•An electroless Ni-W-P alloy was employed in Zn-5Al/Cu as a diffusion barrier.•The growth rate of IMCs in Zn-5Al/Cu fell 97.75% with aid of a Ni-W-P interlayer.•Kirkendall voids in Zn-5Al/Cu can be eliminated by introducing a Ni-W-P layer.•Diffusional formation mechanisms are proposed of IMCs in Zn-5Al/Cu.The operating temperature of high-temperature electronics can significantly promote the growth of intermetallic compounds (IMCs) at solder/substrate interfaces, particularly for low-cost Zn-based solders because of the rapid rate of reaction of Zn with Cu. Thus, a reliable and robust diffusion barrier is indispensable for suppressing the reactions between solder and substrate. In this work, a ternary Ni-W-P alloy was prepared via electroless plating. Its diffusion barrier property was evaluated by comparing the microstructures of IMC layers in Zn-5Al/Ni-W-P/Cu and Zn-5Al/Cu interconnects after liquid-solid reaction for prolonged durations. When the reaction lasted for 30 min, the thickness of the Al3Ni2 produced in the Zn-5Al/Ni-W-P/Cu solder interconnects was only 2.15 μm, whereas the thickness of the interfacial layer of Cu-Zn IMCs (CuZn4, Cu5Zn8 and CuZn) at the Zn-5Al/Cu interface was 94 μm. Because of the unbalanced growth of the IMCs in the Zn-5Al/Cu interconnects, notable numbers of Kirkendall voids were identified at the CuZn4/Cu5Zn8, Cu5Zn8/CuZn and CuZn/Cu interfaces after prolonged liquid-solid reaction. By contrast, the Al3Ni2 layer in the Zn-5Al/Ni-W-P/Cu solder joints remained intact, showing the potential to effectively enhance the mechanical reliability of electronic devices.Download high-res image (243KB)Download full-size image

•SAC305/TiC composite solders were prepared through powder metallurgic route.•Retained ratio of TiC reinforcement in composite solder was quantitatively measured.•Solderability and physical properties of this newly made composite solder were extensively studied.•Microstructural and mechanical characteristics of composite solder joint under isothermal ageing were also investigated.This paper is focused on the effect of TiC nano-reinforcement that was successfully introduced into a SAC305 lead-free solder alloy with different weight fractions (0, 0.05, 0.1 and 0.2 wt%) through a powder-metallurgy route. Actual retained ratios of TiC reinforcement in composite solder billets and solder joints were quantitatively analysed. The obtained SAC/TiC solders were also studied extensively with regard to their coefficient of thermal expansion (CTE), wettability and thermal properties. In addition, evolution of interfacial intermetallic compounds (IMCs) and corresponding changes in mechanical properties under thermal ageing were investigated. Only about 10%–30% of initial TiC nanoparticles added were found retained in the final composite solder joints. With an appropriate addition amount of TiC nanoparticles, the composite solders exhibited an improvement in their wettability. A negligible change in their melting point and a widened melting range were found in composite solders containing TiC reinforcement. Also, the CTE of composite solder alloys was effectively decreased when compared with the plain SAC solder alloy. In addition, a growth of interfacial IMCs in composite solder joints was notably suppressed under isothermal ageing condition, while their corresponding mechanical properties of composite solder joints significantly outperformed those of non-reinforced solder joints throughout the ageing period.

