Ni3Sn4 Intermetallic Compound Research Articles

Microstructural Optimization of Sn-58Bi Low-Temperature Solder Fabricated by Intense Pulsed Light (IPL) Irradiation

In this study, intense pulsed light (IPL) soldering was employed on Sn-58Bi solder pastes with two distinct particle sizes (T3: 25–45 μm and T9: 1–8 μm) to investigate the correlation between the solder microstructure and mechanical properties as a function of IPL irradiation times. During IPL soldering, a gradual transition from an immature to a refined to a coarsened microstructure was observed in the solder, impacting its mechanical strength (hardness), which initially exhibited a slight increase followed by a subsequent decrease. It is noted that hardness measurements taken during the immature stage may exhibit slight deviations from the Hall–Petch relationship. Experimental findings revealed that as the number of IPL irradiation sessions increased, solder particles progressively coalesced, forming a unified mass after 30 sessions. Subsequently, after 30–40 IPL sessions, notable voids were observed within the T3 solder, while fewer voids were detected at the T9-ENIG interface. Following IPL soldering, a thin layered structure of Ni3Sn4 intermetallic compound (IMC) was observed at the Sn-58Bi/ENIG interface. In contrast, reflow soldering resulted in the abundant formation of rod-shaped Ni3Sn4 IMCs not only at the reaction interface but also within the solder bulk, accompanied by the notable presence of a P-rich layer beneath the IMC.

Efficient and robust ultrathin porous carbon encapsulated Ni–Sn intermetallic compounds derived from metal–organic frameworks for in-situ aqueous phase deoxygenation of fatty acid methyl esters

Deoxygenation of fatty acid esters in aqueous phase using in-situ generated H2 has emerged as a promising approach to producing sustainable diesel-like hydrocarbons. In this study, we employed a solvothermal method to synthesize novel Ni–Sn metal–organic frameworks (MOFs) with a homogeneous distribution of Ni and Sn elements. Subsequently, various Ni–Sn intermetallic compounds (IMCs) encapsulated with a porous carbon layer (<3 nm) (Ni–Sn IMC@C) were prepared through the facile pyrolysis of metal–organic frameworks under a controlled N2 atmosphere. Applying methyl palmitate as model reactant and methanol as hydrogen donor, Ni3Sn2 IMC@C gives approximately 100% conversion and 93% yield towards n-C15 and n-C16 alkanes at 330 °C, without significant deactivation during the recycling for five times. The deoxygenation pathway is dominated by decarbonylation (DeCO) on Ni–Sn IMCs. In Ni–Sn IMCs, the oxophilic nature of Sn, along with its charge transfer with Ni, enhances the adsorption of an η2(C, O)-aldehyde configuration. The strategic isolation of adjacent Ni by Sn effectively inhibits methanation and C–C bond hydrolysis, reducing H2 consumption and increasing carbon yield. Moreover, the ultrathin porous carbon encapsulation not only prevents the loss and sintering of Ni3Sn2 IMC under harsh hydrothermal condition but also mitigates mass transfer limitation. Surface oxidation of Ni3Sn2 IMC, particularly the formation of SnO2, is detrimental to deoxygenation. Additionally, Ni3Sn2 IMC@C exhibits high hydrocarbon yields (>89.6%) in in-situ aqueous phase deoxygenation of methyl laurate, methyl stearate and methyl oleate. These findings highlight the remarkable potential of MOF-derived catalysts for hydrothermal deoxygenation.

A first-principles study of the mechanical and physical properties of Ni3Snx intermetallic compounds for high-temperature power device packaging

Ni–Sn intermetallic compound is a highly promising high-temperature packaging material for power devices, and it is very necessary to understand and control the physical and mechanical properties of Ni–Sn intermetallic compounds(IMCs). However, the current research mainly focuses on the rapid preparation of such packaging solder joints, and there is little research on the microstructure, physical and mechanical properties of alloys. In this study, the first-principle and quasi-harmonic Debye models were used to analyze the mechanical characteristics, electronic structure, thermal conductivity, and linear expansion coefficient of three IMCs (Ni3Sn, Ni3Sn2, and Ni3Sn4) of the Ni–Sn system. The results show that Ni3Sn has the highest modulus and fracture toughness and has the best elastic match with Ni. The welded joints of Ni/Ni3Sn/Ni structures can effectively prevent the generation of microcracks under the stress field. They're all mixed-bond compounds, according to the results of electronic structure simulations, and Ni3Sn4 has the most ionic and covalent bonds, so its fracture toughness is the lowest and its anisotropy tendency is the most significant. The thermal conductivity of Ni3Sn is the highest, the linear expansion coefficients of Ni3Sn and Ni3Sn4 are not significantly different, and they match well with the linear expansion coefficients of Ni.

