Growth Kinetics Of Intermetallic Compounds Research Articles

Growth kinetics and thickness prediction of interfacial intermetallic compounds between solid steel and molten aluminum based on thermophysical simulation in a few seconds

The growth kinetics of intermetallic compounds (IMCs) generated by initial interfacial reaction (interaction time<10s) between solid steel and molten aluminum was investigated using a thermophysical simulation method. According to the morphology evolution of the η (Fe2Al5) and θ (Fe4Al13) phase layers in the isothermal experiments in seconds, it is confirmed that the reaction diffusion controls the η phase growth and the θ phase precipitates from the liquid phase during cooling. The growth kinetics models of the η phase maximum and average thickness are proposed as well. The results showed that the maximum growth was governed by the interfacial reaction, whereas the average thickness was governed by a diffusion and interfacial reaction process. Considering the application of the kinetics model in welding-brazing of Fe/Al dissimilar metals, a mathematical model to predict the average thickness of η phase was established using the finite difference method in thermal cycles as well. The predicted results are consistent with the experimental results, indicating that the model is of good accuracy.

Solid-state growth kinetics of intermetallic compounds in Cu pillar solder flip chip with ENEPIG surface finish under isothermal aging

Electroless Ni/electroless Pd/immersion Au (ENEPIG) constitutes a promising metallization to replace the conventional ENIG or Ni/Au for 3D IC integrated module and optoelectronic packaging. Different ENEPIG plating thicknesses with copper pillar solder joints are subjected to thermal aging at 150 °C to investigate the intermetallic compounds (IMCs) formation and growth. Due to low temperature solid-state bonding, both Pd and Au layers do not completely diffused into Sn matrix and participated in the interfacial reactions to form (Pd,Au)Sn4 IMCs phase. (Pd,Au)Sn4 IMCs based on PdSn4 phase with dissolved Au, exhibits high growth rate and substantial consumption of Sn from solder. Upon increasing the aging time, it is found that (Cu,Ni)6Sn5 IMCs and (Pd,Cu,Au)Sn4 phase form at interface and follows a diffusion control mechanism, which weakened the solder joint and caused brittle fracture in a die peel test. (Cu,Ni)6Sn5 IMCs growth rate increases with decreases Pd thickness. Therefore, the types of IMCs formation, growth rate, and reliability of Cu pillar joint strongly depend on bonding temperature, Au and Pd thicknesses, and solder volume. Based on this study, the authors recommend suitable ranges of Au and Pd layer thicknesses for reliable Cu pillar solder joints.

Effect of type of thermo-mechanical excursion on growth of interfacial intermetallic compounds in Cu/Sn-Ag-Cu solder joints

This study reports the effect of different types of thermo-mechanical excursion (TME) on growth of intermetallic compound (IMC) layer formed at the interface of Sn-3.0%Ag-0.5%Cu solder and Cu substrate. 1mm thick solder joints were prepared by reflowing at 270°C for either 60 or 90s. Solder joints were then exposed to one of the following TME: (i) isothermal aging at 60°C for 48, 96 and 144h, (ii) thermal cycling between −25 and 125°C for 100, 200 and 400cycles, and (iii) thermo-mechanical cycling between −25 and 125°C for 100, 200 and 400cycles, wherein a shear strain of 10% per cycle was imposed on the joint. Finite element analysis (FEA) was performed to ascertain the effects of imposed shear strain and volumetric expansion due to the formation of IMC on the stress field in the solder joint. Irrespective of the type of TME, the thickness of the IMC layer increased with time. However, IMC thickness increased relatively more rapidly under thermo-mechanical cycling condition, indicating strain enhanced coarsening of the interfacial IMC layer. FEA showed that high stresses were generated in the IMC layer and near solder-IMC interface due to the formation of IMC layer as well as imposed external strain, which might then not only enhance the IMC growth kinetics, but also affect the morphology of the IMC layer.

