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Growth behavior of intermetallic layers at the interface between Cu and eutectic Sn–Bi by grain boundary diffusion with the grain growth at solid-state temperatures
... Figure 11b illustrates the relationship between the time exponent and the control process for the growth rate of the interfacial IMCs layer. According to the demonstration by Minho O et al. [32] on controlling the solid-state diffusion rate between Sn-Bi and Cu, the time exponent n predominantly reflects the interfacial growth behavior of the IMCs layer. When n = 1.0, the growth of the interfacial IMCs layer is controlled by interfacial reaction; when n = 0.5, the growth is governed by volume diffusion. ...
The demand for high-computing-power AI chips in the consumer electronics market is driving the development of electronic packaging technology toward high-reliability packaging. The application of low melting point solder Sn58Bi in high-reliability 3D packages is currently facing issues of low toughness and poor in-service reliability. A new strategy differentiating the alloying of conventional solders has been proposed, which produced solder joints with improved microstructure and properties through electric pulse auxiliary reflow soldering of Sn58Bi/Cu solder joints. The electric pulse resulted in Bi-rich phases and different morphologies of pre-eutectic Sn in the solder joints via the incubation effect, improving the shear strength of the solder joints. This research provides a new strategy for improving the strength and reliability of Sn58Bi solder joints in advanced electronic interconnection.
... Therefore, the growth behavior of IMCs must be deeply investigated using experimental and numerical methods to understand their kinetics and mechanisms. Recently, there has been a comprehensive investigation into the growth behavior of interfacial IMC layers in lead-free solder joints [24][25][26][27][28][29]. As well known, solid-state diffusion of solder can lead to the formation of a continuous IMC layer at the interface. ...
Zn–30Sn–2Cu–0.5Ni–0.2Al (ZnSnCuNiAl) solder joints were isothermally aged in the temperature range of 100–170 °C for up to 360 h, and the growth behavior and kinetics of the intermetallic compound (IMC) formed between the ZnSnCuNiAl solder and Cu substrate during the aging process were investigated. The results showed that the interfacial reaction between the liquid solder and the solid Cu substrate caused the formation of IMC layers, which were composed of a thin irregular scallop-shaped ε-CuZn5 layer and a thick γ-Cu5Zn8 layer. With the increase in aging temperature and time, the serrated ε-CuZn5 layer gradually peeled off from the IMC interface and entered the solder due to thermal mismatch induced cracks between ε-CuZn5 and γ-Cu5Zn8. The growth of the IMC layer at the solder joint interface was controlled by the bulk diffusion, and its thickness had a linear relationship with the square root of the aging time. Diffusion coefficients and activation energy for the interfacial Cu5Zn8 IMC layer were plotted as a function of aging temperature, and the values were calculated by the Arrhenius equation. The calculated activation energy of the Cu5Zn8 IMC layer was 77.78 kJ/mol, which was higher than that of the similar IMC layer in other lead-free solder joints, indicating a slower growth rate of the IMC layer between the solder and substrate during aging.
... The values of k and n were determined using the least-squares method from the symbols at each annealing temperature, as depicted by the straight lines in Fig. 6(a). The value of n means the gradient, and log 10 k indicates the intercept of the straight line in this figure [38,39]. Based on the evaluation, the value of n for the growth of the DIR region is 0.59. ...
... This improvement effectively suppresses the formation of interfacial intermetallic compounds, particularly Cu 3 Sn. Minho et al. [9] conducted a study on the growth behavior of IMCs at the Cu/(Sn-Bi) interface. The study involved simulating various conditions found in electronic devices, such as repeated reflow bonding and isothermal heat treatment. ...
With the continuous downsizing of solder joints, the influence of the grain characteristics of the under bump metallization (UBMs) on the interfacial reaction is exposed. In this paper, the effect of grain characteristics of Cu UBM on the growth of interfacial intermetallic compounds and the formation of Kirkendall voids in Cu/Sn/Cu micro solder joints during isothermal aging at 150 °C was investigated. Electroplated polycrystalline Cu (EP-Cu), (111) single crystal Cu ((111) Cu), and (111) nano-twinned Cu ((111) nt-Cu) were selected as UBMs. After reflow soldering, the Cu6Sn5 grains formed on (111) nt-Cu were near to the orientation (21-1-3), while those were randomly formed on both EP-Cu and (111) Cu. During isothermal aging, the Cu6Sn5 grains exhibited the highest growth rate on (111) nt-Cu UBM, and the slowest growth rate on (111) Cu UBM, due to the differing number of diffusion routes among the three Cu UBMs with distinct grain characteristics. Furthermore, no Kirkendall voids were detected on the (111) Cu UBM, however, a significant number of Kirkendall voids were identified on the (111) nt-Cu UBM, which can be attributed to the diffusion path and impurities present in the three Cu UBMs. The results will offer theoretical and practical recommendations for the selection of Cu UBMs for micro solder joints in 3D packaging interconnection technology.
... The morphology of the intermetallic compound layer and the effects of reactive diffusion near the interface were observed in more detail with high magnification. Furthermore, the chemical compositions and element distribution map (EDM) were thoroughly analyzed using energy-dispersive X-ray spectroscopy (EDS, EX-37001) with an accelerating voltage of 15 kV to identify the composition by quantitative analysis [33,34]. Transmission electron microscopy (TEM) specimens were obtained from the intermetallic compounds at the interface and analyzed using a JEM-2100 F microscope operating at 200 kV. ...
... Cross-sections of the annealed diffusion couples were meticulously prepared by mechanical polishing using #1500-4000 emery papers and alumina particles measuring 0.05 µm in diameter. Final surface finishing was accomplished using colloidal silica [22,23]. The microstructure of these cross-sections was examined using a differential interference contrast optical microscope (DICOM). ...
This study focuses on the practical relevance of the Al-Ag bonding interface in electronic device fabrication, particularly in wire bonding, which is crucial for enhancing component reliability and performance. Experiments involved Al/Ag/Al diffusion couples, annealed at 703 K, revealing two stable intermediate phases, μ and δ. Characterizing the intermediate phases’ compositions and concentration profiles exposed a vital transition at the δ-Al interface. We used high-voltage electron microscopy (HVEM) to examine crystal structure evolution, identifying a (hexagonal close-packed) hcp structure in the intermediate phase between δ and Al, matching the δ phase. Notably, a substantial microstructural transformation occurred within the Ag-Al diffusion couple, as nano-sized precipitates transitioned from spherical to plate-like, along specific {111} planes, reflecting the evolution from off-stoichiometric, disordered phases to ordered ones. Mapping the concentrations of intermediate phases on the Al-Ag phase diagram revealed shifted and narrower solubility ranges compared to the calculations. This study provides insight into the crystal structure and microstructure changes during diffusion in Al/Ag/Al diffusion couples, holding implications for electronic device fabrication. Understanding intermediate phase behavior and evolution is vital in this context, potentially influencing materials development and process optimization in the electronic components industry, and thus, enhancing device performance and reliability.
This review paper compiles and provides a review of the latest and relevant research on the nanoparticle-reinforced low-temperature SnBi solder. The physical, electromigration and thermomigration, microstructural, interfacial and mechanical properties of a solder are vital to the reliability of the electronic components. Generally, reinforcement of nanoparticles is beneficial to the properties of low-temperature SnBi solder. Literature on the types of nanoparticles added to SnBi focusing on these properties is available but is not outlined in a competent review. This paper provides a significant review on this topic to provide insight into SnBi solder alloy as an alternative solder in the electronic packaging industry.
This study investigates composites and intermetallic, examining their microstructure, characteristics, and applications. The research demonstrates the complicated link between these materials’ fundamental features and their possible practical applications by analysing mechanical strength, thermal conductivity, microstructure, and reinforcement percent. The study discovers a positive relationship between mechanical strength and thermal conductivity, implying the possibility of capitalising on increased strength for enhanced heat transfer. Furthermore, the impact of matrix phases on mechanical properties emphasises the strategic significance of matrix selection. The impact of reinforcement fraction in fine-tuning characteristics highlights the importance of microstructure as a silent architect affecting material behaviour. In conclusion, this study provides insights into the intricate interaction of material characteristics, opening the way for informed material design and application engineering.
