Interfacial Intermetallics Research Articles

Optimization of Joining Parameters in Pulsed Tungsten Inert Gas Weld Brazing of Aluminum and Stainless Steel Based on Response Surface Methodology

Combining aluminum and steel offers a promising solution for reducing structural weight and fuel consumption across various industries. Pulse in tungsten inert gas (TIG) weld brazing effectively suppresses interfacial brittle intermetallics and enhances joint strength by influencing pool stirring and heat input during aluminum-to-steel joining. However, optimizing the pulsed TIG weld brazing process is challenging due to its numerous welding parameters. This study established statistical models for Al/steel joint strength without reinforcement using response surface methodology (RSM) based on central composite design (CCD). The models’ adequacy and significance were verified through analysis of variance (ANOVA). The four welding parameters influence weld strength in the following descending order: pulse on time > base current > pulse current > pulse frequency. Additionally, interactions between pulse current and pulse frequency, and between pulse on time and base current, were observed. Numerical optimization using RSM determined the optimal pulsed GTA weld brazing parameters for aluminum and stainless steel. With these optimized parameters, the joint strength reached 155.73 MPa, and the intermetallic compound (IMC) thickness was reduced to 3.4 μm.

Prototyping Ti2Cu intermetallic grain growth heterogeneously in Ti6Al4V matrix through laser additive manufacturing

The Ti2Cu intermetallics compound possessing an ability to release Cu2+ ions has promising antibacterial properties. This research integrates experimental and computational techniques to delve into the interfacial intermetallic (IMC) formation within the Ti6Al4V/Cu system. Heat transfer analysis is performed to numerically outline the thermal history within the microstructure during laser surface alloying (power = 1000 W and scan speed = 200 mm/s), while the phase field method is employed to describe the Ti2Cu IMC grain growth in the Ti6Al4V matrix. As the IMC and the matrix phases respectively have free energy minima of -68.52 and -61.35 kJ/mol at Taverage=1250.0 K, the growth of Ti2Cu IMC grains at the Ti6Al4V matrix located around the depth of 5 μm from the top surface is thermodynamically favorable. Laser processing can cause the nucleation of IMC precipitates of irregular sizes and curvature profiles, in which case the curvature driven effects between the adjacent grains can additionally affect their growth kinetics. Furthermore, microbiological assays underscore the superior antibacterial efficacy of the Ti6Al4V+Ti2Cu heterostructures as compared to the control (Ti6Al4V) counterpart.

Microstructure and properties of Cu/Si composites for electronic packaging: Effect of tungsten layer on silicon particles

Cu/Si composite are ideal electronic packaging materials, but the intense formation of Cu-Si intermetallics during preparation leads to a significant decrease in their plasticity and thermal conductivity. In this paper, a W layer was formed on Si particles using the sol-gel method, and Cu/Si composites were prepared by hot pressing. The results show that the maximum thickness of the W layer is 1.64 μm, and the minimum thickness is only 809 nm. The W layer acts as a diffusion barrier, effectively inhibiting the Cu-Si interfacial reaction and intermetallics formation, thus preventing the deterioration of matrix and reinforcement properties. Compared with the Cu/Si composite, the elongation of the Cu/Si@W composite improves from 0.2 % to 1.7 %, and the thermal conductivity increases by 7 times to 233.9 W/(m·K). Additionally, among the four samples, the Cu/Si@W composite has the lowest coefficient of thermal expansion (CTE). The W layer enhances the mechanical properties and thermal conductivity while reducing the CTE of Cu/Si composite for improved electronic packaging application.

Characterization of ductile nano-TiC dispersed V4Cr4Ti/RAFMs solid diffusion joint for nuclear fusion reactor application

Reduced activation ferrite/martensite (RAFM) steel and V4Cr4Ti alloy are candidate structural materials applicable for blankets with various coolants for fusion reactor. In order to combine their advantages, a solid-state diffusion bonding bimetallic plate was designed to provide for high-temperature strength and neutron-irradiation resistance from the vanadium alloy and corrosion-resistance from the steel. However, brittle intermetallic and carbide layers can be formed during the high-temperature diffusion process, which is detrimental to the mechanical properties of the joint. To understand the mechanisms for a ductile joint, the diffusion bonding was performed at different temperatures by hot isostatic pressing (HIP). Various methods were used for the interfacial analysis, together with shear mechanical test and Vickers hardness test across the interface. The interfacial intermetallics differ significantly from each other with different bonding temperatures. Brittle σ-(Fe, V) intermetallic and continuous V2C carbide layer could be avoided by optimizing the diffusion parameters. The joint bonded at 850 °C shows the optimal ductility with highest shearing strength value of 240 MPa, resulting from the synergetic effects. The dispersed TiC nanoparticles formed at interface would play an important role to improve the mechanical performances of the V4Cr4Ti/CLF-1 joint. Besides, no continuous thick carbides or σ- (Fe, V) layer, finer grains in the decarburization layer and the ultra-fine recrystallization grains of the vanadium alloy around the interface are all helpful for joint performance improvement.

