An embodiment of the disclosure includes a conductive bump on a semiconductor die. A substrate is provided. A bond pad is over the substrate. An under bump metallurgy (UBM) layer is over the bond pad. A copper pillar is over the UBM layer. The copper pillar has a top surface with a first width and sidewalls with a concave shape. A nickel layer having a top surface and a bottom surface is over the top surface of the copper pillar. The bottom surface of the nickel layer has a second width. A ratio of the second width to the first width is between about 0.93 to about 1.07. A solder material is over the top surface of the cap layer.