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A Sn58Bi composite solder reinforced by Ni nanoparticles was prepared using a mechanical mixing technique, and the thermomigration microstructure and properties of the solder joints were studied. The findings indicate that incorporating an appropriate quantity of Ni nanoparticles can enhance the microstructure of the composite solder and mitigate the coarsening of Bi-phase segregation. At 0.75 weight percent Ni nanoparticle content, the composite solder's tensile strength is 59.7 MPa and its elongation is 54.6%, both of which are noticeably greater than those of the base solder. When the thermal loading time is 576 h, the shear strength of the composite solder joint is 25.5 MPa, which is 30.1% higher than that of the base solder joint. This study reveals that the shear fracture path shifts from the boundary region between the solder seam and the IMC layer to the IMC layer itself. Concurrently, the fracture mode evolves from a mix of brittle–ductile fracture, characterized by quasi-cleavage, to a predominantly brittle fracture, marked by numerous "rock candy-like" cross-sectional features and secondary cracking. Adding Ni nanoparticles to the Sn58Bi composite solder/Cu solder junction can significantly extend its service life. [ABSTRACT FROM AUTHOR]

Zinc nanoparticles, for their potential in drug delivery due to their antibacterial properties and biocompatibility when combined with Sutterby fluid suspended in the sodium alginate acts as a natural biopolymer, and this colloid delivers the drugs more effectively to target sites.The major focus of this study is on the convective and Stefan blowing boundary conditions on the two-dimensional unsteady laminar flow of magnetohydrodynamic nanofluid across a stretched sheet when there is chemical reaction, viscous dissipation, and thermal radiation.The partial differential equation system modelled for the current situation is transformed as a series of nonlinear coupled ordinary differential equations by utilizing appropriately defined transformations. The bvp4c technique is an implicit shooting scheme that solves the highly nonlinear coupled system of partial differential equations. The processing of nanomaterials at high temperatures is pertinent to the current investigation. Using diagrams and tables, the numerical findings of the steady and unsteady solutions for the wall shear stress, heat and solute transfer rates, temperature, velocity, and concentration are explained.The study showed great agreement when compared to previously available research work. It was determined that velocity and magnetic fields had an exact opposite relationship. With an increase in stretching surface temperature, the convective parameter rises. In terms of physical characteristics, the fluid's thermal conductivity rises and rises the temperature. The slow momentum gain in the stretching scenario leads to increased fluid temperature due to the interplay between friction and energy transfer. Increasing boundary layer thickness is found in the fluid velocity curves with increasing power law index values. [ABSTRACT FROM AUTHOR]

Due to the demand for high-performance electronic products, there has been a rapid development in three-dimensional integrated circuit (3D IC) packaging technology. The 3D stacking relies on microbumps and through silicon vias (TSVs) to connect multiple Si dies vertically. These high-performance electronic products usually need to consume high power, combined with limited heat dissipation due to dense packing, resulting in the issue of Joule heating. The problem of Joule heating impacts the reliability of products. This study investigated the factors contributing to Joule heating of microbumps and its subsequent reliability issues through experimental methods and finite element analysis (FEA) simulations. Joule heating was quantified by measuring the temperature increase at stable states using the temperature coefficient of resistance (TCR) equation. We first observed that environmental temperature influences Joule heating. Higher oven temperature can exacerbate the Joule heating effect and lead to higher temperature increase under the same current density. We also examined the effect of intermetallic compound (IMC) formation on Joule heating, with a 12.5% increase in heat generation after IMC formation. Furthermore, the adverse effect of voids induced by sidewall wetting during high-temperature storage (HTS) on Joule heating using FEA was demonstrated. Additionally, we discovered severe Joule heating to cause thermomigration (TM) in the microbumps at a system level. Finally, we proposed structural optimization of the microbump to mitigate Joule heating and enhance reliability.

New types of instabilities associated with crystal anisotropy during thermomigration of rectilinear and curvilinear (annular) zones under the action of a temperature gradient in the silicon-aluminium system are experimentally revealed. A force model of thermomigration is improved, which takes into account vectors of resistance forces to atomic-kinetic processes at the dissolution front. This model explains the observed features of the stable and unstable motion of the linear zones. Reasons and a mechanism of thickenings and kinks of the ends of rectilinear zones, their fragmentation and decay, as well as transformation of annular linear zones into triangular and square zones during thermomigration in the < 111 > and < 100 > directions, respectively, were also explained. [ABSTRACT FROM AUTHOR]

A Sn58Bi composite solder reinforced by Ni nanoparticles was prepared using a mechanical mixing technique, and the thermomigration microstructure and properties of the solder joints were studied. The findings indicate that incorporating an appropriate quantity of Ni nanoparticles can enhance the microstructure of the composite solder and mitigate the coarsening of Bi-phase segregation. At 0.75 weight percent Ni nanoparticle content, the composite solder’s tensile strength is 59.7 MPa and its elongation is 54.6%, both of which are noticeably greater than those of the base solder. When the thermal loading time is 576 h, the shear strength of the composite solder joint is 25.5 MPa, which is 30.1% higher than that of the base solder joint. This study reveals that the shear fracture path shifts from the boundary region between the solder seam and the IMC layer to the IMC layer itself. Concurrently, the fracture mode evolves from a mix of brittle–ductile fracture, characterized by quasi-cleavage, to a predominantly brittle fracture, marked by numerous “rock candy-like” cross-sectional features and secondary cracking. Adding Ni nanoparticles to the Sn58Bi composite solder/Cu solder junction can significantly extend its service life.