Microstructures and mechanical properties of ENIG/Sn-3.5Ag/ENIG joints formed by ultrasonic-assisted solder bonding

The effects of temperature and ultrasonic-assisted time on the microstructures and mechanical properties of the electroless nickel-immersion gold (ENIG)/Sn-3.5Ag/ENIG sandwich structure in ultrasonic chip bonding were investigated. Ultrasonic-assisted solder bonding was performed for 0.5–4 s at 25, 150, and 250 °C. The bonding was successful under all conditions. At 25 °C, Au–Sn intermetallic compounds (IMCs) were formed at the upper interface, and AuSn4 was formed on Ni–Sn IMCs at the lower interface. As the bonding time increased, spalling of AuSn4 was increasingly observed at the upper interface. At 150 °C, a trend similar to that at 25 °C was observed, but as the time increased, the thickness of the joint decreased, Au–Sn IMCs at the upper interface disappeared, and thin Ni–Sn IMCs and Cu6Sn5 were formed. The results observed at 250 °C differed from those observed at 25 and 150 °C because ultrasonic-assisted solder bonding was performed with the solder in a molten state. Ni–Sn IMCs were formed at the upper and lower interfaces, residual Sn and Au–Sn IMCs were present inside, and the thickness of the joint was very small. At 250 °C, the shear strength was high regardless of the bonding time. Observations of the top views and cross-sections of the fracture surfaces showed that, at 25 and 150 °C, the fractures initially occurred at the lower interface but moved to the upper interface as the bonding time increased. At 250 °C, the fractures did not significantly change, regardless of the ultrasonic-assisted time.

Efficient deoxygenation of methyl esters to hydrocarbons on Al2O3 supported Ni-Sn intermetallic compounds

Al2O3 supported Ni-Sn alloy (Ni/Sn atomic ratio of 6) and Ni3Sn, Ni3Sn2 and Ni3Sn4 intermetallic compounds (IMCs) were prepared by co-precipitation method for the deoxygenation of fatty acid methyl esters to produce hydrocarbons. In comparison with metallic Ni, the catalytic evaluation and methyl propionate-TPD/TPSR results reveal that Ni-Sn IMCs remarkably inhibit the decarbonylation/decarboxylation pathway, C-C bond hydrogenolysis and methanation while promote the hydrodeoxygenation pathway, subsequently enhancing the yield of aiming hydrocarbons. Therein, 20%Ni3Sn2/Al2O3 has the best performance, and the hydrocarbons yield reach about 99% in the deoxygenation of methyl octanoate and methyl palmitate. We suggest that the geometric and electronic effects of Sn contribute the excellent performance of Ni-Sn IMCs, and there is synergetic effect between Ni and Sn sites for the activation of C-O/C=O bond, for which a suitable Ni-Sn atomic spacing is very important. Finally, the catalyst stability and deactivation were also investigated.

Interfacial reaction between Sn-Ag solder and electroless Ni-Fe-P diffusion barriers with different internal microstructure

The increasing operation temperature of electronic devices significantly accelerates the growth of Cu-Sn intermetallic compounds (IMCs) at the interfaces of Sn-Ag solder/Cu substrate which deteriorates the device reliability. A robust diffusion barrier is thus required to suppress the interfacial reactions between solders and substrates. In this work, electroless Ni-Fe-P coatings with three different internal microstructure were prepared to compare their interfacial reactions and diffusion barrier properties in Sn-Ag solder interconnects. Ni-Fe-P coatings can effectively suppress the excessive growth of interfacial IMCs in Sn-Ag solder interconnects during reflow process. It has been found that the Ni-Fe-P coatings with crystalline structure had great influence on IMCs formation and morphology at Sn-Ag/Ni-Fe-P interfaces. Among three types of Ni-Fe-P coatings, the crystalline Ni-Fe-P exhibited the optimal diffusion barrier property owing to the presence of a uniform and thinnest FeSn2 layer at the interface, while the amorphous Ni-Fe-P coating was less effective due to the irregular and thickest Ni3Sn4 IMCs formed at the interface.

