Growth Of Kirkendall Voids Research Articles

Phase evolutions and the growth of Kirkendall voids in the V–Zn system

The V-Zn phase diagram became available after 1990. The existence of the VZn16 phase has been questioned mainly. A solid–state diffusion experiment is conducted to reinvestigate the stability of phases. The interdiffusion zone is examined using scanning electron microscopy and energy dispersive spectroscopy. Our analysis confirms that VZn16 is one of the stable phases. Additionally, the growth of the VZn9 product phase is observed in the V/Zn diffusion couple, indicating that this newly discovered phase is also stable. Furthermore, the formation of the Kirkendall voids is observed in the VZn3 phase, which can affect the physico–mechanical reliability of any component during load-bearing application. Moreover, this study's findings could also be beneficial for the ongoing efforts of designing promising Zn–based biomaterials.

Phase field crystal study on the size and strain rate dependent evolution of Kirkendall voids in binary alloy

The evolution of Kirkendall voids in binary alloy under a uniaxial strain rate loading is investigated by the phase field crystal (PFC) model. The simulation results show that model size and the applied strain rate greatly affect the formation and growth of Kirkendall voids. It is found that the grains are prone to rotate when the model size is smaller than 34.7a×34.7a (a is the lattice constant) and the initially set grain boundaries will disappear after relaxation, no Kirkendall voids form in this case. As the model size increases, Kirkendall voids arise and interconnect with each other, and large voids will dissociate to produce small voids. Moreover, the applied strain rate also affects the formation and growth path of Kirkendall voids. Voids only form at the grain boundaries in the absence of strain rate loading, whereas voids form both inside the grains and at the grain boundaries under strain rate loading. The growth of voids can easily form oblique structures caused by dislocation slip, and they tend to grow perpendicular to the direction of applied strain rate.

Forming mechanism and growth of Kirkendall voids of Sn/Cu joints for electronic packaging: A recent review

For the common Cu-Sn interconnection system in microelectronics packaging, a significant concern is the sporadic interfacial void formation within the Cu–Sn interface and intermetallic compound (IMC) layer during the service. The existence of these voids affects the electrical and mechanical properties of the joint and thus deteriorates reliability. Most scholars simply attribute the Kirkendall voids problem to the Kirkendall effect caused by the difference in Cu/Sn inter-diffusion coefficients. However, Kirkendall voids are formed for a combination of many reasons. The plating additives, current density, surface roughness, coating thickness, and substrate types affect the formation of Kirkendall voids. We believe that among the many factors affecting the formation of voids, the unbalanced diffusion of elements (Kirkendall effect) is the root cause, and impurities and Cu microstructure are the dominant factors. The addition of alloying elements to the solder affects the formation of voids by changing the unbalanced diffusion of elements. This paper releases the possible mechanism of Kirkendall voids and gives the perspective summary from three steps of void formation, and the available suppression method is given based on the final solution.

Bifurcation of the Kirkendall marker plane and the role of Ni and other impurities on the growth of Kirkendall voids in the Cu–Sn system

The presence of bifurcation of the Kirkendall marker plane, a very special phenomenon discovered recently, is found in a technologically important Cu–Sn system. It was predicted based on estimated diffusion coefficients; however, could not be detected following the conventional inert marker experiments. As reported in this study, we could detect the locations of these planes based on the microstructural features examined in SEM and TEM. This strengthens the concept of the physico–chemical approach that relates microstructural evolution with the diffusion rates of components and imparts finer understanding of the growth mechanism of phases. The estimated diffusion coefficients at the Kirkendall marker planes indicates that the reason for the growth of the Kirkendall voids is the non–consumption of excess vacancies which are generated due to unequal diffusion rate of components. Systematic experiments using different purity of Cu in this study indicates the importance of the presence of impurities on the growth of voids, which increases drastically for ≥ 0.1 wt% impurity. The growth of voids increases drastically for electroplated Cu, commercially pure Cu and Cu(0.5 at.% Ni) indicating the adverse role of both inorganic and organic impurities. Void size and number distribution analysis indicates the nucleation of new voids along with the growth of existing voids with the increase in annealing time. The newly found location of the Kirkendall marker plane in the Cu3Sn phase indicates that voids grow on both the sides of this plane which was not considered earlier for developing theoretical models.