•Ni-coated graphene (Ni-GNS) composite reinforcement was prepared by electroless plating method.•Ni-GNS/SAC305 composite solders were further prepared through powder metallurgic route.•Microstructures, solderability and mechanical properties of this newly made composite solder were extensively studied.•The existence and distribution of the added reinforcement were confirmed.This paper deals with microstructures and properties of SAC305 lead-free solder reinforced with graphene nanosheets (GNS) decorated with Ni nanoparticles (Ni-GNS). These Ni-coated GNS nanosheets were synthesized by an in-situ chemical reduction method. After morphological and chemical characterization, Ni-GNS were successfully integrated into SAC305 lead-free solder alloy with different weight fractions (0, 0.05, 0.1 and 0.2 wt.%) through a powder metallurgy route. The obtained composite solders were then studied extensively with regard to their microstructures, wettability, thermal, electrical and mechanical properties. After addition of Ni-GNSs, cauliflower-like (Cu,Ni)6 Sn5 intermetallic compounds (IMCs) were formed at the interface between composite solder joint and copper substrate. Additionally, blocky Ni–Sn–Cu IMC/GNS hybrids were also observed homogenously distributed in the composite solder matrices. Composite solder alloys incorporating Ni-decorated GNSs nanosheets showed slightly reduced electrical resistivity compared to the unreinforced SAC305 solder alloy. With an increase in the amount of Ni-GNS, the composite solders showed an improvement in wettability with an insignificant change in their melting temperature. Mechanical tests demonstrated that addition of 0.2 wt.% Ni-GNS would result in 19.7% and 16.9% improvements in microhardness and shear strength, respectively, in comparison to the unreinforced solders. Finally, the added Ni-GNS reinforcements in the solder matrix were assessed with energy-dispersive X-ray spectroscopy, scanning electron microscopy and Raman spectroscopy.

•Cu6Sn5 and Cu3Sn micro-beams were prepared by FIB for cantilever bending test using a nanoindentation system.•The correlation between IMCs crystalline structures and the fracture characteristics of Cu6Sn5 and Cu3Sn was proposed.•Finite element analysis was conducted to clarify the fracture modes of Cu6Sn5 and Cu3Sn under micro-cantilever bending.•The fracture strength of Cu6Sn5 and Cu3Sn were estimated based on simulation and micro-mechanical test.This study focuses on the fracture characteristics of Cu6Sn5 and Cu3Sn micro beams under micro-cantilever bending tests. These micro beams were fabricated by focused ion beam (FIB) from the Sn-rich solder joints aged at 175 °C for 1132.5 h, and then tested using a nanoindenter with a flat tip. Experimental results show that both Cu6Sn5 and Cu3Sn micro beams underwent elastic deformation before their failure. From fractographic analysis, both cleavage fracture and intergranular fracture can be identified from the tested Cu6Sn5 micro beams, while only intergranular fracture was found in Cu3Sn micro beams. Furthermore, based on the experimental results, finite element analysis was carried out to evaluate the tensile fracture strength and strain of Cu6Sn5 and Cu3Sn micro beams. For Cu6Sn, the tensile fracture strength was estimated to be 1.13 ± 0.04 Pa and the average tensile strain was 0.01. The tensile fracture strength and strain of Cu3Sn were evaluated to be 2.15 ± 0.19 GPa and 0.016, respectively.

In this work, SAC305 lead-free solder reinforced with 0.1 wt. % fullerene nanoparticles was prepared using a powder metallurgy method. A lab-made setup and a corresponding Cu/solder/Cu sample for thermo-migration (TM) test were designed and implemented. The feasibility of this setup for TM stressing was further verified with experimental and simulation methods; a temperature gradient in a solder seam was calculated as 1070 K/cm. Microstructural evolution and mechanical properties of both plain and composite solder alloys were then studied under the condition of TM stressing. It was shown that compared to unreinforced SAC305 solder, the process of diffusion of Cu atoms in the composite solder seam was remarkably suppressed. After the TM test for 600 h, Cu/solder interfaces in the composite solder seam were more stable and the inner structure remained more intact. Moreover, the addition of fullerene reinforcement can considerably affect a distribution of Cu6Sn5 formed as a result of dissolution of Cu atoms during the TM test. Hardness data across the solder seam were also found notably different because of the elemental redistribution caused by TM.

Sn99Cu1/Cu solder joints were investigated after isothermal aging at 175°C for different lengths of time under vacuum conditions. The results revealed height reduction of the solder of approximately 1.2 μm after aging for 1132.5 h. This was primarily attributed to growth of a layer of interfacial intermetallic compounds. The reduction was measured by use of a copper block containing a recess filled with solder, which was reflowed then polished flat. Height reduction of the solder joint during aging was found to obey the parabolic law \(\Delta h = -\!\!\hbox{ }0.031\sqrt t\), and was in excellent agreement with theoretical calculation.

