Herein, we report an innovative application of doping conjugated-polypyrrole nanoparticles (PPy NPs) into electrically conductive adhesives (ECAs) to prepare low-electrical resistivity interconnecting materials. PPy NPs were synthesized via a facile one-step chemical oxidative polymerization method at room temperature with the average diameter as small as 86.8 nm. Particles' diameters and dispersity were manipulated under different polymerization conditions through the adjustment of weight percentages and molecular weights of polyvinylpyrrolidone (PVP) as the surfactants. Results showed that higher concentrations of PVP and the longer chains of PVPs resulted in smaller diameters of PPy NPs. We also found that a suitable portion of ethanol in the polymerization mixtures gives rise to a better dispersity than that observed in mixtures without ethanol. When a small amount of PPy NPs was added into traditional epoxy resin-based and silver-flakes-filled ECAs, the resistance measurements showed an enhancement in the electrical conductivity, or in other words, a reduction in resistivity significantly. For example, the electrical resistivity of 70 wt% silver-filled ECAs was reduced from 1.6 × 10−3 Ω cm to 9.4 × 10−5 Ω cm by using only 2.5 wt% PPy NPs as the dopants. Thus, our results confirmed new applications of PPy NPs in the field of ECAs for decreasing the resistance, reducing the dosage of silver in ECAs, and achieving flexible devices. Finally, flexible electrical patterns were printed on paper and polyimide substrates were used as conducting circuits to light LED devices.

In this paper, the electric current density of 1.44 × 104 A cm−2 was imposed to assemble Cu/(30μm) Sn/Cu interconnection systems without flux at ambient temperature to fabricate Cu–Sn interconnects within 180 ms. From the three-dimensional microstructural observation of interfacial intermetallic compounds (IMCs) at different bonding times, the rod-like Cu6Sn5 formed at the initial stage was changed into dendritic Cu6Sn5 due to constitutional supercooling. When the joule heat-induced temperature was increased above the melting point of Cu6Sn5, the dendritic Cu6Sn5 were melted and then totally converted into Cu3Sn, resulting in the formation of homogeneous Cu3Sn intermetallic joints. The ultrarapid microstructure evolution of the interfacial IMCs was caused by enhanced solid–liquid interdiffusion kinetics, which can be attributed to the joule heating effect as well as the solid–liquid electromigration of Cu in molten Sn with the passage of electric current. In addition, the mechanical analysis shows that the microstructure changes of interfacial IMCs can strongly influence the shear strength as well as the fracture mechanism of the resulted joints. The dendritic network of Cu–Sn IMCs enhanced the shear strength of resulted joints due to the interlocking effect, meanwhile, the homogeneous Cu3Sn joint exhibited the highest shear strength of 67.3 MPa.

For deep-space applications under cryogenic temperatures, the mechanical properties of Sn-3.0Ag-0.5Cu (SAC305) solder alloys and SAC305/Cu solder joints have been investigated over the temperature range 77–298 K. The fracture surfaces were analyzed in the plan view to identify the fracture locations in the solder joints, and the fracture mechanisms in solder alloys and solder joints were also explored. Results showed the tensile strength of the solder alloys firstly increased and reached its maximum value at 123 K, and then decreased with descending temperature. Meanwhile, the solder alloys displayed a shift in their fracture mechanism from dimple-ductile fracture to brittle fracture in a mixed model which was combined by transgranular and intergranular. It could be attributed to the asymmetric tetragonal structure of β-Sn which is the main constituent of SAC305 solder. The tensile strength of SAC305/Cu joints showed increment at first and then decrement as the temperature decreased, while increasing the strain rate enhanced the tensile strength. In addition, the fracture location of the joints changed from the solder to the solder/IMC interface or the interfacial IMC layer with the decrement of temperature, resulting in the variation of the fracture mechanism from ductile fracture to brittle fracture.

