•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.

•Ultrasonic brazing of Al2O3/Cu was achieved in air without flux by Zn-14Al filler at 753 K.•Newly formed crystalline Al2O3 aggregated at the ceramic interface.•Overgrowth of intermetallic compounds generated by acoustic pits on Cu was found.•Reaction wetting enhanced by ultrasonics dominated this rapid and high quality brazing.The ultrasonic-assisted brazing of α-alumina to copper was achieved in air without flux using Zn-14wt%Al hypereutectic filler at 753 K within tens of seconds. The effects of ultrasonic time on the microstructures and mechanical properties of joints were investigated. In the joint interlayer, large amounts of intermetallic phases consisted of binary CuZn5 embedded by many ternary Al4.2Cu3.2Zn0.7 particles were formed. At the ceramic interface, newly formed crystalline Al2O3 aggregated. At the Cu interface, acoustic corrosion on the copper resulted in depriving the surface oxides and forming many pits on its surface, which provided saturated Cu in the melted filler alloys during the brazing. The ultrasonic vibrations had distinct effects on the metallurgical reactions of the joints, resulting in intermetallic-phase-filled composite joints with shear strength of 66 MPa. The overgrowth of intermetallic compounds, the newly formed crystalline alumina, and the acoustic pits was probably ascribed to the ultrasonic effects.

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.

•Hybrid interconnects of Au stud bumps/SnCu solder were produced on Cu by flip-chip.•AuSn4 intermetallic with big fraction was the main IMCs in the hybrid joints.•The reliability and fracture mode were influenced by AuSn4 and Kirkendall voids.•The hybrid joints have good electromigration resistance.With miniaturization of the interconnect solder bumps, high current density causes serious reliability issues (stress, electromigration etc.) in electronic packages. Through Au stud bumping on the chips and following reflow of solder to produce hybrid interconnects, the eletromigration resistance may be improved by the intermetallics formed inside them due to their barrier effects on the atoms migration. Here, microstructures and reliabilities of Au stud with serial amounts of Sn-0.7Cu solder paste were studied through controlling size of stencil printing aperture. After reflow, AuSn, AuSn2 and AuSn4 formed from the surface of Au stud bump to the solder. A layer of (Cu,Au)6Sn5 with thickness of 3 μm existed at the interface near the Cu substrate with a scallop shape similar to Cu6Sn5. The fraction of intermetallics to the mixed joints varied with the solder amount. Shear strength decreased slightly when comparing with the sole solder joint due to large amounts of brittle intermetallics. Thermal aging resulted in many Kirkendall voids generated at the interfaces of Au stud and the solder, which further decreased the shear strength. The effect of solder amount on microstructural evolution and fracture modes was discussed. The hybrid interconnects showed a good electromigration resistance.

Solder joint reliability greatly depends on the microstructure of the solder matrix and the morphology of intermetallic compounds (IMCs) in the joints. Addition of strengthening phases such as carbon nanotubes and ceramic particles to solder joints to improve their properties has been widely studied. In this work, ultrasonic vibration (USV) of casting ingots was applied to considerably improve their microstructure and properties, and the resulting influence on fluxless soldering of Cu/Sn-3.0Ag-0.5Cu/Cu joints and their microstructural evolution was investigated. It was demonstrated that USV application during reflow of Sn-based solder had favorable effects on β-Sn grain size refinement as well as the growth and distribution of various IMC phases within the joints. The β-Sn grain size was significantly refined as the ultrasound power was increased, with a reduction of almost 90% from more than 100 μm to below 10 μm. Long and large Cu6Sn5 tubes in the solder matrix of the joints were broken into tiny ones. Needle-shaped Ag3Sn was transformed into flake-shaped. These IMCs were mainly precipitated along β-Sn phase boundaries. High-temperature storage tests indicated that the growth rate of interfacial IMCs in joints formed with USV was slower than in conventional reflow joints. The mechanisms of grain refinement and IMC fragmentation are discussed and related to the ultrasonic effects.

The silver content of lead-free solders affects their microstructure, the interfacial reaction, and the performance of the joints in reliability tests. In this study, Sn3.0Ag0.5Cu (wt.%, SAC305) and Sn1.0Ag0.5Cu (wt.%, SAC105) solder balls of diameter 55 μm were reflowed on gold surface pads by laser-jet soldering. It was found that four types of layered intermetallic compound (IMC) were formed at the interfaces; these were Au5Sn/AuSn, AuSn, AuSn2, and AuSn4 from the pad side to the solder matrix. The Au5Sn/AuSn eutectic region, thickness 400 nm, formed because of the high cooling rate induced by the laser-jet soldering. During high-temperature storage tests, the silver became segregated at the interfaces between the Au–Sn IMC and the solder matrix, resulting in inhibition of IMC growth in SAC305 joints, the shear strengths of which were higher than those of SAC105 joints. In mechanical drop tests, however, percentage failure of the SAC305 joints was twice that of the SAC105 joints.

Bulk metallic glasses (BMGs) have no periodicity in a long range. To adopt BMGs in broader fields of engineering applications, joining between BMGs/BMGs or BMGs/crystalline alloys is crucial. Fe-based amorphous foils are one of the most excellent soft magnetic materials and have been applied in amorphous motors. One big challenge during assembling amorphous stators with aluminum shells is how to obtain a robust joining between them. In this study, ultrasonic-assisted soldering of 100 μm thick Fe-based amorphous foils with aluminum alloys was investigated by using Sn-based fillers in air. To enhance the wetting abilities, ultrasounds with a resonant frequency of about 27 kHz and vibration amplitude of 15 μm were introduced during the soldering process. It was found that both interfaces of the filler/Fe-based amorphous and the filler/aluminum plate were metallurgically joined by the Sn-Zn filler. Both the ultrasonic power and soldering time resulted in the homogeneously distributed and refined Zn-rich phases. Fretting of the filler on the aluminum surface was observed and it was severe when the ultrasonic power and the soldering time were large enough. Ultrasounds may have positive effects on the interfacial wetting and microstructure refinement.

•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.

•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.

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.

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.

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.