Different Diffusion Behavior of Cu and Ni Undergoing Liquid-solid Electromigration

The diffusion behavior of Cu and Ni atoms undergoing liquid-solid electromigration (L-S EM) was investigated using Cu/Sn/Ni interconnects under a current density of 5.0 × 103 A/cm2 at 250 °C. The flowing direction of electrons significantly influences the cross-solder interaction of Cu and Ni atoms, i.e., under downwind diffusion, both Cu and Ni atoms can diffuse to the opposite interfaces; while under upwind diffusion, Cu atoms but not Ni atoms can diffuse to the opposite interface. When electrons flow from the Cu to the Ni, only Cu atoms diffuse to the opposite anode Ni interface, resulting in the transformation of interfacial intermetallic compound (IMC) from Ni3Sn4 into (Cu,Ni)6Sn5 and further into [(Cu,Ni)6Sn5 + Cu6Sn5], while no Ni atoms diffuse to the opposite cathode Cu interface and thus the interfacial Cu6Sn5 remained. When electrons flow from the Ni to the Cu, both Cu and Ni atoms diffuse to the opposite interfaces, resulting in the interfacial IMC transformation from initial Cu6Sn5 into (Cu,Ni)6Sn5 and further into [(Cu,Ni)6Sn5 + (Ni,Cu)3Sn4] at the anode Cu interface while that from initial Ni3Sn4 into (Cu,Ni)6Sn5 and further into (Ni,Cu)3Sn4 at the cathode Ni interface. It is more damaging with electrons flowing from the Cu to the Ni than the other way.