Fast and low-temperature sintering of Ag paste due to nanoparticles formed in situ

Sintering of silver is a popular method for forming interconnections in power electronics. Owing to their large size and spherical shape, micron- and submicron-sized Ag particles synthesized by a polyol method (denoted as polyol Ag particles) are not expected to undergo low-temperature, pressureless sintering. However, previous studies have shown sound bonding with shear strength of more than 40 MPa at 200 degrees C with micron and submicron polyol Ag particles. In this work, to understand the bonding mechanism of polyol Ag particles, the sintering behaviors of two Ag pastes, one with polyol Ag particles and another based on hybrid Ag particles consisting of micron-sized Ag flakes and submicron-sized Ag particles, were investigated without any applied pressure at 175 degrees C via transmission electron microscopy. During the sintering process, Ag nanoparticles formed in situ can significantly accelerate the sintering of the Ag paste, resulting in low electrical resistivity of the sintered Ag paste (9.8 x 10(-6) omega center dot cm) after only 5 min of sintering at 175 degrees C. The Ag nanoparticles were likely generated from the reduction of residual Ag ions or the Ag complex in the paste. The results were also verified by washing the Ag particles or adding Ag ions into the paste.