Size effects on IMC growth of Cu/Ni/Sn-3.5Ag microbump joints during isothermal aging and prediction of shear strength using ANN

Significant downsizing trend is currently directing the advanced microelectronic packaging industry, which poses new challenges to the manufacturing process and reliability of high-density chip interconnections. In this paper, to investigate the size effect of microbump joints on the interfacial structure and bonding strength, the chip-to-chip bonding joints fabricated by microbumps of same Cu/Ni/Sn-3.5Ag construction but of various sizes were experimentally examined. After thermocompression bonding process to fabricate the chip-to-chip microbump interconnections, the intermetallic compound (IMC) growth on the Ni/Sn-3.5Ag interface during 150 °C isothermal aging was compared for joints of 100, 40 and 25 μm in diameter as well as 35, 25 and 15 μm in solder layer height, using field emission electron microscopy and electron probe microanalysis. A clear size effect on the thickness and morphology of Ni3Sn4 IMC was observed, showing thicker IMC in joints of smaller diameter and lower height. The Ni concentration distribution in the solder layer was verified to be dependent on the joint size, with possible reasons being enhanced sidewall diffusion and grain boundary diffusion through IMC grains in small-diametered joints, as well as shorter diffusion path in lower joints. The shear strength of chip-to-chip bonding vehicles was also found to be size-dependent. In addition, artificial neural network (ANN) approach was applied to establish the numerical correlation of structural factors and bonding strength. It was found that a properly constructed ANN model can successfully predict the influence of the complex combinations of geometric and interfacial factors with highly accurate output, providing an effective tool for design and optimization of microbump interconnection structures and processes.

Investigation of interfacial reactions between Sn–Ag–Bi–In solder and (Cu, electroless Ni–P/Cu) substrate

Abstract The growth kinetics of intermetallic compound layers formed between Sn–3Ag–8Bi – 5In solder and (Cu, electroless Ni – P/Cu) substrate were investigated at temperatures between 70 and 170 °C for 1 to 60 days. The layer growth of the intermetallic compounds in the Sn–Ag– Bi – In/Cu and Sn–Ag –Bi – In/electroless Ni –P systems satisfied the parabolic time law. As a whole, because the values of time exponent have approximately 0.5, the layer growth is mainly controlled by diffusion. The apparent activation energies for the growth of the Cu6Sn5 and Ni3Sn4 intermetallic compounds are 81.4 and 67.1 kJ/mol, respectively.

Growth Behavior of Ni-Sn Intermetallic Compounds in Microbumps During Long-Term Aging Process

With downward scaling of bump size in advanced electronic packaging, it is urgent to understand Sn/Ni interfacial reaction in a limited volume. In this paper, the growth behaviors of Ni-Sn intermetallic compounds (IMCs) in Cu/Ni/Sn-Ag microbumps aged at 150 ℃ for 2000 h were investigated. The diffusion of Sn atoms across Ni3Sn4 layer caused constant nucleation of Ni3Sn4 grains near the Ni barrier. The thickening of Ni3Sn4 layer was contributed by grain nucleation and coarsening, leading to a vertical difference of grain size inside the Ni3Sn4 layer. Another kind of IMC, Ni3Sn2, was formed at the Ni/Ni3Sn4 interface. A growth model of Ni-Sn IMCs comprising diffusion, nucleation, grain coarsening and phase transformation was proposed based on experimental results. These findings provided insights into advanced electronic packaging reliability.

