•Bare alumina to Copper was successfully brazed in air by ultrasonic brazing.•Ultrasonic intensity varying with transmitted material affects joint microstructure.•Amount of Cu-Zn intermetallic increased as ultrasonic time increasing.•Better shear strength and thermal dissipation for Cu/alumina joints than alumina/Cu.Fluxless brazing of bare alumina with Cu was conducted with an ultrasonic-assisted brazing technique by a Zn-14wt.%Al filler. The shear strength of Cu/alumina joints (84 MPa) exhibited 27% larger than the alumina/Cu joints (66 MPa) due to different intermetallic phases and their morphologies formed in the seam under the same parameters, which are probably attributed to the transmission rate of ultrasonic energy varying with density of the ultrasonic horn-contacted materials. Overgrowth of stalactite-like CuZn5 contributes to better thermal dissipation of the ultrasonic-assisted brazed Cu/alumina joints.

•We design an experiment to obtain IMCs formed at molten solder/Cu interface.•Round Cu6Sn5 grains with strong texture form at SAC solders/Cu interface.•Interfacial IMC microstructure evolution affects its growth extent.•Ag promotes interfacial IMC growth by affecting growth orientation of Cu6Sn5 grains.The effects of Ag content on the interfacial reactions between Sn–Ag–Cu (SAC) solders and Cu substrates during soldering were studied based on an experiment in which the liquid solder is removed after a long exposure time. This removal of the solder allows for the capture and visualization of the interfacial IMCs formed during soldering and avoids the influence of Cu6Sn5 precipitated from the solder matrix during cooling. The results showed that a Cu6Sn5 scallop type layer with round grains having a strong texture was formed at the interface of the SAC solders/Cu systems. The addition of Ag decreased the IMC/liquid SAC solder interfacial energy and improved the wettability of the solder on the Cu. In addition to the reaction energy, the interfacial microstructure also affected the interfacial IMC growth extent. By changing the growth environment near the interface, Ag affected the orientation of Cu6Sn5 grains formed at the interface, leading to the evolution of a microstructure that generates more Cu6Sn5 grain boundaries at the interface. Consequently, under the same reflow conditions, SAC solders with a higher Ag content in contact with the Cu substrate form thicker IMCs interfacial layers.

Bare α-alumina ceramics were directly bonded together in air through ultrasonic-assisted joining by pure aluminum filler within seconds. It was found that rapid wetting through in-situ reaction of elemental aluminum and oxygen, undercutting of the melted aluminum on ceramic surfaces, and large scale growth of fresh nano-sized alumina particles probably combined successively during the joining, attributing to the acoustic cavitation, the streaming and the micro-jetting by ultrasound effects. Ceramic-composed joints were successfully obtained with fracture strength of 101.5 MPa within 90 s at as low as 973 K.

The formation process and mechanism of the Sn3.5Ag/Cu solder bump during induction heating were investigated to elucidate the basic characteristics of the induction heating soldering process. During induction heating, solder balls quickly melted from the surface to the center, which could be utilized to prepare solder bumps of controlled height. Cu6Sn5 grains with a round scallop shape were generated at the interface as soon as the solder ball was wetted on the Cu substrate. With prolonged heating time, the uniform morphology of the interfacial Cu6Sn5 grains was transformed to a special morphology in which scallop-shaped grains occupy the central area of the interface, surrounded by prismatic grains. Numerous bulk Cu6Sn5 grains were precipitated and regularly distributed in the solder matrix, which affected the hardness and shear strength of the solder bumps.

Highly conductive Cu–Cu interconnections of SiC die with Ti/Ni/Cu metallization and direct bonded copper substrate for high-power semiconductor devices are achieved by the low-temperature sintering of Cu nanoparticles with a formic acid treatment. The Cu–Cu joints formed via a long-range sintering process exhibited good electrical conductivity and high strength. When sintered at 260 °C, the Cu nanoparticle layer exhibited a low resistivity of 5.65 μΩ·cm and the joints displayed a high shear strength of 43.4 MPa. When sintered at 320 °C, the resistivity decreased to 3.16 μΩ·cm and the shear strength increased to 51.7 MPa. The microstructure analysis demonstrated that the formation of Cu–Cu joints was realized by metallurgical bonding at the contact interface between the Cu pad and the sintered Cu nanoparticle layer, and the densely sintered layer was composed of polycrystals with a size of hundreds of nanometers. In addition, high-density twins were found in the interior of the sintered layer, which contributed to the improvement of the performance of the Cu–Cu joints. This bonding technology is suitable for high-power devices operating under high temperatures.Keywords: Cu nanoparticles; high conductivity; low-temperature bonding; power electronic packaging; sintering;