Morphological evolution and growth kinetics of Kirkendall voids in binary alloy system during deformation process—Phase field crystal simulation study

The formation and growth of Kirkendall voids in a binary alloy system during deformation process were investigated by phase field crystal model. The simulation results show that Kirkendall voids nucleate preferentially at the interface, and the average size of the voids increases with both the time and strain rate. There is an obvious coalescence of the voids at a large strain rate when the deformation is applied along the interface under both constant and cyclic strain rate conditions. For the cyclic strain rate applied along the interface, the growth exponent of Kirkendall voids increases with increasing the strain rate when the strain rate is larger than 1.0×10−6, while it increases initially and then decreases when the strain rate is smaller than 9.0×10−7. The growth exponent of Kirkendall voids increases initially and then decreases gradually with increasing the length of cyclic period under a square-wave form constant strain rate.

Interface dynamics in one-dimensional nanoscale Cu/Sn couples

The isothermal metallurgical reaction in two-segmented Cu-Sn nanowires results in the formation of a Sn/Cu6Sn5/Cu3Sn/Cu sandwich structure. In-situ transmission electron microscopy is used to study how Cu6Sn5/Sn and Cu/Cu3Sn interfaces propagate and change shape during the intermetallic compound growth. The Cu6Sn5/Sn interface is observed to evolve from an inclined configuration to a vertical, edge-on configuration with the propagation of the Cu6Sn5 phase towards the Sn segment. The Cu/Cu3Sn interface also becomes less inclined as it propagates toward the Cu segment. This interface evolution is driven by the minimization of the interface energy associated with minimizing the interface area associated with the edge-on interface. The Kirkendall void growth induces the breakage of the Cu segment and results in the Cu3Sn → Cu6Sn5 transformation with the final Sn/Cu6Sn5/void/Cu sandwich structure.

Effect of intermetallic compound layer thickness on the shear strength of 1206 chip resistor solder joint

Purpose – The purpose of this paper is to study the effect of intermetallic compound (IMC) layer thickness on the shear strength of surface-mount component 1206 chip resistor solder joints. Design/methodology/approach – To evaluate the shear strength and IMC thickness of the 1206 chip resistor solder joints, the test vehicles were conventionally reflowed for 480 seconds at a peak temperature of 240°C at different isothermal ageing times of 100, 200 and 300 hours. A cross-sectional study was conducted on the reflowed and aged 1206 chip resistor solder joints. The shear strength of the solder joints aged at 100, 200 and 300 hours was measured using a shear tester (Dage-4000PXY bond tester). Findings – It was found that the growth of IMC layer thickness increases as the ageing time increases at a constant temperature of 175°C, which resulted in a reduction of solder joint strength due to its brittle nature. It was also found that the shear strength of the reflowed 1206 chip resistor solder joint was higher than the aged joints. Moreover, it was revealed that the shear strength of the 1206 resistor solder joints aged at 100, 200 and 300 hours was influenced by the ageing reaction times. The results also indicate that an increase in ageing time and temperature does not have much influence on the formation and growth of Kirkendall voids. Research limitations/implications – A proper correlation between shear strength and fracture mode is required. Practical implications – The IMC thickness can be used to predict the shear strength of the component/printed circuit board pad solder joint. Originality/value – The shear strength of the 1206 chip resistor solder joint is a function of ageing time and temperature (°C). Therefore, it is vital to consider the shear strength of the surface-mount chip component in high-temperature electronics.

Effect of Cu–Sn intermetallic compound reactions on the Kirkendall void growth characteristics in Cu/Sn/Cu microbumps

Growth behaviors of intermetallic compounds (IMCs) and Kirkendall voids in Cu/Sn/Cu microbump were systematically investigated by an in-situ scanning electron microscope observation. Cu–Sn IMC total thickness increased linearly with the square root of the annealing time for 600 h at 150 °C, which could be separated as first and second IMC growth steps. Our results showed that the growth behavior of the first void matched the growth behavior of second Cu6Sn5, and that the growth behavior of the second void matched that of the second Cu3Sn. It could be confirmed that double-layer Kirkendall voids growth kinetics were closely related to the Cu–Sn IMC growth mechanism in the Cu/Sn/Cu microbump, which could seriously deteriorate the mechanical and electrical reliabilities of the fine-pitch microbump systems.