The results of a study of the structural features and electrical properties of the end-to-end thermomigration (ThM) of the p-channels of Si(Al) in a silicon wafer are presented. Structural studies are carried out using X-ray methods of projection topography, diffraction reflection curves, and scanning electron microscopy (SEM). It is shown that the channel-matrix interface is coherent without the formation of misfit dislocations. The possibility is shown of using an array of the ThM of the p-channels of 15 elements for the formation of a monolithic photoelectric solar module in a Si(111)-based silicon wafer of p-channels 100 µm wide with walls in the plane . The monolithic solar module has a conversion efficiency of 13.1%, an open circuit voltage of 8.5 V, and a short circuit current density of 33 mA/cm2. [ABSTRACT FROM AUTHOR]

This review presents promising developments in the use of thermomigration processes to form doped or compositionally modified regions in various semiconductor materials. Methods of capillary formation of flat liquid zones of various compositions, migration of eutectic droplets under pulsed action of electric and temperature fields, and migration of droplets along the atomically clean surface of semiconductors are considered. The use of these methods makes it possible to expand the dimensional limits of device formation over a wide range from centimeter to nanometer scales. New developments in the field of photonics are also presented. The proposed method of recrystallization of silicon fiber core using CO2 laser scanning was combined with thermal migration of melt zone of different composition. This makes it possible to improve the microstructure of core material and form light-emitting GaSb inclusions (and even a Si-GaSb heterostructures) inside the optical fiber. Such integration of semiconductor materials, embedded in the geometry of the optical fiber, is very relevant when creating integrated systems of biomedicine or photon conversion of solar energy. [ABSTRACT FROM AUTHOR]

We report here an approach to inhibit Cu–Sn intermetallic compounds (IMCs) in flip chip solder joints under thermal-gradient annealing. We reflowed two types of solders joints, Cu/SnAg/Cu and Cu/SnAg/Ni, in an oven (isothermal) and on a hot plate (with thermal-gradient) at 260 °C. During the isothermal reflow, the IMC growth rates in the top and bottom sides of the solder joint were ∼4.0 × 10−2 and 8.9 × 10−2 μm/min, respectively. However, the IMC growth rate increased to 7.8 × 10−1, 7.9 × 10−1, and 8.9 × 10−1 μm/min in the cold ends with low, medium, and high thermal gradients, respectively. Yet, the IMC thickness on the hot ends remained almost unchanged. These results indicate that the thermal gradient has facilitated the Cu–Sn IMC formation in the cold ends. But, when a Ni film was sandwiched at the cold end, the IMC growth rates in the cold end were reduced to ∼9.7 × 10−2, 1.4 × 10−1, and 1.5 × 10−1 μm/min with low, medium, and high thermal gradients, respectively, indicating that the thermomigration of IMCs was retarded by the Ni. It is speculated that the Ni layer transformed the IMCs on the hot end into stable (Cu,Ni)6Sn5 ternary IMCs, and thus the thermomigration of Cu was suppressed. A kinetic model is also proposed to address the competitive reactions of Cu-molten solder joints.

The melting behavior and microstructure of bulk Sn-xIn (x = 6, 12 and 24, wt.%) solders and the thermomigration, elemental distribution and intermetallic compound (IMC) growth in Cu/Sn-xIn/Cu micro solder joints during soldering and aging under temperature gradient (TG) were investigated. The results indicate that In addition effectively decreased the melting temperature of the bulk solders. Only the InSn4 phase was detected when In addition was increased to 24 wt.%. During soldering under TG, the growth rate of the interfacial IMC layer at the cold end interfaces gradually decreased as the In content increased. The mechanism of microstructure evolution and elemental distribution in the micro solder joints was revealed based on the TG-induced atomic thermomigration (TM). The chemical potential gradient of atoms was enhanced by TM, and the rapid diffusion of atoms in the liquids resulted in a uniform distribution of In element in both solders and the IMC phase during soldering. While during aging under TG, there was a smaller chemical potential gradient due to the slow atomic diffusion rate. At this time, TG dominated the atomic migration, which resulted in a nonuniform distribution of the In element in the whole joints. This study provides further insight into the application of In-containing solders in electronic packaging. [ABSTRACT FROM AUTHOR]

