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In this study, composite solder pastes with different sizes of Sn and Cu particles were prepared. A large fraction of solid/liquid interfacial contacts adjusted by the Cu particle sizes contributed to an ultra-rapid metallurgical reaction at a high in-situ temperature of 437.1 °C under ultrasonic waves. Thus, the sole IMC of high-melting-point Cu3Sn joints with a tiny proportion of residual Cu were obtained in air at 250 °C for 10 s under the ultrasonic-assisted transient liquid phase (TLP). The intermetallic joints exhibited the average shear strength of 49.96 MPa at ambient temperature. In particular, a promising shear strength of 46.54 MPa was obtained at the high temperature of 350 °C, which exhibits significant potential for the high temperature electronic devices.

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.

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.

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.

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 (200 W) in a high-temperature duration of only 10 s, its shear strength was up to 45.3 MPa. 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.

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.

The time-consuming of transient-liquid-phase is a big challenge for the die bonding. In this paper, a novel rapid bonding method plus alloying design was proposed to cheaply fabricate SnNi intermetallic joint for high temperature electronic packaging. The ultrasonic effect and the evolution of the joint microstructures, shear strength as a function of Ni content were systemically investigated. The amount of Ni3Sn4 in the joint increased with the addition of Ni, and when the Ni content reached 24 wt.%, the joint consisted of nearly sole Ni3Sn4 with a high shear strength of 43.4 MPa was achieved. After aging at 300 °C for 72 h in air, the shear strength of the joint obtained with smaller Ni content solder showed great improvement because of the phase transformation of residual Sn to Ni3Sn4, but for the Sn-24 wt.%Ni and Sn-30 wt.%Ni, it showed a little decrease due to the grains coarsening of Ni3Sn4. The acoustic cavitation and streaming effects dramatically accelerates the reaction of Ni and Sn during the ultrasonic-assisted soldering. The joint obtained by the Sn-24 wt.%Ni solder has the best potential to be applied in the power electronic packaging. Keywords: Ultrasonic-assisted bonding, Die bonding, Tin-nickel compounds, Power electronics, Interconnections, Aging

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 Cu 6 Sn 5 precipitated from the solder matrix during cooling. The results showed that a Cu 6 Sn 5 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 Cu 6 Sn 5 grains formed at the interface, leading to the evolution of a microstructure that generates more Cu 6 Sn 5 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.

The Ni60 and SiC–Ni60 composite coatings were successfully obtained by high-frequency inductive cladding on the AISI 1045 steel surface. The influence of SiC nanoparticles on the morphologies, phase composition, micro-hardness, wear resistance and corrosion resistance of the coatings were investigated systematically. An ultrasonic-assisted method was pre-treated for homogenising coating. The performance tests of coatings indicated that high-quality metallurgical bonding was formed under the optimised parameters. The average micro-hardness of Ni60 and SiC–Ni60 coatings was 765.6 and 1072.4 HV, respectively. The corresponding wear resistance was also measured, and the composite coating showed a much lower friction coefficient and wear rate. The corrosion resistance of the coatings was evidently improved by the addition of SiC nanoparticles.

Rapid formation of complete intermetallic-phases (IPs)-composed joints during die bonding for high temperature applications was achieved by pressureless and fluxless ultrasonic-assisted soldering in air within seconds. The joint consisted of sole (Cu,Ni) 6 Sn 5 plus a thin layer of Cu 3 Sn possessed a rather low thermal resistance of 0.252 mm 2 K W −1 and a high shear strength of 69 MPa. This rapid formation of fully IPs-composed structures was probably accelerated by ultrasonic cavitation and streaming effects through supersaturation of copper in the melted solder. Compared with transient-liquid-phase soldering, this method dramatically reduces the processing time and there was no external force acted on the dies.

Low temperature joining of amorphous alloys has big challenges to prevent them from crystallization. An ultrasonic-assisted soldering method was proposed to join the Fe-based amorphous alloys with the aluminum plates in air by using Sn-based solders. No obvious defects were observed and the joining ratio was more than 90%. At the interface of solder/aluminum, scallop α-Al solid solutions were formed and large fretting (pits) with a size of 50 μm on the aluminum plates was found due to ultrasonic vibrations. At the interface of solder/amorphous, Fe–Zn intermetallic compound (FeZn13) was probably formed. Its formation activation energy was approximately 30.9 kJ/mol between the amorphous iron and the Sn–9Zn during the solid state diffusion, which was much lower compared with crystallized couples. The enhanced diffusion/reaction and the much low activation energy in this work are probably attributed to the amorphous state of the Fe-based foils. Adding nickel in the Sn–Zn solder can improve the hardness of the joints by formation of NiAl3 intermetallic particles and depress the corrosion on the aluminum. Both the initially interfacial wetting and the microstructure refinement during solidification of the filler solder were improved by ultrasonic vibrations. Keywords: Fe-based amorphous alloys, Aluminum, Ultrasonic-assisted soldering, Microstructure, Wetting, Fe–Zn compound

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.

