Growth Kinetics of Reaction Layers in Flip Chip Joints with Cu-cored Lead-free Solder Balls

Abstract

The growth kinetics of the reaction layers formed in the flip chip joints using Cu-cored Sn-5 mass%Ag balls was investigated. In particular, the effect of Ni coating over the Cu core was investigated on the microstructures of flip chip joints with Ni/Au plated Cu pads after reflow soldering and the subsequenct heat exposure at 373 K, 398 K and 423 K. A Ni-Sn reaction layer formed at the Cu core/Sn-5Ag interface in the joint using the Cu-cored Sn-5Ag ball with Ni coating over the Cu core with reflow soldering. A similar Ni-Sn reaction layer formed at the joint interface between the Sn-5Ag solder and the Ni/Au plated Cu pad with reflow soldering. The growth rates of those Ni-Sn reaction layers following the subsequent heat exposure were slower than that of the Cu-Sn reaction layer formed at the Cu core/Sn-5Ag interface in the joint with the Cu-cored Sn-5Ag ball without Ni coating over the Cu core. In this study, the activation energies of the growth of the reaction layers and the shear load of the joint were also investigated.