Growth Kinetics of Intermetallic Compounds During Interfacial Reactions Between SnAgCuGa Lead-Free Solder and Cu Substrate

Sn–Ag–Cu based alloys are well known as an important series of lead-free solders. Gallium has been found to be beneficial for the melting point of the Sn–Ag–Cu solders. In the present work, interfacial reactions between Sn–3.0Ag–0.5Cu–xGa (x = 0, 0.1, 0.5, 1.5 in wt.%) solders and Cu substrates have been examined. The solder and substrate couples were annealed at 80°C for 2, 4, 6, 12, 18, and 24 days, respectively. Various layers of intermetallic compounds have been observed to form using scanning electron microscopy. It has been found that only Cu6Sn5 phase forms when Ga additions are 0, 0.1 and 0.5 in wt.%; but Cu3Sn and Cu9Ga4 form if Ga addition reaches 1.5 wt.%. Thicknesses of these intermetallic compounds have been measured at different annealing stages in order to study the growth kinetics. The growth of all the intermetallic compounds follows the parabolic rule.

Suppression effect of Ni grain size on the Ni3Sn4 growth at the Sn/Ni interface

Effect of Ni grain size on the formation and growth kinetics of intermetallic compounds at the Sn/Ni interface is studied. For a coarse-grained Ni foil, only the Ni3Sn4 phase is formed with a thickness of 28μm after aging at 200°C for 1000h. For a Ni electroplated layer with a fine-grained microstructure, the Ni3Sn4 phase grows at a very sluggish rate and its thickness is only 5μm in the same reaction condition. The suppression effect on the Ni3Sn4 growth is attributable to the diffusion barrier effect of a thin Ni3Sn2 phase formed on the Ni electroplated layer.

Rapid Ag/Sn/Ag transient liquid phase bonding for high-temperature power devices packaging by the assistance of ultrasound

Rapid transient liquid phase (TLP) bonding process on Ag/Sn/Ag system is achieved in air by the assistance of ultrasonic, which has great potential to be applied to high-temperature power devices packaging. In this study, the influence of ultrasonic effect on the morphology and growth kinetics of Ag3Sn grains, and the joint microstructure, mechanical property and thermal reliability were systematically investigated. Experimental results indicated that the rapid consumption of the “dynamic” transient liquid phase was attributed to the accelerated dissolution of Ag substrate and the extrusion of liquid Sn, which were entirely induced by the complex sonochemical effects on the liquid/solid intermetallic compounds (IMCs) interface. An elongated scallop-like morphology of Ag3Sn grains was developed during Ag/Sn interfacial reaction with ultrasonic, accompanied by widening of grooves between neighbored grains. This phenomenon is called as a strengthening thermal grooving, in which the grooves at grain boundaries provide stable molten channels for Ag atoms diffusion from the substrate. Consequently, the improved elemental diffusion was evaluated through the growth kinetics of Ag3Sn IMCs, with conservative estimation of 6–16.5 times faster than the traditional TLP process. In addition, both excellent mechanical property and thermal reliability of the Ag-Sn intermetallic joint were experimentally verified by shear test and high-temperature storage test, respectively.

Electromigration에 의한 Sn-0.7Cu 솔더 금속간화합물 성장 예측

Electromigration에 의한 Sn-0.7Cu 솔더 금속간화합물 성장 예측

Growth Kinetics of Intermetallic Compounds Between Sn and Fe Liquid–Solid Reaction Couples

Growth behavior of the intermetallic compound (IMC), FeSn2, was investigated in the liquid Sn/solid Fe reaction couple over the annealing temperatures from 250 °C to 400 °C. Low-carbon steel AISI 1018 was chosen to make Fe samples. The morphology and thickness of the IMC formed between Sn and Fe were examined using scanning electron microscopy (SEM). In addition, energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were used to confirm that the IMC is FeSn2. The growth kinetics of FeSn2 was modeled by parabolic law and empirical power law. Based on the models, the growth constants, the activation energy, and the time exponents were established at different annealing temperatures. It was found that the time exponent values obtained by fitting with empirical power law deviate from 0.5, meaning that volume (bulk) diffusion is not the only rate-controlling process in the liquid Sn/solid Fe reaction couple. Also, a variation in the time exponent values is indicative that the growth behavior is correlated with grain size growth and irregular grain morphology at different annealing stages. The results of this research show that AISI 1018 steel can readily react with Sn to form IMC on the interface. This is an essential requirement of soldering action using Sn-rich solders.

