Growth Of Cu6Sn5 Research Articles

Strong grain size effect on the liquid/solid reactions between molten solder and electroplated Cu

Electrodeposition of Cu metallization and its soldering reaction with solder alloy are of technological significance in microelectronic packaging industry. This study aims to investigate the effect of Cu grain size on the liquid/solid reactions between molten Sn-3 wt% Ag-0.5 wt% Cu (SAC305) solder alloy and electroplated Cu. Two Cu electroplated films, denominated as fine-grained Cu (FG-Cu) and coarse-grained Cu (CG-Cu), are prepared and reacted with SAC305 at 260 °C for 5–20 min. Microstructural characterization shows that the Cu6Sn5 intermetallic compound (IMC) is formed in these two solder joints, while that in the FG-Cu solder joint grows at a faster rate than that in the CG-Cu solder joints. The Cu6Sn5 IMC in the FG-Cu solder joint exhibits a slender column-like grain morphology, unlike the common scallop-like grain morphology in the CG-Cu solder joints. The strong grain size effect is rationalized by comparing the contribution of two fluxes of ripening and interfacial reaction to the growth of Cu6Sn5.

Applications of Ni and Ag metallizations at the solder/Cu interfaces in advanced high-power automobile interconnects: An electromigration study

An advanced Pb-free solder interconnect, Cu line/Cu pillar/Ni/Sn1.8Ag/Ag/Cu lead frame/Sn3Ag0.5Cu/Au/Ni(P)/Cu trace, using the flip-chip quad flat no‑lead (FCQFN) packaging technology was designed for high-power automobile applications. An electromigration study revealing the Ni and Ag metallization effects on the interfacial reaction was investigated. Current stressing was conducted at a 2 A direct current at 160 °C until the defined 200 %-electrical-resistance-rise failure occurrence. Electromigration induced a two-stage electrical resistance increase mode in the advanced automobile test vehicle with a mechanism transition time of 585 h, as evidenced by the in situ resistance monitoring. Phase transformation from the constituent phases into the predominant (Cu,Ni)6Sn5 and Cu3Sn intermetallic compounds (IMCs) at the interfaces was proposed to explain the rapid resistance rise in the early-stage electromigration. Void formation and coalescence due to the Kirkendall effect and volume shrinkage during the phase transformation were proposed to explain the following steady resistance increase after the critical current stressing time. Furthermore, the phase transformation and growth of the IMCs behaved divergently at different interfaces and electron flow directions. The rapid phase transformation occurred in the downstream Sn1.8Ag solder interconnect due to the accelerated Ni metallization dissolution and formed a complete IMC joint. The anisotropic diffusion driven by the competitive or synergetic effect between the electron wind force and chemical potential gradient was proposed to explain the asymmetric microstructures after the electromigration experiment. The application of Ag metallization benefited the flip-chip soldering process, and the layer-type Ag3Sn IMC served as a diffusion barrier against Sn/Cu interdiffusion at the solder/Cu lead frame interface during the solid-state electromigration. In contrast, the Ni metallization was designed as a diffusion barrier to hinder interdiffusion and the undesirable Cu3Sn growth at the solder/Cu interface. The findings presented in this study could benefit the solder interconnect design with an appropriate metallization combination against undesirable electromigration failure.

Influence of carbon nanotubes on the morphology of Cu6Sn5 in Cu/(Sn–Ag–Cu) solder joints

This study delves into the intricate interaction between multi-walled carbon nanotubes (MWCNTs) and the Sn–Ag–Cu solder system, highlighting its relevance in lead-free soldering applications. Reflow and isothermal aging induce the formation of intermetallic layers at the joint interface, including Cu6Sn5, Cu3Sn, and irregularly shaped Ag3Sn particles. The addition of MWCNTs leads to the flattening of Cu6Sn5 grains, restraining their growth and potentially improving mechanical strength. Notably, MWCNTs exhibit a relatively high affinity with Ag3Sn. Grain size distribution analysis indicates that MWCNTs reduce particle size, effectively suppressing intermetallic compound growth. Over time, grain size increases due to simultaneous coarsening and growth during isothermal aging. In the growth kinetics analysis of intermetallic compounds in SAC305-MWCNTs solder joints, stable behavior is observed for Cu3Sn growth, driven by solid-state diffusion between Cu6Sn5 and Cu. In contrast, Cu6Sn5 exhibits significant variations in growth behavior with temperature changes. Furthermore, the evaluation of activation enthalpy for Cu3Sn growth in the Cu/(SAC305-MWCNTs) and Cu/SAC305 diffusion couples reveals similar n and k values of the power function for the layer growth, indicating a comparable rate-controlling process for Cu3Sn growth. The presence of MWCNTs in the solder does not significantly influence the rate-controlling process or activation enthalpy for Cu3Sn growth, contrasting with the observed effects on Cu6Sn5 growth. This study provides valuable insights into the interactions within the SAC305-MWCNTs solder system, offering significant implications for lead-free soldering applications.

