AgIn2/Ag2In transformations in an In-49Sn/Ag soldered joint under thermal aging

The interfacial reaction between liquid In-49Sn solders and Ag substrates results in the formation of a thicker Ag₂In intermetallic compound accompanied with the development of a thin AgIn₂ layer. Through further aging of the In-49Sn/Ag soldered specimens at various temperatures ranging from room to 100°C, solid/solid trnasitions between Ag₂In and AgIn₂ intermetallic compounds can be observed. When the temperature drops below 75°C, Ag₂In will react with the In-49Sn solder to form the dominant AgIn₂ phase. Conversely, AgIn₂ is consumed at a higher temperature (e.g., 100°C) when reacting with the Ag substrate to create a now dominant Ag₂In phase. Lastly, the different mechanical, electrical, magnetic, and corrosion behaviors of both intermetallic compounds are respectively made known through direct measurements of the material properties of the individual Ag₂In and AgIn₂ bulk samples.     

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.     

Soldering reactions between In49Sn and Ag thick films

The interfacial reactions between In49Sn solders and Ag thick films at temperatures ranging from 200°C to 350°C have been studied. The intermetallic compound formed at the Ag/In49Sn interface is Ag₂In enveloped in a thin layer of AgIn₂. Through the measurement of the thickness decrease of Ag thick films, it has been determined that the reaction kinetics of Ag₂In has a linear relation to reaction time. Morphology observations indicated that the linear reaction of Ag₂In was caused by the floating of Ag₂In into the In49Sn solder as a result of the In49Sn solder penetrating into the porous Ag thick film. A sound joint can be obtained when a sufficient thickness of the Ag thick film (over 19.5 μm) reacts with the In49Sn solder. In this case, the tensile tested specimens fracture in the In49Sn matrix.     

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.     

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.     

Intermetallic formation in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with immersion Ag surface finish

The intermetallic compounds formed in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Ag/Cu pads are investigated. After reflow, scallop-shaped η-Cu₆Sn₅ and continuous planar η-(cu_0.9Ni_0.1)₆Sn₅ intermetallics appear at the interfaces of the Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder joints, respectively. In the case of the Sn3Ag0.5Cu specimens, an additional ε-Cu₃Sn intermetallic layer is formed at the interface between the η-Cu₆Sn₅ and Cu pads after aging at 150°C, while the same type of intermetallic formation is inhibited in the Sn3Ag0.5Cu0.06Ni0.01Ge packages. In addition, the coarsening of Ag₃Sn precipitates also abates in the solder matrix of the Sn3Ag0.5Cu0.06Ni0.01Ge packages, which results in a slightly higher ball shear strength for the specimens.     

Intermetallic reactions in a Sn-20In-2.8Ag solder ball-grid-array package with Au/Ni/Cu pads

The intermetallic compounds formed during the reflow and aging of Sn-20In-2.8Ag ball-grid-array (BGA) packages are investigated. After reflow, a large number of cubic-shaped AuIn₂ intermetallics accompanied by Ag₂In precipitates appear in the solder matrix, while a Ni(Sn_0.72Ni_0.28)₂ intermetallic layer is formed at the solder/pad interface. With further aging at 100°C, many voids can be observed in the solder matrix and at the solder/pad interface. The continuous distribution of voids at the interface of specimens after prolonged aging at 100°C causes their bonding strength to decrease from 5.03 N (as reflowed) to about 3.50 N. Aging at 150°C induces many column-shaped (Cu_0.74Ni_0.26)₆(Sn_0.92In_0.08)₅ intermetallic compounds to grow rapidly and expand from the solder/pad interface into the solder matrix. The high microhardness of these intermetallic columns causes the bonding strength of the Sn-20In-2.8Ag BGA solder joints to increase to 5.68 N after aging at 150°C for 500 h.     

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.     

Electric current effects on Sn/Ag interfacial reactions

The effect of electric current on the Sn/Ag interfacial reaction was studied at 140°C and 200°C, by examining the growth of phase (ε-Ag₃Sn) in the Sn/Ag reaction couples with a constant current density. Only at 140°C was the growth of phase affected by the passage of electric current. The growth rate was enhanced when diffusion of Sn and electron flow were in the same direction, and retarded when they were in the opposite direction. It was found that the diffusion coefficient of Sn through Ag₃Sn was 3.37 μm²/h and the apparent effective charge for Sn in Ag₃Sn was −90, at 140°C.     

Intermetallic reactions in Sn-3.5Ag solder ball grid array packages with Ag/Cu and Au/Ni/Cu pads

During the reflow process of Sn-3.5Ag solder ball grid array (BGA) packages with Ag/Cu and Au/Ni/Cu pads, Ag and Au thin films dissolve rapidly into the liquid solder, and the Cu and Ni layers react with the Sn-3.5Ag solder to form Cu₆Sn₅ and Ni₃Sn₄ intermetallic compounds at the solder/pad interfaces, respectively. The Cu₆Sn₅ intermetallic compounds also appear as clusters in the solder matrix of Ag surface-finished packages accompanied by Ag₃Sn dispersions. In the solder matrix of Au/Ni surface-finished specimens, Ag₃Sn and AuSn₄ intermetallics can be observed, and their coarsening coincides progressively with the aging process. The interfacial Cu₆Sn₅ and Ni₃Sn₄ intermetallic layers grow by a diffusion-controlled mechanism after aging at 100 and 150°C. Ball shear strengths of the reflowed Sn-3.5Ag packages with both surface finishes are similar, displaying the same degradation tendencies as a result of the aging effect.     

