Low temperature solid-state bonding using Sn-coated Cu particles for high temperature die attach

Abstract

This paper describes a low temperature solid-state bonding technique using microscale Sn-coated Cu particles for high temperature die attach applications. The microstructure and shear strength of Cu-Cu joints prepared by solid-state bonding under various bonding conditions were examined. When formic acid vapor was used as the reducing atmosphere, after bonding at 200 °C for 20 min with an applied pressure of 20 MPa, the joints exhibited a microstructure fully comprised of Cu3Sn intermetallic compounds with a dispersion of Cu particles, with a higher shear strength than the conventional Pb-5Sn solder. The phase transformation and microstructure evolution during the bonding process was investigated. The Cu3Sn/Cu mixture microstructure is attributed to the unique structure of Sn-coated Cu particles used in this study. During bonding, the coalescence of adjacent particles and the solid-state reaction between the coated Sn and the core Cu occurred simultaneously. Transient Cu6Sn5 IMC was formed, and then it was completely transformed to Cu3Sn IMC within a short time. Finally, a thermodynamically stable Cu3Sn/Cu composite microstructure was obtained. The results demonstrate that low temperature solid-state bonding using microscale Sn-coated Cu particles has the potential to fulfill the requirements of die attach applications for power device packaging.