A bimodal silver nanoparticle (AgNP) paste has been synthesized via the simple ultrasonic mixing of two types of unimodal AgNPs (10 and 50 nm in diameter). By sintering this paste at 250 °C for 30 min, we obtained an ultrahigh thermal conductivity of 278.5 W m–1 K–1, approximately 65% of the theoretical value for bulk Ag. The shear strength before and after thermal cycling at 50–200 °C for 1000 cycles was approximately 41.80 and 28.75 MPa, respectively. The results show that this excellent performance is attributable to the unique sintered structures inside the bimodal AgNP paste, including its low but stable porosity and the high density coherent twins. In addition, we systematically discuss the sintering behavior of this paste, including the decomposition of the organic layers and the formation of the coherent twins. On the basis of these results, we confirm that our bimodal AgNP paste has excellent potential as a thermal interface material for high temperature power device applications.Keywords: bimodal size distribution; silver nanoparticles; sintering; thermal conductivity; thermal interface material; twin formation;

Silica glass pairs were directly bonded by wet chemical surface activation at a low temperature. A smooth joint interface with no voids and micro cracks was obtained with the assistance of a 250 °C heat treatment and a pressure of ∼30 MPa, and the excellent transmittance of the bonded pair was demonstrated by UV-Vis absorptions. This new method can tolerate a silica glass surface roughness as high as 6 nm. A demo chip with a microfluidic channel was also prepared by this method. A modified model for the glass-to-glass bonding mechanism is proposed based on the surface and interface characterization. Raman scattering analysis showed that Si–O–Si linkages at the silica glass surface were broken, and colloid-like hydrolyzed layers formed on the glass surface after the activation treatment. TEM and EELS results revealed that the 3D glass structure of the Si–O–Si linkages formed again at the interface of the directly bonded silica glass pairs after low-temperature annealing.

•Homogeneous high-melting-point Cu6Sn5 and Cu3Sn intermetallic compound joints were fabricated.•The ultrasonic joining method was utilized to fabricate intermetallic compound joints at room temperature in air.•The processing time was dramatically reduced to only several seconds.•The sonochemical effects of acoustic cavitation were investigated.Homogeneous intermetallic compound joints are demanded by the semiconductor industry because of their high melting point. In the present work, ultrasonic vibration was applied to Cu/Sn foil/Cu interconnection system at room temperature to form homogeneous Cu6Sn5 and Cu3Sn joints. Compared with other studies based on transient-liquid-phase soldering, the processing time of our method was dramatically reduced from several hours to several seconds. This ultrarapid intermetallic phase formation process resulted from accelerated interdiffusion kinetics, which can be attributed to the sonochemical effects of acoustic cavitation at the interface between the liquid Sn and the solid Cu during the ultrasonic bonding process.

•Ultrasonic metal welding of multi-annealed 1100 aluminum foils.•Uniform structure and significant reduction of grain size were obtained.•Dynamic recovery and followed continuous dynamic recrystallization occurred.•Thermal and deformation texture coexisted in the deformed foils.•Sub-grain rotation driven by heat and plastic flow was the mechanism.The annealed 1100 aluminum foils were welded at room temperature with an ultrasonic metal welding (UMW) method. Effects of two key parameters (the oscillation amplitude and the deformation reduction of the welded foils) on the microstructural evolution were investigated. With the increase of oscillation amplitude, the deformation and the grain refinement of the foils in the welded specimen were homogeneous, but the grain size was not less than 25 μm. With the increase of deformation reduction, the microstructures were inhomogenously changed from the initial coarse grains (45 μm) into the dynamically recrystallized fine grains (2 μm) in the upper foil, but they changed little in the lower foil. For both cases, the microstructural evolutions attributed to the grains and/or sub-grains rotation. The dynamic recovery and the followed continuous dynamic recrystallization were the active deformation mechanism during UMW according to the observation of the thermal and the deformation textures. The effects of both ultrasonic amplitude and deformation reduction on the hardness of the builds were measured.

•Silver nanoparticles (Ag NPs) paste is chemically fabricated.•The organic shells outside the Ag NPs flocculated by nitrate are thinner.•Thermal conductivity of Ag NPs sintered at 453 K is above 100 W/K-m.•Large amounts of twin boundaries are observed in the sintered Ag NPs.•Deep crystallization by Ostwald ripening is probably the sintering mechanism.We studied the mechanism and morphological evolution of a thermal interface material prepared by pressureless sintering of a paste containing silver nanoparticles (Ag NPs) flocculated with citrate or nitrate. A high thermal conductivity (>100 W/K-m) was obtained from the material sintered at 453 K for 30 min. Significant crystallization and large amounts of twins were observed in the material. These results could be ascribed to the thinner organic shells surrounded the Ag NPs, resulting in easy activation of Ostwald ripening.

