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

3D structure design of magnetic ferrite cores using gelcasting and pressure-less sintering process

Gelcasting is a well established process for ceramics manufacturing which recently has been proved to be successful for soft ferrites as well. This approach is particularly interesting for power electronics application in which the magnetic components (e.g. transformers and inductors) are three dimensionally integrated on the power module substrate. This paper proposes a gelcasting process adapted to make it more effective for 3D heterogeneous integration. The main novelties in this direction consist of low solid load (65wt%) and gelation without catalyst to improve casting and de-airing steps. The magnetic properties of gelcast samples are compared with commercial materials and correlated with the microstructure

Lepidiota Stigma Scale Video reconstruction

3D reconstruction of plasma-FIB sectioning of Lepidiota Stigma scales.Result of final year project. Stuart Robertson of the Loughborough Materials Characterisation Centre (https://www.lboro.ac.uk/research/lmcc/) performed the sectioning with project student Romy Owen who did all the reconstruction. Simon Martin looked on in admiration. </div

Evaluation of metallurgical risk factors in post-test, advanced 9%Cr creep strength enhanced ferritic (CSEF) steel

9wt.% Cr steels are widely used in the design and fabrication of thick section components in combined cycle or coalfired applications for working temperatures of 600~650°C. This family of materials possesses a martensitic microstructure stabilized by precipitates. The presence of nitrides, inclusions or evolution of second-phase particles may increase the metallurgical risk to creep. The chemical composition and microstructural evolution of 9wt.% Cr steels contribute to thermal stability and long-term performance. In some specialist alloys, Ta is added to the composition which causes the formation of fine MX precipitates which are only present at the nanometre scale in tempered martensite, which hinder the recovery of dislocations and the migration of laths to extend creep life. However, the presence of large Ta-containing particles or inclusions in the 9wt.% Cr steels may have a detrimental effect on its creep performance, as they may act as preferred sites for cavity nucleation. To fully appreciate the development of damage in these steels, it is necessary to link the pre- and post-test conditions, evaluate damage in the parent metal, develop procedures that provide consistency of results, and obtain statistically relevant data. The evolution of the Ta-containing phase has been tracked and quantified using a variety of correlative characterization approaches. Utilizing focused ion beam microscopy and two-dimensional electron-based microscopic characterisation, three-dimensional tomography has identified a strong relationship between creep cavities and Tacontaining phases from the early stages of creep.</p

Defect formation and mitigation in Cu/Cu joints formed through transient liquid phase bonding with Cu-Sn nanocomposite interlayer

 Joining based on transient liquid phase bonding (TLPB) has a great prospect in microelectronic packaging. However, this process is plagued with its low throughput. In this work, we designed and utilised a preformed CuSn nanocomposite interlayer (Cu-Sn NI) to speed up the TLPB process. Comparing with the uses of a Sn interlayer only, the preformed Cu-Sn NI enabled the bonding process to be accelerated by >20 times. Furthermore, instead of columnar Cu6Sn5 grains, Cu-Sn intermetallic compounds joints formed with Cu-Sn NIs were mostly filled by refined equiaxed Cu6Sn5 grains with an average size of ~1.6 μm. As a result, the shear strength of IMC joints achieved with Cu-Sn NI was found to be higher than those bonded with Sn interlayer. However, various kinds of defects, one of the main drawbacks in TLPB, were still found from this unique bonding process, which is likely to deteriorate the performances and long-term reliability of resultant TLPB joints. This study aims to observe and analyse various defects formed during TLPB with Cu-Sn NI, hence propose possible mitigation solutions to prevent their occurrence and consequently improve the reliability of the joints obtained by TLPB with Cu-Sn NIs.  </p

Microstructural and mechanical characteristics of Cu-Sn intermetallic compound interconnects formed by TLPB with Cu-Sn nanocomposite

