Intermetallic Compound Growth Rate Research Articles

Improved Cu/Sn interface reliability using double-layer stacked nano-twinned Cu

The immoderate growth of intermetallic compound (IMC) at the Cu/Sn bonding interface reduces the reliability of bonding joints, leading to the failure of high-density packaging. Herein, we design a double-layer nanotwinned Cu (d-ntCu) structure composed of the perpendicularly aligned nano-twinned Cu (p-ntCu) as the upper layer and horizontally aligned nano-twinned Cu (h-ntCu) as the lower layer. We study the Cu/Sn bonding interface microstructure evolution to reveal the IMC growth inhibition and void suppression ability of d-ntCu. As a comparison, the interface microstructure evolution of single layer h-ntCu/Sn is also studied. After thermal aging, the growth mode of IMC at the Cu/Sn bonding interface can be divided into two stages, which are caused by the in-situ annealing of the p-ntCu. In the initial stage of aging, the main diffusion path of Cu atoms is twin boundary diffusion, causing a rather fast growth of IMC. After 100 h aging, the twins anneal into large grains, which induce a lower growth rate of IMC due to the bulk diffusion of Cu atoms. Compared with the h-ntCu single layer, the IMC thickness of the d-ntCu is suppressed by 20%. Besides, the bonding interface in d-ntCu/Sn is void-free due to the vacancy-sink effect of twin boundaries. Therefore, using d-ntCu as the substrate can not only control the IMC growth rate but also suppress the formation of voids during aging, which is helpful to achieve high bonding reliability.

To suppress thermomigration of Cu–Sn intermetallic compounds in flip-chip solder joints

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

Intermetallic compound evolution and mechanical properties of Cu/Sn58Bi/Cu 3D structures blended with B4C nanoparticles during isothermal aging

The Cu/Sn58Bi/Cu and Cu/Sn58Bi-0.6B4C/Cu joints were prepared by transient liquid phase (TLP) bonding technology to verify the reliability of low-temperature connection joints. Intermetallic compound (IMC) evolution at the interface under different isothermal aging times was analyzed, and the mechanical properties of the joints were tested. The results show that two kinds of IMC, lamellar Cu3Sn and fan-shaped Cu6Sn5, are formed on both sides of the joint. With increasing soldering time, the thickness for both kinds of IMC become thicker, and it is significantly thicker for the IMC near the cold end. Furthermore, the IMC growth rate of the Cu/Sn58Bi-0.6B4C/Cu solder joint is significantly lower. When the isothermal aging time reaches 36 h, the Cu/Sn58Bi/Cu joint has been almost composed of IMC, while it needs a longer time for the Cu/Sn58Bi-0.6B4C/Cu joint. The growth of Cu3Sn IMC is in line with parabolic law, being mainly controlled by volume diffusion near the hot end, while the grain boundary diffusion significantly affects it at the cold end. The joints shear strength decreases gradually with extending bonding time, but it is always higher for the Cu/Sn58Bi-0.6B4C/Cu joint. When the heating time is not long, the fracture mainly appears at the Bi-rich phase in the solder matrix. With increasing bonding time, the location of fracture gradually transformed from the Cu6Sn5 grains to the junction of the two IMC and Cu3Sn grains, along with the intercrystalline fracture and transcrystalline fracture. In summary, B4C nanoparticles can significantly improve the joint reliability and enhance its mechanical properties.

Effect of IMC Thickness on the Mechanical Properties of Microbumps

With the continuous development of electronic products towards high density and small size, the size of interconnecting solder joints also continues to decrease. Under normal service conditions, micro-solder joints will be subjected to multiple loads at the same time, which in turn will cause many reliability problems in the package structure. This paper focuses on the growth pattern of IMC (Intermetallic compounds) during hot-press bonding and analyses its effect on the shear strength and creep properties of copper pillar micro-bumps. Firstly, the solder joints with different IMC thicknesses were obtained by adjusting the bonding parameters during the thermal compression bonding process, and the growth pattern of IMC during the thermal compression bonding process was analyzed. Secondly, shear experiments were conducted on solder joints with different IMC thicknesses, and the effects of IMC on the shear strength and shear fracture mode of solder joints were analyzed. Finally, creep experiments were performed on cylinder-shaped solder joints with different IMC thicknesses under different stress conditions. The results show that throughout the bonding process, copper atoms migrate under the combined effect of temperature gradient (JTM) and concentration gradient (JCM), and IMC shows asymmetric growth, and the growth rate of IMC gradually decreases with the increase of bonding holding time. Under the same temperature conditions, the shear strength of the solder joint shows a trend of increasing and then decreasing with the increase of holding time. In addition, with the increase of IMC thickness, the shear fracture mode of solder joints changes from ductile fracture in the initial stage to tough-brittle combination fracture and finally evolves to brittle fracture. With the continuous increase of stress, the steady-state creep strain rate of all solder joints increases, and the fracture location tends to be closer to the intersection of the solder and the copper pillar from inside the solder joint. With the increase of IMC thickness, the steady-state creep strain rate of solder joints shows a trend of decreasing and then increasing.

