Effect of Ni Coating Over Cu Ball on the Microstructure of Flip Chip Joints With Cu-Cored Solder Balls

Effect of Ni coating over the Cu core of Cu-cored Sn-5mass%Ag balls was investigated on the microstructures of flip chip joints with Ni/Au plated Cu pads after reflow soldering and the subsequent heat exposure at 423 K. For joints using Cu-cored Sn-5Ag balls with Ni coating over the Cu core, a Ni-Sn reaction layer forms at the Cu core/Sn-5Ag interface. A similar Ni-Sn reaction layer forms at the interface between the Sn-5Ag solder and the Ni/Au plated Cu pad. Although the Ni-Sn reaction layers grow with increasing the thermal exposure time, the growth rate is slower than that of a 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.