Enhanced shear strength of Cu–Sn intermetallic interconnects with interlocking dendrites under fluxless electric current-assisted bonding process

Abstract

In this paper, the electric current density of 1.44 × 104 A cm−2 was imposed to assemble Cu/(30μm) Sn/Cu interconnection systems without flux at ambient temperature to fabricate Cu–Sn interconnects within 180 ms. From the three-dimensional microstructural observation of interfacial intermetallic compounds (IMCs) at different bonding times, the rod-like Cu6Sn5 formed at the initial stage was changed into dendritic Cu6Sn5 due to constitutional supercooling. When the joule heat-induced temperature was increased above the melting point of Cu6Sn5, the dendritic Cu6Sn5 were melted and then totally converted into Cu3Sn, resulting in the formation of homogeneous Cu3Sn intermetallic joints. The ultrarapid microstructure evolution of the interfacial IMCs was caused by enhanced solid–liquid interdiffusion kinetics, which can be attributed to the joule heating effect as well as the solid–liquid electromigration of Cu in molten Sn with the passage of electric current. In addition, the mechanical analysis shows that the microstructure changes of interfacial IMCs can strongly influence the shear strength as well as the fracture mechanism of the resulted joints. The dendritic network of Cu–Sn IMCs enhanced the shear strength of resulted joints due to the interlocking effect, meanwhile, the homogeneous Cu3Sn joint exhibited the highest shear strength of 67.3 MPa.