Shrinkage and Sintering Behavior of a Low-Temperature Sinterable Nanosilver Die-Attach Paste

The drying and densification behavior of a nanosilver paste was studied by shrinkage and weight-loss measurements to provide fundamental understanding on the sintering behavior of the nanomaterial for packaging power devices and modules. The measured shrinkage behavior was found to be in good agreement with the weight-loss behavior of the paste as measured by thermogravitational analysis, and the comparison offered direct evidence of ~10% shrinkage contributed by late-stage densification of silver nanoparticles (NPs). It was found that sintered silver joints could be achieved without cracks or delamination under a ramp-soak temperature profile for bonding small-area chips, e.g., 3 mm × 3 mm or smaller. However, for bonding large-area chips, e.g., 5 mm × 5 mm or larger, rapid evaporation of the entrapped organic species caused the chips to delaminate, leading to large longitudinal cracks at the joint interface. Finally, examination of the microstructure evolution of the silver die-attach material revealed that binder molecules inhibited necking of the silver NPs and delayed densification during the sintering process of the nanosilver paste.