Growth kinetics of Al–Mg intermetallics during post-weld annealing treatment

We explored the growth kinetics of intermetallic compound (IMC) layers at the joint interface of the friction stir lap welded Mg alloy to Al alloy joint during post-weld annealing treatment. The intermetallic layer formed between the Al alloy and Mg alloy was composed of continuous β phase and γ phase; the β phase layer grows faster than the γ phase layer. A quantitative analysis of the IMC layer thickness as a function of aging time and temperature was performed. The diffusion coefficients were calculated from the parabolic relationship between the migration of the interface and the annealing time. The activation energies for IMC growth were determined as 113.5 and 69.3 kJ mol−1 for the γ and β phases, respectively.

Size effect on IMC growth in micro-scale Sn-3.0Ag-0.5Cu-0.1TiO2 solder joints in reflow process

The influence of joint size on the interfacial reaction in Sn-3.0Ag-0.5Cu-0.1TiO2 solder joints was investigated in this study. The thickness and grain size of the intermetallic layer were characterized by scanning electron microscope (SEM). Results show that both the thickness and grain size of the intermetallic compound (IMC) increases with the increase in joint size. The growth exponents for both IMC layers and grains are determined by curve-fitting to study the growth kinetics of IMC in wetting reaction. Results reveal that the growth exponents for IMC layer and grains increase with increase in joint size. The size effect might be due to the difference of Cu concentration distribution near the IMC interface in the molten solder. A two dimensional finite element method was adopted to investigate the kinetics of the Cu diffusion behavior in solder joint during the reflow process. Results reveal that the diffusion of Cu atoms might be influenced by the coarsening of the scallop like grains.

Harsh service environment effects on the microstructure and mechanical properties of Sn–Ag–Cu-1 wt% nano-Al solder alloy

This paper investigates the electrical and mechanical performances of eutectic Sn-3Ag-0.5Cu (wt%) solder with the addition of Al nanoparticles. The study revealed that the elastic moduli, electrical resistivity and damping properties of such solder alloy were improved. Further, interfacial reaction phenomena on Au/Ni-plated Cu pad ball grid array substrate during isothermal aging and thermal cycle was evaluated in terms of the formation and growth kinetics of intermetallic compound (IMC) layer. A structural analysis confirmed that at their interfaces a ternary (Cu, Ni)-Sn IMC layer was adhered at the substrate surface. The thickness of this IMC layer was increased with increasing the duration of the isothermal aging and thermal cycle without any defects. In addition, the formation of Ag3Sn, Cu6Sn5, Sn–Al–Ag and AuSn4 IMC phases were evenly distributed in the solder matrix which acts as the second phase reinforcement. The measured shear strength and microhardness indicated that the exposure of the solder joints to the thermal cycles make the joints degraded faster than the situation in isothermal aging.

Interfacial structure and growth kinetics of intermetallic compounds between Sn-3.5Ag solder and Al substrate during solder process

Interfacial reactions between liquid-state Sn-3.5Ag solders and solid-state Al substrates were investigated in this study. The microstructure and growth kinetics of intermetallic compound (IMC) were studied at temperature from 250 °C to 285 °C for 60 min. A layer of Ag2Al IMC was detected forming at the interface and a mass of grains with deep gaps were also observed in the IMC layers. The calculated results of growth kinetics indicated that the growth evolution involves chemical reaction-limited stage (the time exponent n = 1) and the grain boundary diffusion-controlled stage (n = 0.33). When the temperature was elevated from 250 °C to 285 °C, the critical time of transition for the two stages reduced and the corresponding critical thickness increased. The activation energy Q obtained in the chemical reaction-limited stage and the diffusion-controlled stage is 84.105 ± 2.571 kJ/mol and 26.385 ± 3.171 kJ/mol, respectively.