•Combination effects of ultrasonic power and cooling rate on microstructure of SAC305.•Processing depth and time of cavitation and acoustic streaming are limited.•Bigger ultrasonic power makes undercooling smaller and solidification time larger.•Optimizing both ultrasonic power and cooling rate could produce finer alloy.A comparative study on the microstructures of Sn–Ag–Cu alloy ingots grown by ultrasound-assisted solidification was carried out with a specific focus on the limits on the ultrasonic processing depth and time imposed by the cooling rate during the melt solidification. During air-cooling, increasing the ultrasonic power reduced the undercooling temperature and increased the solidification time, leading to β-Sn phase fragmentation from a dendritic shape into a circular equiaxed shape. The grain size was decreased from approximately 300 μm to 20 μm. When the cooling rate was increased from 4 °C/s in air to 20 °C/s in water, the macro-undercooling temperature was more greatly reduced by an increase in ultrasonic power, but the solidification time seemed to change only slightly because only a limited period for ultrasonic processing was permitted in the melt. Under both cooling rates, the microstructures were inhomogeneous along the processing depth. The functional depth and period for ultrasonic cavitation and acoustic steaming contributed to the differences in the solidification microstructures.

•The B content increase results in Bi3 + to Bi5 + conversion rise up the Non-Bridging Oxygen.•The Tg is mainly affected by the Non-Bridging Oxygen.•The coexistence of [BiO3] and [BiO6] is confirmed in Bi2O3–B2O3–SiO2 system.•[BO3] only appeared in high B content samples.The Bi-based lead-free frit for silicon solar cell front contact has a shortage of conversion efficiency compared with traditional products that contain Pb. One feasible method to increase conversion efficiency is by improving the current flow at the interface. The current flow establishment is controlled by the reaction of Bi ions to Ag, SiNx, and the Si wafer. However, the microstructure of Bi ions in the glasses remains unclear. In this work, two groups of Bi2O3–B2O3–SiO2 glasses were studied through differential scanning calorimetry (DSC), X-ray diffraction (XRD), infrared, and Raman and X-ray photoelectron spectroscopies. The results suggest that the Bi3 + to Bi5 + transformation is induced by the increase in B2O3 content in both the 30 mol% and 60 mol% Bi glasses. The coexistence of [BiO3] and [BiO6] is confirmed in all ternary samples. The [BO4] units exhibit in all borosilicate glass. The [BO3] structure only emerges in a high boron concentration. The glass transition temperature (Tg) is strongly affected by the ratio of Non-Bridging Oxygen. The Oxygen packing density also has influence on Tg but not obvious.

With the miniaturization of portable electronic devices, the size of solder joint interconnects is decreasing to micrometer levels. These joints possess only several or even one or two grains, resulting in anisotropy and failure issues. Direct ultrasound-assisted solidification of Cu/SAC305/Cu interconnects for grain refinement and fabrication of isotropic solder joints is presented herein. These joints consist of many β-Sn grains. The average cross-sectional area of the Sn-rich phase is significantly reduced by up to 99% when compared with conventional as-reflowed samples. The ultrasonic power density exhibits a threshold value for affecting the microstructures. Below 200 W cm−2, the β-Sn grains were refined and had circular shape. The Ag3Sn phase grew in a manner similar to branched coral to sizes reaching 30 μm, or as rods aggregated together with Cu6Sn5 tube fragments. Above 200 W cm−2, the microstructures were coarsened and Ag3Sn had plate-like shape. The thickness of Cu6Sn5 intermetallic layers at the Cu/solder interfaces was reduced by more than 26%. The relationships among the ultrasonic power, nucleation rate, local temperature drop, and pressure were identified. At the highest power density of 267 W cm−2, the nucleation rate was about 4.05 × 1014 m−3 s−1, the local temperature drop was 248 K, and the local pressure was on the order of several GPa.

Intermetallic compounds (IMCs) that form at eutectic Sn3.5Ag/Cu and pure Sn/Cu interfaces during solid-state aging are comparatively studied in terms of their respective morphological formations, orientation evolution behaviors, and growth kinetics. During solid-state aging, all the interfacial Cu6Sn5 grains evolve into a layer-type morphology, except for select grains that experience abnormal growth. This abnormal growth is caused by the preferential growth of the Cu6Sn5 at the grain boundary in solder matrix. Meanwhile, textured growth occurs in the Cu6Sn5 layer formed at the eutectic Sn3.5Ag/Cu interface. The morphology of each texture is determined by the initial joint preparation conditions and affects the growth of interfacial IMCs. The results reveal that Sn diffusion occurs faster along the [0001] direction of the Cu6Sn5 crystal than along angles from 25° to 50° relative to the [0001] direction. Additionally, the effects of solder composition on the interfacial IMC growth are evaluated. The results indicate that Ag addition retards IMC growth upon aging by inhibiting diffusion of Cu.

