Thermal stability of high temperature Pb-free solder interconnect characterised by in-situ electron microscopy

The present investigation aimed to use in-situ heating experiment in a transmission electron microscope (TEM) to live characterize the thermal stability of a Cu/Ni-W-P interlayer/ZnAl solder interconnect. It demonstrated the TEM was able to detect live intermetallic compounds (IMCs) growth during heating. In addition, stress building up was evidenced by the progressive evolving of the dislocations at the interface between NiW-P interlayer and the ZnAl Solder. However, due to the μm to nm scale of specimens' dimensions required for electron microscopy, the sample preparation and data interpretation remains a challenge

In-situ micro bend testing of SiC and the effects of Ga+ ion damage

The Young’s modulus of 6H single crystal silicon carbide (SiC) was tested with micro cantilevers that had a range of cross-sectional dimensions with surfaces cleaned under different accelerating voltages of Ga+ beam. A clear size effect is seen with Young’s modulus decreasing as the cross-sectional area reduces. One of the possible reasons for such size effect is the Ga+ induced damage on all surfaces of the cantilever. Transmission electron microscopy (TEM) was used to analyse the degree of damage, and the measurements of damage is compared to predictions by SRIM irradiation simulation

A novel scale-down cell culture and imaging design for the mechanistic insight of cell colonization within porous substrate

At the core of translational challenges in Tissue Engineering is the mechanistic understanding of the underpinning biological processes and the complex relationships among components at different levels, which is a challenging task due to the limitations of current tissue culture and assessment methodologies. Therefore, we proposed a novel scale-down strategy to deconstruct complex bio-matrices into elementary building blocks, which were resembled by thin modular substrate and then evaluated separately in miniaturised bioreactors using various conventional microscopes. In order to investigate cell colonization within porous substrate in this proof-of-concept study, TEM specimen supporters (10-30µm thick) with fine controlled open pores (100~600µm) were selected as the modular porous substrate and suspended in 3D printed bioreactor systems. Non-invasive imaging of human dermal fibroblasts cultured on these free-standing substrate using optical microscopes illustrated the complicated dynamic processes used by both individual and coordinated cells to bridge and segment porous structures. Further in situ analysis via SEM and TEM provided high quality micrographs of cell-cell and cell-scaffold interactions at micro-scale, depicted cytoskeletal structures in stretched and relaxed areas at nano-scale. Thus this novel scaled-down design was able to improve our mechanistic understanding of tissue formation not only at single- and multiple-cell levels, but also at micro- and nano-scales, which could be difficult to obtain using other methods

Formation and homogenisation of Sn-Cu interconnects by self-propagated exothermic reactive bonding

We produced SnCu interconnects by self-propagated exothermic reactions using AlNi NanoFoil at ambient conditions, through the instantaneous localised heat across the interfaces between Sn electroplated Cu substrates. This technique presents a great potential for electronics integration with minimal thermal effects to the components. However, the metastable phases resulted from the non-equilibrium interfacial reactions and solidification were inevitable under a highly transient regime due to a drastic heating/cooling (over 107 K/s). In this study, Finite Element Analysis was performed to predict the temperature profiles across bonding interfaces, which were subsequently correlated with the formation and homogenisation of the bonded structures during the bonding and post-bonding ageing process. It has been revealed that, for nano-sized metastable phases, their formation, morphologies and distribution were primarily attributed to the convective mass transportation, liquid-solid inter-diffusion, and directional non-equilibrium solidification of Sn in molten zone of the bonding interfaces. The non-equilibrium phases initially formed in the SnCu interconnects can be homogenised towards the equilibrium status by accelerated ageing. This was achieved through the coalescing and subsequent growth of the original nano-sized metastable phases, as a result of the solid-diffusion of Cu and Ag atoms at intergranular boundary regions of Sn grains, AlNi NanoFoil/Sn. and Cu/Sn interfaces

Diffusion barrier property of electroless Ni-W-P coating in high temperature Zn-5Al/Cu solder interconnects

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

Synthesis and assembly of gold and iron oxide particles within an emulsion droplet; facile production of Core@Shell particles

Here we report a method for synthesising and assembling nanomaterials at the liquid-liquid interface of an emulsion droplet, resulting in a simple strategy for producing hollow Au shells, or Fe3O4@Au core@shell particles. Mercaptododecanoic acid stabilised Au nanoparticles were added to the aqueous continuous phase, in order to stabilise hexane emulsion droplets formed within a microfluidic chip. The diameters of Au Pickering emulsions could be controlled by varying the flowrates, this produce hollowparticles. The addition of a second nanoparticle, Fe3O4 (average diameter of 12 nm), into the organic phase produced core@shell particles. The diameter of the resultant material was determined by the concentration of the Fe3O4. This report is the first to demonstrate Pickering emulsions within a microfluidics chip for the production of Fe3O4@Au particles, and it is believed that this could be a versatile platform for the large scale production of core@shell particles

