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.     

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.     

Kinetics of intermetallic formation at Sn-37Pb/Cu interface during reflow soldering

The kinetics of the intermetallic layer formation at Sn-37wt.%Pb solder/Cu pad interface during reflow soldering were studied. The growth kinetics were analyzed theoretically by assuming that the mass flux of Cu through channels between scalloplike grains primarily contributes to the growth. Rate-controlling steps considered for the mass flux were the Cu dissolution from the bottom of the channels, diffusion through the channel, and the formation reaction of the intermetallic layer. These results indicated that a transition in the growth rate observed around 120–150 sec of reflow time may be associated with transition of the rate-controlling step from the Cu dissolution to the Cu diffusion through the channel.     

Growth of intermetallic compounds in the Sn-9Zn/Cu joint

We have studied the microstructure of the Sn-9Zn/Cu joint in soldering at temperatures ranging from 230°C to 270°C to understand the growth of the mechanism of intermetallic compound (IMC) formation. At the interface between the Sn-9Zn solder and Cu, the results show a scallop-type ε-CuZn₄ and a layer-type γ-Cu₅Zn₈, which grow at the interface between the Sn-9Zn solder and Cu. The activation energy of scallop-type ε-CuZn₄ is 31 kJ/mol, and the growth is controlled by ripening. The activation energy of layer-type γ-Cu₅Zn₈ is 26 kJ/mol, and the growth is controlled by the diffusion of Cu and Zn. Furthermore, in the molten Sn-9Zn solder, the results show η-CuZn grains formed in the molten Sn-9Zn solder at 230°C. When the soldering temperature increases to 250°C and 270°C, the phase of IMCs is ε-CuZn₄.     

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.     

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.     

Interdiffusion analysis of the soldering reactions in Sn-3.5Ag/Cu couples

Extensive microstructural and kinetic studies on the formation and growth of the intermetallics of Sn-rich solder/Cu couples have been reported. However, experimental data on the interdiffusion mechanisms during soldering reactions are limited and in conflict. The interdiffusion processes for soldering of Sn-3.5Ag alloy/Cu couples were investigated by using the Cr-evaporated surface as a reference line. At the beginning of soldering, Cu was observed to outdiffuse to the molten Sn−3.5Ag alloy until saturation, and the Sn−Ag solder dissolved with Cu collapsed below the reference line. As a result, the scallop-shaped Cu₆Sn₅ intermetallic compound was formed at the newly-formed Sn−Ag−Cu solder/Cu interface below the original Cu surface. When the soldered joint was reflowed at the lower temperature to suppress the Cu dissolution, the Cu₆Sn₅/Cu interface moved into the Cu substrate. Therefore, Sn is the dominant diffusing species for the intermetallic formation during the soldering process, although the extensive Cu dissolution occurs at the early stage of soldering.     

Effect of soldering and aging time on interfacial microstructure and growth of intermetallic compounds between Sn-3.5Ag solder alloy and Cu substrate

The formation and the growth of the intermetallic compound (IMC, hereafter) at the interface between the Sn-3.5Ag (numbers are all in wt.% unless otherwise specified) solder alloy and the Cu substrate were investigated. Solder joints were prepared by changing the soldering time at 250°C from 30 sec to 10 h and the morphological change of IMCs with soldering time was observed. It resulted from the competition between the growth of IMC and the dissolution of Cu from the substrate and IMCs. They were further aged at 130°C up to 800 h. During aging, the columnar morphology of IMCs changed to a more planar type while the scallop morphology remained unchanged. It was observed that the growth behavior of IMCs was closely related with the initial soldering condition.     

Textured growth of Cu/Sn intermetallic compounds

The growth of Cu-Sn intermetallic compounds (IMCs) at the molten Pb-Sn solder/Cu interface was studied over a range of temperatures and for a range of solder compositions. Strong peaks of and planes of η-phase (Cu₆Sn₅) were detected by x-ray diffraction when the Sn content was high. In the low Sn solder (27Sn-73Pb), the η-phase peaks were absent at the two high temperatures, but the (2 12 0) peak of the ε-phase (Cu₃Sn) was prominent. A texture was detected in both layers in and (002) pole figures constructed for the η phase and ε phase, respectively. The growth directions were identified to be 〈101〉 and 〈102〉 for the η phase and 〈102〉 and 〈031〉 for the ε phase, normal to the Cu surface. The growth direction does not change with the morphology and the thickness of the IMC layer. The morphology of the η layer varied gradually from a cellular film with a rugged interface to a dense film with a scalloped interface as the Pb content, temperature, and reaction time increased. The ε layer was always dense and nearly planar.     