Evolutions of the microstructure and properties of Al/Zn–Al/Cu joints exposed to ultrasonic waves during solidification were investigated. An anisotropic microstructure was exhibited in the joint without ultrasonic treatment during solidification. However, a homogeneous microstructure displayed with equiaxial flower-like CuZn5/Al composites interspersed with refined α-Al grains and Zn–Al eutectics was obtained in the joint treated with ultrasound during solidification. The CuZn5 flower showed an isotropic growth from a CuZn5 “shell” on a α-Al nucleus, and the formation was ascribed to the combined effects of ultrasound-induced erosion of base metals and ultrasound-aided grain refinement. Compared with the joint without ultrasonic treatment, the interlayer in the ultrasound-treated joint showed a 26.17% increase in hardness and a 37.99% decrease in the coefficient of thermal expansion (CTE).

A small quantity of adipic acid was found to improve the performance of dicyandiamide-cured electrically conductive adhesive (ECA) by enhancing its electrical conductivity and mechanical properties. The mechanism of action of the adipic acid and its effects on the ECA were examined. The results indicated that adipic acid replaced the electrically insulating lubricant on the surface of the silver flakes, which significantly improved the electrical conductivity. Specifically, one of the acidic functional groups in adipic acid reacted with the silver flakes, and an amidation reaction occurred between the other acidic functional group in adipic acid and the dicyandiamide, which participated in the curing reaction. Therefore, adipic acid may act as a coupling agent to improve the overall ECA performance.

•Thermomechanical response shows a close relationship with the grain structure.•Deformation was frequently confined within the high-angle grain boundaries.•Different grain orientations exhibit different hardness.•Deformation twins can be induced around the indents in SAC105 solder interconnects.The thermomechanical response of Sn-based solder interconnects with differently oriented grains was investigated by electron backscattered diffraction technique under thermal cycling and thermal shock testing in this study. The results showed that deformation and cracking of solder interconnects have a close relationship with the unique characteristics of grain orientation and boundaries in each solder interconnect, and deformation was frequently confined within the high-angle grain boundaries. The micro Vickers hardness testing results showed that the hardness varied significantly depending on the grain orientation and structure, and deformation twins can be induced around the indents by the indentation testing.

In this study, shear tests have been performed on the as-reflowed Sn3.5Ag solder bumps and joints to investigate the deformation behavior of Sn3.5Ag lead-free solder samples. Scanning electron microscopy (SEM) was employed to characterize the microstructures of the samples and orientation imaging microscopy (OIM) with electron backscattered diffraction (EBSD) in SEM was used to obtain crystallographic orientation of grains to provide a detailed characterization of the deformation behavior in Sn3.5Ag solder samples after shear tests. The deformation behavior in solder samples under shear stress was discussed. The experimental results suggest that the dynamic recrystallization could occur under shear stress at room temperature and recrystallized grains should evolve from subgrains by rotation. Compared with that of non-recrystallized and as-reflowed microstructures, the microhardness of the recrystallized microstructure decreased after shear tests.

To improve the electrical conductivity of the dicyandiamide-cured electrically conductive adhesives (ECAs), short-chain difunctional acids with various structures including oxalic acid, succinic acid, adipic acid, phthalic acid and cis-hexahydrophthalic acid were introduced in-situ to replace the commonly used surfactant-stearic acid on the silver filler surface. Oxalic acid, phthalic acid and cis-hexahydrophthalic acid deteriorated the electrical conductivity of ECAs whereas the electrical conductivity was improved significantly with the addition of succinic acid and adipic acid. Moreover, the storage performance, viscosity, curing behavior, bulk resistivity and the shear strength of ECAs with varied content of adipic acid had been further studied. With the increment of adipic acid, the curing temperature of ECAs decreased, while the reaction between dicyandiamide and adipic acid happened as the content reached 0.3 wt%. And the adipic acid not just improved the electrical conductive, but also improved the shear strength without adversely affected the storage performance of ECAs. As the content of adipic acid reached 0.3 wt%, the lowest bulk resistivity 3.8×10−4 Ω cm was obtained, at the content of 0.5 wt%, the shear strength was improved to 18.5 MPa.

•Cu6Sn5 under-bump metallization (UBM) was fabricated via 1 min reflow.•Cu6Sn5 UBM was confirmed to be single-crystal based on the orientation map.•A host-controlled growth phenomenon of Cu6Sn5 phase existed during this process.•The mechanism for this phenomenon was verified to be the grain boundary migration.To inhibit the rapid consumption of the copper substrate at the intergranular regions (grain boundaries or solder channels), a Cu6Sn5 single-crystal layer was fabricated via 1 min reflow at 250 °C. The orientation maps showed that the host-controlled growth behavior of the Cu6Sn5 phase existed during this single-crystal-forming procedure. By combining surface morphologies and kinetic analyses, the physical mechanism behind this behavior was identified as grain boundary migration rather than Ostwald ripening. This study provided a strong foundation for the fabrication of Cu6Sn5 under-bump metallization, as well as other similar intermetallic diffusion barriers.