Transient liquid phase bonding (TLPB) is a promising technology for three-dimensional integration of circuits (3D IC), but it can be slow and less productive. A novel Cu-Sn nanocomposite interlayer (Cu-Sn NI) composed of Sn matrix with an embedded Cu nanowire array prepared by electrodeposition can significantly accelerate the bonding process, approximately by 20 times. Bonding time with a Cu-Sn NI can be as short as ∼2 min to achieve a full Cu-Sn intermetallic compound (IMC) joints, whereas it can take ∼60 min with a pure Sn interlayer of the same thickness under the same bonding conditions (250 °C). Unlike the columnar Cu6Sn5 grains commonly formed with Sn interlayer, refined equiaxed Cu6Sn5 grains with an average size of ∼1.6 µm are found to be formed with Cu-Sn NI. Such grain refinement has significantly contributed to the improvement of shear strength of IMC joints formed with Cu-Sn NI (23.1 ± 3.3 MPa), higher than those bonded with pure Sn interlayer (17.9 ± 2.1 MPa). The underlying mechanisms of the new TLPB process and the formation of finer microstructure when bonding with Cu-Sn NIs are also illuminated and validated based on the experimental observation.</p

Rapid formation of intermetallic joint using Cu-Sn nanocomposite interlayer based on patterned copper nanowire array

This paper presents a novel strategy for transient liquid phase bonding (TLPB) in three-dimensional integrated circuit (3D IC) integration that utilizes a preformed Cu-Sn nanocomposite interlayer based on patterned copper nanowire array. This unique interlayer significantly accelerates the bonding process by ∼10 times compared with traditional TLPB using monolithic Sn layer, thanks to the Cu nanowire array grown through template-assisted electrodeposition. It only takes ∼ 2 min to achieve intermetallic joints with refined microstructures through TLPB at 250 °C with the preformed Cu-Sn nanocomposite interlayers

Microstructure characterisation of electromagnetic pulse welded high strength aluminium alloys

Electromagnetic pulse welding is a high-velocity impact joining process employed with the intention of forming fast and effective solid-state bonds. Electron microscopy techniques, including SEM and TEM, revealed that bonding was not fully accomplished in the solid state; instead, local melting can occur. These locally melted areas likely occur around the point of first contact during the welding process and are associated with a debonded region that runs alongside or through the centre of melted zones. Microstructural characterisation showed dispersoid-free regions, columnar grains, epitaxial growth, and localised increases in O, Fe, Si, and Mn content in locally melted areas. This region contrasts with the solid-state bonded region, in which the interface exhibited sub-micron grains.</p

Thermo-mechanical characteristics and reliability of die-attach through self-propagating exothermic reaction bonding

Self-propagating exothermic reactions (SPER) provide intense localized heat sufficient for bonding metals or alloys with minimal heat excursion to the components, which shows great potential for the die attach in power electronics packaging. However, the reliability of such formed joints is yet to be fully understood owing to a wide range of defects involved in the instantaneous propagating reaction and heating/cooling. In this work, the finite element analysis is performed to understand the thermal transfer and mechanical responses of materials to the SPER bonding for the die attach of Si device onto direct bonded copper (DBC) substrate with Sn-3.0Ag-0.5Cu solder. The simulation has been validated using the temperature distribution in SPER bonding, which shows a good agreement with the actual measured results. Moreover, a systematic investigation on the mechanical responses due to thermal mismatch reveals their effects on the thermal stress of interfaces and bonding reliability

Further enhancement of thermal conductivity through optimal uses of h-BN fillers in polymer-based thermal interface material for power electronics

Due to the demand of miniaturization and increasing functionality in power electronics, thermal dissipation becomes a challenging problem for thermal management and reliability. To enable effective heat transfer across the interconnect interfaces, thermal interface materials (TIMs) are required. Electrically insulating TIMs are primarily polymer-based composites which use conductive fillers to enhance thermal conductivity (TC). In this study, the optimal hybrid filler constituents, achieved through mixing spherical and platelet h-BN particles with different ratios, in polymer-based TIM was predicted using finite element (FE) simulations. The underpinning mechanisms of the variation in TC of the TIMs were analyzed from the temperature distribution patterns and micro heat flux paths. Results showed that with the same total volume fraction of h-BN, mixed spherical and platelet h-BN fillers of a certain ratio can further improve the thermal properties of the TIMs compared with those with spherical or platelet h-BN particles alone