Efficient Hydrothermal Deoxygenation of Methyl Palmitate to Diesel-Like Hydrocarbons on Carbon Encapsulated Ni-Sn Intermetallic Compounds with Methanol as Hydrogen Donor

Efficient Hydrothermal Deoxygenation of Methyl Palmitate to Diesel-Like Hydrocarbons on Carbon Encapsulated Ni-Sn Intermetallic Compounds with Methanol as Hydrogen Donor

Terminal Sinter-Bonding Using Silver Paste for Aluminum Nitride Heater

In this study, a Ag sintered joint was applied to replace the high-Pb solder to bonding materials between Aluminum Nitride (AlN) substrates and terminals used in wafer bake processes. The Ag sintered joint was fabricated using an optimized temperature profile. The high-Pb solder specimens were fabricated using a conventional reflow process. An X-ray analysis and mechanical properties evaluation of the specimens were performed. Subsequently, a high temperature storage test was performed for a high temperature reliability evaluation. The initial void analysis result showed that the Ag sintered joint and high-Pb solder joint had approximately 1%, and 20% voids, respectively. After being stored at a high temperature of 280 ℃ for 1000 h, the shear strength of the high-Pb solder joint decreased by approximately 27% owing to the Pb degradation. However, the shear strength of the Ag sintered joint was maintained. After 1000 h of a high temperature storage test, growth of Ni3Sn4 intermetallic compound and Kirkendall voids were observed at the high-Pb solder interface. Furthermore, a Ag-Au layer was observed at the sintered Ag interface. In addition, the sintered Ag joints densificated after the high temperature storage test.

Effect of Ni3Sn4 on the Thermo-Mechanical Fatigue Life of Solder Joints in 3D IC

In this article, the 3D integration with Ni/Sn/Ni joints was conducted using transient liquid phase (TLP) bonding (250°C, 0.2 N) with different bonding time. After TLP bonding, plane-type Ni3Sn4 intermetallic compound (IMC) was observed, and when the bonding time is 180 min, complete Ni3Sn4 was found. The diffusion coefficient D was determined to be 32.4 μm2/min. Based on the finite element (FE) simulation, the results demonstrated that the shear stress and equivalent creep strain increased obviously with an increase in the IMC thickness; the results calculated show that the IMC thickness impacts the fatigue life of solder joints significantly, and the fatigue life decreases notably with an increase in the Ni3Sn4 thickness.

A low-temperature sealing method for metal-supported oxide fuel cell applications: Ni–Sn transient liquid phase bonding

A low-temperature sealing approach, transient liquid phase bonding with Ni–Sn system, was employed to seal metal-supported solid oxide fuel cell (MS-SOFC) components in this study. The Ni layer was prefabricated on the ferritic stainless steels by electroplating firstly, and then were vacuum annealed to enhance the bonding between the Ni coating and the steel substrate. Afterward, the Ni coated steels were transient liquid phase bonded using a thin interlayer of pure Sn. Effects of bonding temperature (250–400 °C) and holding time (10–60 min) on the interface of joints were investigated. The interfacial microstructure of joints was observed by the scanning electron microscope and the transmission electron microscope. A joint with full intermetallic compounds (IMCs) of Ni3Sn4 and Ni3Sn2 were formed at 400 °C/60 min. These Ni–Sn IMCs were all stable up to 794.5 °C, ensuring that the joint with full IMCs microstructure can be performed at the operating temperature of ∼600 °C for MS-SOFC. Furthermore, the residual Ni coating can also be served as a protective layer for the stack.

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.

Effects of Ni(P) layer thickness and Pd layer type in thin-Au/Pd/Ni(P) surface finishes on interfacial reactions and mechanical strength of Sn–58Bi solder joints during aging