Cemented carbide (WC-15 wt.%Co)/beryllium bronze (BeCu) brazed with Ag-based filler was conducted by using a hybrid ultrasonic resistance brazing method. Based on microstructural analysis (nondestructive and cross-sectional observation) and mechanical evaluation (shear and hardness test), when hybrid ultrasonic vibrations with a frequency of about 30 kHz, it was shown that, not only the well-brazed area of the brazing joint interface increases more than 35% (up to 85%), resulting in that the shear strength increases 40% (about 241 MPa), but also the softened region of the BeCu foil is preserved within the contact area with the WC-Co bar, resulting from restraining the grain growth in the BeCu foil through deceasing the heat input and the high temperature residence time. Due to the acoustic cavitation and the acoustic streaming effects, robust brazed joints of dissimilar alloys or materials could be realized successfully even with no flux, and the properties of the base materials can be sustained as much as possible.

The growth orientations of Cu6Sn5 grains formed at a Sn3.5Ag/polycrystalline Cu interface were investigated. Similar as reported on Cu single crystals, strong textures in Cu6Sn5 layers can also form on polycrystalline Cu, but the texture formation mechanisms differ. The texture formation on polycrystalline Cu occurs during the ripening growth and results from the differences in stability of the interfacial grains with various orientations at different temperatures. A reaction temperature of 240 °C causes the Cu6Sn5 layer to form [0001] texture in the direction normal to the substrate, and a special morphology of interfacial Cu6Sn5 grains can be formed on this layer to reinforce joint properties.Highlights► We find the variation law of Cu6Sn5 growth orientations formed on polycrystalline Cu. ► We explain this phenomenon using the ripening growth theory. ► We develop a special morphology of interfacial Cu6Sn5 to reinforce joint strength.

We report a Cu-to-Cu interconnects fabrication process based on the pressureless low temperature sintering of Ag nanoparticles for electronic packaging. The organic shells of citrates covering the nanoparticles stabilize the Ag nanoparticles. It is not necessary for organic shells to be completely decomposed for sintering to take place. Instead, it is sufficient that the chemical bonds that connect the organic shells with Ag nanoparticles are broken. This new point of view leads to a way to lower bonding temperature. A novel pinecone-like recrystallization morphology of sintered Ag nanoparticles is obtained, which results from the residuals of organic shells by the sintering process. The effect of recrystallization morphology on the thermal conductivity of sintered Ag nanoparticles is discussed. The shear strength of joints reaches 17–25 MPa at temperatures ranging from 423 K to 473 K.Highlights► Pressureless sintered joints are achieved at low bonding temperatures of 423–473 K. ► Sintering can take place so long as the chemical bonds on nanoparticles are broken. ► We find a novel pinecone-like recrystallization morphology of Ag nanoparticles. ► Thermal conductivity of sintered Ag layers is reduced due to the scattering effect.

Orientation imaging microscopy was adopted to characterize the microstructural changes in Sn–Ag-based solder interconnects during thermal cycling and shear testing. The deformation and microstructure evolution of Sn–Ag-based solder interconnects are inhomogeneous, depending on the orientations of β-Sn grains in the as-solidified microstructure. Recovery or recrystallization can take place even under pure shear stress at room temperature, and it tends to occur at high-angle grain boundaries in multi-grained solder interconnects, while it localizes in near-interface region in solder interconnects with only one grain inside. During thermal cycling, the hardness of recrystallized microstructure decreased significantly due to the segregation of Ag3Sn IMC particles towards the newly-formed recrystallized boundaries, increasing the ease of localized deformation in this weakened microstructure. As a consequence, cracks were propagated intergranularly in the recrystallized microstructure.