Interfacial intermetallic compound modification to extend the electromigration lifetime of copper pillar joints

Electromigration is the massive metal atom transport due to electron flow, which could induce a disconnect in electronics. Due to the size of copper pillar bump reduction, the portion of interfacial intermetallic compound in solder joints is increasing obviously. However, there is lack of systematical research on the effects of intermetallic compound on the EM lifetime of solder joints. In this paper, the interfacial intermetallic compound of copper pillar joints is modified to extend the electromigration lifetime. The growth rate of intermetallic compound in solder joints sample is calculated firstly. From 230°C to 250°C, the growth rate of intermetallic compound increases from 0.09 μm/min to 0.19 μm/min. With a longer reaction time, the intermetallic compound layers continuously grow. Then electromigration tests were conducted under thermo-electric coupling loading of 100°C and 1.0 × 104 A/cm2. Compared with lifetime of thin and thick intermetallic compound samples, the lifetime of all intermetallic compound sample improved significantly. The lifetime of thin, thick, and all intermetallic compound samples is 400 min, 300 min, and 1,200 min, respectively. The failure mechanism for the thin intermetallic compound sample is massive voids generation and aggregation at the interface between solder joints and pads. For the thick intermetallic compound sample, the intermetallic compound distance is short between cathode and anode in solder joints, leading to lots of crack create in the middle of solder joints. As the all intermetallic compound sample can greatly reduce the number of voids generated by crystal structure transforming, the lifetime extend obviously.

Evaluation of Growth Rate of Intermetallic Compound during Dissimilar Laser Brazing of Steel and Aluminum alloy using Al-Si Filler Metal

Evaluation of Growth Rate of Intermetallic Compound during Dissimilar Laser Brazing of Steel and Aluminum alloy using Al-Si Filler Metal

Novel Sn-0.7Cu composite solder reinforced with ultrafine nanoscale nickel particles/porous carbon

In this study, the nickel-based metal organic framework (Ni-MOF) was carbonized at high temperature and the nanoscale nickel particles anchored in the carbon skeleton ([email protected]) with flake structure were obtained. The composite solders were subsequently prepared successfully by adding the [email protected] flakes to Sn-0.7Cu solder. The effects of [email protected] on the properties, microstructure and interface reaction of the Sn-0.7Cu solders were systematically investigated. The results showed that the [email protected] flakes had a negligible effect on the melting point of the solders and improved the wettability significantly. The wetting angle decreased by 15.04% when the [email protected] was 0.08 wt% in content. In addition, the size of matrix β-Sn grains were significantly reduced and the matrix β-Sn grains appeared very fine structure after adding the [email protected] flakes. Meanwhile, the elongated (Cu, Ni)6Sn5 phase transformed from rod-shape to dot-shape. During the reflow process, the morphologies of intermetallic compounds (IMCs) gradually changed from shell-shaped to continuously planar, and their thickness decreased by 26.6%. The (Cu, Ni)6Sn5 forming at the interface and the porous carbons distributing in solder matrix effectively hindered the diffusion of Cu and Sn respectively and retarded the growth rate of IMCs.

Surface protrusion induced by inter-diffusion on Cu-Sn micro-pillars

With downward scaling of the micro-bumps in three-dimensional integrated circuits, surface inter-diffusion becomes dominant, changing the kinetic path of intermetallic compounds (IMC) formation and causing serious reliability issues. However, an in-depth understanding of the surface inter-diffusion process and the corresponding influence on the formation mechanism of IMC in a micro-bump remain unclear. We conducted annealing at 170 ℃, over 16 h for pillar type Sn/Cu micro-bumps and observed a unique 2-step sidewall Cu3Sn IMC formation phenomenon on the FIB-cut clean surface of the micro-bumps. It is found the two-step sidewall IMC formation is dominated by the surface inter-diffusion of Sn and Cu atoms. Density functional theory calculations reveal that the activation energy barrier of nucleation for sidewall Cu3Sn IMC is about 1/3 of that of sidewall Cu6Sn5 on corresponding interfacial IMC layers, making the formation of sidewall Cu3Sn dominant. Moreover, we proposed a kinetic model that can predict the mean lateral growth rate of the sidewall IMC which may cause fatal short-circuit failure.

