation energy of 45 kJ/mol. Both 100Sn and 63Sn-37Pb diffusion couples showed a composite intermetallic layer comprised of Cu₃Sn and Cu₆Sn₅. The intermetallic compound layer formed between In and Cu changed from a CuIn₂ stoichiometry at short annealing times to a Cu₅₇In₄₃ composition at longer periods. The growth kinetics were parabolic with an apparent activation energy of 20 kJ/mol. The intermetallic layer growth of the 50In-50Sn/Cu system exhibited extreme variations in the layer thicknesses which prohibited a quantitative assessment of the growth kinetics. The layer was comprised of two compounds: Cu₂₆Sn₁₃In₈ which was the dominant phase and a thin layer of Cu₁₇Sn₉In24 adjacent to the solder.     

Intermetallic compounds formed during diffusion soldering of Au/Cu/Al2O3 and Cu/Ti/Si with Sn/In interlayer

A Si wafer was sequentially sputter-coated with Ti (20 nm), Cu (6 μm), Sn (4 μm), and In (4 μm). The specimen was then diffusion-soldered at temperatures between 150 and 300°C with an alumina substrate deposited with Cu (4 μm) and Au (6 μm). Experimental results showed that a multilayer of intermetallic phases with the compositions of (Cu_0.99Au_0.01)₆(Sn_0.52In_0.48)₅/(Au_0.87Cu_0.13)(In_0.94Sn_0.06)₂/(Au_0.98Cu_0.02) (In_0.95Sn_0.05) formed at the Au/Cu interface. Kinetic analyses revealed that the growth of (Cu_0.99Au_0.01)₆ (Sn_0.52In_0.48)₅ and (Au_0.87Cu_0.13)(In_0.94Sn_0.06)₂/(Au_0.98Cu_0.02)(In_0.95Sn_0.05) intermetallics were diffusion-controlled with activation energies of 21.5 and 31.3 kJ/mol, respectively. Sound tensile strengths of 42 and 48 kg/cm² have been obtained under the bonding conditions of 150°C for 40 min. and 200°C for 30 min., respectively.     

The effect of Cu6Sn5 whisker precipitates in bulk 60sn-40pb solder

Hollow, Cu₆Sn₅ intermetallic rods form within molten 60Sn-40Pb solder when it reacts with Cu. It is suggested that these rods form at the Cu surface by a screw dislocation mechanism and break off into the bulk solder. Hollow hexagonal intermetallics result when the core of the rod dissolves away and fills with molten solder. The mechanical properties of bulk 60Sn-40Pb solder with and without the Cu₆Sn₅ intermetallic rods were tested in tension, at -196° C, 20° C, and 125° C. The intermetallics had no effect on strength, but decreased elongation at the lower temperatures. The intermetallics had a large effect on the fracture characteristics. At -196° C failures initiate by interfacial separation between the intermetallic and solder matrix. At 20° C failures initiate at cleaved intermetallic rods. At 125° C the intermetallic rods appear to have little effect on the mechanical properties.     

Intermetallic compounds and adhesion strength between the Sn-9Zn-1.5Ag-0.5Bi lead-free solder and unfluxed Cu substrate

The intermetallic compounds (IMCs) formed at the interface between the Sn-9Zn-1.5Ag-0.5Bi lead-free solder alloy and unfluxed Cu substrate have been investigated by x-ray diffraction, optical microscopy, scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS). The melting point and melting range of the Sn-9Zn-1.5Ag-0.5Bi solder alloy are determined as 195.9°C and 10°C, respectively, by differential scanning calorimetry (DSC). Cu₆Sn₅ and Cu₅Zn₈ IMCs are formed between the Sn-9Zn-1.5Ag-0.5Bi/unfluxed Cu substrate wetted at 250°C for 10 sec. The interfacial adhesion strength changes from 10.27±0.68 MPa to 8.58±0.59 MPa when soldering time varies from 10 sec to 30 sec at 250°C.