Copper nanowire (CuNW) conductors have been considered to have a promising perspective in the area of stretchable electronics due to the low price and high conductivity. However, the fabrication of CuNW conductors suffers from harsh conditions, such as high temperature, reducing atmosphere, and time-consuming transfer step. Here, a simple and rapid one-step photonic sintering technique was developed to fabricate stretchable CuNW conductors on polyurethane (PU) at room temperature in air environment. It was observed that CuNWs were instantaneously deoxidized, welded and simultaneously embedded into the soft surface of PU through the one-step photonic sintering technique, after which highly conductive network and strong adhesion between CuNWs and PU substrates were achieved. The CuNW/PU conductor with sheet resistance of 22.1 Ohm/sq and transmittance of 78% was achieved by the one-step photonic sintering technique within only 20 μs in air. Besides, the CuNW/PU conductor could remain a low sheet resistance even after 1000 cycles of stretching/releasing under 10% strain. Two flexible electronic devices, wearable sensor and glove-shaped heater, were fabricated using the stretchable CuNW/PU conductor, demonstrating that our CuNW/PU conductor could be integrated into various wearable electronic devices for applications in food, clothes, and medical supplies fields.Keywords: copper nanowires; one-step; photonic sintering; stretchable conductor; wearable device

Cu–Ag hybrid nanowires with well-defined Ag nanoparticles on the surface of Cu NWs were successfully prepared by a mild two-step method. High-resolution transmission electron microscopy (HRTEM) results indicated that the as-prepared silver nanoparticles (Ag NPs) with an average diameter of 8.4 nm were directly grown on the surface of the copper nanowires (Cu NWs) through an in situ substitutional reaction with Cu–Ag metallic bonds formed between Ag NPs and Cu NWs. Growth evolution results showed that the Ag nanoseeds were prone to grow into Ag NPs when the ratio of Cu to Ag was small while they tended to grow into Ag nanowires (NWs) as the ratio of Cu to Ag dramatically increased. Furthermore, the strong surface-enhanced Raman scattering sensitivity (SERS) effect of the as-prepared Cu–Ag hybrid NWs was verified using Rhodamine 6G (R6G) and 4-aminothiophenol (4-ATP) probes.

•High-temperature-stable Cu3Sn interconnects was fabricated within 200 ms.•The dramatic microstructure evolution of CuSn compounds was analyzed.•The formation mechanism of columnar dendritic CuSn compounds was studied.•The Cu3Sn interconnects exhibited a high reliable mechanical property.High-temperature-stable circuit interconnects are highly desirable for the wide band-gap semiconductor devices to operate in harsh environment (>200 °C). In this study, a full Cu3Sn interconnect with the melting point of 679 °C was fabricated by using a micro-resistance spot welding process in an extremely short time of 200 ms and under a low pressure of 0.08 MPa at ambient temperature in Cu/Sn (30 μm)/Cu interconnection system. The microstructure evolution of CuSn intermetallic compounds indicated that the joule heat-induced temperature coupled with the passage of electric current significantly enhanced the interfacial reaction at the liquid Sn/solid Cu metallization interface. Accompanying with the speedy electromigration of Cu atoms in the molten Sn solder, the columnar dendritic Cu6Sn5 compounds were formed due to constitutional supercooling and then were totally transformed into Cu3Sn compounds. The resulted Cu3Sn interconnect exhibited a higher mechanical strength than Sn-based interconnects, offering this type of interconnect a promising application in high-temperature power electronics.

•AgNWs with mean length of 49.4 µm have been synthesized within only 30 min.•Longer AgNWs were achieved at higher concentration of Cu2+ in shorter reaction time.•By adjusting the concentration of Cu2+, the diameter of AgNWs could be adjusted.Long silver nanowires (AgNWs) with mean length of 49.4 µm were synthesized by a rapid one-step polyol method within only 30 min. The relationship between Cu2+ and the size of AgNWs had been studied. AgNWs were longer at higher concentration of Cu2+ because the presence of Cu+ (reduced from Cu2+) could increase the absorption rate of silver atoms (Ag0). By simply controlling the concentration of Cu2+ in reaction process, the diameter of AgNWs ranging from 31 nm to 57 nm could also be adjusted. This rapid method improved the production efficiency of long AgNWs and could be applied to high performance transparent electrodes.