To examine experimentally the kinetics of the reactive diffusion between solid-Cu and solid-Al, sandwich Al/Cu/Al diffusion couples were prepared by a diffusion-bonding technique and then isothermally annealed in the temperature range of T = 693–753 K for various times up to 336 h. Owing to annealing, compound layers of the γ 1, δ, ζ 2, η 2, and θ phases are formed between the Cu and Al specimens. The γ 1, δ, ζ 2, η 2, and θ phases are the only stable compounds at T = 693–753 K in the binary Cu–Al system. At each annealing time, the thickness of the θ phase is much greater than those of the δ, ζ 2, and η 2 phases but smaller than that of the γ 1 phase. Hence, the overall growth of the compound layers is governed by the γ 1 and θ phases. The mean thickness of each compound layer is proportional to a power function of the annealing time. For the γ 1 phase, the exponent m of the power function is 0.5 at T = 753 K. Such a relationship is called a parabolic relationship. As the annealing temperature T decreases, however, m gradually increases and then reaches to 0.66 at T = 693 K. On the other hand, for the θ phase, m is close to 0.5 at T = 723–753 K and becomes 0.42 at T = 693 K. In the γ 1 and θ phases, grain growth occurs at T = 693–753 K. Thus, the layer growth of the θ phase is controlled by volume diffusion at T = 723–753 K but partially by boundary diffusion at T = 693 K. On the other hand, for the γ 1 phase, volume diffusion is the rate-controlling process of the layer growth at T = 753 K, but interface reaction contributes to the rate-controlling process at T = 693–723 K. Consequently, the rate-controlling process varies depending on the annealing temperature in a different manner for each compound.
Sn-58Bi, an eutectic alloy, has been explored for use as a low-temperature lead-free solder alloy. However, the properties of Sn-Bi alloys as well as those of their joints need to be improved significantly so that these alloys can be applicable for practical use. In particular, two drawbacks need to be addressed: the intrinsic brittleness of Bi and the microstructure coarsening of these alloys during aging. In this study, Sn-Bi-Zn (SBZ) and SBZ-In (SBZI) alloys with low Bi contents were examined to elucidate the effects of the addition of Zn and In to the Sn-45Bi alloy on the interface and shear strengths of Cu/Cu joints before and after aging. In the case of the SBZ/Cu and SBZI/Cu joints, Bi coarsening was not observed either near or at the interfaces of the Cu/Cu joints. The shear strengths of the SBZ and SBZI joints remained unchanged after aging for 1008 h, suggesting that the SBZI alloy demonstrated the highest long-term reliability among all the joints examined.
The evolution of the microstructure due to spinodal decomposition in phase separated mixtures has a strong impact on the final material properties. In the late stage of coarsening, the system is characterized by the growth of a single characteristic length scale L ∼ Ctα. To understand the structure-property relationship, the knowledge of the coarsening exponent α and the coarsening rate constant C is mandatory. Since the existing literature is not entirely consistent, we perform phase field simulations based on the Cahn-Hilliard equation. We restrict ourselves to binary mixtures using a symmetric Flory-Huggins free energy and a constant composition-independent mobility term and show that the coarsening for off-critical mixtures is slower than the expected t1/3-growth. Instead, we find α to be dependent on the mixture composition and associate this with the observed morphologies. Finally, we propose a model to describe the complete coarsening kinetics including the rate constant C.
In the big data era, Si chips are integrated more and more to satisfy the fast growing demand from customers. In addition, in the post-COVID-19 virus era, the trend of distance teaching and home office has increased greatly the need of advanced consumer electronic products. To provide more processing tolerance and to build a wider temperature window in manufacturing, it becomes necessary to develop low melting temperature solders because a hierarchy of solders is needed in the packaging technology. Much research has been on Pb-free and Sn-based solders with a melting point from 180 to 230 °C. In this review, we will concentrate on low melting point solder alloys with a melting point lower than 180 °C and even below 100 °C. We review eutectic SnBi, eutectic SnIn, and the alloys with trace addition of a third element to them. Eutectic Sn-Bi solder is too brittle, and Sn-In solder is too soft. However, third element addition can only improve the solder properties partially; thus, we will further review the properties of Sn-Bi-In ternary alloys and the effect on the addition of a fourth element to them. This approach, when we include the Cu substrate, becomes a 5-element system, which leads us to consider high entropy alloys for soldering. With these Sn-, Bi-, and In-based alloys, we hope to develop new ways to design industrial applicable low melting point solders.
Grain boundary structure-property relationship was studied in a Ni-base 602CA coarse-grained alloy using a novel correlative tracer diffusion-analytical microscopy approach. Co-existence of several short-circuit contributions to tracer diffusion was distinguished at higher temperatures. These contributions were related to different families of high-angle grain boundaries with distinct coverages by precipitates and segregation levels as revealed by HAADF-STEM combined with EDX measurements and a detailed atom probe tomography analysis. Annealing at such conditions resulted in Cr23C6-type carbides co-existing with an α-Cr-Mn-enriched phase in addition to sequential segregation layers of Al, Ni, and Fe around them. Curved and hackly grain boundaries showed a high density of plate-like carbides. In contrast, straight grain boundaries were composed of globular carbides with similar chemical composition variations and additionally with alternating layers of Cr and Ni in-between the carbides, similar to spinoidal microstructures. At lower temperatures, hackly interfaces with Cr and Cr-carbide enrichment dominated and the alloy annealed at 403 K contained plate-like Cr23C6-type carbides surrounded by a Ni-rich layer around them. The Ni grain boundary diffusion rates at these relatively low temperatures showed an anomalous character being almost temperature independent. This specific diffusion behaviour corresponds to a metastable grain boundary transition which is explained by a concomitant relaxation of transformation-induced elastic strains occurring on a longer time scale for grain boundary diffusion. Thermodynamic insights into the probable mechanism of decomposition at grain boundaries are provided. The paper provides solid evidences towards the existence of grain boundary phase transitions in the Ni-base multi-component alloy. It underlines the capabilities of the correlated kinetic-structure measurements as a tool to probe such changes.
In total, 0.5 wt% CuZnAl memory particles with the diameters (2–6 μm) were added into Sn–58Bi solder in order to enhance the properties of solder joints in electronic packaging. The interface reaction and the growth kinetics of intermetallic compounds at Sn–58Bi/Cu interface and Sn–58Bi–0.5CuZnAl/Cu interface were studied systematically at 250 °C, 230 °C and 210 °C, the results indicate that the growth rate of intermetallic compounds (IMC) at both interfaces at 250 °C was higher than that at 230 °C, the addition of CuZnAl memory particles can reduce the diffusion coefficient of interfacial IMC, and however, at 210 °C the inhibition effect of CuZnAl particles was so small. Based on the transient liquid phase bonding, it is demonstrated that Cu/Sn–58Bi–0.5CuZnAl/Cu-bonded joints can achieve the chip stacking in 3D packaging with 10 μm thickness with obvious superiority.
Diffusion induced recrystallization (DIR) occurs in Cu of Cu-clad Al (CA) wire during heating at temperatures around 200°C. The kinetics of DIR in the CA wire was experimentally observed in the temperature range of 210–270°C. In this temperature range, CA wires were isothermally annealed for various periods of 3 h to 960 h. Owing to annealing, layers of the α2, γ1, δ, η2 and θ phases form at the original Cu/Al interface, and the DIR region alloyed with Al is produced in the Cu phase from the Cu/α2 interface. The thickness l of the DIR region monotonically increases with increasing annealing time t. The DIR region grows faster at higher annealing temperatures. The new extended-model proposed in a previous study was used to analyze theoretically the experimental result. According to the analysis, l is proportional to t in the early stages but to a power function of t in the late stages. This means that the DIR growth is governed by the interface reaction at the moving boundary in the early stages but by the boundary diffusion across the DIR region in the late stages. Although the shortest annealing time of 3 h at 240–270°C is located in the early stages, most of the annealing times belong to the intermediate stages. Consequently, under the present experimental conditions, both the interface reaction and the boundary diffusion mainly contribute to the rate controlling process of DIR. On the other hand, the interface reaction is the rate controlling process for the shortest annealing time.