To analyze the effects of Ni(P) layer thickness and Pd layer composition on interfacial reactions and the mechanical reliabilities of Sn–58Bi solder joints, we evaluated a phosphorous-contained Ni (Ni(P)) layer thicknesses ranging from 0.3 to 1.0 μm with Au/Pd/Ni(P) or a phosphorous-contained Pd [Au/Pd(P)/Ni(P)] layer in thin-electroless-nickel electroless-palladium immersion gold (ENEPIG) with Sn–58Bi solder joint after aging test. (Pd, Au)Sn4 and Ni3Sn4 intermetallic compounds (IMCs) were dominantly formed at the interfaces of the 0.3 µm to 1.0 μm Ni(P) layers in the thin-Au/Pd/Ni(P) or thin-Au/Pd(P)/Ni(P) joints after aging at 85 °C and 95 °C for 100 h. However, the Ni3Sn4 IMC layer changed to the (Cu, Ni)6Sn5 IMC layer in the 0.3 μm Ni(P) layer contained the Au/Pd/Ni(P) joint after aging at 85 °C for 300 h, because the Cu elements in a Cu pad penetrated through the P-rich Ni layer. Otherwise, the Ni3Sn4 IMC of the 0.3 μm Ni(P) layer in the Au/Pd(P)/Ni(P) joint changed to (Ni, Cu)3Sn4 IMC after aging at 105 °C and 115 °C for 1000 h, due to the P in the Pd layer, which affects the IMC growth rate. The 0.7 µm and 1.0 μm Ni(P) layers in the Au/Pd/Ni(P) or Au/Pd(P)/Ni(P) joints were attributed to the Ni3Sn4 IMC layer for whole aging conditions because the thick P-rich Ni layer suppress Sn and Cu diffusion during aging. In a high-speed shear tests, the shear strength of the 0.3 μm Ni(P) layer in the Au/Pd/Ni(P) joints was relatively low than that of the Au/Pd(P)/Ni(P) joints after aging at 105 °C and 115 °C for 100 h. Ni3Sn4 IMC was observed at the fracture surfaces of the 0.3 μm Ni(P) layer in the Au/Pd(P)/Ni(P) joints after aging at 115 °C for 1000 h, whereas the fracture surface of the Au/Pd/Ni(P) joint was Cu substrate. Therefore, Ni(P) layer thicknesses in excess of 0.7 μm and the P-contain Pd layer in the thin-ENEPIG surface finish with Sn–58Bi solder joints are expected to be highly reliable after long-term aging treatment.

Fast formation of Ni–Sn intermetallic joints using Ni–Sn paste for high-temperature bonding applications

The suitability of Ni–Sn combination paste as a transient liquid phase sintering (TLPS) bonding material for applications in high-temperature power electronic technologies was investigated. The microstructure and mechanical properties of joints bonded through TLPS with a Sn-30 wt% Ni paste were investigated at varying bonding temperatures (250, 270, 300, and 350 °C) and times using field-emission scanning electron microscopy, field-emission transmission electron microscopy, electron probe micro-analyzer, differential scanning calorimetry, and X-ray diffraction analyses. The results indicated that Ni and Sn reacted and bonded quickly to form Ni–Sn intermetallic compounds, which primarily consisted of Ni3Sn4 and residual Ni particles. The mechanical strength analysis results show that the shear strength values of the joints tended to increase as the bonding temperature increased. As Ni3Sn4 reacted with Ni particles, the joints achieved a stable and dense microstructure. The shear strength values were determined to be 47.9 MPa (300 °C) and 41.4 MPa (350 °C) for 15 and 5 min, respectively. Furthermore, the shear strength value saturated at a bonding temperature of 350 °C for a bonding duration of 5 min.

Effect of Co addition into Ni film on shear strength of solder/Ni/Cu system: Experimental and theoretical investigations

Depositing Ni-based films on Cu pads has been accepted as an effective strategy to address the reliability challenges due to the development of plastic ball grid array (PBGA) in electronic packaging technology. Modification mechanisms of Co on Ni film and Sn-37 wt%Pb/Ni/Cu solder joints were researched from experimental and theoretical methods in this work. Experimental results suggested the additional minor Co element (12.46 at.%) led to the formation of Ni–Co solid solution. After reflowing, Ni3Sn4 intermetallic compound (IMC) was formed within Sn–37Pb/Ni/Cu system while (Ni,Co)3Sn4 IMC was produced at solder/Ni–Co interface. Nano-indentation tests indicated that the doping of Co enhanced the mechanical properties of Ni film and Ni3Sn4 IMC. Our single-lap shear tests demonstrated that the shear strength of Sn–37Pb/Ni/Cu reflowing joints increased from 24.68 to 29.72 MPa with the addition of Co. Moreover, theoretical investigations based on first-principles calculations were carried out to uncover the Co strengthening mechanism. The calculated results noted that the improved stability of coating/IMC interface and mechanical properties of coating and IMC phases were the main reasons for the enhanced shear strength of Sn–37Pb/Ni–Co/Cu solder joints.

