G. Ghosh

Dissolution and interfacial reactions of thin-film Ti/Ni/Ag metallizations in solder joints

Abstract The dissolution and interfacial reactions involving thin-film Ti/Ni/Ag metallizations on two semiconductor devices, diode and metal-oxide-semiconductor field-effect transistor (MOSFET), a Sn–3.0Ag–0.7Cu solder, and a Au-layer on the substrates are studied. To simulate the dissolution kinetics of the Ag-layer in liquid solder during the reflow process, the computational thermodynamics (Thermo-Calc) and kinetics (DICTRA: DIffusion Controlled TRAnsformations) tools are employed in conjunction with the assessed thermochemical and mobility data. The simulated results are found to be consistent with the observed as-reflowed microstructures and the measured Ag contents in the solder. In the as-reflowed joints two different intermetallic compounds (IMC) are found near the diode/solder interface. Both are in the form of particles of different morphologies, not a continuous layer, and are referred to as IMC-I and IMC-II. The former corresponds to Ni 3 Sn 4 with Cu atoms residing in the Ni sublattice. It is uncertain whether IMC-II is Cu 6 Sn 5 phase with Ni atoms residing in the Cu sublattice or a Cu–Ni–Sn ternary phase. Near the as-reflowed MOSFET/solder interface, both particles and a skeleton-like layer of Ni 3 Sn 4 are observed. The primary microstructural dynamics during solid state aging are the coarsening of IMC particles and the reactions involving the unconsumed (after reflow) Ni- and the Ti-layer with Sn and Au. While the reaction with the Ni-layer yields only Ni 3 Sn 4 intermetallic, the reaction involving the Ti-layer suggests the formation of Ti–Sn and Au–Sn–Ti intermetallics. The latter is due to the diffusion of Au from the substrate side to the die side. It is postulated that the kinetics of Au–Sn–Ti layer is primarily governed by the diffusion of Au through the Ni 3 Sn 4 layer by a grain boundary mechanism.

Interfacial microstructure and the kinetics of interfacial reaction in diffusion couples between Sn–Pb solder and Cu/Ni/Pd metallization

Abstract The interfacial microstructure and the kinetics of interfacial reaction between eutectic Sn–Pb solder and electroplated Ni/Pd on a Cu substrate have been studied by scanning, transmission and analytical electron microscopies. Besides PdSn 4 and Ni 3 Sn 4 , small grains of Ni 3 Sn 2 with a hexagonal structure are also observed after long-time aging of the diffusion couples at 125°C. The presence of intermetallic phases is correlated with the diffusion paths in the calculated Pd–Pb–Sn and Ni–Pb–Sn isothermal sections. The growth kinetics of the Ni 3 Sn 4 scallops in the submicrometer length scale was analyzed with an Arrhenius type of equation. The thickening kinetics yields a time exponent n =3.1 and an apparent activation energy ( Q h ) of 25,750 J/mol, while the radial growth kinetics data yield a time exponent m ≈6.6 and an apparent activation energy ( Q d ) of 15,300 J/mol. The radial size distributions (RSDs) of Ni 3 Sn 4 scallops were also quantified. The parameters describing RSDs are consistent with the theories of coarsening in two-phase systems containing a very high volume fraction of the second phase. Selective etching of solder revealed the three-dimensional morphology of PdSn 4 and Ni 3 Sn 4 , and also the dynamical phenomena, such as faceting, competitive growth and coalescence of Ni 3 Sn 4 scallops during interfacial reaction. Non-parabolic growth kinetics is discussed in terms of the existing theories and characteristics of the evolving microstructure.