Fig. 3 The mean thickness l of the DIR region versus the annealing time t shown as open rhombuses, squares and circles for T = 483, 513 and 543 K (210, 240 and 270°C), respectively. Fullsize Image
The rate-controlling process of compound growth in the Cu-clad Al (CA) wire was metallographically examined in the temperature range of 483–543 K (210–270°C). CA wires were prepared by a wire drawing technique, and then isothermally annealed for various times up to 3.456 Ms (960 h) in this temperature range. During isothermal annealing, the α2, γ1, δ, η2 and θ phases form as layers at the original Cu/Al interface in the CA wire. However, at 483–513 K (210–240°C), the α2, γ1, δ and η2 phases could not be differentiated from one another in a metallographical manner. The layer of the θ phase is designated layer 1, and that of the α2, γ1, δ and η2 phases is called layer 2. The mean thickness of each layer increases in proportion to a power function of the annealing time. Such a relationship is called a power relationship. The exponent of the power relationship takes values between 0.22 and 0.36 for layer 1 and those between 0.39 and 0.50 for layer 2. The exponent smaller than 0.5 indicates that boundary diffusion as well as volume diffusion contributes to the layer growth. However, the contribution of boundary diffusion is more remarkable for layer 1 than for layer 2. The activation enthalpy for the proportionality coefficient of the power relationship was estimated to be 43 and 73 kJ/mol for layers 1 and 2, respectively. The former value is smaller than the latter value. This is attributed to the contribution of boundary diffusion being greater for layer 1 than for layer 2.
Fig. 3 The total thickness l of the intermetallic layer versus the annealing time t shown as open rhombuses, squares and circles for T = 483, 513 and 543 K (210, 240 and 270°C), respectively. The values of k and n in eq. (2) were estimated by the least-squares method. Fullsize Image
To better understand the annealing mechanisms that occur in Cu-clad Al (CA) wire, the solid-state reactive diffusion between Cu and Al was experimentally examined by a metallographical technique. The hard CA (HCA) wire was prepared by drawing to decrease diameter from 10 mm to 1.5 mm, and then annealed at 250°C (523 K) for 3 h (10.8 ks). The annealed HCA wire is merely called the ACA wire. The HCA and ACA wires were isothermally annealed at temperatures of 150–270°C (423–543 K) for various periods of 12–960 h (43.2 ks to 3.46 Ms). Due to isothermal annealing, the intermetallic layer composed of the θ, η2, δ, γ1 and α2 phases is produced at the original Cu/Al interface in both the HCA and ACA wires. The total thickness of the intermetallic layer is proportional to a power function of the annealing time. The exponent of the power function is 0.23–0.44 for the HCA wire and 0.33–0.53 for the ACA wire. Thus, boundary diffusion as well as volume diffusion contributes to the layer growth. Furthermore, the overall growth rate of the intermetallic layer is slightly greater for the HCA wire than for the ACA wire. Since the exponent is smaller for the HCA wire than for the ACA wire, the contribution of boundary diffusion is greater for the former than for the latter. The greater contribution of boundary diffusion may be the reason why the overall layer growth takes place faster in the HCA wire than in the ACA wire.
Fig. 5 The total thickness l of the IMC layer versus the annealing time t shown as cross, open rhombuses, triangles, squares and circles for T = 423, 453, 483, 513 and 543 K (150, 180, 210, 240 and 270°C), respectively: (a) HCA and (b) ACA. Fullsize Image
The current work investigates the effect of Fe and Bi addition, 0.05 wt% Fe and 1 or 2 wt% Bi, on the mechanical and microstructural properties of the Sn-1Ag-0.5Cu (SAC105) solder alloy after aging at 125 °C for 30 days. Microstructural and mechanical results revealed stabilized properties of Sn-1Ag-0.5Cu-0.05Fe-xBi (x = 1 or 2) after aging treatment. UTS (ultimate tensile strength) and yield strength of SAC105 decreased by aging due to coarsening of intermetallic compounds (IMCs). However, UTS, Yield strength, and total elongation of SAC105-Fe-Bi remained approximately constant by aging treatment. These are due to Bi within the β-Sn matrix reducing the activity of Sn involved in the chemical reaction between Sn and Cu or Sn and Ag, hence reduces the amount of Cu6Sn5 and Ag3Sn IMC particles. Furthermore, partial of Fe atoms solute in the Sn matrix in the eutectic regions reduces the diffusivities of Ag, which is the consequence of the Ostwald ripening process. Therefore, Fe reduces the supply of Ag required for the Ag3Sn coarsening. Moreover, partial substitution of Fe in Cu6Sn5 due to Darken-Gurry ellipse decreased microstructure coarsening rate. The nanoindentation results showed a remarkable increase in Er (reduced elastic modulus) and hardness of β-Sn and IMCs which can be attributed to the solid solution effect and distortion of the lattice parameter of β-Sn by Bi and substitution of Fe with Cu in Cu6Sn5.
To examine the kinetics of the reactive diffusion in the solid-Ni/liquid-Zn system, an isothermal bonding technique was used to prepare semi-infinite Ni/Zn diffusion couples. The diffusion couples were isothermally annealed in the temperature range of T = 773923 K for various periods up to t = 10.8 ks (3 h). In this temperature range, the ¢1 and £ phases are the stable intermetallic compounds in the binary NiZn system. During isothermal annealing, however, only the £ phase forms as a visible compound layer at the original Ni/Zn interface in the diffusion couple. The £ layer grows mainly towards the liquid-Zn and slightly into the solid-Ni. The mean thickness l of the £ layer is described as a power function of the annealing time t. According to the observation, the exponent of the power function is rather close to 0.5 independently of T. Thus, we may consider that the square of l is proportional to t at T = 773923 K: l² = Kt. This relationship is called a parabolic relationship. The temperature dependence of K was evaluated from the experimental results. The evaluation provides the activation enthalpy of 40 kJ/mol for the layer growth.
New types of lead-free solders on the base of tin with addition of metals Ag, Al, Bi, Cu, In, Mg, Sb, Zn were studied. Various alloys were experimentally prepared by smelting of metals in an inert atmosphere or in vacuum in resistance furnace. Purity of metals was minimally 4N. The specimens of solders were studied metallographically including the micro-hardness measurements, complete chemical analysis (ICP-AES, OES), X-ray micro-analysis of alloys by SEM and EDX in order to determine the composition and identification of individual phases. Significant temperatures and enthalpies of phase transformations were determined by DTA. After long-term annealing of selected alloys in vacuum followed by quenching the structural and chemical microanalyses of the present phases and their limit concentrations were carried out. The achieved results were compared with the thermodynamic modelling of the ternary systems (ThermoCalc, Pandat, MTDATA and databases CALPHAD, COST 531 and COST MP0602). Electrical resistivity, density, magnetic susceptibility and wettability of solders were measured as well.
In recent years there has been increased interest in Bi-containing solders to improve the reliability of electronics and drive down processing temperatures. When the Bi content is more than ~ 2 wt% in Sn–Ag–Cu–Bi and Sn–Cu–Ni–Bi solders, (Bi) phase forms by a non-equilibrium eutectic reaction and also precipitates in the solid state. Here, we study the development of bismuth plates during room temperature storage in a range of Bi-containing solders. It is shown that Bi plates precipitate with one of two (Bi)–βSn orientation relationships (ORs). During coarsening the OR with the better lattice match grows at the expense of the other, leading to a single OR. The plate coarsening kinetics scale with the cube root of holding time and are relatively rapid, with plates exceeding 1 µm in length within 1 day at room temperature. Prolonged room temperature storage of more than 1 year resulted in Bi accumulation in 5–20 µm (Bi) particles at βSn grain boundaries.