Microstructural and shear strength properties of RHA-reinforced Sn–0.7Cu composite solder joints on bare Cu and ENIAg surface finish

In this study, the joint effect of rice husk ash (RHA) reinforcement as an alternative silica source and electroless nickel immersion silver (ENIAg) surface finish on the intermetallic compound (IMC) formation and shear strength of the Sn–0.7Cu solder system was investigated. A series of plain and composite lead-free solder systems (Sn–0.7Cu−xRHA; x = 0, 0.01, 0.05 and 0.1 wt%) was successfully developed and subjected to reflow soldering on bare Cu and ENIAg surface finish. After conducting a comprehensive microstructural study using the scanning electron microscopy and energy dispersive spectroscopy techniques, the Cu6Sn5 and Cu3Sn intermetallic compound (IMC) phases were observed at the interface of the Sn–0.7Cu−xRHA/Cu composite solder joints. On the other hand, the (Cu,Ni)6Sn5 and Ni3Sn4 IMC phases dominated the interface of the Sn–0.7Cu−xRHA/ENIAg counterparts. Given the promising potential of the ENIAg surface finish, the Sn–0.7Cu−xRHA/ENIAg exhibited IMC thickness values within a range of 3.81–4.74 µm as compared to the 6.13–9.3 µm range exhibited by the Sn–0.7Cu−xRHA/Cu counterpart. More so, the ENIAg surface finish was effective in improving the shear strength of the plain solder joint, with the Sn–0.7Cu/ENIAg exhibiting 13.44 MPa relative to the 11.5 MPa exhibited by the Sn–0.7Cu/Cu counterpart. Overall, the strengthening effect of the RHA reinforcement was well marked in the Sn–0.7Cu−xRHA/Cu composite solder joints with the composite having 0.1 wt% RHA exhibiting the highest shear strength (14.6 MPa) across the board.

Ultrasonic-accelerated metallurgical reaction of Sn/Ni composite solder: Principle, kinetics, microstructure, and joint properties.

The high-melting-point joints by transient-liquid-phase are increasingly playing a crucial role in the die bonding for the high temperature electronic components. In this study, three kinds of Sn/Ni composite solder pastes composed of different sizes of Ni particles were synthesized to accelerate metallurgical reaction among Sn/Ni interfaces under the ultrasonic-assisted transient liquid phase (U-TLP) soldering. The temperature evolution, microstructure and mechanical property in joints composed by these composite solder pastes with or without ultrasonic energy were systemically investigated. The intermetallic joint consisted of high-melting-point sole Ni3Sn4 intermetallic compound with a little residual Ni was obtained under the conditions of no pressure and lower power (200W) in a high-temperature duration of only 10s, its shear strength was up to 45.3MPa. Ultrasonic effects significantly accelerated the reaction among the interfaces of liquid Sn and solid Ni, which attributed to the temperature rise caused by acoustic cavitation because of large number of liquid/solid interfaces during U-TLP, resulting in accelerated solid/liquid interfacial diffusion and growth of intermetallic compounds. This intermetallic joint formed by U-TLP soldering has a promising potential for applications in high-power device packaging.

Volume Shrinkage-Induced Voiding Mechanism During Electromigration of Cu/Ni/Sn-Ag Microbump.

The effects of annealing, electromigration, and thermomigration on volume shrinkage and voiding mechanisms of Cu/Ni/Sn-2.5Ag microbumps are systematically investigated by using in-situ scanning electron microscopy under current stressing of 1.5×105 A/cm² at 150 °C. The resistance increases rapidly in the initial stage due to formation of intermetallic compounds (IMC)s followed by a gradual increase in resistance. Growth of Ni₃Sn₄ IMCs is controlled by a diffusion-dominant mechanism, and voids and volume shrinkage are closely related to IMC phase transformation of (Au, Ni)Sn₄ to Ni₃Sn₄ in microbumps.