Interfacial reactions between liquid indium and Au-deposited substrates

The morphology and growth kinetics of intermetallic compounds formed during the soldering reactions of liquid indium on Au-deposited substrates in the temperature range of 225°C and 350°C have been investigated. The results show that two types of AuIn₂ intermetallic compounds make their appearance: the continuous-wavy-crystalline type, and the floating-island type. The growth of this intermetallic compound follows the parabolic law, which indicates that the growth is diffusion-controlled. The activation energy of the reaction calculated form the Arrhenius plot of growth reaction constants is 39.42 kJ/mol. Also, the wettability of liquid indium on the surface of the gold-deposited substrate is determined from contact angle measurements. Finally, a mechanism for the interpretation of the wetting behavior of the said Au/In system is proposed, which can be ascertained by SEM observations.     

Intermetallic compounds formed at the interface between liquid indium and copper substrates

The interfacial reactions between liquid In and Cu substrates at temperatures ranging from 175°C to 400°C are investigated for the applications in bonding recycled sputtering targets to their backing plates. Experimental results show that a scallop-shaped Cu₁₆In₉ intermetallic compound is found at the Cu/In interface after solder reactions at temperatures above 300°C. A double-layer structure of intermetallic compounds containing scallop-shaped Cu₁₁In₉ and continuous CuIn is observed after the Cu/In interfacial reaction at temperatures below 300°C. The growth of all these intermetallic compounds follows the parabolic law, which implies that the growth is diffusion-controlled. The activation energies for the growth of Cu₁₆In₉, Cu₁₁In₉, and CuIn intermetallic compounds calculated from the Arrhenius plot of growth reaction constants are 59.5, 16.9, and 23.5 kJ/mole, respectively.     

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.     

A comparative study of the kinetics of interfacial reaction between eutectic solders and Cu/Ni/Pd metallization

A comparative study of the kinetics of interfacial reaction between the eutectic solders (Sn-3.5Ag, Sn-57Bi, and Sn-38Pb) and electroplated Ni/Pd on Cu substrate (Cu/Ni/NiPd/Ni/Pd) was performed. The interfacial microstructure was characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope (SEM). For a Pd-layer thickness of less than 75 nm, the presence or the absence of Pd-bearing intermetallic was found to be dependent on the reaction temperature. In the case of Sn-3.5Ag solder, we did not observe any Pd-bearing intermetallic after reaction even at 230°C. In the case of Sn-57Bi solder the PdSn₄ intermetallic was observed after reaction at 150°C and 180°C, while in the case of Sn-38Pb solder the PdSn₄ intermetallic was observed after reaction only at 200°C. The PdSn₄ grains were always dispersed in the bulk solder within about 10 μm from the solder/substrate interface. At higher reaction temperatures, there was no Pd-bearing intermetallic due to increased solubility in the liquid solder. The presence or absence of Pd-bearing intermetallic was correlated with the diffusion path in the calculated Pd-Sn-X (X=Ag, Bi, Pb) isothermal sections. In the presence of unconsumed Ni, only Ni₃Sn₄ intermetallic was observed at the solder-substrate interface by SEM. The presence of Ni₃Sn₄ intermetallic was consistent with the expected diffusion path based on the calculated Ni-Sn-X (X=Ag, Bi, Pb) isothermal sections. Selective etching of solders revealed that Ni₃Sn₄ had a faceted scallop morphology. Both the radial growth and the thickening kinetics of Ni₃Sn₄ intermetallic were studied. In the thickness regime of 0.14 μm to 1.2 μm, the growth kinetics always yielded a time exponent n >3 for liquid-state reaction. The temporal law for coarsening also yielded time exponent m >3. The apparent activation energies for thickening were: 16936J/mol for the Sn-3.5Ag solder, 17804 J/mol for the Sn-57Bi solder, and 25749 J/mol for the Sn-38Pb solder during liquid-state reaction. The corresponding activation energies for coarsening were very similar. However, an apparent activation energy of 37599 J/mol was obtained for the growth of Ni₃Sn₄ intermetallic layer during solid-state aging of the Sn-57Bi/substrate diffusion couples. The kinetic parameters associated with thickening and radial growth were discussed in terms of current theories.     