The isotropic shear modulus of multicomponent Fe-base solid solutions

Abstract A critical analysis of the available experimental data for the effect of alloying elements on the isotropic shear modulus of bcc (body-centered cube) Fe–X (X=Al, Be, C, Co, Cr, Ge, Ir, Mn, Ni, Pt, Re, Rh, Ru, Si and V) solid solutions is carried out. The total effect of a solute on the shear modulus is decomposed into two contributions: the electronic (or chemical) and the volumetric. A systematic trend of the electronic contribution is demonstrated as a function of electron-to-atom (e/a) ratio and the ground-state electronic configuration of the solute atom. Based on the demonstrated trend, we predict the chemical contribution of the shear modulus of Cu, Mo, N, Nb, Ti and W in ferromagnetic α-Fe (bcc), and that of Ti and V in paramagnetic γ-Fe [face-centered cube (fcc)]. These along with the corresponding volumetric contributions enable us to predict the total effect of a solute on the shear modulus in α-Fe and γ-Fe. In the case of γ-Fe, we derive the chemical and volumetric contributions of Ni and Pt from the experimental shear modulus data of paramagnetic Fe–Ni and Fe–Pt alloys while those of C, Co, Cr, Mn, Mo, N and Si are derived from the shear modulus of paramagnetic Fe–Ni–X alloys. In the case of Al, Be, Cu, Ge, Ir, Nb, Re, Rh , Ru and W, the total effect on the shear modulus is calculated by assuming that the electronic contribution to the shear modulus in γ-Fe is the same as in α-Fe. To calculate the isotropic shear modulus of multicomponent bcc and fcc solid solutions, we propose linear superposition laws. The proposed relationships are validated using the experimental data of a large number of multicomponent alloys having austenitic, ferritic, and lath martensitic microstructures. It is demonstrated that for all three microstructures, in most cases the shear modulus can be predicted with an accuracy of ±3% in multicomponent solid solutions. It is also found that the high dislocation density in lath martensite accounts for a decrease in shear modulus by about 5% compared to the ferritic counterpart. We also demonstrate that the temperature dependence of shear modulus in multicomponent bcc and fcc solid solutions is similar to that of pure α- and γ-Fe, respectively, for up to about 800 K.

Investigation of multi-component lead-free solders

Binary phase diagrams of interest for lead-free solder development have been entered into the THERMO-CALC data base. These may be used directly to calculate multi-component phase relations vs temperature provided there are no ternary or higher order interactions. Such occur in the Sn-Ag-Zn system and are being evaluated. Contact angles of a number of solder-flux combinations on copper were directly measured in spreading tests. With a rosin-isopropyl alcohol flux, the contact angles of binary eutectic solders were in degrees: Sn-Bi at 166°C, 40; Sn-Zn at 225°C, 60; Sn-Ag at 250°C, 45. These angles were little affected by a number of 1% ternary additions to the solder. The contact angles were 20 degrees or less when SnCl2 was used as a flux. The SnCl2 reacts with Cu to form Cu3Sn. The most likely successful approach to obtain satisfactory wetting with lead-free solders appears to be development of a satisfactory flux.

Phase stability, phase transformations, and elastic properties of Cu6Sn5:Ab initio calculations and experimental results

Among many Sn-based intermetallics, Cu 6 Sn 5 (η and η′) is ubiquitous in modern solder interconnects. Using the published structural models of η and η′ and also related structures, the total energies and equilibrium cohesive properties are calculated from first-principles employing electronic density-functional theory, ultrasoft pseudopotentials, and both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy. The accuracy of our calculations is assessed through comparisons between theoretical results and experimental measurements for lattice parameters, elastic properties, and formation and transformation energies. The ambient-temperature experimental lattice constants of η and η′ are found to lie between the LDA and GGA level calculated zero-temperature lattice constants. The Wyckoff positions in the structural models of η and η′ agree very well with the ab initio results. The calculated formation energy of η′ lies between −3.2 and −4.0 kJ/mol, which is more positive by about 3 to 4 kJ/mol compared to reported experimental data obtained by solution calorimetry. Our systematic differential scanning calorimetry (DSC) experiments show that the η′ → η transformation enthalpy is 438 ± 18 J/mol, which is about 66% higher than the literature value. In view of our DSC results on heating and cooling, the nature of η′ → η and η → η′ is discussed. Our experimental bulk modulus of η and η′, and the heat of η′ → η transformation agree very well with the ab initio total energy calculations at the GGA level. Based on these results, we conclude that other isotropic elastic moduli (Young’s modulus, shear, and Poissons ratio) of η and η′ phases measured by pulse-echo technique are representative of their actual properties. The scatter in experimental elastic constants in the literature may be attributed to various factors, such as the measurement technique (pulse-echo versus nanoindentation), type of specimen (bulk, Cu 6 Sn 5 -layer in diffusion couple, thin-film), and anisotropy effects (particularly in Cu 6 Sn 5 -layer in diffusion couples).