Electromigration induced Kirkendall void growth in Sn-3.5Ag/Cu solder joints

Effects of electric current flow on the Kirkendall void formation at solder joints were investigated using Sn-3.5Ag/Cu joints specially designed to have localized nucleation of Kirkendall voids at the Cu3Sn/Cu interface. Under the current density of 1 × 104 A/cm2, kinetics of Kirkendall void growth and intermetallic compound thickening were affected by the electromigration (EM), and both showed the polarity effect. Cu6Sn5 showed a strong susceptibility to the polarity effect, while Cu3Sn did not. The electromigration force induced additional tensile (or compressive) stress at the cathode (or anode), which accelerated (or decelerated) the void growth. From the measurements of the fraction of void at the Cu3Sn/Cu interface on SEM micrographs and analysis of the kinetics of void growth, the magnitude of the local stress induced by EM was estimated to be 9 MPa at the anode and −7 MPa at the cathode.

Simulations of the Kirkendall-Effect-Induced Deformation of Thermodynamically Ideal Binary Diffusion Couples with General Geometries

The Kirkendall effect stems from the imbalance of atomic diffusion fluxes in a crystalline solid. Vacancy generation and annihilation, which compensate for the unbalanced fluxes, result in deformation that is experimentally observed as the motion of fiducial markers in a diffusion couple that can be approximated as a one-dimensional system. In multiple dimensions, such deformation occurs along both the directions parallel to and normal to the primary diffusion direction. In this article, we present a model that couples unbalanced interdiffusion and the resulting plastic deformation. One- and two-dimensional simulations are conducted with the analytically calculated diffusion coefficients of a thermodynamically ideal random alloy; the result shows that the ratio of the diffusion fluxes of the atomic species equals the ratio of atomic hop frequencies, which leads to the final volume ratio also given approximately by the hop frequency ratio if the initial volume ratio is equal. Moreover, the result also demonstrates that the conventional interdiffusion model fails to describe the Kirkendall void growth dynamics. For numerical implementation, we reformulate the diffusion equation to the smoothed boundary form and solve it within the deforming body governed by steady-state Navier-Stokes equation. This work demonstrates that the presented method can be a useful tool for studying Kirkendall-effect-induced deformation.

Secondary IMC formation induced by Kirkendall voiding in Cu/Sn–3.5Ag solder joints

In this investigation on the formation of multiple-layered Kirkendall voids at Cu/Sn–3.5Ag solder joints, Sn–3.5Ag solder balls were reacted with Cu under bump metallurgy (UBM), which was electroplated using bis-sodium sulfopropyl–disulfide, C6H12O6S4Na2 (SPS) additive. The sequence of multilayer Kirkendall voids and Cu–Sn IMC (intermetallic compounds) formations are explained with the aid of cross-sectional scanning electron microscopy (SEM) micrographs and schematic diagrams. During the aging treatment at 150 °C, layers of Cu6Sn5/Cu3Sn formed at the solder joints and Kirkendall voids nucleated at the Cu3Sn/Cu interface as a result of the segregation of residual S originating from SPS. However, with Kirkendall void growth, the net section area of the Cu/Cu3Sn interface decreased and the Cu flux into Cu3Sn was inhibited. As the atomic ratio of Cu against Sn in the Cu3Sn dropped, transformation of Cu3Sn into Cu6Sn5 ensued. Subsequent diffusion of Sn atoms into the remaining Cu UBM through the remaining ligament of the Cu6Sn5/Cu interface precipitated secondary Cu3Sn beneath the primary Cu3Sn/Cu interface, and the secondary Kirkendall voids formed at the new Cu3Sn/Cu interface and so on.

Effects of Electromigration(EM) on the Kirkendall Void Formation in Sn-3.5Ag/Cu Solder Joints