Transient liquid phase bonding under temperature gradient in electronics interconnections yields intermetallic grains at the bonding interface with different morphological features compared with conventional soldering process. However, the interfacial reactions due to the thermal gradient that result in the preferential growth of intermetallics are yet to be fully understood. In this study, incorporating with the thermotransport effect a multiphase field model is developed to elaborate the fundamental growth mechanism of Cu6Sn5 intermetallic in the Sn/Cu solder interconnect under temperature gradient. We particularly account for the effect of orientation and anisotropic thermal conductivity of Cu6Sn5 intermetallic grains in relation to the temperature gradient during their growth, as observed thermal conductivity with the c-axis of Cu6Sn5 intermetallic parallel to the gradient can be 1.6 times of those perpendicular to the gradient. Simulation results show that the temperature gradient can accelerate the growth of the Cu6Sn5 phase, as reported in the experiments. The heat flux is mainly conducted through the intermetallic grain with c-axis parallel to the temperature gradient, causing faster growth of the grain than the grain with c-axis perpendicular to the temperature gradient; the growth rate difference of the two types of grains becomes more pronounced under high temperature gradient. It is revealed that the faster and preferential growth of this type of intermetallic grain is attributed to the higher thermomigration induced diffusion flux and accompanying faster atomic interdiffusion process, especially near the solder/intermetallic interface.

• A novel phase-field model of electromigration incorporating the thermal effect is proposed. • Thermal effect is shown to play a significant role in the morphology evolution of defects. • The coupling effects of the mechanical-electric-temperature fields are resolved. In this paper, the defect evolution caused by electromigration induced surface diffusion in interconnects is investigated using a newly-developed electro-thermo-mechanical coupling phase-field model. The Joule heat and its resulting thermomigration are included into the phase-field model. The governing equation of the phase-field is solved by semi-implicit spectral methods and the accompanied governing equations of applied physics fields are solved by finite volume methods. Comparative investigation into defect evolution with and without the influence of Joule heating is conducted. It is deduced that thermomigration facilitates local elongation of the defect in the "current crowding" region and exerts a substantial influence on the defect morphological evolution. Subsequently, the effect of the inclination angle of the electric field on the void morphology evolution and crack propagation is discussed. We find that the defect achieves the largest characteristic length when the electric field direction is perpendicular to the uniaxial tension direction, implying a higher threat to the circuit safety. This study may help to deepen people's understanding of how the thermal effect functions in electromigration process and sheds light on different modes of defect evolution in interconnects. [Display omitted] [ABSTRACT FROM AUTHOR]

In order to evaluate the vibration resistance of Sn58Bi solder in serving electronic devices, the damping capacities of Sn58Bi solders after thermomigration (TM) test at a temperature gradient of 2000 °C/cm for different time (0, 120, 360, 720, and 1440 h) were characterized under electric current stressing (0, 4.0, and 8.0 A). The results indicate that the phase segregation of TM-tested Sn58Bi solders determines the damping performance of solders. The Bi-rich layer thickens with prolonged TM time and migrates in the direction from high temperature to low temperature. Both the critical strains (the values of dislocation getting rid of pining points) of strain-amplitude-related damping capacity curves increases with prolonged TM time, while decreases with increasing electric current. Moreover, both strain-amplitude-related and temperature-related damping capacity shows a general decreasing trend with prolonged TM time, while increases exponentially with increasing electric current. In addition, the damping mechanism changes from dislocation motion to phase boundary sliding with increasing temperature, and the transition temperature decreases with increasing current but generally increases with TM time. • Strain-amplitude-related damping capacity decreases with Bi-rich layer thickness. • Damping mechanism shifts from dislocation motion to phase boundary sliding. • Damping mechanism transition temperature decreases with increasing current. • Damping capacity increases exponentially with increasing electric current. [ABSTRACT FROM AUTHOR]

The effect of Ga and Sn additives on the kinetics and stability of the process of thermal immigration of liquid zones based on aluminum in silicon is analyzed. The threshold values of the temperature of the beginning of the thermomigration process are determined throughout the range of the composition of the zones. A monotonous increase in the temperature of the onset of motion of liquid bands with increasing concentrations of Ga and Sn was observed. An increase in the rate of thermomigration of the melt zones with increasing Ga concentration in the melt at temperatures above 1473 К and a decrease in the velocity at a lower temperature have been revealed. An increase in the concentration of Sn leads to a decrease in the rate of thermomigration in the entire temperature range. The stability of the thermomigration of the zones with Ga and Sn additions increases with increasing process temperature.