•Ni3P+Ni/(Ni, Cu)3Sn4 double layers were formed after laser soldering.•Ni3P+Ni layer was transformed into Ni3P+ Ni2P after the third hot air reflow.•The thickness of P-rich layer increased with the number reflow times.•The (Ni, Cu)3Sn4 was transformed to (Cu, Ni)6Sn5 after the first hot air reflow.•The (Cu, Ni)6Sn5 was transformed to (Ni, Cu)3Sn2 after the third hot air reflow.In three-dimensional (3D) packaging and assembly technology, Lead-free SnAgCu solder joints inevitably experience multiple reflows. The interfacial compounds of Sn3.0Ag0.5Cu (SAC305)/ Electroless Ni–P/Immersion Au (ENIG) solder bump after laser soldering and subsequent hot air reflows were observed by transmission electron microscopy (TEM), respectively. The initial laser soldering played a key role in interfacial compound evolution during the subsequent multiple reflows. After laser soldering, a thin P-rich layer and an ultrafine scallop-type (Ni, Cu)3Sn4 layer formed at the interface of SAC305/ENIG. The valleys of ultrafine (Ni, Cu)3Sn4 grains served as rapid channels for Ni diffusion, which contributed to the formation of dendritic η-(Cu, Ni)6Sn5+(Ni, Cu)3Sn2 intermetallic compounds after the first hot air reflow. However, with the number of reflow times increasing, the η-(Cu, Ni)6Sn5 totally transformed to noodle-like (Ni, Cu)3Sn2 owing to their similarly hexagonal lattice structure and the limited Cu supply in solder.

Copper nanowire transparent electrodes have received increasing interest due to the low price and nearly equal electrical conductivity compared with other TEs based on silver nanowires and indium tin oxide (ITO). However, a post-treatment at high temperature in an inert atmosphere or a vacuum environment was necessary to improve the conductivity of Cu NW TEs due to the easy oxidation of copper in air atmosphere, which greatly cancelled out the low price advantage of Cu NWs. Here, a high intensity pulsed light technique was introduced to sinter and simultaneously deoxygenate these Cu NWs into a highly conductive network at room temperature in air. The strong light absorption capacity of Cu NWs enabled the welding of the nanowires at contact spots, as well as the removal of the thin layer of residual organic compounds, oxides and hydroxide of copper even in air. The Cu NW TE with a sheet resistance of 22.9 Ohm sq−1 and a transparency of 81.8% at 550 nm has been successfully fabricated within only 6 milliseconds exposure treatment, which is superior to other films treated at high temperature in a hydrogen atmosphere. The HIPL process was simple, convenient and fast to fabricate easily oxidized Cu NW TEs in large scale in an air atmosphere, which will largely extend the application of cheap Cu NW TEs.

Electromigration-induced microstructure evolutions of two types of symmetrical solder joints were investigated. Sn foil sandwiched between Cu plates was reflowed to form two types of joints: a Sn-remained joint with a Cu/Sn/Cu structure and a full-intermetallic compounds (IMCs) joint with a Cu/IMCs/Cu structure. For the Sn-remained joint under current stress (1.0 × 104 A/cm2) at 150 °C, no void propagation was observed until all Sn layer converted to IMC despite the notable polarity effect on IMC growth. However, large voids that are different from Kirkendall one appeared at the cathode side after the formation of a full-IMC structure. In contrast, for the full-IMC joint, the two interfacial Cu3Sn layers kept nearly the same in thickness even under a rather strict condition (1.4 × 104 A/cm2, 200 °C). A few Kirkendall voids nucleated in the Cu3Sn layers but did not concentrate dramatically. Surprisingly, small voids were also present in the Cu6Sn5 layer, and they grew and gathered persistently to cause a potential reliability issue.

•Ultra-long copper nanowires have been prepared in a rich production using a simple hydrothermal method.•Pencil-like copper nanowire with a penta-twinned structure was firstly discovered by changing the amount of glucose.•Pencil-like nanowires experienced growth along both radial and axial directions.Ultra-long copper nanowires with uniform diameters of 45 nm and good dispersity were successfully synthesized in a rich production by a simple hydrothermal method using green raw materials. High resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) results indicate that the ultra-long copper NWs are twinned crystals growing along [1 1 0] direction. A novel morphology of pencil-like nanowires with a penta-twinned structure emerged when reducing the amount of glucose. Results showed that the formation mechanism of pencil-like nanowires is that one dimension (1D) elongation of copper NWs with a five-twinned structure confined by hexadecylamine (HDA) is accompanied by the growth along the radial orientation in the same time.