Effect of indium on microstructure, mechanical properties, phase stability and atomic diffusion of Sn-0.7Cu solder: Experiments and first-principles calculations

Sn-0.7Cu–In ternary alloy solders have received much attention because of their excellent weld stability and wettability. However, how to inhibit or slow down the formation of defects at the welding interface has always been the focus of research. The effects of In on melting point and microstructure of Sn-0.7Cu solders and phase stability and growth rate of intermetallic compounds (IMCs) in the solders were investigated by means of experiments and first-principles calculations. The Rietveld refinement results showed that the lattice constants of β-Sn and η′-Cu6Sn5 (Cu6Sn5) phases increase after In addition. Moreover, In can decrease the melting point, but increase the melting range of Sn-0.7Cu solders. After adding In, the ultimate tensile strength (UTS) increases, while the elongation decreases. From the first-principles calculation results, the stability of Cu6Sn5 increases after In doping, and In doping results in the formation of new chemical bonds between In atom and the neighboring Cu and Sn atoms. Ultimately, the diffusion activation energy and atomic migration barrier of the Cu atom increase after In doping, which in turn decreases the growth rate of Cu6Sn5.

Thermal aging effects on the properties of the Cu/(SAC0307 powder + Zn-particles)/Cu joint ultrasonic-assisted soldered at low-temperature

PurposeThis study aims to evaluate the effect of thermal aging temperature on the properties of Cu/Cu joints.Design/methodology/approachA new method that 1 um Zn-particles and Sn-0.3Ag-0.7Cu (SAC0307) with a particle size of 25–38 µm were mixed to fill the joint and successfully achieved the micro-joining of Cu/Cu under ultrasonic-assisted at low-temperature, and then the effect of thermal aging temperature on the properties of Cu/Cu joints was researched.FindingsThe composition of the intermetallic compounds (IMCs) on the upper and lower interfaces of Cu/SACZ/Cu joints remained unchanged, which was Cu5Zn8 in aging process, and the thickness of the IMCs on the upper and lower interfaces of the Cu/SACZ/Cu joints increased accordingly. Compared with the as-received joints, the thickness of the upper and lower interfaces IMCs of the soldering aged time for 24 h increased by 404.7% and 505.5% at 150ºC, respectively. The IMCs formation tendency and the IMCs growth rate of the lower interface are larger than those of the upper interface because the soldering seam near the IMCs at the upper and lower interfaces of the as-received joints were mostly white SAC0307 balls black Zn-particles, respectively. The growth activation energy of IMCs in the upper and lower interfaces is about 89.21 and 55.11 kJ/mol, respectively. Under the same aging time, with the increase of the aging temperature, the shear strength of Cu/SACZ/Cu joints did not change significantly at first before 150ºC. When the aging temperature reached 150ºC, the shear strength of the joints decreased significantly; the shear strength of the joints was the smallest at 150ºC for 24 h, which was 39.4% lower than that of the as-received joints because the oxidation degree of Zn particles in the joint with the increase of aging temperature and time.Originality/valueCu/Cu joints were successfully achieved under ultrasonic-assisted at low-temperature.

Interfacial structures and mechanical properties of Cu/Sn/Cu containing SiC nanowires under transient liquid phase bonding

In order to achieve low-temperature bonding and high-temperature service, Cu/Sn/Cu and Cu/Sn-0.6SiC/Cu structure were fabricated by transient liquid phase (TLP) bonding, and the microstructure evolution of intermetallic compound (IMC) and mechanical performance of TLP bonding solder joints were investigated. The results demonstrated that the scalloped Cu6Sn5 IMC and lamellar Cu3Sn IMC formed at Sn/Cu interface gradually grew up in both solder joints as the bonding time increased. The asymmetrical growth of IMC was formed on both sides at TLP solder joints interface, whereas the interfacial IMC growth rate of Cu/Sn-0.6SiC/Cu was smaller than that of Cu/Sn/Cu solder. After TLP bonding for 120 min, Cu/Sn/Cu structure was composed of total Cu6Sn5 and Cu3Sn IMC and Cu/Sn-0.6SiC/Cu structure still contained remaining Sn phase. SiC nanowires (SiC NWs) dispersed in the solder matrix could impede the growth path between Sn and Cu, so SiC NWs doped to Sn solder could slow down the IMC growth. Moreover, introducing SiC NWs could promote the shear strength of composite solder compared with Sn solder joints during TLP bonding.