To obtain the interfacial behaviors and the joint strength of Sn–Bi solid solution solder, Sn–2.5Bi and Sn–5Bi solders were selected to be investigated in this work with pure Sn/Cu solder joints as a reference. The results showed that the growth behaviors of intermetallic compound (IMC) at Sn–Bi solder joints were different under reflow soldering and isothermal aging, which was related with the presence of Bi atoms and the diffusion rate of Cu atoms. Furthermore, the Cu6Sn5 particles in solder matrix and the Bi precipitation could change the diffusion path of Cu atoms, which caused the change on the distribution and size of Cu6Sn5 in pure Sn, Sn–2.5Bi and Sn–5Bi solder matrix after isothermal aging. The size and distribution of Cu6Sn5 in solder matrix finally affected the shear strength of solder joints. The shear strength of Sn–2.5Bi/Cu solder joints increased compared with pure Sn/Cu solder joints, and there was no obvious change with the aging times for Sn–2.5Bi/Cu solder joints due to the effect of the reinforcement of Bi atoms. However, the shear strength of Sn–5Bi/Cu solder joints increased firstly and then decreased with the prolongation of aging times. This phenomenon was attributed to the segregation of Bi and the large amounts of Cu6Sn5 particles distributed in solder matrix during the aging condition. Besides, the IMC thickness in Sn–5Bi/Cu solder joint played the important role on the shear strength due to the brittle nature of IMC. In pure Sn solder matrix, the size of Cu6Sn5 particles were larger than that in Sn–Bi solder matrix, which deteriorated the shear strength on pure Sn solder. From the observation on the fracture morphologies, Sn/Cu and Sn–2.5Bi/Cu solder joints showed the ductile fracture occurred in the solder matrix, while Sn–5Bi/Cu solder joints exhibited a mixed fracture.
Fluxless bonding of plateless Cu–Cu substrates at processing temperature lower than 250 °C and low pressure of 0.1 MPa was achieved by transient liquid phase sintering (TLPS) of mixed Cu nanoparticles and Sn–Bi eutectic powders. The effects of mixture composition, and sintering temperature on the shear strength, microstructure, and remelting temperature were investigated. Lowering the sintering temperature of Cu mixed with 65 weight percentage of Sn–Bi (Cu–65SnBi) resulted in decreased shear strength, however, at 200 °C sintering temperature, the obtained highest shear strength was more than 20 MPa. It was found that it is essential to use Cu nanoparticles to accelerate the consumption so that no initial Sn–Bi phases remained after processing. The liquid phase generated at approximately 196 °C during sintering from the reaction between newly formed Cu6Sn5 and Bi-phase was expected to facilitate the densification and strengthening of the joints. Although this newly generated liquid phase was known to solidify as hypereutectic Sn–Bi, by controlling the sintering temperature at 200 °C, the remelting event at 139 °C was not observed by differential scanning calorimetry. It is assumed that the proportion of solidified Sn–Bi eutectic phases in Cu–65SnBi that was sintered at 200 °C were significantly small, hence, when reheated at 150 °C, the obtained shear strength was equivalent to that at room temperature.
The kinetics of reactive diffusion in the Co/Zn system was experimentally examined at solid-state temperatures. In this experiment, sandwich Zn/Co/Zn diffusion couples were prepared by a diffusion bonding technique, and then isothermally annealed in the temperature range of 523–573 K for various times up to 211 h. Owing to annealing, an intermetallic layer consisting of the γ, γ1 and γ2 phases was formed at the original interface in the diffusion couple, where the thickness is much smaller for the γ and γ1 phases than for the γ2 phase. Thus, the γ2 phase predominantly governs the overall growth of the intermetallic layer. The total thickness of the intermetallic layer increases in proportion to a power function of the annealing time. The exponent of the power function takes values of 0.54–0.56 at 523–548 K and that of 0.85 at 573 K. Consequently, volume diffusion mainly controls the layer growth at 523–548 K, but interface reaction as well as volume diffusion contributes to the rate-controlling process at 573 K.
Fig. 3 The total thickness l of the intermetallic layer versus the annealing time t at T = 523, 548 and 573 K shown as open rhombuses, squares and circles, respectively. Fullsize Image
The aim of this study was to apply the transient liquid phase (TLP) bonding technique to low-temperature Sn–Bi-based solders to enable their use in high-temperature applications. The microstructure of the eutectic Sn–Bi solders with and without added Cu particles was investigated with the solders sandwiched between two Cu substrates. The flux of the Cu atoms successfully consumed the Sn phase and resulted in the formation of Sn–Cu intermetallic compounds and a Bi-rich phase in the solder joint. This caused the melting point of the solder joint to increase from 139 to 201 °C. The results of this study show the potential of using low-temperature solders in high-temperature applications. This study also provides new insight into the advantages of using particles in the TLP bonding process.
The thermodynamic compensation law describing an empirical linear relationship between activation enthalpy and activation entropy has seldom been validated for amorphous solids. Here molecular dynamics simulations reveal a well-defined enthalpy-entropy compensation rule in a metallic glass (MG) over a wide temperature and stress range, spanning the glass transition induced by temperature and/or stress. Experiments on other MGs reproduce this law, suggesting that it applies universally to amorphous solids, so we extend it from crystals to amorphous solids. In the glassy state, the compensation temperature is found to agree with the thermal glass transition temperature Tg; whereas in the supercooled liquid region, the compensation temperature matches ∼ 1.4Tg, at which the diffusion kinetics start to feel the roughness of the free-energy surface.
Electroplated and light-induced plated Sn-Bi alloys for interconnection of silicon solar cells are reported. The eutectic 42Sn-58Bi alloys, formed on the interconnection wire by electroplating and on the front metal grid of solar cells by bias-assisted light-induced plating, can melt during a typical lamination process and therefore potentially enable electrical connection between adjacent cells in a solar module without the need to use soldering. Plated alloy composition was analyzed by inductively-coupled plasma optical emission spectrometry and energy dispersive spectrometry, and the morphology of the plated alloy was examined using scanning electron microscopy. Although a eutectic alloy could be electroplated readily using a previously-reported bath composition, light-induced plating of the alloy required a bath with reduced bismuth ion concentration to ensure a eutectic alloy of uniform morphology across the cell surface could be formed at a plating current density at which the solar cell remained forward biased.
The Cu/Sn system is one of the simplest metallurgical options for three-dimensional (3D) microbumps. Even at room temperature, however, intermetallic compounds of Cu3Sn and Cu6Sn5 are formed at the interconnection between Cu and Sn, and voids are produced inside the microbump. The formation of compounds and voids deteriorates mechanical and electrical properties of the microbump and thus causes potential reliability issues. Among various root causes of voids in microbumps, the void formation due to Kirkendall effect was examined in the present study. The Kirkendall effect provides the lower limit of the void formation in the Cu/Sn microbump. In order to develop a criterion for the maximum vacancy concentration in the Cu/Sn system, the growth of intermetallic compounds and the formation of Kirkendall vacancy in the binary C-Sn system were studied by simulation using an analytical diffusion model and experimental results under an assumption of atomic exchange mechanism for diffusion. The fraction of Kirkendall vacancy was calculated and then plotted against the distance representing the Cu/Cu3Sn, Cu3Sn/Cu6Sn5 and Cu6Sn5/Sn interfaces in semi-infinite diffusion couples. Among these three interfaces, a maximum vacancy fraction of about 0.0125 was realised at the location close to the initial Cu/Cu3Sn interface at an annealing temperature of T = 473 K for an annealing time oft = 1 h. The penetration depth of vacancy is much greater on the Cu3Sn side than on the Cu side. This implies that Kirkendall voids may be predominantly formed on the Cu3Sn side of the Cu/Cu3Sn interface. To confirm validity of the simulation, the growth behaviour of intermetallic compounds and the formation of Kirkendall voids were experimentally observed using Cu/Sn diffusion couples prepared by an electroplating technique. The fraction of Kirkendall void in the diffusion couple annealed at T=473 K for t=1 h was measured by binary large objects (Blob) analysis. According to the observation, a row of Kirkendall voids is formed in Cu3Sn along the direction parallel to the Cu/Cu3Sn interface, where the measured value of void fraction is 0.0112. If most of Kirkendall vacancies are used to form Kirkendall voids, the void fraction is almost equal to the vacancy fraction. Thus, the simulation satisfactorily reproduces the experiment. The growth behaviour of the intermetallic compounds in the present Cu/Sn diffusion couple well coincides with that observed for a semi-infinite Cu/Sn diffusion couples in a previous study.
