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 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.     

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.     

Interfacial reactions between liquid indium and silver substrates

The morphology and growth kinetics of intermetallic compounds formed at the interface between liquid indium and silver substrates in the temperature range from 473 K to 673 K were studied. Electron microprobe (EPMA) analysis and x-ray diffraction (XRD) revealed that the intermetallic compound Ag₂In formed as scallops. The growth followed parabolic kinetics, suggesting that the growth was diffusion-controlled. Moreover, the activation energy calculated from the Arrhenius plot of growth reaction constants was 41.55 KJ/mole. Also, the relationship between the wettability of liquid indium on the surface of silver substrates and their interfacial reactions was clarified.     

Morphology of intermetallic compounds formed between lead-free Sn-Zn based solders and Cu substrates

The morphologies of intermetallic compounds formed between Sn-Zn based solders and Cu substrates were investigated in this study. The investigated solders were Sn-9Zn, Sn-8.55Zn-0.45Al, and Sn-8.55Zn-0.45Al-0.5Ag. The experimental results indicated that the Sn-9Zn solder formed Cu₅Zn₈ and CuZn₅ compounds on the Cu substrate, while the Al-containing solders formed the Al_4.2Cu_3.2Zn_0.7 compound. The addition of Ag to the Sn-8.55Zn-0.45Al solder resulted in the formation of the AgZn₃ compound at the interface between the Al_4.2Cu_3.2Zn_0.7 compound and the solder. Furthermore, it was found that the cooling rate of the specimen after soldering had an effect on the quantity of AgZn₃ compound formed at the interface. The AgZn₃ compound formed with an air-cooling condition exhibited a rougher surface and larger size than with a water-quenched condition. It was believed that the formation of the AgZn₃ compound at the interface occurs through heterogenous nucleation during solidification.     

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 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.     

Intermetallic compounds formed during the reflow of In-49Sn solder ball-grid array packages

The intermetallic compounds formed at the interfaces between In-49Sn solder balls and Au/Ni/Cu pads during the reflow of In-49Sn solder, ball-grid array (BGA) packages are investigated. Various temperature profiles with peak temperatures ranging from 140°C to 220°C and melting times ranging from 45 sec to 170 sec are plotted for the reflow processes. At peak temperatures below 170°C, a continuous double layer of intermetallics can be observed, showing a composition of Au(In,Ni)₂/Au(In,Ni). Through selective etching of the In-49Sn solders, the intermetallic layer is made up of irregular coarse grains. In contrast, a number of cubic-shaped AuIn₂ intermetallic compounds appear at the interfaces and migrate toward the upper domes of In-49Sn solder balls after reflow at peak temperatures above 200°C for longer melting times. The upward floating of the AuIn₂ cubes can be explained by a thermomigration effect caused by the temperature gradient present in the liquid solder ball. The intermetallic compounds formed under various reflow conditions in this study exhibit different types of morphology, yet the ball shear strengths of the solder joints in the In-49Sn BGA packages remain unaffected.     

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.     

Intermetallic compound layer growth by solid state reactions between 58Bi-42Sn solder and copper

Solid state intermetallic compound layer growth was examined following ther-mal aging of the 58Bi-42Sn/Cu couple for a temperature range of 55 to 120°C and time periods of from 1 to 400 days. The intermetallic compound layer was comprised of sublayers that included the traditional Cu₆Sn₅ stoichiometry as well as one or more complex Cu-Sn-Bi chemistries. The number of sublayers increased with aging temperature and time. Time-dependent layer thickness computations based upon the empirical expression, Atⁿ + B, revealed a time exponent, n, that decreased with increasing temperature from a maximum of 0.551 at 70°C to 0.417 at 120°C. The apparent activation energy for growth (at 100 days) was 55± 7 kJ/mol. The Bi-Sn/Cu data, together with that from the other solder/copper systems, suggested that at a given homologous temperature, the quantity of Sn in the solder field determines the intermetallic compound layer thickness as a function of time.     

Intermetallic embrittlement of thin unsupported tin/copper specimens

Thin, unsupported vacuum-cast tin samples were prepared and, after half of the samples were coated with a thin layer of evaporated copper, stressed to failure using a specialized tensile testing apparatus. The samples were examined optically and by analytical scanning and transmission electron microscopy. Samples coated with copper exhibited very different mechanical behavior than did the uncoated tin samples which exhibited simple ductile behavior. Postfailure examination of the specimens shows that the differences in mechanical behavior are not due to the copper coating, but, in fact, due to copper-tin intermetallics formed within the bulk of the tin.     

固液界面纳米气泡的研究

在经典热力学理论中，室温下水中纳米气泡被认为是不能稳定存在的。近年来随着对疏水表面研究的深入，越来越多的现象暗示固液界面存在纳米气泡，并引起疏水长程作用力。目前直接探测固液界面纳米气泡的最有力手段是AFM，但它只能观察纳米气泡的形貌，无法对其进行直接定性，很有必要提供纳米气泡存在和来源的更直接证据。在云母表面进行乙醇和水替换形成纳米气泡，从成像条件和脱气对纳米气泡的影响两方面进行系统研究。不同成像模式和成像条件下AFM观察到的差异在一定程度上证明了观察到的就是纳米气泡。脱气实验结果表明，经脱气后乙醇和水形成纳米气泡的数量和概率明显降低，表明乙醇和水中溶解的气体是纳米气泡的来源，并为固液界面存在纳米气泡提供了更直接的证据。     