Board-level drop properties of Sn–49Bi–1Ag/Sn–3.0Ag–0.5Cu hybrid solder joints assembled by low-temperature soldering

The Cu/Sn–49Bi–1Ag/Sn–3.0Ag–0.5Cu/Ni–P hybrid solder joints for customer electronics were assembled by low-temperature soldering at 170–200°C. The relationship between microstructural evolution and board-level drop failure mechanism of these solder joints during aging at 110°C was investigated. The Bi-rich zone increased from 24.98to 78.85% with increasing soldering temperature from 170 to 200°C. The growth rate of intermetallic compound (IMC) in 170-SAC/SB solder joints was approximately 3 times faster than that in 200-SAC/SB solder joints. The drop failure mechanism was explained that the drop-induced crack propagation path shifted from Bi phases in the as-soldered joints to the solder/Cu6Sn5 interface in the aged joints, since the interfacial Cu6Sn5 IMC grew thicker with increasing aging time.

Electrical and thermal stability of Al-Cu welds: Performance benchmarking of the hybrid metal extrusion and bonding process

Advances in joining processes for aluminum and copper are sought after to facilitate the greater adoption of aluminum in electrical applications. Aluminum's chemical affinity to copper causes the joining and lifetime of Al-Cu welds to be vulnerable to the formation of various intermetallic compounds. Intermetallic compounds and the resulting weld structure are known to reduce the structural integrity and increase the electrical resistance of Al-Cu welds. In this study we evaluate the novel joining process, Hybrid Metal Extrusion and Bonding, for butt welding aluminum and copper. The weld structure was examined using scanning and transmission electron microscopy, and the weld resistance was measured using four-point measurements forecast to the weld interface. Energy dispersive spectroscopy and electron diffraction zone axis patterns were analysed to identify intermetallic compounds. Weld samples were examined pre and post heat treatment at 200 °C, 250 °C and 350 °C for total durations of over 1000 h. The results are compared to existing Al-Cu joining processes, and a new metric, weld interface resistivity, is proposed to compare the electrical properties of bimetallic welds. The Hybrid Metal Extrusion and Bonding process was found to form a thin, consistent and straight intermetallic layer with negligible impact on electrical resistance in the as-welded condition. Artificial ageing of samples by heat treatment established the overall growth rate of intermetallic compounds. The growth rate was used to evaluate the weld's operational lifetime versus temperature. The intermetallic growth rate of Hybrid Metal Extrusion and Bonding was quantified at 200 °C and compared to alternative processes. The Hybrid Metal Extrusion and Bonding process showed a significant performance advantage requiring the longest time to reach 2 μm thickness. Furthermore, the growth of intermetallic compounds did not increase the electrical resistance of the weld interface. The negligible impact on electrical resistance and slow intermetallic growth are promising results of the potential functional performance. This study is the first characterisation of the Hybrid Metal Extrusion and Bonding process for electrical applications showcasing its exciting potential for the joining of aluminum and copper.

Controlling the distribution of porosity during transient liquid phase bonding of Sn-based solder joint

The rapid growth of (Cu,Ni)6Sn5 intermetallic compounds (IMCs) when Sn reacts with Cu containing 6 wt%Ni (Cu-6Ni) offers huge potential for resolving the lengthy transient liquid phase (TLP) processing time. However, the uncontrolled formation of voids at the centre of the joint remains a serious issue. Therefore, we have studied the influence of different Ni concentrations (0, 2 and 6 wt%) in the Cu substrate and the relative location of the porosity when reacting with Sn-0.7Cu solder in the sandwich structure of the joint. Through in-situ X-ray microradiography, the changes in the void positions and the reaction time required to form the IMC joints was interpreted with reference to growth kinetics equations. The results indicate that the different growth rates of (Cu,Ni)6Sn5 and Cu6Sn5 IMCs due to the different Ni concentrations of the Cu-Ni substrates can control the location of the porosity and influence the shear strength of the TLP lap joint. This discovery is a potential gateway to developing reliable electronic interconnects for the power device industry.

