A preformed lead frame includes a metallic substrate, a plurality of spaced-apart conductive lead frame units and intersecting trenches, a molding layer, and a plurality of conductive pads. The lead frame units and the molding layer are formed on the substrate. Each of the lead frame units includes a die supporting portion, a plurality of lead portions surrounding and spaced apart from the die supporting portion, and a gap formed among the die supporting portion and the lead portions. The trenches are formed among the conductive lead frame units. The molding layer fills the gaps and the trenches. Each of the conductive pads is formed on a top surface of the die supporting portion of a respective one of the lead frame units.