The article describes a method of detecting the predicted theoretically transfer local perturbation effect on one of the interface flat liquid zone to another when it thermomigration in the crystal. The results of experimental studies of the features of the induced perturbation and its impact on the stability of the flat zone as a whole. It was found that the effect of induced instability could be the basis for a method of forming semiconductor wafers in volume of regular structures in the form of bands of deep electron-hole transitions and through the conducting channels. A method of using induced instability effect for silicon device structures.

The article presents research results of the crystal structure of the silicon regions, locally recrystallized in the process of thermomigration liquid silicon-aluminum areas of a silicon substrate. Selective etching of silicon is found that recrystallized in the channel, there are areas that have increased defects. Determined that these areas are located at the surfaces of the substrate, the depth of these areas does not exceed the thickness of the liquid zone. Out defective areas, the channel has low densities of dislocations and a high degree of monocrystalline silicon. Using the methods of x-ray diffractometry and transmission electron microscopy high resolution revealed that on the border of the channel and the substrate are dislocation half-stitch, which lie in the surface layers of the front and back sides of the substrate. In recrystallized areas are detected the {311} - defects.

The study identifies the main types of defects occurring in the formation of discrete zones on the basis of aluminium selective surface wetting of silicon wafers. The dependence of defect formation on the surface microrelief, and the thickness of the protective coating of silicon oxide, the process temperature, the velocity of the melt, the height of the melt and the concentration of additives of gallium to aluminum have been investigated. Statistical processing of a lot set of experimental data allowed to establish the optimal conditions of the process offormation zones and to reduce the total relative amount of all types of defects to values not exceeding 5 %.

The motion of liquid droplets is studied across many physics subfields and manipulation of nanoscale droplets on demand holds great promise in, e.g., nanotechnology. In this study, AuGe droplets on a germanium substrate are manipulated by an electron beam in a scanning electron microscope. The electron beam exposure creates a local temperature gradient in a substrate and induces a directional thermomigration of droplets nearby. To quantitatively analyse this phenomenon and to reveal the mechanism behind it, experimental observations under different conditions (beam current, sample temperature, scanning strategy) and simulations are combined. The obtained insights suggest that the droplet motion is limited by the dissolution of the substrate below the droplet. In addition, it is shown that the electron tractor beam is not only able to control the droplet motion but can also be used to split the droplets, thus opening new possibilities for droplet manipulation inside an SEM. [ABSTRACT FROM AUTHOR]

The paper analyzes the reasons and factors that allow avoiding faceting of non-oriented linear zones. It is shown that in the manufacture of semiconductor chips with a large perimeter and a reverse voltage of 2000 V, the conditions sine qua non to create isolating walls on silicon wafers with an orientation different from (111) are to form an ensemble of linear zones by the method of high-temperature selective forced wetting (HSV) and to fulfill a number of requirements to the “thermomigration” photomask and zones immersion stage during TM at high temperatures. It is shown that these factors provide a stable migration of an ensemble of linear zones through wafers (110) even in a stationary temperature gradient field. For the first time in the world, the authors practically demonstrate the possibility of stable migration of an ensemble of non-oriented linear zones through silicon (110) in a stationary temperature gradient conditions, outlining the conditions and factors necessary for this process. It is assumed that when the conditions for the formation of linear zones and their immersion are met, the crystallographic orientation of the silicon wafers does not matter at all.

Abstract The motion of liquid droplets is studied across many physics subfields and manipulation of nanoscale droplets on demand holds great promise in, e.g., nanotechnology. In this study, AuGe droplets on a germanium substrate are manipulated by an electron beam in a scanning electron microscope. The electron beam exposure creates a local temperature gradient in a substrate and induces a directional thermomigration of droplets nearby. To quantitatively analyse this phenomenon and to reveal the mechanism behind it, experimental observations under different conditions (beam current, sample temperature, scanning strategy) and simulations are combined. The obtained insights suggest that the droplet motion is limited by the dissolution of the substrate below the droplet. In addition, it is shown that the electron tractor beam is not only able to control the droplet motion but can also be used to split the droplets, thus opening new possibilities for droplet manipulation inside an SEM.

Sn-rich solder joints in three-dimensional integrated circuits and their reliability issues, such as the electromigration (EM), thermomigration (TM), and thermomechanical fatigue (TMF), have drawn attention related to their use in electronic packaging. The Sn grain orientation is recognized as playing an important role in reliability issues due to its anisotropic diffusivity, mechanical properties, and coefficient of thermal expansion. This study reviews the effects of the Sn grain orientation on the EM, TM, and TMF in Sn-rich solder joints. The findings indicate that in spite of the failure modes dominated by the Sn grain orientation, the size and shape of the solder joint, as well as the Sn microstructures, such as the cycling twining boundary (CTB), single crystals, and misorientations of the Sn grain boundary, should be considered in more detail. In addition, we show that two methods, involving a strong magnetic field and seed crystal layers, can control the Sn grain orientations during the solidification of Sn-rich solder joints. [ABSTRACT FROM AUTHOR]