Kirkendall voids and the micro voids induced by electromigration result in the serious failure in the solder interconnect. Interfacial reactions between Sn-3.5Ag solder and the electroplated Cu using SPS additive in the bath were investigated under high current condition. During subsequent isothermal aging at 150°C, the formation of Kirkendall voids was accelerated by S segregation at the void surface and Cu/Cu3Sn interface, and then the voids were preferentially localized at the interface. And under high current flow (1X104A/cm2) at the cathode side, electromigration caused the micro voids at the Cu6Sn5/Sn interface. The growth of Kirkendall voids and the micro voids were both affected by aging time and current density. Effects of current flow on the growth of Kirkendall voids and the micro voids were examined by applying high electric current during isothermal aging at 150°C. From the measurements of the fraction of voids at the Cu/Cu3Sn and Cu6Sn5/Sn interfaces on SEM micrographs and analysis of the kinetics of void growth, which voids are more dominant was examined according to the time and the current density. Furthermore, under current flow the intermetallic compounds (IMC) thickening as well as voids growth was affected by polarity. Even though Cu was consumed remarkably, the growth of both Cu3Sn and Cu6Sn5 IMCs was suppressed at the cathode side. On the other hands, the thickness of Cu6Sn5 at the anode side was increased by reaction with Cu flux atoms which migrate towards the anode from the cathode Cu line.

Effect of isothermal aging on intermetallic compounds and Kirkendall void growth kinetics of Au stud bumps

Intermetallic compounds (IMCs) and Kirkendall void growth kinetics at various interfaces in Au stud bumps were studied in terms of isothermal aging at 120 °C, 150 °C, and 180 °C for 300 h. Phases of AuSn, AuSn2, and AuSn4 form at the interface of Au studs and Sn after a reflow. The thickness of an Au-Sn IMC was quantified with an image analyzer as a function of aging temperature and time. The growth of the Au-Sn IMC increases linearly with the square root of the aging time at 120 °C, 150 °C, and 180 °C. The growth of the Au-Sn IMC at 180 °C differs from the growth of the Au-Sn IMC at 120 °C and 150 °C because of excessive Au consumption by the Au studs. Kirkendall voids form at the interface of the Au stud and the Au-Sn IMC and increase linearly with the square root of the aging time. The apparent activation energies for the growth of the Au-Sn IMC and the Kirkendall voids were determined to be 0.57 eV and 0.2 eV, respectively, from measurement of the thickness of the Au-Sn IMCs and the width of the Kirkendall voids in relation to the aging temperature and time.

Condensation and Growth of Kirkendall Voids in Intermetallic Compounds

A model for the simulation of Kirkendall voiding in metallic materials is presented based on vacancy diffusion, elastic-plastic and rate-dependent deformation of the material. Starting with a phenomenological explanation of the Kirkendall effect we briefly discuss the consequences on the reliability of microelectronic components. Then, a constitutive model for void nucleation and growth is introduced, which can be used to predict the temporal development of voids in solder joints during thermal cycling. We present numerical studies and discuss the potential of the results for the failure analysis of joining connections.

Continuum simulations of the formation of Kirkendall-effect-induced hollow cylinders in a binary substitutional alloy

We examine binary substitutional diffusion in cylindrical diffusion couples in which free surfaces are considered explicit vacancy sources and sinks. The central region of the cylinder is initially occupied by an atomic species with a larger hop frequency, while the outer region is occupied by another atomic species with a smaller hop frequency. Equilibrium vacancy concentration is maintained at free surfaces that serve as vacancy sources and sinks. In the crystal, diffusion is governed by the standard diffusion equations with analytically evaluated diffusion coefficients. The void growth dynamics and hollow cylinder formation stemming from the Kirkendall effect are simulated. Our results show that the Kirkendall void growth involves two competing factors. One is the net inward vacancy flux that favors void growth. The other is the Gibbs–Thomson effect that favors void shrinkage. We compute the critical initial radius for void growth above which the Kirkendall effect dominates over the Gibbs–Thomson effect. The fully grown void radius and the elapsed time to the fully grown size are also predicted for different fast-diffuser volume fractions and fast-to-slow diffuser atomic hop frequency ratios.

Effects of sulfide-forming element additions on the Kirkendall void formation and drop impact reliability of Cu/Sn–3.5Ag solder joints

Ternary Pb-free solders, Sn–3.5Ag–X, containing 0.5 wt.% of Zn, Mn and Cr, were reacted with Cu UBM, which was electroplated using SPS additive. Characteristics of Cu–Sn IMCs and Kirkendall void formation at the Cu/Sn–3.5Ag solder joints were significantly affected by the third element, and the potency to suppress Kirkendall voids at the solder joint increased in the order of Cr, Mn, Zn, which was indeed the order of the drop reliability improvement. From the AES analyses, it was suggested that the sulfide-forming elements in the solder diffused into the Cu UBM and reduced the segregation of S atoms to the Cu/Cu 3Sn interface by scavenging S, which led to the suppression of Kirkendall void nucleation at the Cu/Cu 3Sn interface and the drop reliability improvement. In the case of the Zn-containing solder joint, Cu 3Sn phase, known to be a host of Kirkendall voids, did not form at all even after extended aging treatments. The magnitude of the tensile stress at the Cu 3Sn/Cu interface which drove the Kirkendall void growth was estimated to be 10–100 MPa.

