EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Power electronic modules are integral for energy conversion in applications like renewable energy generation and transport. 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 talk is about how three-dimensional (3D) X-ray tomography is used to carry out cradle-to-grave studies of these attachments under operation. 3D tomography provides perspectives otherwise infeasible, such as virtual cross-sections in the lateral plane of the attachment. These perspectives have been key in pointing towards the degradation mechanisms at play. They demonstrate, for example, how the structure of the sintered attachment continues to densify under operation, and a consequence of this is the formation of shrinkage cracks which resemble mud-cracks in the most porous regions.

Focused ion beam (FIB) and electron backscatter diffraction (EBSD) imaging have also 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. As-sintered and cycled sintered Ag layers both exhibit heterogenous porous structures consisting of randomly oriented equiaxed grains whose size vary depending on the local density of the region sampled. Power cycling brings about grain growth and the loss of twin boundaries, and these are more pronounced within more dense regions of the Ag attachment. The copper substrate undergoes some grain refinement. Cracks, which appear to initiate within the Ag layer, propagate across the Ag-Cu boundary and transgranularly through fine grained regions within the copper with little tortuosity.

This work was supported by Mentor Graphics and the UK Engineering and Physical Sciences Research Council projects EP/K035304/1 & EP/R004366/1 through the Centre for Power Electronics.