Natural network-structured hydrogels (e.g. bacterial cellulose (BC)) can be synthesised with specific artificial hydrogels (e.g. poly(2-hydroxyethyl methacrylate) (PHEMA)) to form a tougher and stronger nanofibre-reinforced composite hydrogel, which possesses micro- and nano-porous structure. These synthetic hydrogels exhibit a number of advantages for biomedical applications, such as good biocompatibility and better permeability for molecules to pass through. In this paper, the mechanical properties of this nanofibre-reinforced hydrogel containing BC and PHEMA have been characterised in terms of their tangent modulus and fracture stress/strain by uniaxial compressive testing. Numerical simulations based on Mooney-Rivlin hyperelastic theory are also conducted to understand the internal stress distribution and possible failure of the nanofibre-reinforced hydrogel under compression. By comparing the mechanical characteristics of BC, PHEMA, and PHEMA-based nanofibre reinforced hydrogel (BC-PHEMA) under the compression, it is possible to develop a suitable scaffold for tissue engineering on the basis of fundamental understanding of mechanical and fracture behaviours of nanofibre-reinforced hydrogels.

•Entire Cu–Sn IMCs joint has been prepared through TLP soldering at 533 K.•Different microstructure as Cu6Sn5 surrounding Cu3Sn phase was observed.•Flux-driven ripening process of Cu3Sn IMC joint was revealed through EBSD results.•Microstructural evolution of Cu–Sn IMCs joints was demonstrated systematically.•Porous Cu3Sn IMC joint was confirmed to be mechanically robust.This study focuses on the mechanism of phase transformation from Cu6Sn5 into Cu3Sn and the homogenization process in full intermetallics (IMCs) micro-joints, which were prepared by soldering the initial Cu/Sn/Cu structure through high temperature storage in vacuum environment as the Transient Liquid Phase (TLP) process. From the microstructural observation by electron backscatter diffraction (EBSD), a mixture of IMCs phases (Cu6Sn5 and Cu3Sn) has been found to constitute the sandwich-structured Cu/IMCs/Cu joints. With the dwell time increasing at 533 K, there were two layers of Cu3Sn emerging from both sides of copper substrates with the depletion of Cu6Sn5 layer, toward merging each other in the IMCs interlayer. Then the Cu3Sn grains with various sizes became more homogenous columnar crystallites. Meanwhile, some equiaxial ultra-fine grains accompanied with the Kirkendall voids, were found only in adjacent to the electroplated copper. In addition, a specific type of micropillar with the size ∼5 μm × 5 μm × 12 μm fabricated by focus ion beam (FIB) was used to carry out the mechanical testing by Nano-indentation, which confirmed that this type of joint is mechanically robust, regardless of its porous Cu3Sn IMC interconnection.

A two-dimensional cross-sectional poly-lattice kinetic Monte Carlo (2DCSP-KMC) model has been developed for simulation of the electrodeposition of polycrystalline copper on either a copper or gold substrate. The model has proved capable of capturing the effects of the deposition parameters, including the applied electrode potential, the concentration of Cu2+ ions and the temperature on the resultant deposit microstructure, the evolution of the grain density, the grain size, the variance of grain size and the grain boundary misorientation distribution. Three unit shapes namely the funnellike, columnar and pyramidal shapes, resulting from the competition between the growth of a grain and its neighbouring grains, are abstracted from a variety of morphology of individual grains to describe the effects of the deposition parameters on the deposit microstructure. The fundamentals of the dependence of these effects on substrate have been discussed and ascribed to the competition amongst the possible KMC events at each KMC step.

