Asymmetrical growth of Cu6Sn5 intermetallic compounds due to rapid thermomigration of Cu in molten SnAg solder joints

We observed asymmetrical growth of Cu6Sn5 intermetallic compounds (IMCs) on the two interfaces of Cu/SnAg/Cu solder joints during reflow at 260 °C on a hot plate. The IMCs grew to 12.3 μm on the cold end and 3.5 μm on the hot end after reflow for 40 min. However, the consumption of Cu on the cold end is less than that on the hot end. We propose that rapid thermomigration of Cu is responsible for the asymmetrical growth of the IMCs. With the simulated thermal gradient of 51 °C/cm across the liquid solder, the heat of transport of Cu is calculated as 20 kJ/mol.