In the field of semiconductor electronic packaging, lead-based eutectic solder alloy is prevailing and cost friendly. However, lead soldering contacts cannot operate at high temperatures and its toxicity also has significant environmental impact. The functionality and mechanical integrity of a lead-free, nano-silver die-attachment has been demonstrated when it is sintered under high temperature and pressure. Nevertheless, the pressure-assisted sintering process increases the cost and it remains challenging to prevent the silicon die from cracking under pressure when the die is thinner and with rougher surface. It is thus desirable to utilize a pressure-less and lower-temperature sintering process. However, mechanical properties of the nano-silver die-attachment sintered at lower-temperature and pressure-less conditions tends to fail mechanically near the interfaces between the silver di-attachment and the substrate, in particular when gold is used as a metallization layer. To understand the underlying failure behaviors and mechanisms, we visualized and quantified the 3D structure of sintered nano-silver for die-attachment using x-ray ptychography and full field x-ray nano-tomography. The feature size distribution of the different phases, porosity, and the shape of the silver phase were determined as a function of metallization substrates, sintering pressure, types of nano-silver particles and thermal aging conditions. This analysis and the die shear test provided understanding in the failure mechanism within the sintered nano-silver powder structure.