Effect of thermal cycling on interfacial IMCs growth and fracture behavior of SnAgCu/Cu joints

The morphological features and growth kinetics of interfacial IMCs have shown significant effect on the mechanical properties, and hence, on the reliability of lead free solder joints. The growth behavior of the interfacial IMCs of SnAgCu/Cu solder joint, in reflow and thermal cycling, was investigated with the focus on the influence of reflow dwell time and the cyclic parameters on the growth kinetics. The three-dimensional IMCs feature was explored by etch the solder matrix out of the SnAgCu/Cu interface. The phases of IMCs were identified by energy dispersive X-ray (EDX). The thickness of the IMCs was measured by element mapping and phase constitution analysis. The thermal cycling of SnAgCu/Cu soldered joints was performed within the temperature region of -25degC to 125degC and -40degC to 125degC respectively. The corresponding IMCs growth rates were formulated according to the data from various thermal cycles. The growth kinetic of the IMCs was analyzed in the framework of diffusion principles. The shear strength of the joint was evaluated and the fracture mechanism was analyzed in accordingly. It was found that Cu₆Sn₅ was formed and followed by rapid coarsening at the solder and Cu interface during reflowing. During thermal cycling, however, IMCs grain coarsening and breaking were noted. The thickness of IMCs was found increases with the thermal cycles; however, the growth rate is less than that of thermal aging. The dwell time in high temperature portion of a thermal cycle was found has significant influence on the growth rate of the IMCs. The growth of the IMCs, both for reflowing and thermal cycling, was found follows diffusion model. The shear strength of the joints was found decreases both with the increase of the thermal cycles as well as with the decrease of the cooling dwell temperature in the thermal cycles.