In order to solve the thermal migration problem caused by the large temperature gradient during the service of micro solder joints, a thermal migration test device was designed to study the microstructure evolution and mechanical properties of the composite solder/Cu soldering joint during thermal migration. The results show that the designed test device can meet the single thermal migration test condition. Compared with the non-thermal loading, after 200 h of Ni-GNSs reinforced Sn2.5Ag0.7Cu0.1RE composite solder/Cu soldering joint thermal migration, the hot end of the joint Cu6Sn5 intermetallic compound (IMC) is greatly reduced and microvoids appear at the interface. The thickness of the thick scallop-shaped IMC at the cold end interface is increased significantly, and the layered IMC Cu3Sn with an average thickness of 1 µm is formed between the cold end Cu/Cu6Sn5 interface. The shear strength of solder joints is decreased by 33% after 200 h of heat loading. The fracture position of composite brazing joints migrates from the transition zone of the hot end interface IMC/soldering seam to the interface IMC. The fracture mechanism changes from ductile fracture to ductile fracture dominated by ductile-brittle mixed fracture, then with increasing in heat loading time, transforms into ductile-brittle mixed fracture dominated by brittle fracture. The addition of Ni-GNSs reinforcing phase can inhibit the thermal migration of composite solder/Cu soldering joints. [ABSTRACT FROM AUTHOR]

Si(111) wafers patterned with an array of vertical 100 µm‐wide Al‐doped (1 × 1019 cm−3) p‐channels extending through the whole wafer were studied by X‐ray Laue diffraction techniques. The X‐ray techniques included projection topography, and measurement of rocking curves and cross sections in the vicinity of the 02 reciprocal space node in the double‐ and triple‐crystal geometry, respectively. The channels are uniform along the depth of the wafer, and their structural perfection is comparable to that of the silicon matrix between the channels. Simulation of the rocking curves was performed using the methods of the dynamical theory of X‐ray diffraction. The rocking‐curve calculations both taking into account and neglecting the effect of the instrumental function were carried out using the Takagi–Taupin equations. The calculated angular dependences of intensities of both diffracted and transmitted X‐rays correspond well to the experimentally obtained rocking curves and demonstrate their high sensitivity to the structural distortions in the channel. An unambiguous reconstruction of strain and structural distortions in the Si(Al) channel using the Laue diffraction data requires further development of the theoretical model. [ABSTRACT FROM AUTHOR]

The continuous improvement of electronic device performances and the advancement of packaging technology have brought greater challenges to the reliability of solder joints in integrated circuits (ICs). In view of the increase of chip integration and the reduction of micro-bump size in 3D ICs, the high current density and excessive Joule heat will result in serious reliability issues of electromigration (EM) and thermomigration (TM) respectively, which evidently shorten the lifetime of solder joints. This article reviewed the influence of EM, TM and their coupling effect on microstructure evolution and properties of different joints soldered with pure Sn, SnAgCu, SnBi and SnZn solder. What's more, the increase of EM and TM resistances of micro-joints by microalloying, particle reinforcement and substrate improvement was explored. Finally, the shortcomings of current studies and the future research directions were summarized to further improve the EM and TM reliability of micro-joints, which promotes the application of lead-free solders in 3D ICs package.

The mechanisms of densification in spark plasma sintering (SPS) were investigated both analytically and numerically for a model system of two spherical metallic powder particles. From the microscopic temperature distribution, the possibility of a micro-local overheating of the particle–particle contacts was analysed for different particle sizes, contact geometries, materials, and electrical loads. It is shown that, for particles below the size of one millimetre, local overheating is below one Kelvin. Subsequently, the material transport by thermomigration, electromigration, and diffusion driven by surface curvature and external pressure was derived from microscopic field distributions obtained from analytical calculations and finite-element simulations. The results show that, while the mechanical pressure accelerates material transport by orders of magnitude, the electrical current and the temperature gradients do not. It is also shown that pulsing the current has no significant influence on the densification rate. [ABSTRACT FROM AUTHOR]

The equation of mean-time-to-failure (MTTF) for electromigration has been reevaluated from the viewpoint that in irreversible processes, entropy production is the controlling factor. We have justified that the power factor on current density is $n = 2$ , as given in Black’s equation. Furthermore, on the basis of entropy production, we provide a unified model of the equations of MTTF for thermomigration and stress migration. The effect of thermal conduction on the physical link between entropy production and the failure of void formation is discussed. [ABSTRACT FROM AUTHOR]

A technique that allows one to form thick thermomigration silicon layers heavily doped with gallium for prospective power electronic devices is proposed. The dependences of the perfection of structure and the composition of the layers on the temperature of their formation were studied by the methods of X-ray topography, X-ray reflection curves, and secondary-ion mass spectrometry. It is established that the formed layers are single-crystal layers and do not contain misfit dislocations at the interface with the silicon substrate. It is shown that the concentration of gallium in the layers can be varied in the range of (1.6–4.8) × 1019 cm–3, which is higher than the concentration achieved in the case in which silicon is doped with aluminum. [ABSTRACT FROM AUTHOR]