Trace of Ag atoms migration induced by temperature gradient in Cu/Sn-5.0Ag/Cu solder joints

The growth of interfacial IMC in Cu/Sn/Cu and Cu/Sn-5.0Ag/Cu micro solder joints and the trace of Ag atoms migration in Cu/Sn-5.0Ag/Cu micro solder joint during reflow with and without temperature gradient (TG) were investigated. The growth of interfacial IMC during isothermal reflow was symmetrical and controlled by grain boundary diffusion; whereas that during reflow under TG was asymmetrical and controlled by reaction rate. In addition, the Cu and Ag atoms could migrate from the hot end to the cold end driven by TG and react with Sn atoms to form Cu6Sn5 and plate-type Ag3Sn at the cold end in the Cu/Sn-5.0Ag/Cu solder joint. As the reflow time prolonged, the plate-type Ag3Sn would embed in the Cu6Sn5 phase. Due to the larger interfacial energy of Cu6Sn5/Ag3Sn and the smaller absolute value of the enthalpy of formation for Ag3Sn, the Ag3Sn would dissolve and release Sn atoms to support Cu6Sn5 growth by consuming the surplus Cu atoms generated by the additional TG, leaving Ag atoms in the full IMC joint. Specifically, the Ag element remained in the Cu6Sn5 phase with Ag content of 0.55 wt%.

Nucleation and growth of Cu6Sn5 during the aging process of Cu/Sn interface in electronic packaging-by in-situ TEM

The rapid nucleation of Cu6Sn5 during the soldering process, as well as its growth at the solid-state interface system (Cu/Sn) during chip service, present significant challenges. To study this, we introduced a small amount of Co element into the Cu-Sn system in order to reduce the formation energy of Cu6Sn5. Through transmission electron microscopy, we performed in situ observations of the nucleation and growth of Cu6Sn5 at the Cu/Sn solid phase interface. Our experimental findings confirm that the diffusion of Cu element at the interface plays a dominant role in the nucleation and growth process of Cu6Sn5. During this diffusion process, once the local concentration of Cu surpasses a threshold, Cu6Sn5 begins to nucleate around the Cu-rich region. The driving force behind this phase transition arises from two factors: non-uniformity of atomic flux at grain boundaries leading to stress accumulation, and localized bonding between a pair of valence electrons on Cu and adjacent Sn atoms, resulting in the formation of a shared orbital. Based on the curvature of the nucleation solid interface, three distinct growth modes have been observed: spiral growth, layer-layer growth, and island growth. These variations stem from changes in the dynamic mechanism associated with the morphology coefficient linked to the interface arc length. The nucleation and growth of Cu6Sn5 are governed by anisotropic attachment kinetics, with the growth process transitioning to diffusion-controlled continuous growth.