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.

With the dramatic requirements of high power and high integrity in field of power devices, the study of novel interconnection materials and packaging methods has been paid an extremely attention. Presently, the most candidates for the chip bonding are Zn-based, Bi-based, noble-metal-based (such as Au-Sn, Au-Ge, Au-Si), nano-metal particles (Ag or Cu nanoparticles) and transient-liquid-phase (TLP) bonding. However, a high bonding temperature and poor ductility for Zn-based solder (more than 420°C) causing irreversible damages or accelerated aging affects to the electronic devices; Bi-based solder alloys are still under way mainly due to their inferior thermal and electrical conductivity as well as poor workability; It's worth noting that nano-silver sintering and TLP bonding technology have been proven to be the potential solutions to the power electronic packaging, because their processing temperatures are relatively low and the produced joints are of high re-melting temperatures. However, Ag is expensive and easy to electromigration, which will influence the stability/reliability of the joints and hinder to universal industry applications. For the TLP bonding, the process to produce full intermetallic compounds (IMCs) is time-consuming, commonly more than 60 min, due to relatively low reaction velocity. To solve the problem of time-consuming of the TLP, an enhanced and low-cost auxiliary energy is necessary. Ultrasonic-assisted soldering has been proven able to solder dissimilar metals at low temperatures within a very short time. In our previous work, a full Cu6Sn5/Cu3Sn IMCs joint through TLP assisted by ultrasound was obtained. Because of the acoustic cavitation and streaming effect created by ultrasonic waves propagating in molten alloys, the atom diffusion and physical-chemical interactions were accelerated at the liquid/solid interfaces. However, a sole IMC joint with single phase has not been fabricated so far. High re-melting temperature joint consisted of nearly sole Ni 3 Sn 4 with a high shear strength of 43.4 MPa was achieved with Sn-24 wt.%Ni by ultrasonic-assisted soldering for 10 s. This rapidly formed intermetallic joint was phase-stable as isothermally aged at 300 °C for 72 h in air and its shear strength was 33.4 MPa due to the Ni 3 Sn 4 grains coarsening. These results demonstrate that the less time-consuming ultrasonic-assisted soldering has wide potentials for high-temperature power electronic packaging.

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.

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.

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 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.

Ultrasound-assisted brazing of Cu/Al dissimilar metals was performed using a Zn–3Al filler metal. The effects of brazing temperature on the microstructure and mechanical properties of Cu/Al joints were investigated. Results showed that excellent metallurgic bonding could be obtained in the fluxless brazed Cu/Al joints with the assistance of ultrasonic vibration. In the joint brazed at 400 °C, the filler metal layer showed a non-uniform microstructure and a thick CuZn 5 IMC layer was found on the Cu interface. Increasing the brazing temperature to 440 °C, however, leaded to a refined and dispersed microstructure of the filler metal layer and to a thin Al 4.2 Cu 3.2 Zn 0.7 serrate structure in the Cu interfacial IMC layer. Further increasing the brazing temperature to 480 °C resulted in the coarsening of the filler metal and the significantly growth of the Al 4.2 Cu 3.2 Zn 0.7 IMC layer into a dendrite structure. Nanoindentation tests showed that the hardness of the Al 4.2 Cu 3.2 Zn 0.7 and CuZn 5 phase was 11.4 and 4.65 GPa, respectively. Tensile strength tests showed that all the Cu/Al joints were failed in the Cu interfacial regions. The joint brazed at 440 °C exhibited the highest tensile strength of 78.93 MPa.

This paper describes a method to achieve rapid pressureless low-temperature sintering of Ag nanoparticles for bonding. Organic shells adsorbing on the surface of Ag nanoparticles to stabilize them were thinned to create a sparse protecting layer. The numerous coherent twin boundaries formed in sintered Ag nanoparticles with a grain size of 21 nm induce ultrahigh thermal conductivity (229 W m−1 K−1), which overcomes the intrinsic defect that metals with nanosized grains generally exhibit a significantly reduced thermal conductivity because of the grain boundary scattering effect.