Interaction of lead-free solders with copper substrate represents an important phenomenon in the issue of reliability of solder joints. New experimental data describing phase equilibria in the Cu-In-Sn system after long-time diffusion annealing at the 400°C/50 hours, 600 °C/310 hours and 600°C/48 hours will be presented. The composition of solders was: 100% Sn, 75 % Sn + 25 % In, 50 % Sn + 50 % In, 25 % Sn + 75 % In, 100 % In. The fast quenching method was employed to freeze thermodynamic equilibrium after annealing, followed by metallography, microhardness measurements, SEM (Scanning Electron Microscope) and WDX (Wave Dispersive X-ray) analysis. New phase equilibrium data, together with the data from literature, represent the best existing experimental description of phase equilibria in the system in question. The obtained experimental results of the phase equilibria were compared with the thermodynamic modelling by the CALPHAD (Calculation of Phase Diagrams) method and with other authors.
The electrical resistances of Cu/Sn57BiSbNi/Ni solder joints were continuously monitored during current stressing at temperatures between 95 and 125°C, and current densities up to 7kA/cm2, for times approaching 3000 h. Scanning electron microscopy was utilized to characterize corresponding changes in solder joint microstructure. The statistics of failure based upon a 20% increase in electrical resistance (correlated with the growth of a continuous layer of Bi at the anode) were analyzed for groups of nominally identical samples, while either current density or temperature was independently varied. At longer times, after prodigious, diffusion-limited growth of a Cu6Sn5-based phase, catastrophic failure (an increase in electrical resistance of 20 mΩ) was observed and correlated with crack propagation across the width of the sample. The statistics of catastrophic failure were analyzed during current stressing of these SnBi-based solder joints at a relatively high temperature (125°C) and current density (4kA/cm2).
Reduced activation ferritic martensitic steel JLF-1 (Fe-9Cr-2 W-0.1 C) corroded in liquid Sn at 673–873 K due to the formation of intermetallic compound layers. The outward growth of FeSn2 layer at 773 K was rate-controlled by a boundary diffusion. The inward growth of FeSn-like layer was recognized in the exposure for more than 250 h. The FeSn growth progressed according to the diffusion of Sn along the boundaries of martensite structure. Thin Cr2O3 layer formed by a pre-oxidation treatment did not function as an anti-corrosion layer in liquid Sn, though the sintered specimens of Fe2O3 and Cr2O3 revealed corrosion resistance.
The formation of microporosity during solidification of Sn-Bi alloys is investigated through experiments and simulations. Samples of varying composition with 20, 30, 47 and 58 wt.% Bi are solidified in a copper mould and the pore fraction is measured through scanning electron microscopy. Finite element models are derived from X-ray micro computed tomography data and a material model is implemented to capture the effect of liquid pressure and volumetric swelling. The experimental assessment of solidification porosity shows a strong dependency on alloy composition. The round morphology of porosity found in the Sn-20 wt.% Bi alloy indicates its formation in the terminal stage of solidification. Microporosity formation is computed by means of Finite Element simulations and the results correlate with experimental findings. The analysis confirms volumetric shrinkage in the liquid state of confined eutectic domains as the mechanism for pore formation and determines the composition dependency of Sn-Bi alloys.
The growth behavior of compounds for the reactive diffusion between solid Cu and molten binary Bi-Sn alloys was experimentally examined using semi-infinite diffusion couples prepared by an isothermal bonding technique. Isothermal annealing of (Biy-Sn1-y)/Cu diffusion couples with Bi mol fraction of y = 0.27-0.46 was conducted in the temperature range of 453-603 K (180-330 oC) for various times up to 345.6 ks (96 h). In the diffusion couple with y = 0.44, an intermetallic layer composed of Cu6Sn5 with irregular scallop shapes and Cu3Sn with relatively uniform thickness forms at the (Bi-Sn)/Cu interface at 453-523 K (180-250 oC). In contrast, at 563-603 K (290-330 oC), the intermetallic layer consists of only Cu3Sn. Isothermal sections of the equilibrium phase diagram in the ternary Bi-Cu--Sn system were calculated at 453-603 K (180-330 oC) by a CALPHAD (calculation of phase diagrams) method. For the diffusion couple with y = 0.44, the diffusion path passes through the three-phase tie-triangles containing Cu6Sn5 and/or Cu3Sn in the isothermal section at 523 K (250 oC), but through only that containing Cu3Sn in the isothermal section at 603 K (330 oC). As a result, for y = 0.44, the intermetallic layer is composed of Cu6Sn5 and Cu3Sn at 453-523 K (180-250 oC), whereas it consists of only Cu3Sn at 563-603 K (290-330 oC). The mean thickness of the intermetallic layer is proportional to a power function of the annealing time, and the exponent of the power function is mostly smaller than 0.5. Such smaller values of the exponent mean that the layer growth is controlled by boundary diffusion across the intermetallic layer. Adopting the common value of 0.30 as the exponent of the power function for the Cu3Sn layer at 563-603 K (290-330 oC), we obtain a value of 17.9 kJ/mol for the activation enthalpy of the proportionality coefficient of the power function.
Low reflow temperature solder interconnect technology based on Sn-Bi alloys is currently being considered as an alternative for Sn-Ag-Cu solder alloys to form solder interconnects at significantly lower melting temperatures than required for Sn-Ag-Cu alloys. Microstructural evolution after reflow and aging, especially of intermetallic compound (IMC) growth at solder/pad surface finish interfaces, is important to understanding fatigue life and crack paths in the solder joints. This study describes intermetallic growth in homogeneous solder joints of Sn-Bi eutectic alloy and Sn-Bi-Ag alloys formed with electroless nickel-immersion gold (ENIG) and Cu-organic surface protection (Cu-OSP) surface finishes. Experimental observations revealed that, during solid state annealing following reflow, the 50nm Au from the ENIG surface finish catalyzed rapid (Ni,Au)Sn4 intermetallic growth at the Ni-solder interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to significant solder joint embrittlement during fatigue testing. Intermetallic growth of (Ni,Au)Sn4 was decreased by Ag alloying of eutectic Sn-Bi solder and was completely eliminated by changing the metallization from ENIG to Cu-OSP on the board side of the assembly. The reduction in (Ni,Au)Sn4 growth rate with Ag additions is attributed to changes in grain boundary wetting of the IMC by Bi with Ag alloying.
In accounting for concentration dependent grain boundary diffusion, an extra term is derived and added to the boundary condition in Fisher’s Model for atomic transport in fast diffusing boundaries under the condition of low diffusant concentration resulting in an expanded model. The validity of this is evaluated using an implicit finite difference model which shows that a minimum of around 0.5-0.6 order of magnitude difference between the minimum and maximum grain boundary diffusion values is necessary for a significant effect to be observed. This was investigated for a grain boundary diffusion coefficient that exponentially increased and decreased with concentration. Whereas effects on measured grain boundary diffusion coefficients are evaluated to be negligible, the concentration dependency of the grain boundary diffusion substantially affects the elemental distribution in the surrounding of the grain boundary. The latter directly influences grain boundary related metallurgical phenomena such as grain growth or phase transformations in nanocrystalline materials.
A compact 2.0 T superconducting magnet has been developed for use in photoelectron microscopy. The magnet was required to be compact and magnetically well shielded with low stray fields. Because the magnet is for use with a microscope, the working volume can be small. A small volume implies that the stored magnetic energy is low, and with low stray fields, it makes the magnet safe while operating and during quench events. The magnet is a cryogen free design that uses a diamond loaded vacuum grease for current lead encapsulation and cooling. To make as small a coil as possible, a new coil winding method was developed that does not require solder joints between pancake windings. We show that a low temperature Sn/Bi/Ag eutectic solder can be used for connecting the input leads in this application.