Effect of CoSn3 nanocrystals on Sn3Ag plating for electronic packaging

Plating Sn3Ag on copper substrates represents a crucial electronic packaging technique. In this study, we propose a novel composite plating approach, wherein CoSn3 nanocrystals are deposited within the Sn3Ag coating. The resulting reflowed Sn3Ag joints exhibit a range of distinctive properties. Notably, CoSn3 nanocrystals dissolve in Sn during the reflow process, thereby lowering the supercooling required for Sn nucleation. Consequently, Sn crystals grow in six-fold cyclic twins. Additionally, the dissolution of Co atoms in Sn leads to a reduced solubility of Cu atoms in Sn, consequently lowering the supercooling required for the nucleation of Cu6Sn5. Simultaneously, this phenomenon promotes the nucleation of Cu6Sn5, resulting in a considerable precipitation of Cu6Sn5 nanoparticles within the joints. Therefore, the mechanical properties of the joints are significantly enhanced, leading to a notable 20% increase in shear strength. Furthermore, the presence and distribution of Co elements within Sn induce changes in the growth pattern of interfacial Cu6Sn5. The growth process of Cu6Sn5 is dominated by the interfacial reaction, leading to its growth in a faceted shape. During the aging process, the dissolution of Co elements in Sn impedes the continuous growth of Cu6Sn5 at the interface, causing Cu6Sn5 to be distributed in the form of islands inside the joint. Remarkably, elemental Co acts as an inhibitor for the development of Cu3Sn and reduces the occurrence of Kirkendall voids.

Alteration of Cu3Sn growth and retention of multi-orientation Cu6Sn5 during thermal aging in Cu-xNi-yZn/Sn3.5Ag/Cu transient liquid-phase bonding

With the development of the artificial intelligence (AI) industries, electronic packaging is developing in the direction of high-density, high efficiency and multi-functionality. To realize the high density and tiny scale interconnection, application of microbumps is inevitable. Thus, the mechanical and thermal properties of intermetallic compounds (IMCs) become quite important as the proportion of IMC in the bump is greatly increased. In this study, three types of full IMCs bump, Cu/Sn-3.5Ag(SA)/Cu, Cu18Ni/SA/Cu and Cu18Ni18Zn/SA/Cu, were fabricated through transient liquid phase (TLP) bonding. The evolution of orientation and the sizes of the Cu6Sn5 grain before and after long-term thermal aging were analyzed and compared. After long-term thermal aging, the full IMCs bump with Ni and Zn added into Cu substrate simultaneously maintained consistent structure. In addition, the unusual growth of network-like Cu3Sn was observed in Cu18Ni/SA/Cu. Furthermore, the mechanical test and fracture cross-section analysis were conducted to discuss the crack paths. The study focused on establish the correlation between microstructure and mechanical performance. Network-like Cu3Sn was considered to enhance the strength of interfaces. Finally, the full IMCs bump which demonstrated the extraordinary thermal stability provided a reliable microstructure for application.

Study on Ni-GNSs enhanced Sn2.5Ag0.7Cu0.1RE/Cu solder joints β-Sn grain orientation and interfacial IMC growth kinetics under constant temperature thermomigration

Aiming at the thermomigration problem caused by the large temperature gradient (TG) of micro-soldering joints, a constant temperature thermomigration device with temperature control function was designed and manufactured. This paper studied the polarity phenomenon, crystallographic characteristics, and interfacial intermetallic compound (IMC) growth kinetics of Ni-GNSs reinforced Sn2.5Ag0.7Cu0.1RE/Cu solder joints under thermomigration. The results indicate that Ni-GNSs reinforced Sn2.5Ag0.7Cu0.1RE/Cu solder joints exhibited a significant thermomigration polarity phenomenon under the conditions of TG ≥ 1000 °C/cm and θ ≤ 43.5° between the c-axis and TG of β-Sn grains. At the cold end of the solder joint, the Cu6Sn5 phase at the interface gradually thickens and forms Cu3Sn phase on the substrate side, while microcracks expanded and gradually developed into macrocracks. At the hot end of the interface, the Cu6Sn5 phase gradually dissolved. The growth of the Cu3Sn phase was accompanied by “Kirkendall voids” that formed cracks at the interface between the joint and the solder until a “fully IMC solder joint” was formed. The growth of Cu6Sn5 IMC at the solder joint interface came before that of Cu3Sn IMC. The average temperature and temperature gradient of the solder joint were correlated with the growth of Cu3Sn IMC, leading to the formation of interface Cu3Sn IMC due to the oversaturation of Cu atoms. The addition of 0.05 wt% Ni-GNSs refines the grain structure and increases the activation energy for the growth of Cu6Sn5 and Cu3Sn IMC at the cold end of the solder joint, suppressing the thermomigration polarity phenomenon.