Based on the weak formulation for combined surface diffusion and evaporation-condensation,a governing equation of the finite element is derived for simulating the evolution of intergranular microcracks in copper interconnects induced simultaneously by stressmigration,electromigration and thermomigration. Unlike previously published works,the effect of thermomigration is considered. The results show that thermomigration can contribute to the microcrack splitting and accelerate the drifting process along the direction of the electric field. The evolution of the intergranular microcracks depends on the mechanical stress field,the temperature gradient field,the electric field,the initial aspect ratio and the linewidth. And there exists a critical electric field χc,a critical stress field ■,a critical aspect ratio βc and a critical linewidth ■. When ■or ■,the intergranular microcrack will split into two or three small intergranular microcracks. Otherwise,the microcrack will evolve into a stable shape as it migrates along the interconnect line. The critical stress field,the critical electric field and the critical aspect ratio decrease with a decrease in the linewidth,and the critical linewidth increases with an increase in the electric field and the aspect ratio. The increase of the stress field,the electric field or the aspect ratio and the decrease of the linewidth are not only beneficial for the intergranular microcrack to split but also accelerate the microcrack splitting process. [ABSTRACT FROM AUTHOR]

Highlights • Anisotropic diffusion of Cu in Sn grains affects formation of Cu 6 Sn 5 compounds. • Rapid growth of Cu 6 Sn 5 at the cold-end occurs in low α angle Sn grains during TM. • Slow growth rates of Cu 6 Sn 5 at the cold-end in high α angle Sn grains during TM. • Formation of Cu 6 Sn 5 near grain boundaries in solder when Cu fluxes is blocked. Abstract The effect of Sn orientation on thermomigration was investigated in Cu/Sn2.3Ag/Ni microbumps with diameter of 20 μm. Results showed that anisotropic diffusion of Cu in Sn grains affected both formation rates and locations of Cu 6 Sn 5 intermetallic compounds (IMCs). Rapid growth of Cu 6 Sn 5 IMCs at the cold-end interface was observed when the c-axis of Sn grains was parallel to the temperature gradient. In addition, diffusion of Cu atoms was blocked by a Sn grain with a high angle between the Sn c-axis and the thermal gradient, resulting in formation of Cu 6 Sn 5 IMCs near the grain boundary inside the solder. [ABSTRACT FROM AUTHOR]

Abstract Sn-Bi based alloys are widely used as thermal interfacial materials, solders in electronic packaging, thermal fuse and low temperature resistant performance electronic components. However, the inherent tendency of microstructure coarsening and phase segregation in Sn-Bi alloys under thermal loading can easily lead to the inhomogeneous mechanical and thermal response behavior, which can promote crack initiation and expansion near the interface between two major phases (i.e., Bi-rich and Sn-rich phases), and this has brought about serious reliability concern for the above applications and thus attracted increasing attention in recent years. In this study, a phase field model is developed to simulate the microstructural evolution and phase segregation behavior of the eutectic Sn58Bi solder in a line-type Cu/Sn58Bi/Cu interconnect under the condition of isothermal aging and temperature gradient respectively. Results show that the Bi-rich phase and Sn-rich phase distribute inhomogeneously in the Sn58Bi alloy matrix of the solder interconnect during thermal aging; the large size Bi-rich phase particles grow up at the expense of the small size ones, and the value of coarsening exponent n is 0.17. Under temperature gradient, Bi atoms migrate along the direction of the heat flux, and consequently a Bi-rich phase segregation layer forms on the cold end, while a Sn-rich phase area is left on the hot end. The relationship between the temperature gradient distribution and the microstructural characteristics is well revealed. Moreover, the phase segregation under temperature gradient induces the decrease of thermal conductivity, which in turn deteriorates the heat transfer performance of solder interconnects. In addition, comparing the coarsening behavior of the Bi-rich phase under temperature gradient with that during isothermal aging, it is clear that the temperature gradient leads to faster coarsening of the Bi-rich phase, and there is a linear relationship between the mean equivalent radius of the Bi-rich phase and nondimensional time. Finally, the study of thermomigration kinetics of the Bi-rich phase shows that the thickness of the Bi-rich phase segregation layer increases almost linearly with nondimensional time, which is consistent with the theoretical analysis. Highlights • Temperature gradient is coupled to phase field model to simulate thermomigration of Sn58Bi alloy. • Bi-rich phase coarsens faster under temperature gradient than isothermal aging. • Revealing the relationship between temperature gradient and microstructural characteristics • Thermal conductivity of solder interconnects decreases during thermomigration. • Clarifying interactions among microstructure, thermomigration and temperature gradient [ABSTRACT FROM AUTHOR]

