A receptacle connector includes an insulative housing having an upper wall and a lower wall with a plurality of vertical walls linked therebetween to form corresponding passageways alternately arranged with the vertical walls. A plurality of contacts are disposed in the corresponding passageways, respectively. A plurality of upper apertures are formed in the upper wall to communicate with the corresponding passageways. A plug connector is adapted to be mated with the receptacle connector and includes an insulative body with opposite top wall and bottom wall. A plurality of through openings are formed in the top wall. A plurality of grooves are formed in an interior face of the top wall to communicate the through openings of the plug connector with the corresponding apertures of the receptacle connector.