Electromigration Characteristics of Flip Chip Sn-3.5Ag Solder Bumpsunder Highly Accelerated Conditions

The e ects of severe current crowding and Joule heating on the damage morphology and the electromigration parameters were evaluated for ip chip Sn-3.5Ag solder bumps with Cu underbump metallurgy. in-situ electromigration testing in a scanning electron microscope was performed to correlate the statistical lifetimes with the detailed microstructural characteristics. The highly accelerated test conditions used led to an activation energy of 1.63 eV and a current density exponent of 4.6, which can be attributed to severe current crowding and Joule heating e ects. Real-time microstructural analysis revealed that interfacial voids nucleated around corners of the under-bump metallurgy layer, where electrons entered from the chip or the substrate interconnect line to the solder bump and then grew along the Cu6Sn5/solder interface, irrespective of the current ow's direction. Accelerated growth of Kirkendall voids was also observed at the Cu3Sn/Cu interface and within Cu3Sn. However, electrical failure of the bump resulted from electromigration-induced interfacial void propagation along the Cu6Sn5/solder interface, which can be explained in terms of the large di erence in Cu di usion ux between the Cu-Sn intermetallic compound layer and the solder itself.

Effect of Copper-Tin Intermetallic Compound on Reliability of Fine Bump Joint Structures

Mechanical and electrical reliability of fine bumps with diameter and height of tens of μm was studied considering the growth of the intermetallic compound (IMC) at the interface between a bump and a copper thin-film interconnection. It was found that the increase of the thickness of the IMC change the stress and strain field around the interface significantly, and thus, the fracture mode from the fatigue crack of solder to fatigue crack of the copper interconnection or delamination between the IMC and the copper interconnection. This is because that the mechanical properties of the grown IMC differ from those of copper and tin and that a large amount of Kirkendall voids appeared around the interface. In addition, the resistance of the bumps increased drastically with the increment of the thickness of the IMC layer because of the growth of Kirkendall voids. Therefore, it is very important to minimize the growth of the IMC to assure the reliability of the bump joint structures.

Kirkendall voids in the intermetallic layers of solder joints in MEMS

Abstract Microelectronic systems consist of the functional chip unit itself and a surrounding package which includes several electro-mechanical connections, e.g., solder joints between different metal layers. Experimental observation shows that aging of the solder alloy as well as the formation and growth of so-called Kirkendall voids significantly contributes to the degradation of the joining capability. Starting with a phenomenological explanation of the Kirkendall effect, we present in this contribution a constitutive model for void nucleation and growth. The model accounts for the effects of vacancy diffusion, surface tension and rate-dependent plastic deformation on ensembles of spherical voids. It can be applied to predict the temporal development of voids in solder joints. Numerical simulations on the material point level provide insight into the (not yet completely understood) mechanisms of failure by formation and growth of Kirkendall voids and show the potential of the model for the failure analysis of joints.

Effects of residual S on Kirkendall void formation at Cu/Sn–3.5Ag solder joints

Solder joints of Cu/Sn–3.5Ag were prepared using Cu foil or electroplated Cu films with or without SPS additive. With a high level of SPS in the Cu electroplating bath, voids tended to localize at the Cu/Cu 3Sn interface during subsequent aging at 150 °C, which was highly detrimental to the drop impact resistance of the solder joints. In situ Auger electron spectroscopy of fractured joints revealed S segregation on the Cu/Cu 3Sn interface and void surfaces, suggesting that segregation of S to the Cu/Cu 3Sn interface lowered interface energy and thereby the free energy barrier for Kirkendall void nucleation. Once nucleated, voids can grow by local tensile stress, originating from residual stress in the film and/or the Kirkendall effect. Vacancy annihilation at the Cu/Cu 3Sn interface can induce tensile stress which drives the Kirkendall void growth.