Robust shear strength of Cu–Au joint on Au surface-finished Cu disks by solid-state nanoporous Cu bonding

The metallization process has received significant attention in the electronic industry. It is one of the best method to enhance bonding strength via interfacial reaction between the back side metallization layer of the SiC chip and an insert material while facilitating superior electrical and thermal conductivity along with excellent chemical resistance. This study is aimed at understanding the effect of the bonding temperatures (200–400°C) on the shear strength of the joints formed between the nanoporous Cu (NPC) sheets and Ni(P)/Au surface-finished Cu disks for the replacement of the high-Pb-containing solder joint in high-temperature applications (> 300°C). We observed that the mechanical properties depend not only on the porosity of the NPC layer but also on the interfacial reaction between the NPC and Au layer of NPC bonding joints. The shear test confirmed the shear strength of the NPC bonding joints above 300°C to be 28 MPa, which was higher than that of conventional high-Pb-containing solder and sintered Cu nanoparticle joints. To understand this promising result, the bonding microstructure was investigated via scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA), and transmission electron microscopy (TEM) analyses. The present study revealed that the shear strength of NPC bonding is closely related to the microstructural characteristics, which are driven by the densification reaction of the NPC layer through the surface diffusion of Cu atoms and interdiffusion reaction between the NPC and Au layer according to the bonding temperature. Based on these results, a mechanism was proposed to explain the superiority of the NPC bonding joints on Ni(P)/Au surface-finished Cu disks achieved using this method.