Phase field crystal simulation of morphological evolution and growth kinetics of Kirkendall voids at the interface and in the intermetallic compound layer of Sn/Cu soldering system under cyclic loading

Kirkendall voids can cause weakening in mechanical properties at the wire bonding and soldering interconnect on integrated circuit packaging, assembly and interconnections. These voids may form at the Cu/Cu3Sn interface and in the Cu3Sn layer of the solder interconnect consisting of the Sn-based solder and Cu substrate (i.e., Sn/Cu soldering system). Excessive formation and growth of Kirkendall voids may increase the potential for brittle interfacial fracture, and the existence of voids will reduce the thermal conductivity. Thus, characterization of formation and growth of Kirkendall voids is very important for the evaluation of performance and reliability of solder interconnects. In this paper, a phase field crystal model is utilized to study the morphological evolution of Kirkendall voids in a typical Sn/Cu soldering system subjected to a periodic tensile stress with different forms. Firstly, a constant tensile stress is applied to the solder joint in the first half cycle, while without imposing stress in the second half cycle. The simulation results show that Kirkendall voids may nucleate and grow at the interface under the cyclic tensile stress, and the coalescence growth of the voids is not obvious. With increasing the strain rate, the growth exponent and void size increase. Secondly, a square-wave form periodic stress with various lengths of cyclic period and a high strain rate is applied perpendicular to the interface, the simulation results show that no significant difference can be observed regarding the growth process of Kirkendall voids. Moreover, with increasing the length of cyclic period, the growth exponent decreases at first and then increases gradually.