The ultrasound-assisted brazing of 1060 Al alloy using a Zn–14Al hypereutectic filler metal was investigated at different temperatures. The effects of brazing temperature on the bonding ratio, shear strength and microstructure of the joints were studied. Cavities and discontinuous cracks were found in the joint ultrasonically brazed at 410 °C, and the joint showed a low bonding ratio and poor shear strength. Excellent bonding ratios and high shear strength were obtained in the joints ultrasonically brazed at 440 °C and 470 °C. The primary α-Al phase showed a refined spherical shape in the joint ultrasonically brazed at 440 °C, but showed a coarse dendritic structure in the joint ultrasonically brazed at 470 °C and that brazed at 440 °C without ultrasonic vibration. The refined spherical microstructure shown in the joint ultrasonically brazed at 440 °C was attributed to cavitation-aided grain refinement effects.

The wide band-gap semiconductor devices, such as SiC, GaAs power devices, provide great opportunities to develop power electronic systems with increased power densities, high reliability in extreme environments and higher integration density. High-temperature resistance is needed for the thermal interface materials. In this work, the intermetallic joints consisted of nearly sole Ni3Sn4 during die bonding for high temperature applications were achieved with Sn-24 wt.%Ni by pressureless ultrasonic-assisted soldering in air for 10 s. This high re-melting temperature joint possessed high shear strength of 43.4 MPa and good microstructural stability as isothermally aged at 300 °C for 72 h in air. After aging, the IMCs phase did not change and the shear strength dropped (33.4 MPa) due to the Ni 3 Sn 4 grains coarsening. These results demonstrate that the less time-consuming ultrasonic-assisted soldering has shown its potential for high temperature power electronic packaging.

Wide band gap semiconductors, such as SiC, GaN, etc. can provide power electronic systems with increased power densities, high reliability in extreme environments and higher integration density, and have been receiving considerable attentions to replace Si power semiconductor devices. High power, high service temperature are the characteristics of these semiconductors, however, these make big challenges for the interconnections between the semiconductor die and the substrate. Novel interconnection materials without lead have been developing, including novel solder alloys, such as Au-Sn, Au-Ge, Bi-Ag, Zn-Al, etc. and metallic nanoparticles (NPs), such as Ag NPs, Cu NPs, etc. Corresponding interconnection methods can be divided into soldering, transient liquid phase (TLP) bonding and sintering. Indeed, materials research and development obviously is effective for solving the present challenges. Novel techniques are another important way for wide band gap semiconductors packaging. Thus, different from the traditional Sn-based soldering (commonly resulting in an interconnect possessing a low-melting temperature similar to the soldering temperature), this work reports a fabrication method of forming intermetallic compound (IMC) interconnect through ultrasonic-assisted soldering with traditional Sn-0.7wt.%Cu solder alloy. A (Cu, Ni) 6Sn5 ternary IMC joint was fabricated at 250°C for 10s. The thermal conductivity of the IMC joint was 75.8W/(m·K), which was approximately 3 times higher than the joint obtained by traditional reflow [20W/(m·K)]. The IMC joint can be served at the temperature less than 332°C (0.8 times of the melting point).

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.

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.

This paper describes a new method to achieve rapid formation of intermetallic phases joints for hightemperature packaging application. A high-melting-point joint which consists of (Cu,Ni) 6 Sn 5 and Cu 3 Sn was formed without flux in air with a high shear strength of 67 MPa was obtained at the conditions of 500 W ultrasonic power for 10 s. 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 and there was no external force acted on chips.

At ambient temperature, Al–1%Si wire of 25 μm diameter was bonded successfully onto the Au/Ni/Cu pad by ultrasonic wedge bonding technology. Physical process of the bond formation and the interface joining essence were investigated. It is found that the wire was softened by ultrasonic vibration, at the same time, pressure was loaded on the wire and plastic flow was generated in the bonding wire, which promoted the diffusion for Ni into Al. Ultrasonic vibration enhanced the interdiffusion that resulted from the inner defects such as dislocations, vacancies, voids and so on, which ascribed to short circuit diffusion.

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.