The influence of isothermal annealing on mechanical properties of the Cu-clad Al (CA) wire was experimentally examined to better understand the annealing mechanisms that occur in the CA wire. In the experiment, the CA wire was prepared by drawing to decrease the diameter d from 10 mm to 2.9, 1.5 and 0.5 mm. The CA wires with these three different diameters were isothermally annealed at temperatures of 423–603 K (150–330°C) for various periods between 10.8 ks and 3456 ks (3 and 960 h). At room temperature, tensile tests were performed on the CA wire using an Instron type testing machine, and hardness tests were made on the Cu layer and the Al core of the CA wire utilizing a micro Vickers hardness testing machine. For the CA wire without annealing, the ultimate tensile strength su is 231, 215 and 198 MPa for d = 0.5, 1.5 and 2.9 mm, respectively, and the elongation eu is 0.4, 0.4 and 1.2% for d = 0.5, 1.5 and 2.9 mm, respectively. Due to isothermal annealing, recovery and recrystallization take place in both the Cu layer and the Al core of the CA wire, and an intermetallic layer consisting of various Cu–Al compounds forms at the Cu/Al interface. The intermetallic layer promotes formation of cavity at the Cu/Al interface during the tensile test. As the thickness l of the intermetallic layer reaches to the critical thickness lm, su attains to the minimum value, and eu takes the maximum value. Here, lm = 1–2 µm for d = 0.5–2.9 mm, respectively. For l > lm, however, su is insensitive to l, but eu decreases with increasing thickness l. Thus, the appropriate combination of the annealing temperature and time is essentially important to realize the optimal mechanical properties of the CA wire.
Fig. 9 The results in Fig. 8(b), 8(d) and 8(f) are represented as open circles, triangles and rhombuses, respectively. The corresponding results reported by Hug and Bellido⁷⁾ are also shown as solid squares. Fullsize Image
The kinetics for the isothermal carburization of pure iron (Fe) at a temperature of 1073 K (800°C) for times between 1.8 ks (0.5 h) and 32.4 ks (9 h) was experimentally observed by Togashi and Nishizawa. According to the observation, the austenite (γ) phase with the face-centered cubic (fcc) structure is produced on the Fe specimen of the ferrite (α) phase with the body-centered cubic (bcc) structure, and gradually grows into the α phase. The carbon (C) concentration in the γ phase at the moving γ/α interface is greater than that of the γ/(γ + α) phase boundary in a phase diagram of the binary Fe–C system. Although the former one gradually approaches to the latter one with increasing annealing time, their difference hardly vanishes even at the longest annealing time of 32.4 ks (9 h). The molar Gibbs energy of the γ phase was described by a two-sublattice model to evaluate the chemical driving force working at the moving γ/α interface. The evaluation provides that the chemical driving force monotonically decreases with increasing annealing time. This annealing time dependence of the chemical driving was used to calculate the migration distance of the γ/α interface as a function of the annealing time. The observation for the interface migration was satisfactorily reproduced by the calculation. According to the calculation, the migration of the γ/α interface is controlled by the interface reaction at the moving interface in the early stages, but it is governed mainly by the volume diffusion of C across the γ phase and partially by the interface reaction in the late stages. The experimental annealing times mostly belong to the transition stages between the rate-controlling processes of the interface reaction and the volume diffusion.
Fig. 5 The thickness l of the γ layer versus the annealing time t. Open circles show the observation reported by Togashi and Nishizawa,⁷⁾ a solid curve and a dashed line indicate the calculation from eq. (11), and a thin dashed line represents the extrapolation of the dashed line in the early stages. Fullsize Image
In this work, we studied the effect of the reaction between the depositing atoms and the substrate element on the morphological evolution of the each deposition of the Sn–Bi–Sn multilayer structure on the Cu substrate. For the deposition of the single Sn layer on the Cu substrate, the reaction between the depositing Sn atoms and the Cu substrate is very minimal. TEM study shows only a very thin interfacial Cu–Sn compound layer (17 nm) formed between the depositing Sn layer and the Cu substrate. Yet, the latent heat of the vaporized Sn (296.1 kJ/mol) melts the Sn deposition layer on the Cu layer. Owing to the surface tension of the molten Sn, the molten Sn could agglomerate as the Sn clusters for reducing the surface energy.
For the subsequent deposition of Bi and Sn layers, the eutectic reaction would occur between the depositing Bi atoms and the Sn clusters. The energy required for the eutectic reaction between the depositing atoms and the substrate elements is provided by the latent heat released from the vaporized Bi (or Sn) atoms arriving on the deposition surface. The eutectic liquid formed on the deposition surface serves as a medium to (1) receive and redistribute the depositing atoms and (2) dissolve and transport the substrate elements. The above process (1) and (2) explain the morphological evolution of the each deposition of the Sn–Bi–Sn multilayer structure and the mechanism of morphological evolution is proposed in this work.
A composite solder was prepared by adding different amount Ni modified multi-walled carbon nanotubes (MWCNTs) into the Sn-3.0Ag-0.5Cu (SAC305) solder. Experiments were conducted to investigate the thermal behavior, mechanical properties of composite solder alloys, shear fracture behavior of solder joints were evaluated as well. The modification process was carried out on the MWCNTs using electroless modification method to synthesis the Ni modified carbon nanotubes (Ni-CNTs), which was manually mixed into SAC305 solder powder with different amount. The melting point of the composite solder alloys was slightly increased with the increment of the amount of Ni-CNTs. The nanoindentation test results confirmed that the hardness and modulus of composite solder alloys were enhanced after the addition of Ni-CNTs. The aging experiment was implemented for the joints and the results indicated that the Ni-CNTs could effectively inhibit the growth of IMC layer as an additive in SAC305 solder. The shear strength of SAC305-xNi-CNTs/Cu (x = 0, 0.1 and 0.2) joints was reinforced with the increment of Ni-CNTs amount. Furthermore, the SAC305-0Ni-CNTs/Cu and SAC305-0.1Ni-CNTs/Cu solder joints were broken partially between the solder and IMC, mostly between the solder matrix, and SAC305-0.2Ni-CNTs/Cu solder joint was broken completely inside solder matrix. It was suggested that the fracture mode of the solder joints changed from mix fracture mode to ductile fracture mode as the amount of Ni-CNTs increased.
Co has been studied extensively by many research groups as an alternative material for underbump metallization, since Co–Sn compounds show better mechanical properties than Cu–Sn compounds. Information on reactive diffusion at the solid/liquid interface is considerably important to form mechanically and electrically reliable solder joints. In the present study, the kinetics of the reactive diffusion between solid Co and liquid Sn was experimentally examined using semiinfinite Co/Sn diffusion couples prepared by an isothermal bonding technique. Isothermal annealing of the diffusion couple was conducted at temperatures in the range of 523 K to 583 K for various times up to 96 h. An intermetallic layer formed at the original Co/Sn interface in the diffusion couple during annealing. One or two intermetallic compounds among α-CoSn3, β-CoSn3, and CoSn2 were identified, depending on the annealing temperature. The total thickness of the intermetallic layer was proportional to a power function of the annealing time. The overall growth rate of the intermetallic layer did not increase with increasing annealing temperature but was dependent on the kind of compound formed at the interface. The overall growth rate at 583 K was much slower than at lower annealing temperatures, since two compounds (CoSn2 and CoSn3) were identified at the interface, while only CoSn3 formed at 523 K to 563 K. This indicates that the interdiffusion coefficient of CoSn2 is much smaller than that of CoSn3. Based on the exponent of the power function and the microstructure evolution at the moving interface, the layer growth of the compounds was controlled by volume diffusion with spheroidal growth.
The diffusion reaction kinetics for Sn-Ag alloys with pure Cu has been examined experimentally to determine the effects of adding Ag to Sn on the growth behavior of compounds at a heated interface between a Sn-base solder and a Cu-base conductor. A solid-state diffusion bonding technique was utilized to make sandwich (Sn-Ag)/Cu/(Sn-Ag) diffusion couples with Ag mol fraction of 0.011 to 0.033. The diffusion couples were isothermally annealed at 433 K to 473 K for up to 1944 h. During annealing, Cu6Sn5 and Cu3Sn layers formed at the bonded (Sn-Ag)/Cu interface in the diffusion couple. The layer thickness of Cu3Sn (l3) was less than that of Cu6Sn5 (l6), and the concentration of Ag was negligible in both Cu6Sn5 and Cu3Sn. The total thickness (l = l3 + l6) was proportional to a power function of the annealing time with an exponent n of 0.30 to 0.40. Because n is less than 0.5, boundary diffusion as well as volume diffusion must contribute to the layer growth. The ratios of l3 and l6 to l (r3 and r6, respectively) were insensitive to the annealing time and the Ag concentration. Addition of Ag to Sn barely affects the layer growth. The value of r3 was 0.34, 0.34, and 0.30 at temperature of 433 K, 453 K, and 473 K, respectively, with corresponding value of r6 of 0.66, 0.66, and 0.70, respectively.