In this study, the microstructure, orientation evolution and failure modes of SnPb, SnAgCu, and mixed (SnAgCu bump + SnPb paste) solder interconnects were studied by thermal cycling between 0 °C and 100 °C (with 15 min dwell times and 15 min ramps). The cross-sectioned ball grid array (BGA) solder interconnects were characterized by scanning electron microscopy (SEM) equipped with an electron backscattered diffraction (EBSD) detector to observe the orientation, microstructure and damage evolution under thermal cycling. The results show that the as-reflowed SnPb solder interconnect is composed of a limited number of grains with similar crystallographic orientations, while the mixed and SnAgCu solder interconnects are composed of a few β-Sn grains or even only one grain. Significant recrystallization behavior was observed in strain concentration regions in all these interconnects after thermal cycling. Small subgrains with low angle grain boundaries could finally evolve into recrystallized grains by rotation and coalescence as thermomechanical fatigue continues. Due to strain enhanced diffusion, dramatic coarsening of Pb phase was observed in the recrystallized regions of SnPb and mixed solder interconnects; similarly, the coarsening of Ag3Sn intermetallic compound (IMC) particulates occurred in the recrystallized microstructures of mixed and SnAgCu solder interconnects after thermal cycling. Cracks were initiated and propagated mainly within the recrystallized microstructures by intergranular cracking in the solder bulk. Interphase sliding was frequently observed along the crack path of SnPb solder interconnects. Vickers microhardness results show that SnAgCu and mixed solder interconnects degrade significantly as thermal cycling continues, and hardness correlates well with the accumulated damage in these solder interconnects.Graphical abstract(a) EBSD orientation map and enlarged area of subgrains in mixed solder interconnect and (b) grain boundary map of enlarged area in (a).Highlights► A limited number of grains with similar orientation were found in SnPb solder joints. ► Recrystallization occurred in strain concentration region by subgrain rotations. ► Strain enhanced coarsening of Pb and Ag3Sn occurred in recrystallization region. ► Recrystallized microstructure is significantly degraded. ► Fine grains produced after recrystallization facilitate grain boundary sliding.

The growth orientation of Cu6Sn5 intermetallic compounds (IMCs) formed at the eutectic Sn37Pb/polycrystalline Cu interface during solid-state aging was investigated. The results indicate that the interfacial Cu6Sn5 grains exhibit textured growth under solid-state conditions, and their preferred orientations are affected by the initial joint preparation conditions. Cu6Sn5 grains in the [0001] direction normal to the interface are stable in solid and molten Sn37Pb solder at 200 °C, but are rapidly consumed at 280 °C. This effect leads to the formation of different textures in the Cu6Sn5 layer during the solid-state aging treatment of joints formed at 200 and 280 °C. In addition, the influence of texture evolution on the growth of interfacial IMCs was evaluated. The results indicate that Sn diffusion is faster along the [0001] direction of the Cu6Sn5 crystal than along an angle of 25–45° to the [0001] direction; therefore, more IMCs are generated at the interface of the joints formed at 200 °C than at those formed at 280 °C under the same solid-state reaction conditions.Highlights► We observe textured growth of interfacial Cu6Sn5 layer during solid-state aging. ► We explain the texture formation mechanism of interfacial Cu6Sn5 layer. ► The textural morphologies are determined by the initial joint preparation conditions. ► The textures of interfacial Cu6Sn5 grains affect the growth of interfacial IMCs.

The growth behaviors of the intermetallic compounds (IMCs) formed at the eutectic Sn3.5Ag/polycrystalline Cu and pure Sn/polycrystalline Cu interfaces are comparatively studied based on an experiment in which the liquid solder is removed before the end of soldering. This removal of the solder allows for the capture and visualization of the interfacial IMCs formed during liquid-state soldering and avoids the influence of Cu6Sn5 precipitated from the solder matrix during cooling. The results show that round, scallop-type Cu6Sn5 grains with a strong texture form at the molten solder/Cu interface and that their growth is controlled more by grain boundary (GB) diffusion at the beginning of the reaction followed by volume diffusion, whereas the growth of Cu3Sn is only volume-diffusion-controlled. In addition, in contrast to the predictions of some studies, Ag does not inhibit interfacial IMC growth. Instead, by changing the interfacial energy between the molten solder and the interfacial IMC, the addition of Ag affects the growth orientation and coarsening behavior of interfacial Cu6Sn5 grains. These changes lead to more Cu6Sn5 GBs at the interface and therefore greater IMC formation and Cu consumption in the Sn3.5Ag/Cu reaction than in the Sn/Cu reaction under the same reflow conditions.Highlights► We design an experiment to obtain IMCs formed at molten solder/Cu interface. ► Round Cu6Sn5 grains with strong texture form at molten solder/Cu interface. ► Growth orientations of Cu6Sn5 grains affect their coarsening behaviors. ► Coarsening behaviors of Cu6Sn5 grains affect their growth thickness. ► Ag does not inhibit the interfacial IMC growth.

The growth behavior of Cu6Sn5 grains formed at an Sn3.5Ag/Cu interface was investigated. During soldering, Cu6Sn5 grains formed at the interface, showing a flattened ovoid shape. During solidification, Cu precipitated from molten solder in the form of Cu6Sn5, forming faceted surfaces on existing interfacial grains. The interfacial Cu6Sn5 morphology was unrelated to its crystal orientation, which was primarily dependent on reaction temperature. A reaction temperature of 240 °C led to an increase in (002) growth and a decrease in (101) growth with time. However, the (002) plane peak was not detected in the interfacial grains formed at a higher reaction temperature (280 °C).