•The orientation of interfacial Cu6Sn5 grains was obtained by EBSD technology.•Two types of hollowed Cu6Sn5 strips were found at different temperatures.•The formation mechanism of hollowed Cu6Sn5 was elaborated based on Bravais law.•The relationship between Cu6Sn5 grain orientations and morphologies was clarified.The morphologies and orientations of Cu6Sn5 intermetallic compounds in the Sn3.0Ag0.5Cu solder joints both on polycrystalline and single crystal Cu pads under different peak reflow temperatures and times above liquids were investigated. The relationship between Cu6Sn5 grain orientations and morphologies was clarified. At the interface of Sn3.0Ag0.5Cu/polycrystalline Cu pad, scalloped Cu6Sn5 intermetallic compounds formed at 250 °C and roof shape Cu6Sn5 formed at 300 °C. Both scalloped Cu6Sn5 and roof shape Cu6Sn5 had a preferred orientation of {0001} plane being parallel to polycrystalline Cu pad surface. Besides, the percentage of large angle grain boundaries increased as the peak reflow temperature rose. At the interface of Sn3.0Ag0.5Cu/(111) single crystal Cu pad, the Cu6Sn5 intermetallic compounds were mainly scallop-type at 250 °C and were prism type at 300 °C. The prismatic Cu6Sn5 grains grew along the three preferred directions with the inter-angles of 60° on (111) single crystal Cu pad while along two perpendicular directions on (100) single crystal Cu pad. The orientation relationship between Cu6Sn5 grains and the single crystal Cu pads was investigated by electron backscatter diffraction technology. In addition, two types of hollowed Cu6Sn5 intermetallic compounds were found inside the joints of polycrystalline Cu pads. The long hexagonal Cu6Sn5 strips were observed in the joints reflowing at 250 °C while the hollowed Cu6Sn5 strips with the ‘’ shape cross-sections appeared at 300 °C, which was attributed to the different grain growth rates of different Cu6Sn5 crystal faces.

Morphologies and grain orientations of Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) at the interface of Sn3.5Ag0.5Cu lead-free solder alloy and copper substrates were investigated by Electron Backscatter Diffraction (EBSD) technology combined with Scanning Electron Microscopy (SEM). Scalloped Cu6Sn5, Cu6Sn5 planes and six-prism shaped Cu6Sn5 IMCs were observed at the interface after 48 h of soldering at 250 °C. Moreover, Cu6Sn5 planes emerged in pairs on top and beside the scalloped Cu6Sn5, and two adjacent Cu6Sn5 planes were with an angle of 120°. Cu3Sn and scalloped Cu6Sn5 IMCs had preferred orientations, and they were Cu6Sn5 (0001)∥substrate surface, and Cu3Sn (1¯00) and (100)∥substrate surface.Highlights► Grain orientations of Cu–Sn IMCs with long time soldering were investigated. ► Preferred orientation of Cu6Sn5 was Cu6Sn5 (0001)∥substrate surface. ► Preferred orientations of Cu3Sn were Cu3Sn (11¯00) and (100)∥substrate surface. ► Cu6Sn5 planes emerged in pairs on top and beside the scalloped Cu6Sn5.