In-situ fusion process and Cu–Sn compounds evolution mechanism of nano-intermetallic compound mixed solder joints

This study investigated the fusion and phase-transformation processes of nano-intermetallic compound (nano-IMC) mixed solder pastes. In particular, the interfacial reaction with the Cu substrate, the kinetics and mechanism of Cu6Sn5 growth inside the solder joint, and the shear strength of the solder joint were investigated. In-situ SEM and XRD results showed that the SAC305 and Sn–10Cu powders fused and the liquid solder ball completely melted at 227 °C. A higher initial Cu concentration in the mixed solder joints resulted in a lower Cu concentration gradient that inhibited the diffusion of Cu atoms. The growth activation energy of the IMC layer of a SAC305 + 20 wt% Sn–10Cu solder joint was 104.81 kJ/mol, which was higher than that of the SAC305 IMC layer (75.75 kJ/mol). During aging, Cu6Sn5 rods within the mixed solder joint formed gullies on the outer facets, and the Cu6Sn5 rods continued to grow along the [0001] crystal direction. The number of Sn grains and the proportion of high-angle grain boundaries in the mixed solder joints were higher after reflow. After aging at 150 °C for 1000 h, the shear strength of the SAC305 + 20 wt% Sn–10Cu solder joints was 34.25 MPa, slightly higher than that of SAC305 (33.12 MPa).

Kinetics of Cu6Sn5 and Cu3Sn intermetallic compounds growth and isothermal solidification during Cu-Sn transient liquid phase sintering process

The kinetics of Cu-Sn intermetallic compounds (IMCs) growth relates to the materials preparation, process regulation and reliability prediction in the new energy field and electronic packaging industry. As a typical instance that Cu-Sn IMCs are involved with, this work investigated the kinetics of Cu-Sn IMCs growth and isothermal solidification of Cu-Sn transient liquid phase sintering (TLPS) process for high temperature resistant packaging of the third-generation semiconductors power devices. A three-dimensional kinetics model of Cu/Sn solid/liquid interfacial reaction and the corresponding kinetics equations of Cu6Sn5 and Cu3Sn growth was established. Based on Cu-Sn IMCs growth kinetics, a parameter ξSn called the residual rate of Sn was defined to characterize the isothermal solidification kinetics of Cu-Sn TLPS process. By measuring the volume fraction of Cu and Sn, the quality fraction of residual Cu and Sn and produced Cu-Sn IMCs were determined, and thus the kinetics of Cu-Sn IMCs growth and the isothermal solidification of Cu-Sn TLPS process were experimentally studied to validate the theoretical model. The results showed that the theoretical results predicted by the model were well-matched with experimental results. Besides rising temperature, the kinetics of Cu-Sn IMCs growth and the isothermal solidification can be remarkably accelerated by decreasing Cu particle size. Reducing Sn content does not affect the kinetics of Cu-Sn IMCs growth but helps to shorten the isothermal solidification time of Cu-Sn TLPS process.

Failure Analysis of Printed Circuit Board Solder Joint under Thermal Shock

Investigating the failure mechanism of solder joints under different temperature conditions is significant to ensure the service life of a printed circuit board (PCB). In this research, the stress and strain distribution of a PCB solder joint was evaluated by high- and low-temperature thermal shock tests. The cross-section of the solder joint after thermal shock testing was measured using a 3D stereoscopic microscope and SEM equipped with EDS. The microstructure of the lead-free solder joint and the phase of the intermetallic compound (IMC) layer were studied by XRD. The working state of the PCB solder joint under thermal shock was simulated and analyzed by the finite element method. The results show that thermal shock has a great effect on the reliability of solder joints. The location of the actual crack is consistent with the maximum stress–strain concentration area of the simulated solder joint. The brittle Cu6Sn5 and Cu3Sn phases at the interface accelerate the failure of solder joints. Limiting the growth of Cu6Sn5 and Cu3Sn phases can improve the reliability of solder joints to a certain extent.

