Micro-Silver Sinter Paste Developed for Pressure Sintering on Bare Cu Surfaces under Air or Inert Atmosphere

Abstract

Die attach on power semiconductor using leadfree technique has attracted considerable interest. Silver sinter has demonstrated significant develop over the past years to be considered one of frontrunner non-lead containing die attach solution. Pressure silver sintering by far offers superior thermal and electrical conductivity properties which enables power electronics applications to operate at high temperature. Eliminating precious metal finishing on substrate would represent significant compatibility to present supply chain and lower the entry barrier to adopt silver sinter solution. This paper explores the development of a safe-to-use micro-Ag sinter paste for pressure sintering on bare Cu for power electronics packaging. We attached Ag metallized mechanical Si dies on silicon nitride active metal brazed copper substrates with Ag and Au metallization as well as without metallization by silver sintering at 230°C with a pressure of 10 MPa for 3 min. We observed that the average initial die shear strength for Ag metallized substrate is higher than that for Au metallized and bare Cu substrates. This observation points to the self-diffusion of Ag is faster than the silver/gold and silver/copper interdiffusion. The average die shear strength for all the samples increased remarkable after temperature cycling test with a condition of -40°C/+150°C and after a long term storage at 250°C. It is highly likely that the sintering process is not yet completed under the mild sintering process conditions we used in this study and consequently Ag, Au and Cu continued to diffuse during temperature cycling test and high temperature storage and as a result strengthen the sintered joint. It is strongly believed that the sintering process is completed after a certain time of storage at 250°C as we observed no further increase in die shear strength after 250 h storage. The bending test results further confirm the increase of bonding strength by thermal cycling. It is worth noting that cohesive break in the Cu layer was observed for Ag metallized and bare Cu substrates after 1000 h storage at 250°C. Elemental analysis by energy dispersive X-ray spectroscopy demonstrates that interdiffusion between Ag and Cu occurred during high temperature storage in which Cu from the substrate diffused into the silver sintered layer and concurrently Ag from the silver sintered layer diffused into the substrate. In contrast, we observed cohesive break in the sintered layer after 1000 h storage at 250°C for Au metallized substrate indicating that Au metallized layer acts as a barrier to prevent Cu from the substrate from diffusion into the silver sintered layer.