A leadframe for semiconductor packages is provided. The leadframe includes a die pad, a side rail, a tie bar, and a plurality of leads. The side rail is around the die pad. The tie bar connects the die pad and the side rail. The leads extend from the side rail to close proximity to the die pad. The leads includes a first lead and a second lead being at opposite locations of the leadframe relative to a center line through the die pad. The first and second leads are substantially asymmetrical with each other relative to the center line and have different impedance values. The plurality of leads are disconnected from each other.