Ultrasound-induced interinhibitive dissolution-precipitation evolution and significantly improved mechanical properties of Kovar/SnSb10 assembly

Ultrasound-assisted Kovar/SnSb10 assemblies are promising candidates for high-precision electronic devices; however, limited research has been conducted on the effect of ultrasonication on its interfacial evolution and mechanical properties. In this study, the microstructure evolution, orientation relationships (ORs), interinhibitive dissolution-precipitation mechanism, and mechanical properties of a Kovar/SnSb10 assembly with/without ultrasonication were systematically investigated. The morphology of the intermetallics (IMCs) evolved from a newly-formed, lash-type to a block-type morphology with an increasing reflow duration, and dentation-type IMCs appeared after ultrasonication. The interfacial IMC without ultrasonication mainly contained FeSn2 and a minority of Ni3Sn2, whereas the strong disturbance caused by ultrasound led to the formation of FeSn in the peripheral FeSn3 and NiSn IMCs. Furthermore, high-resolution transmission electron microscopy (HRTEM) analysis revealed that the ORs of IMC/solder and IMC/Kovar transformed from coherent and semi-coherent to incoherent relationships after ultrasonic treatment. The ORs of the IMC/solder growth front after ultrasonication were [0 1¯ 1]Sn∥[1 1 0]FeSn3, (0 1 1)Sn∥(0 0 1¯)FeSn3 and [0 1¯ 1]Sn∥[5¯ 1 0]NiSn, (0 1 1)Sn∥(0 1¯ 0)NiSn. Additionally, 60 s ultrasonic treatment promoted the growth of IMCs, and increased the ultimate shear strength by approximately 20 MPa, despite the emergence of incoherent interfaces. This phenomenon was primarily attributed to the dispersion strengthening effect of the abundant ultrasound-induced nano-Ni3Sn2 precipitates. This study substantiates the feasibility of ultrasonication-assisted Kovar/SnSb10 assemblies and introduces a novel concept for low-cost, high-efficiency, and high-performance packaging.

Laser assisted fabrication of mechanochemically robust Ti3Au intermetallic at Au-Ti interface

Intermetallics such as Ti3Au are characterised by improved mechanical properties and reduced susceptibility to corrosion. This study extensively blends the techniques of process simulation by finite element analysis, light microscopy, scanning electron microscopy, energy dispersive spectroscopy and structural roentgenography analysis to investigate the interfacial intermetallics (IMC) in Au-Ti system. Ti was electrochemically coated with a micrometric layer of gold and then laser-treated in a wide range of parameters. Variability included both the laser beam power and the beam feed rate. The experimental results confirm that the formation of an intermetallic Ti3Au phase is possible. Finite element method was employed to numerically estimate the transient temperature values at the Au-Ti interface during laser treatment. At laser power 100 W and scan speed of 1.5 mm/s, the interfacial temperature reached a magnitude larger than the melting point of Ti for a duration less than 5.0 ms. The decrease of scan speed to 1.0 mm/s for constant P, increased this duration up to 97.0 ms where both provided sufficient energy for convection-based reaction at the interface. Through nanoscale molecular dynamics simulation, it has been revealed that during nanoindentation, Ti and Au atoms of Ti3Au IMC exert alarger resistance force to the indenter as compared to the atoms of pure Ti material. The IMC material not only offers amore robust mechanical response during indentation but also exhibits superiority over pure Ti material in a corrosion test.

