Influence of sintering environment on silver sintered on copper substrate

Sintered silver joint has been reported as the front-runner for Pb-free die attach joints for high temperature applications. In this study, micron-Ag and nano-Ag paste were sintered in either air or nitrogen (N₂) atmosphere. While sintered silver joints have excellent mechanical properties, their performance depends on density and porosity which in turn are dependent on sintering environment. In this study, micron-Ag and nano-Ag paste were sintered in either air or N₂ gas environment. Micron-Ag joint failed to sinter in N₂ atmosphere but registered die shear strength of 13 MPa when sintered in air despite thick Cu oxides at the Cu-sintered micron-Ag interfaces. Sintering nano-Ag in air promoted an inter-diffused region of Ag–Cu (200 nm) at the Cu-substrate with nano-cracks visible under transmission electron microscope. This nano-cracks reduced the die-shear strength in spite of the finer grain-size found in air-sintered nano-Ag joint. In addition, a nano-indentation method based on "constant-load" was used to differentiate the creep mechanism and elastic-properties of sintered Ag joints produced in air and N₂ gas from those of Sn-3.5Ag solder. This simplified approach confirmed the familiar relationship between density and creep resistance observed in other sintered materials; low density sintered Ag was less resistant to creep failure and had a lower elastic modulus than the high density sintered Ag joint.