•We prepare Ag–PEDOT:PSS inks using in situ synthesis and ultrasonically dispersion.•The Ag–PEDOT:PSS inks are inkjet printed to be thin films and characterized.•Ag:PSS bond formed in in situ synthesis is detrimental to the conductivity of films.•Dispersing silver nanoparticles into pristine PEDOT:PSS improves film conductivity.•Factors that can influence the properties of the films are analyzed and discussed.Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)/silver nanoparticles composite inks have been prepared through in situ synthesis and ultrasonic dispersion. The developed inks were proved to be suitable for various inkjet printing trials to deposit the thin films which were subsequently characterized to assess their electric and optical properties. The results have indicated that the dedoping of PSS from PEDOT during the in situ synthesis can be detrimental to the conductivity of the deposited composite films. However, the addition of silver nanoparticles to pristine PEDOT:PSS has significantly enhanced the conductivity of the thin films, with an inevitable loss in transparency. The various factors that can influence the properties of the thin films have also been analyzed and discussed. This study provides an insight into the effect of silver nanoparticles on PEDOT:PSS thin films deposited using inkjet printing process, and their properties due to the methods of ink formulation.

PEDOT:PSS is a conductive polymer blend which is widely used as a thin functional film in the fabrication of electronic devices. Modification of PEDOT:PSS properties is usually required to enhance their performance which demands further research to gain a complete understanding. This paper is concerned with the optimization of PEDOT:PSS thin films produced by inkjet printing which is deemed as a very promising fabrication technology. Both thermal and solvent annealing were utilized to optimize the ink-jetted PEDOT:PSS thin films. It has been found that thermal annealing is relatively ineffective for the improvement of conductivity and transparency of the thin films. However, solvent annealing is introduced for inkjet printing combined with subsequent thermal annealing has induced a further increase of conductivity in the thin film.Graphical abstractHighlights► We optimize printed PEDOT:PSS thin films through thermal and solvent annealing. ► Thermal annealing is ineffective for the improvement of the thin film properties. ► Solvent annealing makes the PEDOT:PSS solution more suitable for jetting. ► Conductivity and transmittance of the films can be improved by solvent annealing. ► Subsequent baking after solvent annealing can further improve conductivity.

Sn–3.8 wt.% Ag–0.7 wt.% Cu solder was applied to Al–1 wt.% Cu bond pads with an electroless nickel (Ni–P) interlayer as an under bump metallisation (UBM). The microstructure and micromechanical properties were studied after ageing at 80 °C and 150 °C. Two types of intermetallic compounds (IMCs) were identified by electron back-scattered diffraction (EBSD), these being a (Cu, Ni)6Sn5 formed at the solder–UBM interface and Ag3Sn in the bulk solder. The (Cu, Ni)6Sn5 layer grew very slowly during the ageing process, with no Kirkendall voids found by scanning electron microscopy (SEM) after ageing at 80 °C. Nano-indentation was used to analyse the mechanical properties of different phases in the solder. Both (Cu, Ni)6Sn5 and Ag3Sn were harder and more brittle than the β-Sn matrix of the Sn–Ag–Cu alloy. The branch-like morphology of the Ag3Sn IMC, especially at the solder–UBM interface, could ultimately be detrimental to the mechanical integrity of the solder when assembled in flip-chip joints.

Electrochemical impedance spectroscopy (EIS) is a powerful analysis technique, which can provide a wealth of information on the corrosion reactions, the mass transport and the electrical charge transfer characteristics of physical vapour deposition (PVD) ceramic coated steels in an aqueous solution. Although a huge amount of potentially useful data can be generated using the EIS technique, these data need to be carefully interpreted. This is usually done using an ‘equivalent circuit’ which comprises an assembly of electrical circuit elements that model the physicoelectric characteristics of the electrode/solution interface. A systematic study of PVD ceramic (TiN and CrN) coated mild steel and AISI 316L stainless steel was carried out using the EIS technique as the coated systems were immersed in 0.5 N NaCl solution. The relevant equivalent circuits (ECs) are developed by non-linear least square curve fitting to the exponential data to build up a description of the influence of different coatings deposited on steels on the temporal evolution of corrosion in such systems. Constant phase elements describing the non-ideal (e.g. capacitive) characteristics of the electrochemical interface, designated as Q, are introduced to achieve a more accurate simulation of electrochemical corrosion. The mass transport behaviour is also dealt with, through the introduction of diffusion-related elements such as Warburg (designated as W) and cotangent–hyperbolic (designated as O) impedance. The use of these elements significantly improves the quality of fit of the simulation to the EIS data. Finally, the physical validity of the proposed models is discussed in terms of the current–frequency response of the coated steel electrode, during extended corrosion degradation over a period of immersion of up to one week.