In this study, the effect of mechanical stresses on the recrystallization behavior of Sn-3.0Ag-0.5Cu Pb-free solder interconnects was studied by four-point cyclic bending, shear tests, and tensile tests. Scanning electronic microscopy with electron backscattered diffraction was used to characterize the microstructure and crystallographic orientation of the solder interconnects. The results show that recrystallization occurs under the different mechanical stresses in these tests at room temperature, and that recrystallized grains evolve from subgrains by rotation. Microhardness measurements after shear tests show that the hardness of the recrystallized microstructure was decreased by 15% and 41%, respectively, compared with that of the nonrecrystallized and as-solidified microstructures. Furthermore, the fine-grained microstructure produced after recrystallization facilitated grain boundary sliding. Therefore, the deformation and cracking behaviors were localized in the degraded recrystallized microstructures once recrystallization occurred, accelerating failure of the solder interconnects.

The evolution of microstructures and grain orientations of a Pb-free solder interconnect during thermal cycling significantly affects its mechanical properties and failure modes. Thus, Sn-3.0Ag-0.5Cu ball grid array assemblies were subjected to thermal cycling to study the thermomechanical responses of the solder interconnects. The orientations and microstructures of the solder interconnects were studied by optical microscopy with cross-polarized light and scanning electron microscopy with an electron backscattered diffraction analysis system. Localized recrystallization behavior was observed in Pb-free solder interconnects during thermal cycling. Closer examination of the very early stage of recrystallization in the same solder interconnect revealed that the subgrains appeared before the formation of the recrystallized grains, and the orientations of the small recrystallized grains separated by high-angle grain boundaries evolved from the initial orientations by subgrain rotation. The localized recrystallization produced fine-grained microstructures during thermal cycling, providing an additional deformation mechanism for the solder interconnects, i.e., grain boundary sliding, which would have been impossible prior to recrystallization. The grain orientation has a strong effect on damage generation and the subsequent failure mode; initiation and propagation of cracks could be facilitated by the intrinsic anisotropic thermomechanical responses of the differently oriented grains, leading to a change in the crack propagation path and corresponding failure mode.

The morphologies of Cu6Sn5 grains formed at the interface between Sn-3.5Ag (wt.% unless otherwise specified) and Cu substrates were studied in this work. Reflow experiments were performed for 60 s at peak temperatures of 513 K, 533 K, 543 K, and 553 K. Two morphologies of interfacial Cu6Sn5 grains were observed in wetting reactions: prism type, above 543 K, and scallop type, below 533 K. During aging, the two morphologies gradually transitioned to layer type. These three morphologies could be transformed into each other as long as the corresponding condition changed. The morphology transition of Cu6Sn5 in the wetting reaction was explained by the change in Jackson’s parameter with temperature. In addition, the effect of the Cu content in molten solder on interfacial Cu6Sn5 grains was examined. Significant differences in shear strength were observed for solder joints with different interfacial Cu6Sn5 morphologies in the case of a lower shear height. Joint strength is discussed in terms of the microstructure of the solder matrix and the morphology of interfacial Cu6Sn5 grains.

Interconnection joints are the signal and power carriers for chip-to-package, and their electrical property determines the whole component/device performances. With the process parameters (P, F and t) varying, the bond resistance was in situ measured during ultrasonic bonding. The influence of the process parameters on the bond resistance was obvious. The measured bond resistance changed in the range from 64.5 mΩ to 72.5 mΩ with the ultrasonic power (P) increasing. The maximum change of the single bond resistance was about 4 mΩ. The causation was analyzed in two aspects, evolution of the bond interface and deformation of the bond wire. Interfacial resistance (RI) and deformation resistance (RD) were two primary parts of the variance value.

A rapid yet simple methodology to form the solder joints used in electronic packaging has been demonstrated through an induction heating system in this paper. The objective is to control the shapes of the solder joints precisely and form the hourglass-shaped solder joint using local melting phenomenon. For Sn–Ag eutectic solder bullet-shaped solder bumps and hourglass-shaped solder joints can be obtained easily within 2.3 s. The reason for the formation of hourglass-shaped solder joints has been analyzed using different morphologies of Ag3Sn intermetallic compound (IMC), and is explained by the non-uniform temperature distribution caused by a local melting phenomenon and skin effect.

