Effect of thermomigration on the growth kinetics of Cu6Sn5 at liquid-solid interfaces in Cu/Sn/Cu solder joints

Abstract

With the continuous miniaturization of electronic packaging, micro bumps for chip interconnects are smaller in size, and thus the reliability of interconnects becomes more and more sensitive to the formation and growth of intermetallic compounds (IMCs) at liquid-solid interface during soldering. Thermomigration (TM) is one of the simultaneous heat and mass transfer phenomena, and occurs in a mixture under certain external temperature gradient. In the process of interconnection, micro bumps usually undergo multiple reflows during which nonuniform temperature distribution may occur, resulting in TM of metal atoms. Since the interdiffusion of atoms between solders and under bump metallization (UBM) dominates the formation of interfacial IMCs, TM which enhances the directional diffusion of metal atoms and induces the redistribution of elements, will markedly influence the growth behaviors of interfacial IMCs and consequently the reliability of solder joints. The diffusivity of atoms in liquid solder is significantly larger than that in solid solder and in consequence a small temperature gradient may induce the mass migration of atoms. As a result, the growth of interfacial IMCs becomes more sensitive to temperature difference between solder joints in soldering process. So far, however, few studies have focused on liquid state TM in solder joints, and the growth kinetics of interfacial IMCs under TM during soldering is still unknown to us. In this study, Cu/Sn/Cu solder joints are used to investigate the migration behavior of Cu atoms and its effect on the growth kinetics of interfacial Cu6Sn5 under temperature gradients of 35.33℃/cm at 250℃ and 40.0℃/cm at 280℃, respectively. TM experiments are carried out by reflowing the Cu/Sn/Cu interconnects on a hot plate at 250℃ and 280℃ for different durations. For comparison, isothermal aging experiments are conducted in a high temperature chamber under the same temperatures and reaction durations. During isothermal aging, the growth of interfacial Cu6Sn5 follows a parabolic law and is controlled by bulk diffusion. Under the temperature gradient, asymmetrical growth of interfacial Cu6Sn5 is observed between cold and hot ends. At the cold end, the growth of the interfacial Cu6Sn5 is significantly enhanced and follows a linear law, indicating a reaction-controlled growth mechanism; while at the hot end, the growth of the interfacial Cu6Sn5 is inhibited and follows a parabolic law, indicating a diffusion-controlled growth mechanism. The dissolved Cu atoms from the Cu substrate at the hot end are driven to migration toward the cold end by temperature gradient, providing the Cu atomic flux for the fast growth of the interfacial Cu6Sn5 at the cold end. With the variation of the measured thickness of Cu6Sn5 IMC at the cold end and the simulated temperature gradients, the molar heat of transport Q^* of Cu atoms in molten Sn is calculated to +14.11 kJ/mol at 250℃ and +14.44 kJ/mol at 280℃. Accordingly, the driving forces of thermomigration in molten solder FL are estimated to be 1.62×10-19 N and 1.70×10-19 N, respectively.