Direct Bonded Copper (DBC) is now commonly applied as heat transfer substrates for high power electronics. The metallizaed surfaces are designed for improving the heat conduction and joining with copper heat sink. However, due to the mismatch of coefficient of thermal expansion, the joining area is usually the failure regions. In the present work, the alumina ceramic substrate and the copper substrate were directly brazed by using an Zn-Al filler with an ultrasonic-assisted method. A high quality of joints without obvious drawbacks at the both interfaces was obtained under optimized brazing parameters. With the increase of ultrasonic power, the amount of intermetallic compounds (IMCs) increased evidently. At a constant brazing time, the shear strength of joints increased firstly then decreased. When the ultrasonic power was 200W at a brazing temperature of 480 °C and a brazing time of 30s, the average shear strength reached 80MPa. With such a rapid and lower temperature brazing by the ultrasonic-assisted, we can acquire high-performance joints than traditional methods.

Cu-to-Cu interconnects were achieved by pressureless low temperature sintering of Ag nanoparticles for electronic packaging. Ag nanoparticles could be sintered so long as the chemical bonds by which organic shells connected with Ag nanoparticles were broken with no necessity that organic shells completely decomposed, which provided a way to lower bonding temperature. The pinecone-like recrystallization morphology of sintering Ag nanoparticles was observed by transmission electron microscopy, which resulted from the residuals of organic shells by sintering process. The thermal conductivity of sintered Ag nanoparticles was affected strongly by the recrystallization morphology because of grain boundary scattering effect. The shear strengths of joints reached 17–25 MPa at temperatures ranging from 150 °C to 200 °C.

One model experiment in this study was conducted to investigate the influences of ultrasonic vibration (USV) on solder joints by fabricating Cu/SAC305/Cu sandwich structure samples. The samples were first heated to 270°C and then cooled in air to 250°C, at which temperature USV with five different powers were applied to samples during their cooling procedure. Undercooling is significantly decreased as the power increase, while the soldering quality improved. The grain size is significantly refined under the ultrasonic treatment, with a reduction of almost 90% from 100μm to 10μm. And Cu6Sn5 length scope decreases from more than 200μm (0W) to 10~100μm as the ultrasound power increases. However, the Sn-rich phase changed from dendritic type to equiaxed cell as the power increase. The average width of β-Sn dendrite is about 5 μm, while the diameter of Sn-rich cell grows to almost 50 μm after 267W USV treatment. Besides, the Ag3Sn phase translates from needlelike to plate form as the ultrasound power increases.

Ultrasonic additive manufacturing (UAM) is now extensively used in advanced materials processing since it applies ultrasonic frequency vibrations locally pressed on the metal parts to make them deformed and/or joined. In the field of electronic packaging, wirebonding is such a kind of type, especially such as ultrasonic wedge bonding, which combines ultrasonic and pressure only, without heating during the bonding process. Through high frequency vibrations, the metal ribbons or wires were interconnected with substrates within mille-seconds. Without ultrasounds, commonly it needs more heat or pressures. One of the most important mechanisms is focused on the effects of ultrasonic-induced deformation structures in the metals. Previously, we have observed the evolution of the textures and grains, but there have not been found any data on the report of nanoscale features of the metal itself. Therefore, in the present study, by using high-resolution transmission electron microscope (HR-TEM) pure coarse-grained aluminum was chosen for investigating the deformation microstructures and mechanism when exposed under ultrasonic vibrations at room temperature. It was found that deformed pure aluminum wire has equiaxial grain shape, and recrystallization grains are evident since there has little defects in them. But within some grains, sub-structures can be found. HR-TEM observation reveals that these sub-structures can be defined as sub-grains with diameter of a few nanometers. Further, it is the first that large stacking faults (SFs) with a width longer than 15 nm and even microtwins can be seen clearly. Their distribution mainly aligns parallelly or with an angle of about 70 degree. Increasing ultrasonic power seems enlarge these SFs. Cyclic vibrations contribute to the formation of these SFs/microtwins.

Multi-annealed 1100 Al alloy foils were welded to form monolithic samples using the ultrasonic additive manufacturing (UAM) process at room temperature. 40%, 60%, 80% of the oscillation amplitudes were investigated at 20 kHz frequency and a constant contact pressure. Deformed microstructures were analyzed using low-resolution electron backscattered diffraction (EBSD). Evolution of fine equiaxed grains from an initial coarser grain structure was observed in the volume of Al alloy foils, and the extent of the grain refinement is proportion to oscillation amplitude. Additionally, the orientation image maps (OIM), polar figures (PF), and orientation distribution functions (ODF) of the samples are obtained. The characteristics of the crystallographic orientation were analyzed to find the effect of ultrasonic amplitude on the microtexture of the matrix material.