A novel lead-free bumping technique using an alternating electromagnetic field (AEF) was investigated. Lead-free solder bumps reflowed onto copper pads through AEF have been achieved. A comparison was conducted between the microstructures of the lead-free solder joints formed by the conventional thermal reflow and AEF reflow. Keeping the substrate temperature lower than that of the solder bumps, AEF reflow successfully created metallurgical bonding between the lead-free solders and metallizations through an interfacial intermetallic compound (IMC). The AEF reflow could be finished in several seconds, much faster than the conventional hot-air reflow. Considering the morphology of the interfacial Cu6Sn5 IMC, a shorter heating time above the melting point would be a better choice for solder joint reliability. The results show that AEF reflow is a promising localized heating soldering technique in electronic packaging.

The effect of the anode and cathode on the electrochemical corrosion behavior of lead-free Sn-Ag-Cu and Sn-Ag-Cu-Bi solder joints in deionized water was investigated. Corrosion studies indicate that SnO crystals were generated on the surfaces of all lead-free solder joints. The constituents of the lead-free solder alloys, such as Ag, Cu, and Bi, did not affect the corrosion reaction significantly. In contrast to lead-free solders, PbOx was formed on the surface of the traditional 63Sn-37Pb solder joint in deionized water. A cathode, such as Au or Cu, was necessary for the electrochemical corrosion reaction of solders to occur. The corrosion reaction rate decreased with reduction of the cathode area. The formation mechanism of SnO crystals was essentially a galvanic cell reaction. The anodic reaction of Sn in the lead-free solder joints occurred through solvation by water molecules to form hydrated cations. In the cathodic reaction, oxygen dissolved in the deionized water captures electrons and is deoxidized to hydroxyl at the Au or Cu cathode. By diffusion, the anodic reaction product Sn2+ and the cathodic reaction product OH− meet to form Sn(OH)2, some of which can dehydrate to form more stable SnO·xH2O crystals on the surface of the solder joints. In addition, thermodynamic analysis confirms that the Sn corrosion reaction could occur spontaneously.

Ultrasonic wedge bonding is one of the most important interconnection techniques for IC dies inner circuits with the outer world for the power and signal transportation. There is a significant effect of the bonding parameters to the joint quality and reliability. Via the chemical etching method, the bond configuration and interface characteristics were observed. It was found that the bond interface outline changed from the shell-nut to the circular pie shape with the increase of the ultrasonic power, furthermore, the actual joining position developed from the bond peripheral to the central, then to the whole bond interface. From the point of view of the stress distribution and the deformation energy, the essence of the bond interface formation was given.

Ultrasonic gold and aluminum wire wedge bonding are widely used for electrical and signal interconnections of the integrated circuit chip packages. In this paper, based on the metallurgical theories and thermal aging test methods, the long-term thermal reliabilities of gold and aluminum wire wedge bonding on aluminum and Au/Ni/Cu pads, were investigated, respectively. At 200 °C, the Au/Al bond interfaces evolved little when the storage time was less than 48 h; with the aging time increasing, the interfacial intermetallic compounds (IMC) grew up from the pad (vertical growth); the primary compounds were Au5Al2 near the bond toe and heel, and Au2Al at the periphery. Then, the thickness of IMC was unchanged, and extended horizontally (lateral growth), Au5Al2 transformed into more stable Au2Al phase, furthermore, cracks ran through the interface of the gold and IMC because of severe Kirkendall voids. However, Al/Au bond was more stable, and the IMC grew slowly. The purple plague AuAl2 resulted in interfacial cracks. Moreover, the bond wire was filled with cavities.

Solder wetting behavior on pad was studied under continuous and ultrasonic modulated laser with an 8 Pa low vacuum condition. Based on these results, a novel ultrasonic modulated laser flux-less soldering method was put forward and its mechanism was also investigated in detail. Ultrasonic oscillation of temperature field, which resulted in an ultrasonic vibration of the surface of the solder droplet, was generated at the surface of the solder droplet induced by the heating of the ultrasonic modulated laser. The ultrasonic cavitation phenomenon was appeared at the solder wetting interface when the surface ultrasonic vibration was transmitted to the inside of the solder droplet, which force the molten solder to wet on the pad.

•We prepared pre-tinned Cu substrates with various oriented Cu6Sn5 layer.•The oriented Cu6Sn5 layer accelerates the orientation concentration during aging.•The oriented Cu6Sn5 layer changes the orientation distribution range during aging.•Different orientation distributions affect interfacial IMC growth by affecting Sn diffusion.An effective method for inhibiting the growth of interfacial intermetallic compounds during solid-state aging has been proposed in this study. Pre-tinned Cu substrates containing oriented interfacial Cu6Sn5 grains were prepared and reacted with a low-Ag Pb-free solder. The results show that the initial oriented interfacial grains were retained after the subsequent reflow. In addition, the presence of the oriented grains accelerated their orientation concentration and significantly affected the ultimate orientation distribution range during solid-state aging. As a result, the growth of the interfacial intermetallic compounds was inhibited due to the slower diffusion of Sn species through the solder/substrate interface.