Phase field modelling combined with data-driven approach to unravel the orientation influenced growth of interfacial Cu6Sn5 intermetallics under electric current stressing

Microstructure of interfacial intermetallics plays an important role in determining the service performance and reliability of interconnects especially due to the anisotropic properties of intermetallic grains, preferential growth of intermetallics induced by the electric current is observed in experiments, but the exact mechanisms for this have not been understood completely. For this endeavor, a phase field model considering the free energy arisen from the applied electric current is developed to tackle the intermetallic grain growth behavior in Sn/Cu interconnects, the focus falls on the influence of anisotropic electric conductivity. Simulation results show that electric current stressing preferentially accelerates the intermetallic growth, and the orientation effects are more pronounced with higher value of electric current density. Due to the competitive growth of multiple Cu6Sn5 grains in the presence of electric current, most region of the intermetallic layer is occupied by the grains whose c-axis is along the direction of the electric current. The intermetallic grain with higher electric conductivity along the electron flow holds smaller current field, while the phase boundaries own higher electric field. It is found that the higher local electric field concentrating at phase boundary drives faster phase boundary migration. The data generated from physics-based phase field simulations are utilized to train a neural network model that functionally maps the area of a particular grain with the applied current density, simulation time, grain identification, orientation of the grain and its neighboring grains. The prediction model reveals that that electric current stressing preferentially accelerates the intermetallic growth, and the orientation effects are more pronounced with higher value of electric current density. The results from phase field simulation and physically informed machine learning model further deepen understandings of the microstructure evolution and selective intermetallic growth in the context of electric current, and shed light on the in-silico studies and design route of interconnects under other types of loadings.

Influence of Pre-Tinning Process on Coating Morphology and Interface Structure of Low Carbon Steel Dipped in Molten 6061 Al Alloy

Pre-treated low carbon steel specimens with flux or flux + tin mixture were coated by hot-dip aluminizing process. Al alloy (6061) was melted and hold at 750 °C. Fluxed and pre-tinned low carbon steel samples were dipped in a molten bath for time intervals of 0.5, 1, 2.5 and 3.5 min. Applying double coating processes via tinning-aluminizing techniques facilitated the formation of Fe-Al intermetallic interface and increasing the thickness of homogenous coating layer over the substrate material. The presence of Sn facilitates to great extent the formation of a better interlayer-free bond of residual flux and/or oxides. The fluxed–dipped steel substrates have inhomogeneous distribution of Al alloy coating as well as an interface with residual flux and oxides for dipping time up to 2.5 min. A homogenous distribution with good thickness morphology of the Al alloy coating and homogeneous thin intermetallic interface was achieved for tinned steel substrate at all applied dipping times. The comparison between the pre-tinning and pre-fluxing processes on steel substrates showed a significant effect of tinning over fluxing treatment acting on the thickness layer of Al-coating and interface using a short time dipping. For dipping time up to 2.5 min, the hardness of pre-tinning substrates is greater than that of fluxed ones due to the presence of residual flux and void interface in fluxed steel.

Properties of Sn-3 wt%Ag-5 wt%Cu alloys with Cu6Sn5 intermetallics grain refined by Mg

This paper investigates the influence of Mg additions on the microstructure, thermal properties, electrical properties, and joint strength of bulk and ball grid array (BGA) reflowed Sn-3 wt%Ag-5 wt%Cu solder alloy and the joint intermetallics (IMCs). The cross-sectional microstructure analysis revealed that the addition of Mg effectively triggered the grain refinement of primary Cu6Sn5 IMCs in the bulk solder alloys. In addition, the thickness of the interfacial Cu6Sn5 IMCs layer was suppressed. The optimal addition for refinement is in the vicinity of 0.10 wt%Mg. Furthermore, synchrotron tomography three-dimensional reconstructed imaging reveals that the grain refinement of primary Cu6Sn5 IMCs is also effective even in BGA scale solder joint at normal reflow conditions. These primary Cu6Sn5 IMCs showed a non-faceted morphology. 0.10 wt%Mg addition reduced the number and size of the Cu6Sn5 IMCs. The grain refinement of primary Cu6Sn5 IMCs could be attributed to the presence of Mg, Mg2Sn, and MgO phases that potentially serve as heterogenous nucleation sites. The thermal analysis showed that the degree of undercooling values was significantly reduced to the minimum with 0.10 wt%Mg addition, which increased when the amount of Mg increased to 0.15 wt%. The mechanical properties showed that the ductility of the solder joint at high shear speed was improved.

