Electric-current-assisted sintering of nanosilver paste for copper bonding

Nanosilver paste is a promising lead-free die-attach material suitable for power electronic packaging, especially for high-temperature applications. Compared with conventional hot-pressing method, electric-current-assisted sintering (ECAS) can greatly improve the efficiency and properties of sintered nanosilver joint. In this paper, the rapid sintering behaviors and mechanical properties, including temperature profile, removal of organics, shrinkage, pore size, particle size, and shear strength, of nanosilver joint at different stages of ECAS were studied to help understand the fundamental mechanism. Based on the results, rapid sintering by high direct current, e.g., 6 kA, can sinter nanosilver for copper bonding at low temperature within 1200 ms under uniaxial load, e.g., 10 MPa. And the ECAS process can be divided into three stages. At the initial stage, the removal of most organics and the rearrangement of nanosilver particles cause rapid shrinkage of the joint. After initial stage the nanosilver particles come into direct contact with each other, and the shear strength of the joint increases quickly due to the diffusion of atoms through the melted region between the particles. At the final stage, further shrinkage of the joint proceeds by plastic deformation under loading and high temperature (above 400 °C). The elimination of crystal defects also contributes to the shrinkage at the final stage. The sintering of nanosilver finally increases the shear strength of the joint to about 50 MPa.