Interfaces in lead-free soldering

Structural integrity of circuits is greatly dependent on interfacial microstructure. In this paper, the status of the current understanding of various interfaces appearing in lead-free soldering is reviewed, and recent data on interfaces in electronic interconnections, primarily analyzed by transmission electron microscopy (TEM), is presented. The compound Cu₆Sn₅ is formed, as localized precipitates attach to the interface of a Cu substrate with Sn plating, even in an as-received condition. After long-time exposure at room temperature, it grows into a Cu₆Sn₅ layer along the interface. When the temperature is raised slightly or Sn in a plating layer is consumed by the reaction, a Cu₃Sn layer can grow between a Cu₆Sn₅ layer and a Cu substrate. In soldering, most Sn alloys involving pure Sn, Sn-Ag, or their ternary alloys form two intermetallic compounds, e.g., Cu₆Sn₅ and Cu₃Sn, on a Cu substrate, with the former much thicker than the latter. The Ni plating forms Ni₃Sn₄/Ni₃Sn₂ double layers at the interface with Sn alloys in soldering with the latter layer very much thinner. In contrast, Fe-42Ni alloy forms (Fe,Ni)Sn₂ double layers by the reaction with Sn and Sn-Ag(-Cu). When Zn becomes one of the elements of the solder, Zn first reacts with a substrate. Thus, the Sn-Zn alloy forms different intermetallic compounds at an interface with Cu, i.e., the CuZn/Cu₅Zn₈ double layers. The Sn-Zn alloy also forms a thin AuZn layer when thin Au plating is on a substrate.     

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.     

Interfacial reactions between In 10Ag solders and Ag substrates

The morphology and growth kinetics of intermetallic compounds formed during the reaction of liquid In 10Ag on Ag substrates in the temperature range between 250°C and 375°C are studied. The results indicate that the Ag₂In intermetallic compounds that appear at the interface are in the columnar shape, enveloped by thin AgIn₂ shells. The growth kinetics of intermetallic compounds are parabolic, indicating that the reaction is diffusion-controlled. The Arrhenius reaction activation energy was found to be 44.9 kJ/mol. Also, the wetting behavior of the In10Ag on Ag substrates was studied. The results show that there exists a transient plateau of the contact angle variation. Such a phenomenon can be explained by the intermetallic compound precursor halo formation preceding the edge of the solder drop.     

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.     

Solid state intermetallic compound growth between copper and high temperature,tin-rich solders—part I: Experimental analysis

An experimental study was performed which examined the solid state growth kinetics of the interfacial intermetallic compound layers formed between copper and the high temperature, tin-rich solders 96.5Sn-3.5Ag (wt.%) and 95Sn-5Sb. These results were compared with baseline data from the 100Sn/copper system. Both the 96.5Sn-3.5Ag and 95Sn-5Sb solders exhibited the individual Cu₃Sn and Cu₆Sn₅ layers at the interface; the thickness of the Cu₃Sn layer being a function of the aging time and temperature. The total thickness of the intermetallic compound layer formed in the 96.5Sn-3.5Ag solder/copper couple showed a mixture of linear and √t dependencies at the lower temperatures of 70,100, and 135°C, and a t^0.42 dependence at 170 and 205°C. The combined apparent activation energy was 59 kJ/mol, the Arrhenius plot showed a knee between the low and high temperature data. The total layer thickness of the 95Sn-5Sb/copper system exhibited √t dependence at the three lower temperatures and t^0.42 growth kinetics at 170 and 205°C. The combined apparent activation energy was 61 kJ/mol.     

Effect of Cross-Interaction between Ni and Cu on Growth Kinetics of Intermetallic Compounds in Ni/Sn/Cu Diffusion Couples during Aging

The solid-state, cross-interaction between the Ni layer on the component side and the Cu pad on the printed circuit board (PCB) side in ball grid array (BGA) solder joints was investigated by employing Ni(15 μm)/Sn(65 μm)/Cu ternary diffusion couples. The ternary diffusion couples were prepared by sequentially electroplating Sn and Ni on a Cu foil and were aged isothermally at 150, 180, and 200°C. The growth of the intermetallic compound (IMC) layer on the Ni side was coupled with that on the Cu side by the mass flux across the Sn layer that was caused by the difference in the Ni content between the (Cu_1−x Ni _x )₆Sn₅ layer on the Ni side and the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side. As the consequence of the coupling, the growth rate of the (Cu_1−x Ni _x )₆ Sn₅ layer on the Ni side was rapidly accelerated by decreasing Sn layer thickness and increasing aging temperature. Owing to the cross-interaction with the top Ni layer, the growth rate of the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side was accelerated at 150°C and 180°C but was retarded at 200°C, while the growth rate of the Cu₃Sn layer was always retarded. The growth kinetic model proposed in an attempt to interpret the experimental results was able to reproduce qualitatively all of the important experimental observations pertaining to the growth of the IMC layers in the Ni/Sn/Cu diffusion couple.     

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 .     

Sn0.7Cu-xEr/Cu焊点界面反应及其化合物层生长行为研究

通过回流焊工艺制备了Sn0.7Cu-x Er/Cu(x=0,0.1,0.5)钎焊接头,研究钎焊温度及等温时效时间对接头的界面金属间化合物(IMC)的形成与生长行为的影响。结果表明:Sn0.7Cu钎料中微量稀土Er元素的添加,能有效抑制钎焊及时效过程中界面IMC的形成与生长。在等温时效处理过程中,随着时效时间的延长,界面反应IMC层不断增厚,在相同时效处理条件下,Sn0.7Cu0.5Er/Cu焊点界面IMC层的厚度略小于Sn0.7Cu0.1Er/Cu焊点界面的厚度。通过线性拟合方法,得到Sn0.7Cu0.1Er/Cu和Sn0.7Cu0.5Er/Cu焊点界面IMC层的生长速率常数分别为3.03×10–17 m2/s和2.67×10–17 m2/s。