Three‐dimensional damage morphologies of thermomechanically deformed sintered nanosilver die attachments for power electronics modules

© 2019 The Authors. Journal of Microscopy published by John Wiley & Sons Ltd on behalf of Royal Microscopical Society. A time-lapse study of thermomechanical fatigue damage has been undertaken using three-dimensional X-ray computer tomography. Morphologies were extracted from tomography data and integrated with data from microscopy modalities at different resolution levels. This enables contextualization of some of the fine-scale properties which underpin the large-scale damage observed via tomography. Lateral views of crack development are presented, which show networks analogous to mud-cracks. Crack fronts which develop in the most porous regions within the sintered attachment layer travel across the boundary into the copper substrate. The propagation characteristics of these cracks within the substrate are analysed. Evidence is provided of heterogeneous densification within the sintered joint under power cycling, and this is shown to play a major role in driving the initiation and propagation of the cracks. Examination of the texture (differing levels of X-ray absorption) of virtual cross-sectional images reveals the origins of the nonuniformity of densification. Finally, cracks within the sintered joint are shown to have a negligible impact on the conduction pathway of the joint due to their aspect ratio and orientation with respect to the assembly. Lay Description: This paper concerns the use of three-dimensional (3D) X-ray tomography, a nondestructive technique, to perform cradle-to-grave studies of sintered nanosilver die-attachments under operation. Sintered nanosilver die-attachments have been proposed as a more reliable and environmentally friendly alternative to solder alloy joints for emerging power electronics module designs. However, their degradation mechanisms are not as well understood. This same sample-study is about observing how the fine-scale structure of a sintered attachment evolves and degrades over time. Using 3D tomography affords otherwise infeasible perspectives, such as virtual cross-sections in the lateral plane of the attachment. These perspectives provide qualitative information which elucidates the degradation mechanisms. They demonstrate, for example, that the structure of the sintered attachment densifies under operation, and a consequence of this is the formation of shrinkage cracks in the most porous regions, much like mud-cracks. Other imaging techniques (metallographic etching and scanning electron microscopy) have been used in correlation with 3D renderings of these cracks to analyse their propagation and reveal their relationship both with the internal structure of the sintered attachment itself, and the structure of the substrate to which it is joined. It is shown that the cracks develop within the sintered attachment layer and eventually cross over into the substrate. A comparison of two sintered attachments with contrasting bulk porosities allows the effect of initial bond quality on crack development to be examined

Self-assembled titanium-based macrostructures with hierarchical (macro-, micro-, and nano) porosities : a fundamental study

This study details the novel self-assembly of sodium titanate converted Ti-based microspheres into hierarchical porous 3D constructs, with macro-, micro-, and nanoporosity, for the first time. Ti6Al4V microspheres were suspended into 5 M NaOH (60 °C/24 h) solutions, with extensive variations in microsphere:solution ratios to modify microsphere interaction and initiate self-assembly through proximity merging of titanate surface dendritic growth. The formed structures, which either produced 1) unbonded, sodium titanate-converted microspheres; 2) flat (non-macroporous) scaffolds; or 3) open, hierarchically porous scaffolds, were then assessed in terms of their formation mechanism, chemical composition, porosity, as well as the effect of post-heat treatments on compressive mechanical properties. It was found that specific microsphere:solution ratios tended to form certain structures (3 flat non-macroporous, >8 powder) due to a combination of microsphere freedom of movement, H2 gas bubble formation, and exposed surface reactivity. This promising discovery highlights the potential for lower temperature, simplistic production of 3D constructs with modifiable chemical properties due to the ion-exchange potential of titanate structures, with clear applications in a wide-range of fields, from medical materials to catalysts

Self-assembled titanium-based macrostructures with hierarchical (macro-, micro-, and nano) porosities: A fundamental study