Influence of Temperature and Current Stressing on Cu‐Sn Intermetallic Compound Growth Characteristics of Lead‐Free Microbump

AbstractA numerical analysis of the Cu flux on Cu/Sn/Cu is successfully used to establish kinetic models that are verified with reported data. Kinetic models are adopted to discuss the polarity effect of intermetallic compounds (IMCs) growth at different alloying stages. The models reveal that, before Sn solder is depleted during thermal aging, the net thermodiffusion Cu flux in Cu6Sn5 is over three times larger than that in Cu3Sn. While coupling with current stressing, the IMCs thickness increases from parabola‐like curves to a linear‐like relationship. The degree of influence decreases in the order of Cu6Sn5 on anode, Cu6Sn5 on cathode, Cu3Sn on anode, and Cu3Sn on cathode. Electromigration Cu flux in Sn is the critical factor that accelerates the anode's Cu6Sn5 growth, and its influence on the growth rate over 1000 times that on the anode's Cu3Sn. After Sn solder is depleted, Cu6Sn5 gradually converts into Cu3Sn, and its thickness is linearly decreases with square root of annealing time. When coupling with a current density of 1.0 × 105 A cm−2, the thickness ratio of Cu3Sn/Cu6Sn5 reduces from 1:2 to 1:6. Remarkably, irrespective of whether current exists or not, the depletion of Cu6Sn5 always takes much longer than that of Sn solder.

Interfacial Reaction and Electromigration Failure of Cu Pillar/Ni/Sn-Ag/Cu Microbumps under Bidirectional Current Stressing.

The electromigration behavior of microbumps is inevitably altered under bidirectional currents. Herein, based on a designed test system, the effect of current direction and time proportion of forward current is investigated on Cu Pillar/Ni/Sn-1.8 Ag/Cu microbumps. Under thermo-electric stressing, microbumps are found to be susceptible to complete alloying to Cu6Sn5 and Cu3Sn. As a Ni layer prevents the contact of the Cu pillar with the solder, Sn atoms mainly react with the Cu pad, and the growth of Cu3Sn is concentrated on the Cu pad sides. With direct current densities of 3.5 × 104 A/cm2 at 125 °C, the dissolution of a Ni layer on the cathode leads to a direct contact reaction between the Cu pillar and the solder, and the consumption of the Cu pillar and the Cu pad shows an obvious polarity difference. However, with a bidirectional current, there is a canceling effect of an atomic electromigration flux. With current densities of 2.5 × 104 A/cm2 at 125 °C, as the time proportion of the forward current approaches 50%, a polarity structural evolution will be hard to detect, and the influence of the chemical flux on Cu-Sn compounds will be more obvious. The mechanical properties of Cu/Sn3.0Ag0.5Cu/Cu are analyzed at 125 °C with direct and bidirectional currents of 1.0 × 104 A/cm2. Compared with high-temperature stressing, the coupled direct currents significantly reduced the mechanical strength of the interconnects, and the Cu-Sn compound layers on the cathode became the vulnerable spot. While under bidirectional currents, as the canceling effect of the electromigration flux intensifies, the interconnect shear strength gradually increases, and the fracture location is no longer concentrated on the cathode sides.

Optimal doping elements for inhibiting surface-diffusion of adatoms on Cu3Sn

Downward scaling of the micro-bumps in three-dimensional integrated circuits (3D-ICs) aggravates the formation of lateral Cu3Sn causing serious reliability issues. Inhibiting the continuous surface diffusion of Cu and Sn during solid-state aging is critical to suppress the growth of lateral Cu3Sn. Herein, through comprehensive density functional theory calculations, we found Ti and Si are the ideal surface doping elements for suppressing surface diffusion. The doping Ti and Si not only significantly increase the diffusion barriers of Cu and Sn adatoms in their vicinity but also reduce the mobility of the surface atoms. Through electronic structure and crystal orbital Hamilton population analysis, we reveal the underlying mechanism is the enhanced binding of Cu and Sn adatoms in neighboring adsorption sites and strong covalent bonding of Ti and Si with the surface. Moreover, it is found doping Ti impedes the surface diffusion of Sn adatoms much more greatly than Cu adatoms while doping Si hinders the surface diffusion of Cu adatoms rather than Sn adatoms. This work sheds light on the possibility of utilizing doping elements on intermetallic compound surface to effectively inhibit surface diffusion and reduce reliability risks in 3D-ICs.