Interfacial reaction and microstructural evolution between Au-Ge solder and electroless Ni-W-P metallization in high temperature electronics interconnects

© 2017 IEEE. The elevated working temperature of high temperature electronics can inevitably cause potential excessive growth of interfacial intermetallic compounds (IMCs), which can significantly deteriorate the mechanical integrity of electronic devices. Therefore, a robust diffusion barrier that can operate reliably under elevated temperature is highly demanded to retard the interfacial reaction between solder and substrate. In this work, a ternary Ni-W-P alloy was deposited through electroless plating and applied as an Under Bump Metalisation (UBM) to Au-Ge solder joints. The interfacial reaction in Au-Ge/Ni-W-P solder joints after reflow and prolonged ageing durations was investigated. We found NiGe and Ni5Ge3 layers formed after reflow, however only NiGe was observed after 1000h aging at 300°C. The thickness of NiGe increases linearly with the square root of ageing time up to 1500h, indicating that the growth mechanism of NiGe is diffusion-control process when Ge atoms are sufficient. After ageing for 2000h, although Ge atoms from Au-Ge solder was fully consumed, the Ni-W-P coating remained stable and exhibited excellent diffusion barrier property. During various ageing durations, the top-view morphology of NiGe IMC grains changed from pyramid-like and polygon-like shape at as-built stage to granulate-like (up to 1500h), and finally a polygon-like shape (after 2000h)

Combined effects of surface oxidation and interfacial intermetallic compound growth on solderability degradation of electrodeposited tin thin films on copper substrate due to isothermal ageing

© 2018 Elsevier Ltd We report new insights into the solder wettability degradation of Sn thin films on Cu under 155 °C isothermal ageing. A multiscale wettability degradation model was established, reflecting quantitatively the surface oxidation and interfacial intermetallic compound (IMC) growth, on the basis of solder wetting behaviour. The thermal oxidation of Sn exhibited heterogeneous inward thickening, lateral expanding and outward platelet-like growth, forming nanocrystalline, oxygen-deficient SnO2with pronounced voiding/cracking propensity. Unlike a commonly held belief that the initial wettability loss is due to surface-exposing and oxidation of IMCs, it was found from dual combined effects of inward surface oxidation and outward IMC growth

Focused ion beam preparation of microbeams for in situ mechanical analysis of electroplated nanotwinned copper with probe type indenters

A site‐specific xenon plasma focused ion beam preparation technique for microcantilever samples (1 μm ‐ 20 μm width and 1:10 aspect ratio) is presented. The novelty of the methodology is the use of a chunk lift‐out onto a clean silicon wafer to facilitate easy access of a low‐cost probe type indenter which provides bending force measurement. The lift‐out method allows sufficient room for the indenter and a line of sight for the electron beam to enable displacement measurement. An electroplated nanotwinned copper (NTC) was cut to a 3 × 3 × 25 μm micro‐beam and in‐situ mechanically tested using the developed technique. It demonstrated measured values of Youngs modulus of 78.7 ± 11 GPa and flow stress of 0.80 ± 0.05 GPa, which is within the ranges reported in the literature

Mesoscopic structure features in synthetic graphite

The mesocopic structure features in the coke fillers and binding carbon regions of a synthetic graphite grade have been examined by high resolution transmission electron microscopy (TEM) and Raman spectroscopy. Within the fillers, the three-dimensional structure is composed of crystal laminae with the basal plane dimensions (La) of hundreds nanometres, and thicknesses (Lc) of tens of nanometres. These laminae have a nearly perfect graphite structure with almost parallel c-axes, but their a-b planes are orientated randomly to form a “crazy paving” structure. A similar structure exists in the binding carbon regions, with a smaller La. Significantly bent laminae are widely seen in quinoline insoluble inclusions and the graphite regions developed around them. The La values measured by TEM are consistent with estimates from the intensity ratios of the D to G Raman peak in these regions. Atomistic modelling finds that the lowest energy interfaces in the crazy paving structure comprise 5, 6 and 7 member carbon rings. The bent laminae tend to maintain the 6 member rings, but are strained elastically. We suggest that a 7 member carbon ring leaves a cavity representing an arm-chair graphite edge contributing to the Raman spectra D peak