A device includes a chip attached to a substrate. The chip includes 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 conductive trace and a mask layer overlying the conductive trace, wherein the mask layer has an opening exposing a portion of the conductive trace. An interconnection is formed between the conductive pillar and the exposed portion of the conductive trace. The opening has a first dimension (d1) measured along the long axis of the conductive pillar and a second dimension (d2) measured along the short axis of the conductive pillar. A ratio of L to d1 is greater than a ratio of W to d2.