The microscopic failure mechanisms contributing to the interface fracture toughness of two different pressureless sintered silver interconnects during mixed-mode delamination tests have been studied. Two sintered silver materials are used, one containing nanoparticles (NP) and one containing microflakes (μF). The adhesives are sintered between two 35 × 5 mm2 copper plates electroplated with a silver backside metallisation layer. The fracture behaviour has been monitored under in-situ optical microscopy using the miniature mixed-mode bending setup, combined with post-mortem Scanning Electron Microscopy (SEM) fractography analysis. The significant difference in microstructure between the two interconnect materials results in a pronounced difference in failure behaviour, and resulting mixed-mode interface fracture toughness. The two main competing failure mechanisms are interface delamination and bulk fracture. The NP-interconnect exhibits the typical increase in interface fracture toughness with increasing mode angle, whilst the μF-interconnect shows a never reported before dependency, having a global minimum. The fracture morphologies of the delaminated samples are analysed to explain the difference in failure behaviour using Scanning Electron Microscope (SEM) images. This study offers valuable insights into the complex interplay between the microstructures of the interconnect, failure mechanisms, and the resulting interface fracture toughness.