Influence of sintering environment on silver sintered on copper substrate

Abstract

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.