Thermomigration-induced asymmetrical precipitation of Ag3Sn plates in micro-scale Cu/Sn-3.5Ag/Cu interconnects

Understanding the microstructural changes induced by thermomigration during thermal bonding process is important to micro-scale solder interconnects. In the present work, the precipitation of Ag3Sn plates in 50 mu m Cu/Sn-3.5Ag/Cu interconnects under thermomigration has been in situ characterized using synchrotron radiation real-time imaging technology. The precipitation of Ag3Sn plates is found to be asymmetrical, i.e., Ag3Sn plates preferentially precipitate on the cold end rather than the hot end in the interconnects; even if precipitate on the hot end, the Ag3Sn plates tend to dissolve into the molten solder during the subsequent cooling. It is shown that the asymmetrical precipitation of Ag3Sn plates is induced by directional thermomigration of Ag atoms toward the cold end under the temperature gradient across the interconnects, and the simulated temperature gradient is 172 degrees C/cm in the present work. The asymmetrical precipitation of Ag3Sn plates is analyzed and discussed from the viewpoint of thermomigration-affected nucleation and flux-controlled growth. (C) 2015 Elsevier Ltd. All rights reserved.