Precipitation of paraequilibrium cementite: Experiments, and thermodynamic and kinetic modeling

The precipitation of cementite prior to the precipitation of the strengthening M2C phase is investigated using two model ultra-high strength (UHS) steels. The structure, microstructure and chemical composition of cementite are studied by analytical electron microscopy techniques. The structure of cementite precipitated during early stages of tempering at 755 and 783 K was confirmed by convergent beam electron diffraction. In an alloy containing 0.16 mass% C, the cementite particles were primarily plate shaped and interlath type, whereas in an alloy containing 0.247 mass% C both inter- and intralath particles were observed. Consistent with the earlier studies on tempering of Fe-C martensite, lattice imaging of cementite suggests microsyntactic intergrowth of M5C2 (Hagg carbide). Quantification of the substitutional elements in cementite confirms its paraequilibrium state with ferrite at the very early stage of tempering. Computational thermodynamic and kinetic tools, Thermo-Calc and dictra (diffusion controlled transformation) software, respectively, are used to model the precipitaton of paraequilibrium cementite in several multicomponent alloys. A thermodynamic model parameter describing the effect of Si on the stability of cementite is proposed. The model parameter is consistent with the following results: (a) that Si does not partition to cementite in Fe–Si–C and Co–Si–C alloys under orthoequilibrium conditions, and (b) there is a large driving force for the precipitation of paraequilibrum cementite in an Fe–0.41C–3Mn–2Si alloy where it has been experimentally verified. The nucleation driving forces for the precipitation of paraequilibrium cementite, and the two-phase (ferrite and cementite) paraequilibrium boundaries for multicomponent alloys are calculated using the Thero–Calc software systems. The results of growth simulations of cementite under paraequilibrium condition in multicomponent systems using the dictra software are also presented.

Thermodynamic Modeling of the Pd-X (X = Ag, Co, Fe, Ni) Systems

A set of self-consistent thermodynamic model parameters is presented to describe the phase equilibria of the Ag-Pd, Co-Pd, Fe-Pd, and Ni-Pd systems. In most cases, the calculated values using the optimized model parameters agree very well with the experimental data. The FePd and FePd3 phases with large homogeneity ranges are described by the compound energy formalism. At present, insufficient thermodynamic data are available for these two phases. Therefore, experimental data on the heat of formation and/or the first-principle calculation of cohesive energies will be very useful for further refinement of thermodynamic parameters of FePd and FePd3 phases.

Thermodynamic and kinetic modeling of the Cr-Ti-V system

A synergistic approach of thermodynamic and kinetic modeling is applied to the Cr-Ti-V system. To assist the design of (α+β) and β titanium alloys for structural applications and vanadium alloys for fusion reactor applications, a set of self-consistent and optimized thermodynamic model parameters is presented to describe the phase equilibria of the Cr-Ti, Cr-V, Ti-V, and Cr-Ti-V systems. The Laves phases, α-Cr2Ti, β-Cr2Ti, and γ-Cr2Ti, are described by a two-sublattice model assuming antistructure atoms on both sublattices. The calculated thermodynamic quantities and phase diagrams are in good accord with the corresponding experimental data. To assist the simulation of the kinetics of diffusional transformations in bodycentered cubic (bcc) alloys, the atomic mobilities of Cr, Ti, and V are modeled. A set of optimized mobility parameters is given. Very good agreement between the calculated and experimental diffusivities was found.

Thermodynamics and kinetics of stable and metastable phases in the Ni-Zr system

The thermodynamic parameters of all stable phases in the Ni-Zr system are reported in this paper. The available experimental data are taken into account in deriving the interaction parameters of various phases. The Ni 7 Zr 2 , Ni 21 Zr 8 , Ni 11 Zr 9 , NiZr, and NiZr 2 phases are treated as stoichiometric. The solid solubility of the Ni 5 Zr, Ni 3 Zr, and Ni 10 Zr 7 phases are described using a two sublattice model. The calculated thermodynamic properties and the phase diagram are in good agreement with the experimental ones. The thermodynamic parameters, along with the classical nucleation and crystal growth theory, have been used to explain the formation of metastable noncrystalline and crystalline phases upon rapid quenching of the liquid alloys. It has been demonstrated that even without any prior knowledge of the physical property, such as glass transition or crystallization temperature and thermodynamic property, such as the enthalpy of amorphous → crystal transformation of the amorphous alloy , it is possible to make a reasonably good “first order prediction” of the glass-forming range in a binary system. The experimentally determined glass-forming range in the Ni-Zr system is found to be in good agreement with the predicted one.

Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T0 temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (Ms) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.