Preparation of large Cu3Sn single crystal by Czochralski method

Cu3Sn was recently predicted to host topological Dirac fermions, but related research is still in its infancy. The growth of large and high-quality Cu3Sn single crystals is, therefore, highly desired to investigate the possible topological properties. In this work, we report the single crystal growth of Cu3Sn by Czochralski (CZ) method. Crystal structure, chemical composition, and transport properties of Cu3Sn single crystals were analyzed to verify the crystal quality. Notably, compared to the mm-sized crystals from a molten Sn flux, the cm-sized crystals obtained by the CZ method are free from contamination from flux materials, paving the way for the follow-up works.

Effects of Mo nanoparticles addition on the evolution of microstructure in Cu58Bi-xMo/Cu solder joints during aging

A novel composite lead-free solder was prepared by adding Mo nanoparticles in Sn58Bi solder. Microstructure evolution of Sn58Bi-xMo(x = 0, 0.5%,1%)/Cu solder joints were investigated during aging process at different temperature. The results reveal that Mo nanoparticles significantly inhibited the growth of Cu6Sn5 compounds layer in solder joints, but had little influence on Cu3Sn compounds layer. The diffusion coefficients of atoms at different aging temperature were calculated. It was observed that several Cu6Sn5 compounds layer began to separate and diffuse into solder when Sn58Bi-0.5%Mo/Cu and Sn58Bi-1%Mo/Cu joints were aged for 240 h at 130 ℃. The influence mechanisms of Mo nanoparticles on the growth of Cu6Sn5 and Cu3Sn compounds were analyzed.

Predicting the Cu6Sn5 Growth Kinetics During Thermal Aging of Cu-Sn Solder Joints Using Simplistic Kinetic Modeling

Predicting the Cu6Sn5 Growth Kinetics During Thermal Aging of Cu-Sn Solder Joints Using Simplistic Kinetic Modeling

Influences of Impurity Incorporation in Electroplated Cu on the SnAgCu and Ni-Containing SnAgCu Solder Joints

SnAgCu and Ni-containing SnAgCu alloys are Pb-free solders widely used to join with Cu to construct the solder joints. Electrodeposition is a technology commonly used to fabricate Cu but co-deposition of organic impurities originating from additives is an inevitable reliability issue. This study investigates the impurity effect on the voiding propensity in the two solder joints (SnAgCu/Cu and SnAgCu-Ni/Cu) subjected to thermal aging at 200 °C. Results show that a high level of impurity incorporation causes massive void propagation along the SnAgCu/Cu and SnAgCu-Ni/Cu interface. Reduction of the impurity concentration by precise control of the additive formulas can weaken the impurity effect and effectively suppress the void propagation. The weakening phenomenon of the impurity effect is more pronounced in the SnAgCu-Ni/Cu joint, indicating that suppression of the Cu3Sn growth as well as Kirkendall voids by Ni addition is also helpful in reducing the influences of impurities.

New phenomenon: Cellular boundary during the isothermal stage in the Sn-3Ag/(0 0 1)Cu and Sn-3.5Ag/(0 0 1)Cu joint

The Cu6Sn5 intermetallic, which commonly forms at the solder interconnects, is a critical component contributing to the reliability of solder joints. In this paper, the growth behavior during the isothermal stage of Cu6Sn5 grains in the Sn-xAg/(001)Cu (x = 0, 3, 3.5) joint was investigated. A new phenomenon, the Cu6Sn5 with cellular boundary, is found in the isothermal stage of Sn-3Ag/(001)Cu and Sn-3.5Ag/(001)Cu joint, but not found in the Sn/(001)Cu joint. The result suggests that the Ag3Sn play an important role in the formation of the cellular boundary. High-pressure air blowing was used to uncover the morphology of Cu6Sn5 at the isothermal stage. Scanning electron microscopy and electron backscatter diffraction were employed to characterize the morphology and grain orientation, respectively. The result has a great meaning in understanding the growth of Cu6Sn5 during the isothermal stage and improving the reliability of solder joints.

Kinetics of Cu6sn5 and Cu3sn Intermetallic Compounds Growth and Isothermal Solidification During Cu-Sn Transient Liquid Phase Sintering Process

Kinetics of Cu6sn5 and Cu3sn Intermetallic Compounds Growth and Isothermal Solidification During Cu-Sn Transient Liquid Phase Sintering Process