In Part I, of this work the equivalent circuits for electrochemical impedance spectroscopy (EIS) modelling of PVD coated steels in 0.5 N NaCl solution were established. In this paper, Part II, the EIS spectra of such coated systems are modelled using the equivalent circuits. The circuit parameters obtained are correlated with the dielectric characteristics, and microstructure of steels and PVD hard coatings. Coating porosity and localised corrosion with exposure time have also been determined using the corrosion potential difference (ΔEcorr) between mild steel and PVD coatings and polarisation resistance Rp, which was obtained through EIS modelling using equivalent circuits. In addition, diffusion rates of the reactants (e.g. oxygen) through ‘permeable’ defects (e.g. pores) are studied by introducing the diffusion impedances W and O in EIS modelling. It has been found that the usage of impedances W and O is closely related to the crystallite features of PVD coatings. Warburg impedance (W) is most suitable for columnar crystallites, while the co-tangent-hyperbolic diffusion impedance (O) is best for the equiaxed crystallite structure. Finally, visual inspection, SEM examination, and the scanning reference electrode technique were employed to observe the corrosion progress of PVD coated steels with immersion time, in order to validate the EIS interpretation.

Replacing the femoral head and the acetabular socket with an artificial prosthetic component is practiced throughout the world to restore painless joint function for patients suffering from disabling hip joint disease. Moving and anchored parts of the prosthesis are exposed to different mechanical stresses and chemical actions; thus, various mechanical, chemical, tribological, and corrosion processes are experienced by the implants. TiN coatings were deposited on Ti–6Al–4V substrates using a plasma-assisted electron beam PVD technique, with the intention of enhancing the tribological and electrochemical performance of the femoral head. The thickness, microhardness, surface roughness, and interfacial adhesion of the TiN coatings were evaluated by means of ball-crater testing, micro-indentation, surface profilometry, and scratch testing, respectively. Impacting under a dynamic repetitive impact load, and multi-pass scratching with a 0.2 mm radius diamond indentator on the coated surface under a load of 50% of the critical load for debonding, were carried out to study wear and contact fatigue failure of the TiN coated systems. The corrosion performance was also evaluated by electrochemical testing including d.c. potentiodynamic and a.c. impedance spectroscopic (EIS) techniques in 0.5 N NaCl solution. It was found that adhesive failure during impact was observable after 5×104 cycles. The number of traversals Pc at which the coatings started to fail during multi-pass test was in a range of 300–400. The PVD TiN coatings significantly reduced the corrosion rate, icorr by approximately 2 orders of magnitude. The coated systems also exhibited superior pitting resistance, which greatly suppressed the localised deterioration. In addition, the polarisation resistance of the TiN coated Ti–6Al–4V systems, determined through the EIS modelling with an equivalent circuit, was 2–4 MΩ cm2 after 15 days of immersion.

AC electrochemical measurement techniques such as electrochemical impedance spectroscopy (EIS) are finding their usefulness in evaluation of the corrosion behaviour of physical vapour deposition coated steel systems. In this work, EIS has been employed to study the corrosion performance of two commercially available hard coatings (TiN and CrN) on mild steel substrate. In order to improve corrosion resistance, a four-layer deposit of each coating material was used. EIS data were acquired at the corrosion potential for each coating material (TiN or CrN) on mild steel in 0.5 N NaCl solution. Interpretation of these spectra reveals the details of the corrosion process as the coated system is immersed in the solution by correlating the coating microstructure. Finally, visual inspection as immersion time increases provides the comparison on pit initiation and progression between TiN and CrN coated systems, as supportive evidence to the EIS analysis.