Au–Sn bonding material for the assembly of power integrated circuit module

Insulated gate bipolar transistor (IGBT) chips are the key components in high-temperature power electronic modules, which have to efficiently convert electricity between direct and alternating current. In this study, the eutectic Au–Sn (20 wt.% Sn) is successfully used to assemble IGBT chips and direct-bond-copper substrates by using solid liquid interdiffusion (SLID) bonding. During subsequent isothermal aging at 150, 200, and 240 °C, the microstructure evolution and growth kinetics of intermetallic compounds are investigated. Excellent thermal stability and mechanical strength are observed. It is concluded that the eutectic Au–Sn solder is ideal to assemble high-temperature IGBT by using the SLID process.

Interface formation and properties of friction spot welded joints of AA5754 and Ti6Al4V alloys

This study aims to understand the influence of dwell time on the microstructure of the interface and lap shear strength of friction spot welded joints of AA5754 and Ti6Al4V alloys. The interface reaction layer consists mainly of TiAl3 intermetallic compound. It was noticed that dwell time significantly influences the diffusion process during the friction spot welding, thereby modifying the thickness of the interface and thus affecting the mechanical performance of the joints. Minimizing or optimizing the brittle TiAl3 phase was demonstrated to be the key issue to achieve high strength Ti/Al dissimilar joints. The intermetallic compound growth kinetics was also examined showing that an incubation time of approximately 2.7s is necessary before it nucleates at the Ti/Al interface and grows laterally to form a continuous layer. Afterwards, the TiAl3 layer grows toward the Al during thickening with a corresponding growth rate of k=2.92×10−7m/s. These results are critical to understand the microstructure–properties relationship and contribute with additional improvements on the joint performance.

Effect of Joint Scale and Processing on the Fracture of Sn-3Ag-0.5Cu Solder Joints: Application to Micro-bumps in 3D Packages

In 3-dimensional (3D) packages, a stack of dies is vertically connected to each other using through-silicon vias and very thin solder micro-bumps. The thinness of the micro-bumps results in joints with a very high volumetric proportion of intermetallic compounds (IMCs), rendering them much more brittle compared to conventional joints. Because of this, the reliability of micro-bumps, and the dependence thereof on the proportion of IMC in the joint, is of substantial concern. In this paper, the growth kinetics of IMCs in thin Sn-3Ag-0.5Cu joints attached to Cu substrates were analyzed, and empirical kinetic laws for the growth of Cu6Sn5 and Cu3Sn in thin joints were obtained. Modified compact mixed mode fracture mechanics samples, with adhesive solder joints between massive Cu substrates, having similar thickness and IMC content as actual micro-bumps, were produced. The effects of IMC proportion and strain rate on fracture toughness and mechanisms were investigated. It was found that the fracture toughness GC decreased with decreasing joint thickness (hJoint). In addition, the fracture toughness decreased with increasing strain rate. Aging also promoted alternation of the crack path between the two joint–substrate interfaces, possibly proffering a mechanism to enhance fracture toughness.