Formation of Fe5Si3 precipitate in the Fe2Al5 intermetallic layer of the Al/steel dissimilar arc welding joint: A transmission electron microscopy (TEM) study

The further strengthening of the Al/steel dissimilar joint is significantly meaningful for car lightweight development in automotive industry. It has been experimentally proved that the use of Si containing aluminum alloy filler material such as ER4043 grade welding wire can effectively improve the joint strength by reducing the FeAl intermetallic interface thickness. However, the specific functioning mechanism of Si has not been clarified explicitly thus controversy still exists. In the present research, an Al/steel dissimilar joint is generated using the variable polarity cold metal transfer (VP-CMT) technique. To address the suppression function of Si on the FeAl intermetallic interface growth clearly, the focused ion beam (FIB) is conducted to extract the testing sample right at the interface, which is subsequently investigated under TEM. According to the obtained results, the intermetallics formed at the FeAl interface are generally Fe4Al13 phase on the aluminum side and Fe2Al5 phase adjacent to the steel side. Also, rod-shaped Fe5Si3 precipitates are confirmed in the Fe2Al5 matrix near the steel side, rather than Si solid solutions or ternary Fe-Al-Si phases stated in the previous researches. As indicated by the dislocation distribution in the Fe2Al5 matrix, the generation of Fe5Si3 precipitates considerably suppresses the brittle Fe2Al5 fracture. Such phenomenon clarifies the Si element functioning mechanism of improving the Al/steel dissimilar joint quality.

Formation of Die Soldering and the Influence of Alloying Elements on the Intermetallic Interface.

Die soldering of die castings is a serious problem in the aluminum casting industry. The precise mechanism, the influence of the alloy composition, and the options for prevention have not yet been fully elaborated. A well-established solution for alloys with low iron content is the addition of manganese. However, up to 0.8 wt.% is necessary, which increases the amount of brittle phases in the material and consequently reduces ductility. Immersion tests with 1.2343 tool steel and pure aluminum as well as a hypoeutectic AlSi-alloy with Mn, Mo, Co, and Cr additions were carried out to systematically investigate the formation of die soldering. Three different intermetallic layers and a scattered granular intermetallic phase formed at the interface between steel and Al-alloy after immersion into the melt for a duration of 6 min at 710 °C. The combined presence of the irregular, needle-shaped β-Al5FeSi phase and the surrounding alloy was responsible for the bond between the two components. Mn and Mo inhibited the formation of the β-phase, and instead promoted the αC-Al15(Fe,X)3Si2 phase. This led to an evenly running boundary to the AlSi-alloy and thus prevented bonding. Cr has proven to be the most efficient addition against die soldering, with 0.2 wt.% being sufficient. Contrary to the other elements investigated, Cr also reduced the thickness of the intermetallic interface.

Interfacial Reactions between Ga and Cu-10Ni Substrate at Low Temperature.

Ga alloys have been attracting significant renewed attention for low-temperature bonding applications in electronic packaging. This study systematically investigates the interfacial reaction between liquid Ga and Cu-10Ni substrates at 30 °C. In addition to CuGa2 formed from binary Ga/Cu couples, a layer of nanocrystalline Ga5Ni and CuGa2 formed between the Cu-10Ni substrate and the blocklike micrometer scale CuGa2 layer. The growth of interfacial intermetallics (IMCs) on the Cu-10Ni substrate was substantially accelerated compared to the IMC growth in binary Ga/Cu couples. Reaction kinetics study shows the IMC growth from the Cu-10Ni substrate was controlled by reaction and volume diffusion, while the IMC growth from the Cu substrate was controlled by volume diffusion. It is also found that the presence of Ni within the CuGa2 phase resulted in improved thermal stability and a smaller coefficient of thermal expansion during heating from 25 to 300 °C, using synchrotron XRD analysis. There was least thermal expansion anisotropy among most of the IMCs that form in conventional Sn-based solder alloys, including Cu6Sn5 and so forth. It is concluded that using a Cu-10Ni substrate as opposed to a Cu substrate could achieve sufficient metallurgical bonding within shorter processing time. The results have implications for broadening the application temperatures when using Ga as a low-temperature joining material.

Retardation Effect of Tin Multilayer on Sn-3.0Ag-0.5Cu (SAC305)-Based Solder Joint Interface

Transient liquid phase-like soldering technique has been successfully applied to prepare lead-free solder joints at 230 °C. Multiple interlayers of tin thin films in combination with a Sn-3.0Ag-0.5Cu (SAC305) solder paste have been used as filler for joining. Microstructural analysis was carried out by electron microscopy and energy-dispersive spectroscopy. For interlayer containing samples (Cu-Sn-SAC305-Sn-Cu), interfacial intermetallics layers found to be more homogeneous compared to the samples without interlayers (Cu-SAC305-Cu). Quality of the joints has been investigated nondestructively by ultrasonic method. On measuring the electrical resistivity by four-probe method, Cu-Sn-SAC305-Sn-Cu solder joint shows lower electrical resistivity compared to that of the Cu-SAC305-Cu solder joint, wherein nonlinear ultrasonic parameters confirm the superiority of former over later.