Considering two kinds of the most widely used wire bonding pads of the COB and BGA packaging styles, the characters of the bond interfaces in Al-Ni and Al-Au systems were compared after high temperature storage test through SEM observation. It was found that, according to the appearance, the Al-Au bond aged at 250degC in air for more than 40 days was expanded badly, and surface cracks were severe, but the Al-Ni bond changed little. At bond interface, AuAl2 IMC was formed in Al-Au system, furthermore, many inner cracks initial the boundary of the wire and the IMC connected with the cracks in the wire, however, there was no such thick IMC formed in Al-Ni system. The causation was analyzed based on the metallurgical theory. Experimentally, the Al-Ni system was more reliable than the Al-Au system.

Al+1%Si wire with 25mum diameter and three kinds of pads (Au/Ni/Cu pad , Cu pad and Ni/Cu pad) are bonded by ultrasonic wedge bonding. The influence of the parameters (ultrasonic power, bonding time and bonding force) on the formation of joint are studied. Surface plot and contour plot are protracted with MINITAB, and the processing parameters are also optimized. A special circuit to test joint resistance is designed. As the variable, the parameters are used to analyse the tendency of interfacial resistance. The factors are investigated by the curve of resistance

Ultrasonic wedge bonding, which can be done at room temperature, is widely used in microelectronic package. It is primarily used to bond either Al wire or Au wire. After bonding and aging, the joint cross-section of 25mum Au wire bonded on Al metallization pad was analyzed by SEM with EDX. It is found that, there was evident diffusion of Au and Al, and Au5Al2 intermetallic compound (IMC) formed first after aging for 10 days at 200 degC in atmosphere, and there was also some Au2Al formed at the edge of the interface. After aging for 20 days, the IMC developed slightly, and also Kirkendall voids generated at the interface. After aging for 30 days, the IMC began to transform to other Au-Al IMCs, and it became Au2 Al IMC finally after aging for 40 days

The joining mechanism of ultrasonic wedge bonding was investigated by chemical etching method. Compared to peeling-off method, the advantage of chemical etching method not only reflected the characters of the bond interface, but also indicated the physical process and bonding mechanism of the interconnection between the metal wire and metallization. It was found that the evolution and characters of the bond interface were affected heavily by the bonding parameters. The lateral and longitudinal joint marks were observed. The former were perpendicular to the direction of the ultrasonic vibration, and distributed at the periphery of the bond interface with some periodicity, which were the metallurgical joining parts. However, the later were parallel to the vibration directions locating at the bond center. The features above had important influence on the bond resistance. The mechanism was ascribed to the stress evolution and distribution during bonding process.

Ultrasonic wire bonding is one of the most important techniques for die interconnection in microelectronic packaging. It is widely used for power devices, microwave devices and photoelectron devices packaging. At room temperature, ultrasonic energy and plastic deformation energy, generated by metal wire plastic deformation under the wedge tool pressure, make the wire and metallization join together. The ultrasonic bonds, after bonding and aging, of Al+1%Si wire with 25/spl mu/m diameter bonded on the Au/Ni/Cu pad, are analyzed by scanning electronic microscopy (SEM) with energy dispersive x-ray spectrometer (EDX). The joints begin at the bond periphery where it is the location of the greatest plastic flow. It is found that the mechanism of ultrasonic bonding is, both the plastic flow of metal wire generated by wedge tool pressure which results in the diffusion of Ni into Al wire; and the effect of ultrasound is that, on the one hand, ultrasonic vibration enhances the metal wire ability of plastic flow and, on the other hand, it generates many defects inside the metal wire which are the fast diffusion channels. The diffusion type is likely the short-circuit diffusion, which is more prominent than the crystal diffusion when the temperature is low. After high temperature storage at 170 /spl deg/C for 10 days, there is evident diffusion of Ni into Al wire, but the microstructure is the same with the bonds after bonding, there is no evident change. Aged for 30 days, the bond interface forms a cloud-like structure, and the major composition is Al and Ni with weight percent of 78.82% and 15.55% respectively. However, the diffusion is not even and some parts of the bond interface are absence of Ni diffusion. When the aging time is 40 days, the cloud-like structure transforms into rectangular island-like structure and there are many cavities inside the bond wire, which are different from the Kirkendall voids because of the shape and dimension.