Systematic investigation of the effect of Ni concentration in Cu-xNi/Sn couples for high temperature soldering

High temperature solders have experienced a marked increase in demand due to the boom of high power interconnects in electric vehicles and the miniaturisation of electronic devices. Transient liquid phase (TLP) bonding is a promising technology for these applications. Alloys from the Cu-Sn system widely used in conventional soldering processes have potential for high temperature applications if the relatively low melting point Sn phase can be consumed in a TLP process. This process results in a joint composed of Cu6Sn5 or Cu3Sn intermetallic compounds (IMCs) which have higher melting points than Sn. Alloying Ni to the Cu substrate promotes the growth rate of the (Cu,Ni)6Sn5 IMCs, significantly reduces the TLP processing time and costs, while suppressing the formation of (Cu,Ni)3Sn. However, the properties of the (Cu,Ni)6Sn5 formed also change significantly depending on the Ni concentration in the system. In this work, the composition, morphology, grain sizes and crystal structure of the (Cu,Ni)6Sn5 formed via the reaction of Sn and Cu-xNi substrates of different Ni concentrations are studied in detail via electron microscopy and synchrotron powder X-ray diffraction to provide a basis for further understanding of the physical, electrical and mechanical properties. The findings are further verified by density functional theory calculations. Additionally, the complex interplay of grain boundary diffusion rates and the IMC nucleation rate which accelerate with increasing Ni concentration, along with the Cu-xNi dissolution rate and the crystal growth rate which decrease with increasing Ni concentration are identified to be the main factors affecting the IMC growth rates.

Effects of amorphous Co W and Ni W barrier layers on the evolution of Sn/Cu interface

In advanced microelectronic packaging, a high-performance diffusion barrier layer is urgently required for Sn/Cu joint, to hinder the formation of CuSn intermetallic compounds (IMCs). In this work, the barrier performance of electrodeposited Ni, Co, amorphous NiW and amorphous CoW were studied. During 150 °C aging test, amorphous CoW exhibited the best barrier performance among the four barrier layers with the slowest IMC growth rate. Compared with the reaction-controlled IMCs in Sn/Co/Cu sample, the IMCs' growth rates in the other three samples were limited by different diffusion species. Due to the larger diffusion flux of Ni diffusing from NiW layer than that of Co from CoW layer, IMCs in Sn/Ni-W/Cu sample grew faster than Sn/Co-W/Cu sample. The nanoindentation test indicated that the IMCs formed in Sn/Co-W/Cu sample were less sensitive to brittle fracture than Sn/Ni-W/Cu sample. This research provides a scientific basis for the selection of barrier layers with excellent barrier performance and mechanical properties in electronic packaging.

Factors Controlling the Defect Formation in Cu-Sn Micro Joints in 2.5D and 3D Interconnects

The Cu-Sn reaction upon soldering has been thoroughly studied and the sporadic formation of Kirkendall voids within the intermetallic bonds between ball grid array (BGA) or surface mount technology (SMT) solder joints and Cu pads has been demonstrated as well-known concern. However, in the realm of ever developing device miniaturization, there is very limited information on the impact of space confinement on the dynamics of the Cu/solder joint interface. It is known that no voiding has been found in solder joints with high-purity Cu but this phenomenon presents itself, albeit to a different extent, when the joint is realized with electroplated Cu. Also, it is believed, that the voiding propensity in the Cu3Sn intermetallic compounds (IMC) is more likely to be higher in micro joints than in BGA/CSP scale (macro) joints for a given quality of electroplated Cu. Along with that it has been shown that the IMCs on Cu surfaces seem to grow much faster when the solder becomes very thin, and this is accompanied with a greatly enhanced risk of Kirkendall void nucleation.The emphasis of this work is on understanding the voiding behavior within the intermetallic bonds of Cu-Sn micro joints (5 – 30 µm) with the eventual goal of learning how it can be controlled or prevented. Our systematic studies have corroborated the association of voiding with the incorporation of small concentrations of impurities resulting from organic additives degradation during the Cu electroplating as already illustrated for macro joints. Our results also show lack of any voiding not only in macro- but also in micro joints when the interconnection is realized with high purity Cu, likely because no organic impurities are present in sufficient amount to prompt defect nucleation and growth. Our research further demonstrates that the IMC growth rate varies with factors and parameters such as solder volume to pad area ratio, solder composition, pad finish, initial assembly process parameters, subsequent heat treatments, and specifics of the further thermomechanical history, thus impacting differently the overall voiding level. Following this lead, the analysis of micro joints subjected to different synthetic and processing treatments clearly demonstrates the propensity for voiding to be substantially higher, for a given quality of electroplated Cu, in very small solder joints (5 – 30 µm), such as those found in 3D assemblies, unlike in macro joint counterparts. The risk and severity of voiding in these thinner joints is confirmed to depend on the type and amount of incorporated organic impurities in the electroplated Cu. Therefore, in the present report we show how the bath additive chemistry and various Cu electroplating parameters play a systematic role in the type and quantity of impurity incorporation, and thus the propensity for voiding. Approaches to control the electroplating Cu quality in a way that leads eventually to void-free micro joints are also discussed.