This study details the novel self-assembly of sodium titanate converted Ti-based microspheres into hierarchical porous 3D constructs, with macro-, micro-, and nanoporosity, for the first time. Ti6Al4V microspheres were suspended into 5 M NaOH (60 °C/24 h) solutions, with extensive variations in microsphere:solution ratios to modify microsphere interaction and initiate self-assembly through proximity merging of titanate surface dendritic growth. The formed structures, which either produced 1) unbonded, sodium titanate-converted microspheres; 2) flat (non-macroporous) scaffolds; or 3) open, hierarchically porous scaffolds, were then assessed in terms of their formation mechanism, chemical composition, porosity, as well as the effect of post-heat treatments on compressive mechanical properties. It was found that specific microsphere:solution ratios tended to form certain structures (3 flat non-macroporous, >8 powder) due to a combination of microsphere freedom of movement, H2 gas bubble formation, and exposed surface reactivity. This promising discovery highlights the potential for lower temperature, simplistic production of 3D constructs with modifiable chemical properties due to the ion-exchange potential of titanate structures, with clear applications in a wide-range of fields, from medical materials to catalysts

Suitable thicknesses of base metal and interlayer, and evolution of phases for Ag/Sn/Ag transient liquid-phase joints used for power die attachment

Both real Si insulated gate bipolar transistors (IGBT) with conventional Ni\Ag metallization and a dummy Si die with thickened Ni\Ag metallization have been bonded on Ag foils electroplated with 2.7 m and 6.8 m thick Sn as an interlayer at 250ºC for 0 min, 40 min and 640 min. From microstructure characterization of the resulting joints, suitable thicknesses are suggested for the Ag base metal and the Sn interlayer for Ag/Sn/Ag transient liquid phase (TLP) joints used in power die attachment, and the diffusivities of Ag and Sn in the Ag phase are extracted. In combination with the kinetic constants of Ag3Sn growth and diffusivities of Ag and Sn in Ag reported in the literature, the extracted diffusivities of Ag and Sn in Ag phase are also used to simulate and predict the diffusion-controlled growth and evolution of phases in the Ag/Sn/Ag TLP joints during an extended bonding process and in service

Characterization and solderability of cold sprayed Sn-Cu coatings on Al and Cu substrates

Cold sprayed Sn-Cu coatings approximately 40 and 25 μm in average thickness were deposited on aluminium and direct bonded copper (DBC) substrates respectively. Both a statistical analysis of coating thickness and a roughness analysis of the coating/substrate interface and the coating surface were carried out for the as-sprayed coatings using scanning electron microscope images. The results obtained can be related to substrate types and spraying conditions. Tin oxide on the surfaces of the as-sprayed coatings was revealed by employing X-ray photoelectron spectroscopy analyses and transmission electron microscopy. It came from an oxide shell around feedstock powder particles and was only locally broken down during cold spraying. Although the tin oxide inhibited fluxless soldering, flux-supported reflow of cold sprayed Sn on the DBC substrate produced Cu/Sn/Cu solder joints that were acceptable for application in electronic packaging and interconnects. In general, measures which can avoid or remove the tin oxide are needed to achieve improved solder joints using cold sprayed Sn coatings as the solder layers

A Risk Assessment and Management Framework to Support Project Delivery

Risk factors associated with construction projects have a major impact on issues related to cost, time and quality of project delivery. Unexpected events result in either positive or negative outcomes often causing deviations from project plans and making construction projects particularly prone to risk. Despite the extensive research on risk management in the construction industry, there is limited literature dealing specifically with project risk in developing countries. It is against this background that this study investigates the use of risk assessment and management processes in organisations. Data were collected from construction professionals working with construction clients (both private and public), consultants, and contractor organisations within the Ghanaian construction sector. The survey data were used to examine the differences in the levels of agreement between the three stakeholders. The research findings indicated that although project stakeholders are aware of risk factors associated with construction projects, there is a need for the benefits of using risk management processes to be emphasised and communicated. Based on the research findings a risk assessment and management framework was established to support future successful delivery of construction projects in developing countries

Applications-Based Design of SiC Technology

The adoption of SiC devices as a viable technology depends crucially on maximising the potential advantages of the material. This is best achieved by the adoption of co-design techniques in which the optimisation of the SiC device is performed in parallel to that of the package and the overall application. This paper considers suitable techniques for this co-design and describes new approaches to the development of SiC technology for practical applications