Measurements of electrical resistance and temperature distribution during current assisted sintering of nanosilver die-attach material

Low-temperature sintering of nanosilver is emerging as a promising lead-free die-attach solution for packaging of semiconductor devices for high-power density or high-temperature applications. However, the bonding process requires either a relatively long heating cycle up to one hour under zero pressure or hot pressing at which the parts are under several MPa of uniaxial stress for tens of seconds to a few minutes at the sintering temperature. In this study, we investigated the use of current assisted sintering technology to simplify and speed-up the bonding process for joining copper parts. We found that strong joining strength could be achieved under one second. To understand this ultra-rapid bonding process and optimize the processing parameters, the electrical resistance across and surface temperature distribution around the silver bond-line during the current heating were characterized in relation to the applied current and current-on time. The electrical resistance was measured by a four-point method, and the surface temperature distribution was measured by an infrared camera equipped with a macro lens. We found that the electrical resistance decreased with increasing current-on time to an asymptotic value. The peak temperature and duration at the joint changed rapidly with alternating current (AC) and current-on time. When the current-on time was for 8 00 ms at an AC of 7 kA, the peak temperature around the bond-line reached about 500 degrees C. Since there should be temperature difference between the surface of the sintered joint and the joint inside, finite element method was used to analyze the temperature distribution of the whole nanosilver joint based on the measured surface temperature distribution of nanosilver joint.