Rapid Sintering Nanosilver Joint by Pulse Current for Power Electronics Packaging

Abstract

Sintering of nanosilver paste has been extensively studied as a lead-free die-attach solution for bonding semiconductor chips. The bonding process typically consists of a low-temperature drying step to remove organic solvents in the paste followed by sintering at around 250°C . Normally, a soak time of several minutes at the sintering temperature is necessary to establish strong bond strength by the conventional heating method. In this paper, we tested the feasibility of applying pulses of alternating electrical current through the nanosilver bonding layer to achieve strong joints in less than a second, not minutes. Experiments were carried out by joining rectangular copper blocks that were either coated with a layer of electroplated silver or without. A layer of nanosilver paste was stencil printed on one block, dried at temperature below 100°C, before the other copper block was placed on. The bonding members were then inserted under an alternating-current spot-welding machine for rapid joining with current pulses. Die-shear test was used to quantify the joint strength. Investigated processing variables on the joint strength were current level, current-on time, nanosilver bondline thickness, predrying temperature and time, and copper surface finish. Scanning electron microscopy was used to characterize the joint microstructure. It is suggestive that the current sintering of nanosilver paste could be used for rapid joining of metal-to-metal connection, such as bonding copper bus bars onto power electronics modules.