Interfacial compounds growth of SnAgCu(nano La2O3)/Cu solder joints based on experiments and FEM

SnAgCu alloys are promising as the lead-free solders for replacing traditional SnPb solders, 0.1wt.% La2O3 nano-particles are selected as the additives into these solders for further improve the property. The interface reaction and the growth kinetics of intermetallic compounds (IMC) at SnAgCu/Cu and SnAgCu-nano La2O3/Cu interface were investigated during thermal cyclic loading. The growth rate of IMC in SnAgCu-nano La2O3/Cu is lower than that of SnAgCu/Cu, the La2O3 nanoparticles can reduce the diffusion coefficient and activation energies of IMC layers. And the finite element simulation demonstrates that von Mises stress concentrates at the Cu3Sn layer near Cu6Sn5 layer, the crack can be found in the stress concentrated area in the experiment, and SnAgCu-nano La2O3/Cu shows small length of crack comparing with SnAgCu/Cu, which demonstrates the La2O3 nanoparticles can enhance the reliability of SnAgCu/Cu solder joints.

Evaluation of Electroplated Co-P Film as Diffusion Barrier Between In-48Sn Solder and SiC-Dispersed Bi2Te3 Thermoelectric Material

The diffusion barrier property of Co-P film as a buffer layer between SiC-dispersed Bi2Te3 bulk material and In-48Sn solder was investigated. A Co-P film with thickness of ~6 µm was electroplated on SiC-dispersed Bi2Te3 substrate, joined with In-48Sn solder by a reflow process, and annealed at 100°C for up to 625 h. The formation and growth kinetics of intermetallic compounds (IMCs) at the interface between the In-48Sn and substrate were studied using transmission electron microscopy and scanning electron microscopy with energy-dispersive x-ray spectroscopy. The results showed that crystalline Co(In,Sn)3 formed as an irregular layer adjacent to the solder side at the solder/Co-P interface due to diffusion of Co towards the solder, and a small amount of amorphous Co45P13In12Sn30 appeared at the Co-P side because of diffusion of In and Sn into Co-P. The growth of Co(In,Sn)3 and Co45P13In12Sn30 during solid-state aging was slow, being controlled by interfacial reaction and diffusion, respectively. For comparison, In-48Sn/Bi2Te3-SiC joints were prepared and the IMCs in the joints analyzed. Without a diffusion barrier, In penetrated rapidly into the substrate, which led to the formation of amorphous In x Bi y phase in crystalline In4Te3 matrix. These IMCs grew quickly with prolongation of the annealing time, and their growth was governed by volume diffusion of elements. The experimental data demonstrate that electroplated Co-P film is an effective diffusion barrier for use in Bi2Te3-based thermoelectric modules.

Effect of thermomigration on the growth kinetics of Cu6Sn5 at liquid-solid interfaces in Cu/Sn/Cu solder joints

With the continuous miniaturization of electronic packaging, micro bumps for chip interconnects are smaller in size, and thus the reliability of interconnects becomes more and more sensitive to the formation and growth of intermetallic compounds (IMCs) at liquid-solid interface during soldering. Thermomigration (TM) is one of the simultaneous heat and mass transfer phenomena, and occurs in a mixture under certain external temperature gradient. In the process of interconnection, micro bumps usually undergo multiple reflows during which nonuniform temperature distribution may occur, resulting in TM of metal atoms. Since the interdiffusion of atoms between solders and under bump metallization (UBM) dominates the formation of interfacial IMCs, TM which enhances the directional diffusion of metal atoms and induces the redistribution of elements, will markedly influence the growth behaviors of interfacial IMCs and consequently the reliability of solder joints. The diffusivity of atoms in liquid solder is significantly larger than that in solid solder and in consequence a small temperature gradient may induce the mass migration of atoms. As a result, the growth of interfacial IMCs becomes more sensitive to temperature difference between solder joints in soldering process. So far, however, few studies have focused on liquid state TM in solder joints, and the growth kinetics of interfacial IMCs under TM during soldering is still unknown to us. In this study, Cu/Sn/Cu solder joints are used to investigate the migration behavior of Cu atoms and its effect on the growth kinetics of interfacial Cu6Sn5 under temperature gradients of 35.33℃/cm at 250℃ and 40.0℃/cm at 280℃, respectively. TM experiments are carried out by reflowing the Cu/Sn/Cu interconnects on a hot plate at 250℃ and 280℃ for different durations. For comparison, isothermal aging experiments are conducted in a high temperature chamber under the same temperatures and reaction durations. During isothermal aging, the growth of interfacial Cu6Sn5 follows a parabolic law and is controlled by bulk diffusion. Under the temperature gradient, asymmetrical growth of interfacial Cu6Sn5 is observed between cold and hot ends. At the cold end, the growth of the interfacial Cu6Sn5 is significantly enhanced and follows a linear law, indicating a reaction-controlled growth mechanism; while at the hot end, the growth of the interfacial Cu6Sn5 is inhibited and follows a parabolic law, indicating a diffusion-controlled growth mechanism. The dissolved Cu atoms from the Cu substrate at the hot end are driven to migration toward the cold end by temperature gradient, providing the Cu atomic flux for the fast growth of the interfacial Cu6Sn5 at the cold end. With the variation of the measured thickness of Cu6Sn5 IMC at the cold end and the simulated temperature gradients, the molar heat of transport Q^* of Cu atoms in molten Sn is calculated to +14.11 kJ/mol at 250℃ and +14.44 kJ/mol at 280℃. Accordingly, the driving forces of thermomigration in molten solder FL are estimated to be 1.62×10-19 N and 1.70×10-19 N, respectively.