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 Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes

The intermetallic compounds (IMCs) formed during the reflow and aging of Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes were investigated. After reflow, the thickness of (Cu, Ni, Au)₆Sn₅ interfacial IMCs in Sn3Ag0.5Cu0.06Ni0.01Ge was similar to that in the Sn3Ag0.5Cu specimen. The interiors of the solder balls in both packages contained Ag₃Sn precipitates and brick-shaped AuSn₄ IMCs. After aging at 150°C, the growth thickness of the interfacial (Ni, Cu, Au)₃Sn₄ intermetallic layers and the consumption of the Ni surface-finished layer on Cu the pads in Sn3Ag0.5Cu0.06Ni0.01Ge solder joints were both slightly less than those in Sn3Ag0.5Cu. In addition, a coarsening phenomenon for AuSn₄ IMCs could be observed in the solder matrix of Sn3Ag0.5Cu, yet this phenomenon did not occur in the case of Sn3Ag0.5Cu0.06Ni0.01Ge. Ball shear tests revealed that the reflowed Sn3Ag0.5Cu0.06Ni0.01Ge packages possessed bonding strengths similar to those of the Sn3Ag0.5Cu. However, aging treatment caused the ball shear strength in the Sn3Ag0.5Cu packages to degrade more than that in the Sn3Ag0.5Cu0.06Ni0.01Ge packages.     

Intermetallic compounds formed during interfacial reactions between liquid Sn-8Zn-3Bi solders and Ni substrates

The morphology and growth kinetics of intermetallic compounds (IMCs) formed at the interfaces between liquid Sn-8Zn-3Bi solders and nickel substrates in the temperature range from 225°C to 400°C are investigated for the applications in bonding recycled sputtering targets to their backing plates. The results show that a continuous single layer of Ni₅Zn₂₁ IMC appears at temperatures below 325°C, while a double layer containing Ni₅Zn₂₁ and Ni₃₅Zn₂₂Sn₄₃ IMCs is formed at temperatures above 325°C. In both cases, the growth kinetics of IMCs is interface-controlled. During the growth of IMCs, their reaction fronts migrate in the direction of the solder much more rapidly than toward the nickel substrate, and erosion of the Ni substrate is quite slight.     

Influence of initial morphology and thickness of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> and Cu<Subscript>3</Subscript>Sn intermetallics on growth and evolution during thermal aging of Sn-Ag solder/Cu joints

Intermetallic-layer formation and growth in Pb-free solder joints, during solder reflow or subsequent aging, has a significant effect on the thermal and mechanical behavior of solder joints. In this study, the influence of initial intermetallic morphology on growth rate, and kinetics were examined in a Sn-3.5Ag solder reflowed on Cu. The initial morphology of the intermetallic was tailered by cooling in water, air, or furnace conditions. Solder aging was conducted at 100°C, 140°C, and 175°C and aged for 0–1,000 h. Cooling rate, aging temperature, and aging time played an important role on microstructure evolution and growth kinetics of Cu₆Sn₅ (η) and Cu₃Sn (ɛ) intermetallic layers. Prior to aging, faster cooling rates resulted in a relatively planar Cu₆Sn₅ layer, while a nodular Cu₆Sn₅ morphology was present for slower cooling. Intermetallic-growth rate measurements after aging at various times, indicated a mixed growth mechanism of grain-boundary and bulk diffusion. These mechanisms are discussed in terms of the initial intermetallic thickness and morphology controlled by cooling rate, diffusion kinetics, and the competition between Cu₆Sn₅ and Cu₃Sn growth.     

Intermetallic reactions in Sn-8Zn-20In solder ball grid array packages with Au/Ni/Cu and Ag/Cu pads

During the reflow process of Sn-8Zn-20In solder joints in the ball grid array (BGA) packages with Au/Ni/Cu and Ag/Cu pads, the Au and Ag thin films react with liquid solder to form γ₃-AuZn₄/γ-Au₇Zn₁₈ and ε-AgZn₆ intermetallics, respectively. The γ₃/γ intermetallic layer is prone to floating away from the solder/Ni interface, and the appearance of any interfacial intermetallics cannot be observed in the Au/Ni surface finished Sn-8Zn-20In packages during further aging treatments at 75°C and 115°C. In contrast, ε-CuZn₅/γ-Cu₅Zn₈ intermetallics are formed at the aged Sn-8Zn-20In/Cu interface of the immersion Ag BGA packages. Bonding strengths of 3.8N and 4.0N are found in the reflowed Sn-8Zn-20In solder joints with Au/Ni/Cu and Ag/Cu pads, respectively. Aging at 75°C and 115°C gives slight increases of ball shear strength for both cases.     

Effects of Co addition in eutectic Sn-3.5Ag solder on shear strength and microstructural development

In this study, the approach of composite solder using eutectic Sn-3.5Ag solder and Co was tried. Co particles and Sn-3.5Ag solder paste were mechanically mixed at Co weight fractions from 0.1% to 2.0%. For the Co-mixed Sn-3.5Ag solder pastes, their melting temperatures and spreading areas were measured. The solder pastes were stencil printed on test substrates and reflowed to form solder bumps. Ball shear test was performed to examine shear strength of Co-reinforced Sn-3.5Ag solder bumps. As a result, Co addition up to 2 wt.% did not alter the melting temperature under heating but reduced undercooling. The maximum shear strength of Co-reinforced Sn-3.5Ag solder bumps increased by 28% compared to normal ones. The increase in shear strength can be attributed to the (Cu,Co)₃Sn₂ intermetallic compounds.