A package structure includes a chip attached to a substrate. The chip includes a bump structure including a conductive pillar having a length (L) measured along a long axis of the conductive pillar and a width (W) measured along a short axis of the conductive pillar. The substrate includes a pad region and a mask layer overlying the pad region, wherein the mask layer has an opening exposing a portion of the pad region. The chip is attached to the substrate to form an interconnection between the conductive pillar and the pad region. The opening has a first dimension (d1) measured along the long axis and a second dimension (d2) measured along the short axis. In an embodiment, L is greater than d1, and W is less than d2.