Effects of PCB ENIG and OSP Surface Finishes on the Electromigration Reliability and Shear Strength of Sn-3.5Ag PB-Free Solder Bump

The effects of printed circuit board electroless nickel immersion gold (ENIG) and organic solderability preservative (OSP) surface finishes on the electromigration reliability and shear strength of Sn-3.5Ag Pb-free solder bump were systematically investigated. In-situ annealing tests were performed in a scanning electron microscope chamber at 130, 150, and <TEX>$170^{\circ}C$</TEX> in order to investigate the growth kinetics of intermetallic compound (IMC). Electromigration lifetime and failure modes were investigated at <TEX>$150^{\circ}C$</TEX> and <TEX>$1.5{\times}10^5A/cm^2$</TEX>, while ball shear tests and failure mode analysis were conducted under the high-speed conditions from 10 mm/s to 3000 mm/s. The activation energy of ENIG and OSP surface finishes during annealing were evaluated as 0.84 eV and 0.94 eV, respectively. The solder bumps with ENIG surface finish showed longer electromigration lifetime than OSP surface finish. Shear strengths between ENIG and OSP were similar, and the shear energies decreased with increasing shear speed. Failure analysis showed that electrical and mechanical reliabilities were very closely related to the interfacial IMC stabilities.

Growth kinetics of Ag3Sn in silicon solar cells with a sintered Ag metallization layer

The interfacial reactions and growth kinetics of intermetallic compounds at the solder/sintered Ag metallization interface in silicon solar cells were investigated during solid-state aging. Three common solders (Sn37Pb, Sn36Pb2Ag, and Sn3Ag0.5Cu) were used to conduct several experiments testing the quality of the solder joints in this study. Accelerated tests were performed to determine the long-term reliability of the solder joints. The storage temperatures were 100, 120, 150, and 180±1°C for various aging durations. The results showed that the sintered Ag electrode gradually transformed into an Ag3Sn intermetallic compound (IMC), and the growth kinetics of Ag3Sn remained a diffusion-controlled process during isothermal aging. The formation of Ag3Sn increased the series resistances (RS) of the cell modules because Ag3Sn had a higher electrical resistivity than sintered Ag did. In addition, the results revealed that Sn3Ag0.5Cu PV ribbon effectively slowed down the growth kinetics of Ag3Sn. The apparent activation energies for Ag3Sn at Sn37Pb/sintered Ag, Sn36Pb2Ag/sintered Ag, and Sn3Ag0.5Cu/sintered Ag interfaces were calculated to be 104, 108, and 121KJ/mole, respectively. These kinetics data were established for the first time for the solder alloys commonly used in PV module assembly industry.