Analysis and control of Cu6Sn5 preferred nucleation on single crystal (0 0 1)Cu

Fully oriented-intermetallic interconnects (FOI) of high melting point and service performance have highly considerable potential application in the current generation of 3D electronic packaging. The primary process of fabrication of this structure is the Cu6Sn5 preferred nucleation. In this article, the orientations of 59 successive Cu6Sn5 grains along the radius of (0 0 1)Cu/Sn-3.0Ag solder interface were investigated. Finally, an analysis and control model of Cu6Sn5 preferred nucleation determined by chemical potential gradient of Cu migration was established and applied to address and manage the orientation growth of Cu6Sn5 under different reflow temperatures and solder compositions. The results pave the way for orientation controlling of interfacial intermetallics in 3D packaging technologies.

Effect of helium-argon shielding gas in laser-metal inert-gas hybrid welded-brazed Al/steel dissimilar joint

Laser-metal inert-gas welding hybrid welding was applied to dissimilar butt joining of 304 stainless steel and 6063 aluminum alloy using three different shielding gas: 100% argon, 25% helium + 75% argon, and 50% helium + 50% argon. The influence of helium–argon shielding gas on the weld appearance and interfacial intermetallics layer of Al/stainless steel welding-brazing joint was discussed. The addition of helium can shrink arc, suppress laser plasma, and increase the stability of the droplet transfer process. Helium could not change the composition of the interface layer, which consists of θ-Fe(Al, Si)3 layer and τ5-Al7.2Fe1.8Si layer in the top, middle, and bottom regions of the joints. However, the interfacial intermetallics layer becomes thinner and the morphology was more uniform as the helium content changed from 0% to 50%. By analyzing the tensile fracture, it was found that the thicker θ-Fe(Al, Si)3 easily caused the fracture at the interface layer when using 100% argon. The highest tensile strength and the best ductility can be obtained by using 50% helium + 50% argon compared with that of joints obtained with 100% argon and 25% helium + 75% argon. Compared with joints welded in the pure argon atmosphere, the strength and ductility of joints increased by 18.4% and 191%, respectively.

The Growth of Interfacial IMC Layer in SAC0307 Solder Joints with Specific Grain Orientation Under Electrical and Thermal Coupling Fields

We reported the reliability of Sn0.3Ag0.7Cu (SAC0307) solder joints under electrical and thermal coupling fields, where the c-axes of Sn grains were all nearly parallel to the direction of current and thermal flows. In these studies, individual electromigration (EM), individual thermomigration (TM), and thermoelectric coupling migration behaviors were investigated by applying a current density of 104 A/cm2, and a temperature gradient of 103°C/cm at Cu/SAC0307/Cu solder joints, respectively. The SEM and EBSD were utilized to characterize the morphologies and crystal orientations of each solder joint. The results indicated that the Cu atoms tended to migrate to the anode side under EM. The thickness of an interfacial intermetallic (IMC) layer at the anode increased 3.93 μm. In addition, the Cu atoms diffused toward the lower temperature side under TM, and the thickness of the IMC layer at cold side increased 1.72 μm. When the solder joints were experiencing the coupling fields with the same directions of current and thermal flows, the thickness of IMC layer at anode side (cold side) could increase up to 7.67 μm. The results showed that the effect of thermal flow could assist to accumulate the IMC at the anode side, and accelerate the migration of Cu atoms. This research could help to further understand the reliability behaviors of SAC0307 solders under the service environment.