Eutectic Sn58Bi (SnBi) solder paste mixed with 0 wt.%, 3 wt.%, 5 wt.%, 8 wt.% and 15 wt.% of Sn-3.0Ag-0.5Cu (SAC) paste was prepared by mechanical mixing. The effects of SAC paste additions on the microstructure evolution of SnBi-SAC/Cu composite solder joints during isothermal aging were investigated. The results indicated that the number of large Bi-rich phases decreased and the relative areas of β-Sn increased with increasing SAC content. Moreover, 1-μm Bi-rich particles were found near the Bi-rich phases. During the isothermal aging process, the diameter of the 1-μm Bi-rich particles in the solder bulk increased by about 50% with aging time by Ostwald ripening. The thickness of the interfacial intermetallic compound in all the solder joints increased slightly during the aging process. The formation of Cu6Sn5 was suppressed by the Bi-rich phases above the Cu6Sn5 layer with the aging time increasing. In addition, the solder bulk showed many cracks along the β-Sn grain boundaries after isothermal aging when the content of SAC paste was 5 wt.%. With 8 wt.% or 15 wt.% SAC, fractures were more obvious near the interface than away from the interface.
The interfacial microstructures of Sn-3.0Ag-0.5Cu (SAC305) solder systems with thin Ni(P) mono-coatings and Cu-Ni(P) dual-coatings were investigated after reflowing and isothermal aging. The ultrathin mono-Ni(P) plating of the SAC305/Ni(P) solder joint was found to rapidly decompose and then transform into a Ni2SnP phase. An intermetallic compound (IMC) formed at the plating/substrate interface, indicating that the ultrathin mono-Ni(P) plating was not an effective diffusion barrier. However, only a single IMC layer ((Cu,Ni)6Sn5) formed at the solder/plating interface in the SAC305/Cu/Ni(P)/Cu system. The (Cu,Ni)6Sn5 IMC effectively suppressed atomic diffusion, protecting the Ni(P) plating and Cu substrate. Although P-Sn-O pores formed in the root of the (Cu,Ni)6Sn5 IMC layer, the dual-Cu/Ni(P) plating protected the solder system for an extended period. The IMC growth rate constants of the SAC305/Cu, SAC305/Ni(P), and SAC305/Cu/Ni(P)/Cu solder joint systems were 0.180, 0.342, and 0.068 μm/h1/2, respectively. These results indicate that the application of dual-Cu/Ni(P) plating can effectively hinder the growth of IMC.
A highly selective and durable electrocatalyst for carbon dioxide (CO2) conversion to formate is developed, consisting of tin (Sn) nanosheets decorated with bismuth (Bi) nanoparticles. Owing to the formation of active sites through favorable orbital interactions at the Sn‐Bi interface, the Bi‐Sn bimetallic catalyst converts CO2 to formate with a remarkably high Faradaic efficiency (96%) and production rate (0.74 mmol h−1 cm−2) at −1.1 V versus reversible hydrogen electrode. Additionally, the catalyst maintains its initial efficiency over an unprecedented 100 h of operation. Density functional theory reveals that the addition of Bi nanoparticles upshifts the electron states of Sn away from the Fermi level, allowing the HCOO* intermediate to favorably adsorb onto the Bi‐Sn interface compared to a pure Sn surface. This effectively facilitates the flow of electrons to promote selective and durable conversion of CO2 to formate. This study provides sub‐atomic level insights and a general methodology for bimetallic catalyst developments and surface engineering for highly selective CO2 electroreduction. Orbital interactions of Bi‐Sn lead to the electrocatalytic conversion of CO2 to formate with high selectivity, activity, and durability. This is attributed to the electronic states of Sn upshifting away from the Fermi level due to the coupling with Bi, making the HCOO* intermediate adsorb more favorably on Bi‐Sn than on pure Sn surfaces.
Semi-infinite Cu/Sn diffusion couples prepared by an isothermal bonding technique were used to examine experimentally the kinetics of reactive diffusion in the solid-Cu/liquid-Sn system. Isothermal annealing of the diffusion couple was conducted in the temperature range of T = 753–793 K for various periods up to t = 144 ks (40 h). Owing to annealing, an intermetallic layer composed of ε-Cu3Sn with scallop morphology and δ-Cu4Sn with rather uniform thickness is formed at the original Cu/Sn interface in the diffusion couple. The total thickness of the intermetallic layer is proportional to a power function of the annealing time, and the exponent of the power function is close to unity at all the annealing temperatures. Such a power relationship holds also for the ε-Cu3Sn scallop and the δ-Cu4Sn layer. This means that volume diffusion controls the growth of the ε-Cu3Sn scallop and the morphology of the Cu3Sn/Sn interface influences the rate-controlling process. In contrast, the growth of the δ-Cu4Sn layer is governed by the interface reaction at the moving Cu4Sn/Cu interface. Adopting a mean value of 0.87 for the exponent, we obtain a value of 129 kJ/mol for the activation enthalpy of the intermetallic growth.
The Cu/Sn system is one of the most fundamental and important metallic systems for solder joints in electric devices. To realize reliable solder joints, information on reactive diffusion at the solder joint is very important. In the present study, we experimentally investigated the kinetics of the reactive diffusion between solid Cu and liquid Sn using semi-infinite Cu/Sn diffusion couples prepared by an isothermal bonding technique. Isothermal annealing of the diffusion couple was conducted in the temperature range of 533–603 K for various times up to 172.8 ks (48 h). Using annealing, an intermetallic layer composed of Cu6Sn5 with scallop morphology and Cu3Sn with rather uniform thickness is formed at the original Cu/Sn interface in the diffusion couple. The growth of the Cu6Sn5 scallop occurs much more quickly than that of the Cu3Sn layer and thus predominates in the overall growth of the intermetallic layer. This tendency becomes more remarkable at lower annealing temperatures. The total thickness of the intermetallic layer is proportional to a power function of the annealing time, and the exponent of the power function is close to unity at all the annealing temperatures. This means that volume diffusion controls the intermetallic growth and the morphology of the Cu6Sn5/Sn interface influences the rate-controlling process. Adopting a mean value of 0.99 for the exponent, we obtain a value of 26 kJ/mol for the activation enthalpy of the intermetallic growth.
The impurity diffusion of Hf and Zr in Gd-doped CeO2 was examined by means of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Dense, polycrystalline samples with a dopant site fraction of nGd = 0.5% were subjected to diffusion anneals in the temperature range 1400 ≤ T/K ≤ 1600 in ambient air. Isothermal values of the bulk diffusion coefficient, Db, were similar for the two isovalent impurity cations. The activation enthalpies of bulk diffusion were also similar, ∆HDb(Zr) = (5.5 ± 0.4) eV and ∆HDb(Hf) = (5.3 ± 0.3) eV. Fast grain-boundary diffusion of Hf was observed (the high background of Zr prevented the analysis of Zr grain-boundary diffusion). The activation enthalpy of the grain-boundary diffusion product, ∆HωDgb(Hf) = (5.9 ± 0.8) eV, is, surprisingly, similar to the activation enthalpies for bulk diffusion. Attention is drawn to possible problems that may arise when using chemical tracers instead of true (stable or radioactive) tracers.
The kinetics of the solid-state reactive diffusion between Sn-Ag alloys and pure Ni was experimentally observed to examine effects of addition of Ag into Sn on the growth behavior of compound at the interconnection between the Sn-base solder and the multilayer Au/Ni/Cu conductor during energization heating. In this experiment, sandwich (Sn-Ag)/Ni/(Sn-Ag) diffusion couples with Ag concentrations of y= 0.011-0.033 were prepared by a diffusion bonding technique, and then isothermally annealed at temperatures of T= 453-473 K for various periods up to 3169 h, where y is the mol fraction of Ag. After annealing, an intermetallic layer consisting of Ni3Sn4 was recognized between the Sn-Ag and Ni specimens in the diffusion couple. Here, the concentration of Ag in Ni3Sn4 is negligible. The mean thickness of the intermetallic layer is proportional to a power function of the annealing time, and the exponent of the power function takes values of 0.33-0.43 at T= 453 K and those of 0.54-0.62 at T= 473 K. Thus, the growth of the intermetallic layer is controlled by boundary and volume diffusion at T= 453 K. In contrast, at T= 473 K, interface reaction and interdiffusion contribute to the rate-controlling process of the intermetallic growth. The addition of Ag into Sn accelerates the intermetallic growth within the experimental annealing times.
