In-situ reduction enabled anti-oxidation sintering of copper paste in air for power module packaging

With the development of power electronic devices fabricated by wide-bandgap semiconductors, whose working temperature can reach over 250 °C, the corresponding die-attach material also should be able to withstand such a high temperature. Cu paste is an effective material to interconnect the chip with the substrate, but oxidation of Cu particles during storage and sintering now inhibits its application. In order to avoid oxidation, a Cu paste with self-reducibility and antioxidation ability was prepared. The Cu paste was mixed with two reductants and pre-oxidized Cu nanoparticles. The reductants can not only reduce the oxide layer but also provide a reducing atmosphere during sintering in air. Therefore, the in-situ reduced Cu nanoparticles can promote the necking growth without oxidation and lead to a higher strength. The maximum shear strength of 46.44 MPa was achieved by the joints that sintered at 300 °C for 30 min. In addition, the grain size, grain orientation and dislocation density of Cu joints sintered for various time were analyzed by Electron Backscattered Diffraction (EBSD) to illustrate the sintering mechanism. The initial grain boundary misorientation and the deformation strain from assisted pressure drive the grain boundary migration, leading to the formation of coherent interface, grain growth, dislocation accumulation and twin grain growth with the sintering time increased to 30 min. Therefore, the high shear strength was due to dislocation strengthening and twin grain strengthening.