Effect of annealing method and applied stress on aging behavior of copper-aluminum bimetals

The purpose of this study was to investigate the effect of aging method and the presence or absence of external stress on the aging behavior of Cu/Al diffusion couples. For this purpose, three processes including constant temperature furnace annealing (CTFA), constant temperature electrical annealing (CTEA) and variable temperature electrical annealing (VTEA) were performed as aging cycles.The samples were subjected to tensile stresses of 0, 65, 325 and 650 kPa during aging. After the aging, the microstructure of the samples interfaces was examined by optical and scanning electron microscopy. After identifying the intermetallic phases, the thickness of individual phases and the total thickness of the interfacial intermetallics were measured.The results showed that after 250 h of aging without applying external stress, the thickness of the intermetallic phases for VTEA samples was about 3–4 times the thickness of the layers formed in the CTFA samples and about 2 times the thickness of the intermetallic layers in CTEA samples. The results also showed that if external tensile stresses were applied during the aging besides the use of a constant electric current, the increase in the thickness of the intermetallic phases would be much more significant. Applying the stress increases the thickness of the intermetallic layers as well as their numbers.The results of this study show that the investigation of the aging behavior of bimetals and diffusion couples used in the electrical, electronics or other industries that their aging occurs due to the flow of electric current by conventional methods does not have the desired accuracy. Therefore, a fundamental review of these methods, which are established and utilized by most researchers and scientific institutions based on the assumption of constant temperature of the components during the aging, is proposed.

Interface microstructure evolution of a novel CuW/Al composite fabricated by an infiltration method

In the present work, a novel CuW/Al composite was fabricated using an infiltration method by designing a porous W structure on the CuW surface. The evolution and growth kinetics of interfacial intermetallics compounds (IMCs) were studied. The metallurgical reaction between the CuW alloy and Al lead to the formation of five transition zones, which were layer-like, hypereutectic, eutectic, hypoeutectic and needle-like zones from the CuW side to the Al side. Three kinds of IMCs formed at the CuW/Al interface, which were identified as AlCu, Al2Cu and Al4W based on SEM, EDS and TEM analysis. In addition, the effects of temperature (690–740 °C) and holding time (10–60 min) on interfacial microstructure were investigated. The thickness of interface increased with increasing temperature and holding time. The growth kinetics of interface obeyed linear behavior and could be expressed as: y = 3.27 × 108 exp (-163/RT)t. Ab-initio calculations showed that Al2Cu formed first because of the low formation enthalpy. The results provide a guide for controlling the interfacial microstructure and promote the engineering application of the novel composite.

Infrared Brazed Joints of Ti50Ni50 Shape Memory Alloy and Ti-15-3 Alloy Using Two Ag-Based Fillers.

The wettability, microstructures, and bonding strength of infrared brazing Ti-15-3 and Ti50Ni50 shape memory alloy using 72Ag-28Cu (wt.%) and 68.8Ag-26.7Cu-4.5Ti (wt.%) filler metals have been investigated. Only Ticusil® active braze readily wets both Ti50Ni50 and Ti-15-3 substrates. Wetting of eutectic 72Ag-28Cu melt on Ti50Ni50 base metal is greatly ameliorated by adding 4.5 wt.% Ti into the alloy. The brazed Ti-15-3/BAg-8/Ti50Ni50 joint consists of Cu-Ti intermetallics in the Ag-rich matrix. The formation of interfacial Cu(Ti,V) and (CuxNi1−x)2Ti intermetallics next to Ti-15-3 and Ti50Ni50 substrates, respectively, is attributed to the wetting of both substrates. The brazed Ti-15-3/Ticusil®/Ti50Ni50 joint shows a vigorous reaction, which results in the formation of a large amount of Ti2Ni intermetallics in the joint. The maximum joint strengths using BAg-8 and Ticusil® filler metals are 197 MPa and 230 MPa, respectively.

Evolution of microstructure and effects on crack formation of Sn3.0Ag0.5Cu/Cu solder joints under accelerated thermal cycling

Microstructure evolution of Sn3.0Ag0.5Cu/Cu solder joint and their effects on crack formation were investigated under accelerated thermal cycling. Isothermal aging experiments show that the thickness of interfacial compounds layer was proportional to the square root of aging time. A few large Ag3Sn compounds were observed along solder/interfacial compounds layer even into the interfacial Cu6Sn5 compounds layer after a long aging time. The accelerated thermal cycling tests were carried out for the solder joints with different microstructure. Experimental results showed that crack formation mechanism was greatly dependent on the microstructures of solder joint. For as-reflowed joint, stress concentration mainly appeared the grain boundaries of β-Sn due to the anisotropy of thermal expansion coefficients. The movements of dislocations were hindered by pinning on the fine Ag3Sn precipitates. Microcracks initiated and propagated in β-Sn. However, with the increase of volume fraction of interfacial intermetallics compounds layer and the disappearance of Ag3Sn cellular structure, brittle fracture occurred in the coarsened Cu6Sn5 compounds layer.