The kinetics of the solid-state reactive diffusion between pure Cu and Zn was experimentally examined using sandwich Zn/Cu/Zn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range of 523-623 K for various times up to 49 h. Owing to annealing, an intermetallic layer consisting of the gamma and epsilon phases was formed at the original interface in the diffusion couple, where the thickness is much smaller for the e phase than for the gamma phase. The total thickness of the intermetallic layer increases in proportion to a power function of the annealing time. The exponent of the power function takes values of 0.60-0.62 at 523-623 K. These values of the exponent indicate that volume diffusion predominantly controls the layer growth and interface reaction partially contributes to the rate-controlling process.
The effect of isothermal aging on the microstructure and shear strength of Sn3Ag0.5Cu/Cu (SAC305/Cu) solder joints were studied systematically. The single-lap shear samples of SAC305/Cu solder joint were prepared and aged up to 456 h at 150 and 180 °C, respectively. The interfacial intermetallic compound layer of the aged solder joint gradually thickened with increasing both aging time and aging temperature, while, the interfacial intermetallic compound layer morphology transformed from scallop-type to layer-type after the aging treatment. The growth of the interfacial Cu-Sn interfacial intermetallic compounds layer of aged SAC305/Cu solder joints exhibited a linear function of the square root of aging time, indicating that the formation of the interfacial Cu-Sn interfacial intermetallic compounds during aging treatment was mainly controlled by the diffusion mechanism. The diffusion coefficient (D) values of interfacial intermetallic compound layer were 4.64×10−17 and 1.06×10−16 m2 s−1 for aging temperatures of 150 °C and 180 °C, respectively. Single-lap shear tests results revealed that the shear strength of SAC305/Cu solder joints decreased continuously with an increase in interfacial intermetallic compound layer thickness and aging time. The main reason for these characteristics was the excessive increase in the interfacial intermetallic compound thickness of solder joints, causing a change in the stress concentration of the shear load from the protruding region to the inside of the interfacial intermetallic compound layer at the same tested condition. In addition, the shear fractures in as-reflowed and short time aged solder joints were shown to be ductile in nature and confined in the bulk solder rather than through the interfacial intermetallic compound layer. However, the shear fracture locations transferred from the solder bulk to the interfacial Cu-Sn interfacial intermetallic compound layer, and finally to the Cu3Sn/Cu interface with increasing aging time.
The kinetics of the reactive diffusion between solid Ni and liquid Sn was experimentally examined using Ni/Sn diffusion couples. The diffusion couples were prepared by an isothermal bonding technique and then immediately annealed in the temperature range of T=533–623 K for various times up to t=14.4 ks (4 h). During annealing, a compound layer of Ni3Sn4 is formed at the original Ni/Sn interface in the diffusion couple and grows mainly into the liquid Sn specimen. The mean thickness of the compound layer is proportional to a power function of the annealing time. The exponent n of the power function takes values between 0.31 and 0.43. Since there is no systematic dependence of n on T, we may consider that n is insensitive to T within experimental uncertainty. When growth of a compound layer with uniform thickness is controlled by volume diffusion, n is equivalent to 0.5. If boundary diffusion contributes to the layer growth and grain growth occurs in the compound layer, however, n becomes smaller than 0.5. Since grain growth practically takes place in the compound layer, it is concluded that the layer growth of Ni3Sn4 is mainly controlled by boundary diffusion at T=533–623 K.
Ternary Cu-Sb-Zn system has been thermodynamically assessed by using CALPHAD method and experimentally by DSC and SEM-EDS methods. The liquidus projection, invariant equilibria, several vertical sections and isothermal sections at 450 degrees C and 25 degrees C were predicted using COST 531 Thermodynamic Database. Phase transition temperatures of alloys along three predicted vertical sections of the Cu-Sb-Zn ternary system with molar ratios Cu:Zn = 1: 3, Cu: Zn = 1 and Sb:Zn = 1, were determined by DSC analysis. Predicted isothermal sections at 450 degrees C and 25 degrees C were compared with the results of the SEM-EDS analysis. The calculated values were found to be in a good agreement with the experimentally obtained values and literature data.
Pb-base high-temperature solders (mass % Sn=5-10, melting point (m.p.)=300-310°C) are widely applied under severe conditions, although the harmful nature of Pb is recognized. Bi-base alloys (m.p. of Bi =270°C), Zn-base alloys (m.p. of Zn=420°C), and several Au-base eutectic alloys (m.p. of Au-20Sn and Au-3.6Si=280 and 363°C, respectively) are proposed as candidates for Pb-free high-temperature solders. This paper reviews the features of Bi-base composite solders containing reinforcement particles of a superelastic Cu-Al-Mn alloy in a Bi matrix to relax thermal stress and to prevent the propagation of cracks, and Zn-Al base solders, which have high stability and high reliability enough to be utilized in practical applications under severe thermal cycle tests between -40 and 230°C more than 2000 cycles.
To examine growth behavior of alpha-CoSn3 at solid-state temperatures, kinetics of reactive diffusion between Co and Sn was experimentally observed using sandwich Sn/Co/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range of 433-473 K for various times up to 744h. Owing to annealing, an intermetallic layer consisting of CoSn3 was formed at the original interface in the diffusion couple. The mean thickness of the intermetallic layer increases in proportion to a power function of the annealing time. The exponent of the power function takes values of 0.67 and 0.62 at 433-453 and 473 K, respectively. These values of the exponent indicate that volume diffusion predominantly controls the layer growth and interface reaction partially contributes to the rate-controlling process.
A hypothetical binary system composed of one intermetallic compound and two primary solid-solution phases has been considered in order to examine the kinetics of the reactive diffusion controlled by boundary and volume diffusion. If a semi-infinite diffusion couple initially consisting of the two primary solid-solution phases with solubility compositions is isothermally annealed at an appropriate temperature, the compound layer will be surely produced at the interface between the primary solid-solution phases. In the primary solid-solution phases, however. there is no diffusional flux. Furthermore, we suppose that the compound layer is composed of a single layer of square-rectangular grains with an identical dimension. Here, the square basal-plane is parallel to the interface, and hence the height is equal to the thickness of the compound layer. Under such conditions, the growth behavior of the compound layer has been analyzed numerically. In order to simplify the analysis. the following assumptions have been adopted for the compound layer: there is no grain boundary segregation; and volume and boundary diffusion takes place along the direction perpendicular to the interface. When the size of the basal-plane remains constant independently of the annealing time, the thickness of the compound layer is proportional to the square root of the annealing time. In contrast, the growth of the compound layer takes place in complicated manners, if the size of the basal-plane increases in proportion to a power function of the annealing! time. Nevertheless. around a certain critical annealing time, the thickness of the compound layer is approximately expressed as a power function of the annealing time. For each grain, the layer growth is associated with increase in the height, and the grain growth is relevant to increase in the size of the basal-plane. The exponent for the layer growth almost linearly decreases with increasing exponent for the gain growth.
Pb-based solders have been the cornerstone technology of electronic interconnections for many decades. However, with legislation in the European Union and elsewhere having moved to restrict the use of Pb, it is imperative that new Pb-free solders are developed which can meet the long established benchmarks set by leaded solders and improve on the current generation of Pb free solders such as SAC105 and SAC305. Although this poses a great challenge to researchers around the world, significant progress is being made in developing new solder alloys with promising properties. In this review, we discuss fundamental research activity and its focus on the solidification and interfacial reactions of Sn-based solder systems. We first explain the reactions between common base materials, coatings, and metallisations, and then proceed to more complex systems with additional alloying elements. We also discuss the continued improvement of substrate resistance to attack from molten Sn which will help maintain the interface stability of interconnections. Finally, we discuss the various studies which have looked at employing nanoparticles as solder additives, and the future prospects of this field.