An electrical connector defining a mating direction, includes an insulative housing a plurality of contacts retained in the insulative housing. The insulative housing includes several side walls and a bottom wall connecting the side walls and a mating space surrounded by the side walls and bottom wall. Each contact comprises a contacting portion received in the mating space and a mounting portion protruding perpendicularly from to run through the bottom wall. One of the side walls defines at least an opening and the opening extends along a direction parallel to the contacts and runs through the bottom wall. The opening defines a retaining surface facing to the bottom wall.