Printed circuit boards that use fine pitch technology have a greater risk of open-circuit failure, due to void formations caused by the growth of intermetallic compounds. This failure mode is reported to be a result of electromigration (EM) damage. Current stressing occurs when current flows in a solder bump, thereby producing EM. Joule heating is also a significant occurrence under current stressing conditions, and induces thermomigration (TM) in solder bumps during EM. This study investigated the intermetallic compound (IMC) growth kinetics for Sn-0.7Cu solders, modeled by EM, TM, and chemical diffusion. The modeling results concurred with the observed kinetics of IMC growth. Electromigration influenced the growth of IMCs most significantly for a current density of 10 kA/cm2. The effect of TM on the IMC growth had to be considered for a thermogradient of 870°C/cm. However, the effect of chemical diffusion was insignificant on IMC growth, specifically for a current density of 10 kA/cm2. [ABSTRACT FROM AUTHOR]

Abstract: The interfacial behaviors of Cu/molten Sn-58Bi/Cu solder joints under the coupling effect of a temperature gradient and the current stressing have been investigated. The most obvious change of the interfacial behaviors under the individual electromigration (EM) and thermomigration (TM) in molten solder was the asymmetrical growth of interfacial Cu-Sn intermetallic compounds (IMCs), which grew rapidly as the stressing time prolonged. The growth rates of the interfacial IMCs induced by TM under a temperature gradient of 40 °C/cm were slightly faster than EM under the current density of 0.5 × 104 A/cm2. However, the microstructure evolution and interfacial behaviors changed obviously when the thermal was distributed unevenly across the entire solder stripe under current stressing. It was found that there was a Bi-rich layer adhered to the anode side and a distributed Cu6Sn5 phase existed in the solder matrix when the anode and the cold end were at the same end. Additionally, a large number of Bi-blocks and Cu-Sn IMCs were dramatically observed in the solder matrix when the anode and the hot end were on the same side. The main reason for this result may be attributed to the significant change of the diffusion of Bi atoms under the thermo-electric coupling conditions. In the initial melting stage of solder, the Bi atoms in molten Sn-58Bi solder rapidly diffused to the anode and then began to be reversely dissolved into the solder matrix. The experimental results proved the additional temperature gradient played a positive or negative role on the reverse dissolution of Bi atoms in the heat preservation process.Graphical Abstract: [ABSTRACT FROM AUTHOR]

Synchrotron radiation real-time imaging technology was used to in situ study the liquid-solid interfacial reactions in Cu/Sn-9Zn/Ni solder interconnects with and without temperature gradient. It was clarified that a Cu 5 Zn 8 intermetallic layer adhering to the interfaces of the solder interconnects acted as an effective barrier against Cu or Ni diffusion, and thus retarded the consumption of the underneath Cu or Ni substrate and suppressed the coupling effect of Cu-Ni cross-interaction and thermomigration. However, both Cu 6 Sn 5 -type and Ni 3 Sn 4 -type intermetallic compounds had little effect on protecting Cu or Ni substrate. The barrier mechanism of the Cu 5 Zn 8 intermetallic layer has been discussed. In addition, the massive spalling of interfacial Cu 5 Zn 8 was in situ observed during reflow with Cu as the hot end. The massive spalling was caused by the Zn depletion in the liquid solder, the fast dissolution of hot end Cu or Cu 6 Sn 5 -type intermetallic compound, and the poor-soluble nature of Cu 5 Zn 8 layer. [ABSTRACT FROM AUTHOR]

Grain-boundary migration, void formation as well as associated hillock formation are important mechanisms which lead to the failure of interconnects in the microelectronic packages. An understanding of the underlying physics of each of the phenomena can allow better design of interconnects. In this paper, we formulate a new phase-field model based on a grand-potential formalism for studying the phenomena of grain-boundary grooving under the combined influence of pure diffusion controlled transport, electric current and thermal gradient. We separately investigate the contributions of each of the stimuli towards the process of grain-boundary migration and hillock formation, by performing phase-field simulations as well as comparing with analytical theories. Additionally, we qualitatively reproduce the phenomena observed in experiments on polycrystalline metals, wherein electromigration and thermomigration may act in unison or against each other towards their contributions in grooving, hillocking and void growth. [ABSTRACT FROM AUTHOR]

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In this work we provide an extended Cahn–Hilliard equation describing separation processes in binary alloys affected by temperature gradients coupled with a heat equation. The three important material parameters used in the model, namely the Gibbs’ configurational free energy density, the mobility of atoms and their heats of transport, are modelled as sufficiently smooth function in mole fraction and absolute temperature for Sn–Pb alloys. The model consists of two nonlinear fourth order partial differential equations. Consequently, the variational formulation of the problem mandates approximation functions which are at least C 1 -continuous. In order to fulfil this requirement, a NURBS based finite element (FE) scheme is employed. Here we provide only a brief overview of the used discretization techniques. Details on the numerical treatment of the model, in particular on the implementation of essential boundary conditions within NURBS spaces, can be found in our work. Concluding computational studies of two- and three dimensional thermomigration events within Sn–Pb alloys will demonstrate the quality of our model. [ABSTRACT FROM AUTHOR]