Thermomechanical reliability of a Cu/Sn-3.5Ag solder joint with a Ni insertion layer in flip chip bonding for 3D interconnection

Thermal compression bonding (TCB) has been widely used in flip chip bonding processes for three-dimensional integrated packaging. Therefore, studying the TCB process and solder joint structure is significant for improving reliability. In this study, a Ni layer was deposited between the Cu/Sn-3.5Ag solder bumps of upper and lower chips, and TCB process tests were carried out at different bonding temperatures, forces and times. The solder height at the top and bottom of the copper pillar were recorded. Through the establishment of a mathematical analysis model, the relationship between the solder joint height and maximum stress was analyzed theoretically, and the trend of the shear stress and solder joint height was obtained from die shear tests. Then, the solder joint morphology, strength and microstructure composition of the microbumps were observed after aging. Our results showed that the growth rate of intermetallic compounds (IMCs) for Cu/Sn-3.5Ag microbumps with a 25 μm diameter was 2.5 times those with a 100 μm diameter and that the Ni barrier layer prevented the diffusion of Cu in Sn-3.5Ag solder. In addition, the Ni layer improved the strength of the Cu/Sn-3.5Ag joints after aging. Finally, with extended aging time, the fracture site was found to move from the solder region to the Cu6Sn5 layer for the Cu/Sn-3.5Ag samples, while it remained at the Sn-3.5Ag/IMC interface for the Cu/Ni/Sn-3.5Ag samples.

Effect of CNTs on the intermetallic compound growth between Sn solder and Cu substrate during aging and reflowing

Sn is the main interconnect material for three-dimensional (3D) Packaging of chip stacking in electronic packaging. In this paper, the intermetallic compound (IMC) produced through an interfacial reaction between Sn-xCNTs (x = 0, 0.075 wt%) solder and a Cu substrate was evaluated at 130 °C, 150 °C, and 170 °C for 30, 50, 100 h and after multiple reflows (3, 6, 9). In Sn-0.075CNTs system, CNTs inhibited the growth of Cu6Sn5 and refine the microstructure of solder joint. The growth rate of IMC decreased after reflowing and aging for 100 h. Compared to pure Sn/Cu system, the thickness of Cu6Sn5 and Cu3Sn was the thinner when the CNTs addition amount was 0.075 wt%. Some voids and cracks were formed in solder joints after reflowing and thermal aging. At this time, the IMC growth activation energies of Sn solder is 33.256 kJ/mol, and that of Sn-0.075CNTs composite solder is 58.19 kJ/mol.

Study of electromigration in Sn-Ag-Cu micro solder joint with Ni interfacial layer

This paper reports the extreme sensitivity of the microstructure evolution in Sn-Ag-Cu (SAC) micro-solder joint, consisting of 15–20 µm thick solder alloy and electrolytic Ni plated Cu electrodes, to electromigration (EM) conditions resulting from a kinetic competition between the growth of Ni3Sn4 intermetallic compound (IMC) and EM of Sn in solder matrix. Microstructural analysis of samples tested at various conditions reveals that the voiding in the solder joint develops to the full extent only when the EM rate of Sn in the solder alloy is sufficiently greater than the growth rate of Ni3Sn4 IMC because the IMC phase is EM-inactive. The decisive evidence for such behavior is found from the samples tested at 160 and 170 °C under 35 kA/cm2. The samples tested at 170 °C show the classic solder joint microstructure that typically develops under the influence of EM, that is the exaggerated growth of Ni3Sn4 at the anode as a result of Ni-EM and voids at the cathode interface of the joint as a result of Sn-EM. Contrarily, such characteristic features are not found in samples tested at 160 °C, instead, the joint is fully converted to Ni3Sn4 without sign of its biased growth and voiding activities. The near absence of biased EM microstructure at 160 °C is believed to result from the stress-gradient developed by Sn EM counteracting the EM force on Ni. No such effect is at work at higher temperatures due to the significantly higher initial Sn EM rate which leads to voids and reduces the stress gradient.