Phase field simulations of intermetallic compound growth during soldering reactions

Phase field simulations of the microstructural evolution of the intermetallic compound (IMC) layer formed during isothermal soldering reactions between Sn-Cu solder alloys and a Cu substrate are presented. The simulation accounts for the fast grain boundary (GB) diffusion in the IMC layer, the concurrent IMC grain coarsening along with the IMC layer growth, and the dissolution of Cu from the substrate and IMC layer. The simulation results support the previous suggestions that the growth kinetics of the IMC layer during soldering is predominantly governed by the fast GB diffusion and the concurrent coarsening rate of the IMC grains. The IMC grain coarsening is initiated by a competitive growth of the IMC grains at the solder/IMC interface. It is also shown that the dissolution of Cu into an unsaturated solder reduces the coarsening rate of the IMC grains, consequently decreasing the temporal growth exponent of the IMC layer.     

Reliability of In-48Sn solder/Au/Ni/Cu BGA packages during reflow process

The interfacial reactions and shear properties of In-48wt.%Sn/Au/Ni/Cu solder joints were investigated in terms of reflow conditions, i.e., reflow temperature and duration time. The thickness of an AuIn₂ intermetallic compound (IMC) layer, formed at the solder/substrate interface, slightly increased with the duration time. The spalling of the AuIn₂ intermetallics in the solder led to the formation of a Ni₃(Sn,In)₄ IMC layer between the solder and exposed Ni layer. The longer duration time resulted in the spalling and grain growth of Ni₃(Sn,In)₄ intermetallics. The higher reflow temperature accelerated the interfacial reactions between the solder and substrate. From the ball shear test results, the formation and growth of a continuous plate-shaped AuIn₂ IMC layer increased the shear force of the solder joints, whereas the spalling and grain growth of cubic-shaped AuIn₂ intermetallics significantly decreased the shear force. The formation and spalling of cubic-shaped Ni₃(Sn,In)₄ intermetallics increased the shear force, whereas the spalling and grain growth of polyhedron-shaped Ni₃(Sn,In)₄ intermetallics decreased the shear force. The crack propagated at the Au-rich/AuIn₂/solder interface in the initial reflow stage, then toward the AuIn₂ intermetallics dispersed in the solder matrix, and finally along the Ni₃(Sn,In)₄ intermetallics spalling off in the solder.     

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。     

Thin-film reactions during diffusion soldering of Cu/Ti/Si and Au/Cu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with Sn interlayers

The multilayer thin-film systems of Cu/Ti/Si and Au/Cu/Al₂O₃ were diffusion-soldered at temperatures between 250°C and 400°C by inserting a Sn thin-film interlayer. Experimental results showed that a double layer of intermetallic compounds (IMCs) η-(Cu_0.99Au_0.01)₆Sn₅/δ-(Au_0.87Cu_0.13)Sn was formed at the interface. Kinetics analyses revealed that the growth of intermetallics was diffusion-controlled. The activation energies as calculated from Arrhenius plots of the growth rate constants for (Cu_0.99Au_0.01)₆Sn₅ and (Au_0.87Cu_0.13)Sn are 16.9 kJ/mol and 53.7 kJ/mol, respectively. Finally, a satisfactory tensile strength of 132 kg/cm² could be attained under the bonding condition of 300°C for 20 min.     

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.     

Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with Ti/Cu/Ni under bump metallization during reflow soldering

Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with the Ti/Cu/Ni under bump metallization (UBM) during 350°C reflow for durations ranging from 50 sec to 1440 min were investigated. A thin intermetallic layer of only 0.4 μm thickness was formed at the 95Pb5Sn/UBM interface after reflow for 5 min. When the reflow was extended to 20 min, the intermetallic layer grew thicker and the phase identification revealed the intermetallic layer comprised two phases, (Ni,Cu)₃Sn₂ and (Ni,Cu)₃Sn₄. The detection of the Cu content in the intermetallic compounds indicated that the Cu atoms had diffused through the Ni layer and took part in the intermetallic compound formation. With increasing reflow time, the (Ni,Cu)₃Sn₄ phase grew at a faster rate than that of the (Ni,Cu)₃Sn₂ phase. Meanwhile, irregular growth of the (Ni,Cu)₃Sn₄ phase was observed and voids formed at the (Ni,Cu)₃Sn₂/Ni interface. After reflow for 60 min, the (Ni,Cu)₃Sn₂ phase disappeared and the (Ni,Cu)₃Sn₄ phase spalled off the NI layer in the form of a continuous layer. The gap between the (Ni,Cu)₃Sn₄ layer and the Ni layer was filled with lead. A possible mechanism for the growth, disappearance, and spalling of the intermetallic compounds at the 95Pb5Sn/UBM interface was proposed.     

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 .     

Selective formation of intermetallic compounds in Sn-20In-0.8Cu ball grid array solder joints with Au/Ni surface finishes

After Sn-20In-0.8Cu solder balls are reflowed on a ball grid array (BGA) substrate (substrate A) with an Au/Ni surface finish, scallop-shaped intermetallic compounds with a composition of 0.83[Cu₆(Sn_0.87In_0.13)₅] + 0.17[Ni₃(Sn_0.87In_0.13)₄] are formed at the solder/pad interface. The distribution of the intermetallics is not altered by gravity or by multiple reflows of the solder joints. As another substrate (substrate B) is further attached onto the primary reflowed BGA assembly to form a sandwich structure subjected to subsequent multiple reflows, the Cu₆(Sn_0.87In_0.13)₅ interfacial intermetallic scallops remain still on the side of substrate A while many Au(In_0.91Sn_0.09)₂ intermetallics of cubic shape appear near the solder/Ni interface on the side of substrate B. When the Sn-20In-0.8Cu solder balls are assembled simultaneously in between two substrates (A and B), Au(In_0.91Sn_0.09)₂ intermetallic cubes of equal proportion are observed to form on both sides of the assembly. In summarizing the results, it is proposed that the diffusion of Cu atoms in the Sn-20In-0.8Cu solder toward the Ni layers after Au thin-film dissolution on Au/Ni surface finishes led to the formation of Cu₆(Sn_0.87Zn_0.17)₅ intermetallic compounds, which prevailed over the gravitational effect so that no intermetallic sedimentation in the liquid solder would occur. The appearance of Au(In_0.91Sn_0.09)₂ at the Ni/Sn-20In-0.8Cu interfaces was hindered by the preferential formation of Cu₆(Sn_0.87Zn_0.17)₅ until the Cu atoms in the Sn-20In-0.8Cu solder matrix were consumed to a lower content via the attachment of a second substrate to the assembly.     

Isothermal solidification of Cu/Sn diffusion couples to form thin-solder joints

Isothermal solidification of conventional Cu/Sn diffusional couples was performed to form thin (30 μm) joints consisting of Cu-Sn intermetallics. During initial stages of isothermal solidification, both Cu₆Sn₅ and Cu₃Sn phases grow, even though the former is the dominant. After consumption of all available Sn, the Cu₃Sn phase grows reactively at the expense of Cu and Cu₆Sn₅. Finally, we obtain solder joints that consist of only Cu₃Sn. Indentation fracture-toughness measurements show that Cu₃Sn is superior to Cu₆Sn₅. Furthermore, indentations of Cu₃Sn exhibit the presence of shear bands, which are not observed in Cu₆Sn₅, implying that the former is more ductile than the latter. Ductile intermetallic-based joints formed by isothermal solidification are promising candidates to form thin (as thin as 5–10 μm or less) solder joints, as they are thermally and thermodynamically stable compared to conventional solder joints. Excess copper in the interconnect provides ductility to the interconnect.     

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.