In this paper, a Cu/SnAg 3.0 Cu 0.5 /Cu solder joint is current stressed under 6 A at 120 °C. After current stressing for 300 h, a serrated interface and voids are observed at the cathode. Current-crowding and thermal-crowding in solder grooves between adjacent Cu 6 Sn 5 grains are found to be responsible for the back-diffusion of Sn and Cu atoms, eventually leading to the formation of a cathodic serrated interface and voids. In addition, the Sn orientation of the solder grooves may also play an important role in the robustness of the cathodic Cu 6 Sn 5 grains and electrode. Our study may provide understanding of electromigration/thermomigration-induced evolution of the Cu/Sn interface under current stressing and provide visual data for interpreting early open-circuit failure of solder joints. [ABSTRACT FROM AUTHOR]

The thermodynamics and kinetics of the thermomigration of molten zones based on Al–Ga and Al–Ga melts in the preparation of silicon epilayers have been studied in detail. We have determined the threshold thermomigration temperature for zones of various compositions. The migration onset temperature has been shown to increase monotonically with increasing Ga or Sn concentration in the liquid phase. The thermomigration rate of Si–Al–Ga zones decreases with increasing gallium concentration at temperatures below 1473 K and increases at higher temperatures. The thermomigration rate of Si–Al–Sn zones decreases with increasing Sn concentration over the entire temperature range studied. No chemical compounds have been detected in the Si–Al–Ga or Si–Al–Sn system, which simplifies the use of the thermomigration method in these systems. [ABSTRACT FROM AUTHOR]

This work established the correlation between the location of temperature gradients and the positions where breakdown is observed on different Pt filament layouts in cantilever and full membrane type micro-hotplates. Focusing on practical aspects like in real operation, self-heating was applied to investigate the limitations of high temperature application and to reveal the fatal failure mechanisms. Besides electromigration, another phenomenon playing dominant role in the breakdown of the filaments, the temperature gradient driven thermomigration of Pt was identified. This limits the local allowable temperature gradient to < 0.4 °C/μm for operation temperature above 700 °C. [ABSTRACT FROM AUTHOR]

Interfacial reactions in Ni/Sn/Ni and Ni/Sn-9Zn/Ni micro solder joints during thermomigration (TM) have been studied by reflowing solder joints on a hot plate. Asymmetrical growth and transformation of interfacial intermetallic compounds (IMCs) were clearly observed. The growth of the NiSn IMC in the Ni/Sn/Ni solder joints was always fast at the cold end and relatively slow at the hot end. Only asymmetrical growth of the NiZn IMC in the Ni/Sn-9Zn/Ni solder joints occurred at the beginning because Zn was the dominant TM species; however, asymmetrical transformation of the NiZn IMC also occurred under the combined effect of Zn depletion and Ni dissolution and migration, resulting in formation of a thin τ-phase layer at the hot end and a thick τ-phase/NiZn/τ-phase sandwich structure at the cold end. TM of Ni and Zn atoms was identified towards the cold end, being responsible for the abnormal IMC evolution. Addition of Zn was found to slow the TM-induced IMC growth and Ni dissolution. [ABSTRACT FROM AUTHOR]

• The addition of Zn@Sn avoids coarsening and segregation of the Bi-rich phase. • The heat transfer Q* of Bi atoms can be calculated as 22.5 kJ/mol. • The Zn-rich phase acts as a hindrance to the diffusion of the Bi phase. • Sn-58Bi-6%Zn@Sn enhances the long-term reliability of Sn58Bi solder joints. In the field of electronic packaging, Sn-58Bi solder paste is a promising low-temperature option for step soldering. However, the tendency for the Bi-rich phase to coarsen and segregate poses a significant challenge to the long-term reliability of the solder joints. The Sn coated Zn (Zn@Sn) core-shell particles were prepared through a chemical plating process and incorporated into the Sn-58Bi low-temperature solder paste to enhance its reliability. The microstructural and mechanical characteristics of the solder joints were evaluated under thermal cycling and thermomigration conditions to assess their long-term service reliability. The addition of Zn lowers the nucleation energy and promotes the refinement of the eutectic structure, avoiding a significant reduction in shear strength after thermal cycling. In thermomigration, the heat transfer Q* of Bi is calculated to be 22.5 kJ/mol. The Zn-rich phases hardly move and are evenly distributed within the solder joints, acting like a barrier to prevent the Bi phase diffusing from the hot end to the cold end. [Display omitted] [ABSTRACT FROM AUTHOR]