Embodiments of mechanisms for forming a package structure are provided. The package structure includes a semiconductor die and a substrate. The package structure includes a pillar bump and an elongated solder bump bonded to the semiconductor die and the substrate. A height of the elongated solder bump is substantially equal to a height of the pillar bump. The elongated solder bump has a first width, at a first horizontal plane passing through an upper end of a sidewall surface of the elongated solder bump, and a second width, at a second horizontal plane passing through a midpoint of the sidewall surface. A ratio of the second width to the first width is in a range from about 0.5 to about 1.1.