Growth kinetics of CuSn intermetallic compound in Cu-liquid Sn interfacial reaction enhanced by electric current.

In this paper, electric currents with the densities of 1.0 × 10 A/cm and 2.0 × 10 A/cm were imposed to the Cu-liquid Sn interfacial reaction at 260 °C and 300 °C with the bonding times from 15 min to 960 min. Unlike the symmetrical growth following a cubic root dependence on time during reflowing, the CuSn growth enhanced by solid-liquid electromigration followed a linear relationship with time. The elevated electric current density and reaction temperature could greatly accelerate the growth of CuSn, and could induce the formation of cellular structures on the surfaces because of the constitutional supercooling effect. A growth kinetics model of CuSn based on Cu concentration gradient was presented, in which the dissolution of cathode was proved to be the controlling step. This model indicates that higher current density, higher temperature and larger joint width were in favor of the dissolution of Cu. Finally, the shear strengths of joints consisted of different intermetallic compound microstructures were evaluated. The results showed that the CuSn-based joint could achieve comparable shear strength with Sn-based joint.