To improve the ultrasonic bondability and antioxidation property of pure Cu pad, multi-layer structures of Ti/Cu/TaN/Ag are deposited on Si substrate with magnetron sputtering technology. The as-deposited multi-layers had uniform thicknesses and smooth surface with the roughness of 2.098 nm. X-Ray diffraction analysis indicated that both Ag and Cu layers had deposited with (111) preferred orientation, and the grain size of Ag layer was 29.5 nm. The phase composition of TaN layer was Ta2N.For comparison of antioxidation properties, traditional pad structure Ti/Cu/Ag was also fabricated with the same process. Obvious Cu oxidation occurred on the surface of the Ti/Cu/Ag structure after heating at 150 °C for 30 min. However, no Cu element was detected on the surface of Ti/Cu/TaN/Ag structure even after heating at 300 °C, which indicated that the Ta2N layer in Ti/Cu/TaN/Ag structure can prevent the underneath Cu pad from oxidation. 100% Au bondability was achieved on Ti/Cu/TaN/Ag structure under both room temperature and 150 °C. The shear fractures mainly occurred in Au bonds, indicating the multi-layers fabricated by magnetron sputtering technology had strong bond interface with Cu pad.Highlights► Uniform, smooth Ti/Cu/TaN/Ag layers can be fabricated by magnetron sputtering. ► Ag layer deposited with (111) as the preferred orientation. ► 100% bondability achieved on Ti/Cu/TaN/Ag structure. ► TaN between Cu pad and Ag layer acts as the barrier layer for Cu atom diffusion.

This paper describes features of a three-dimensional finite element model to simulate the temperature field of a large complicated Al alloy structure during electron beam welding (EBW), aiming to control the final distortion of the welded structure. The actual workpiece is about 1 m in length, with over 8 m aggregate weld length. Because a much finer mesh was required to describe the electron beam heat source, computational work would be substantially increased due to the three-dimensional model. In order to improve calculation speed and quality of simulation, parallel calculation was performed by establishing a computer cluster system composed of four PCs. At the same time, a dynamic three-dimensional keyhole was applied in this model to simulate the heat generation in the cavity. Following the heat source, the keyhole moved along the weld line, allowing a more complex expression to describe the heat source of EBW. Several welding process parameters including input energy and welding speed were studied systematically, as well as the influence of pre-deformation before welding on the ultimate distortion. The results show that the input energy and welding speed have a direct effect on the temperature field, especially on the shape and dimensions of the weld pool, and they seriously influence the final distortion. Pre-deformation also has an effect on distortion, but not apparently as strong as the parameters mentioned above.

Copper wire is attracting more and more attention in wire bonding technology due to its advantages in comparison with gold or aluminum wire. This paper presents an achievement of ultrasonic wedge bonding with 25 μm copper wire on Au/Ni plated Cu substrate at ambient temperature. A detailed investigation from the aspects of process optimization, bonding mechanism, interdiffusion, ultrasonic effects on microstructure and microhardness of the bonding materials were performed. The results show that it is possible to produce strong copper wire wedge bonds at room temperature, and the thinning of the Au layer was found directly below the center of the bonding tool with the bonding power increasing. Interdiffusion between copper wire and Au metallization during the wedge bonding at ambient temperature was assumed negligible. The wedge bonding was achieved by wear action induced by ultrasonic vibration. The ultrasonic power did contribute to enhancing deformation of the copper wire due to ultrasonic softening effect which was then followed by the strain hardening of the copper wedge bond, and the dynamic recovery or recrystallization of the copper wire caused by ultrasonic vibration during wedge bonding was also found.

Copper nanowire transparent electrodes have received increasing interest due to the low price and nearly equal electrical conductivity compared with other TEs based on silver nanowires and indium tin oxide (ITO). However, a post-treatment at high temperature in an inert atmosphere or a vacuum environment was necessary to improve the conductivity of Cu NW TEs due to the easy oxidation of copper in air atmosphere, which greatly cancelled out the low price advantage of Cu NWs. Here, a high intensity pulsed light technique was introduced to sinter and simultaneously deoxygenate these Cu NWs into a highly conductive network at room temperature in air. The strong light absorption capacity of Cu NWs enabled the welding of the nanowires at contact spots, as well as the removal of the thin layer of residual organic compounds, oxides and hydroxide of copper even in air. The Cu NW TE with a sheet resistance of 22.9 Ohm sq−1 and a transparency of 81.8% at 550 nm has been successfully fabricated within only 6 milliseconds exposure treatment, which is superior to other films treated at high temperature in a hydrogen atmosphere. The HIPL process was simple, convenient and fast to fabricate easily oxidized Cu NW TEs in large scale in an air atmosphere, which will largely extend the application of cheap Cu NW TEs.