The present invention relates to a connector device, and more particularly to an improved wire to board connector of reduced height and improved reliability. 
In communications equipment for transferring large volumes of information, such as portable telephones, personal computers and the like, coaxial cables are used for transferring high-frequency signals.  A cable-attached plug connector is formed by arranging a large number of such coaxial cables in parallel into a flat shape and attaching a plug connector to an end of the coaxial cables.  These cables are terminated, by way of the plug connector, to electronic components mounted on a substrate, such as a circuit board, for processing signals transiting the cables. 
There has been a trend toward the miniaturization of communications equipment such as portable telephones, personal computers, and the like.  In order to realize miniaturization, there has been an increasingly stringent demand for reduced height, size, and weight of a connector as a component of those equipment.  This reduced size may lead to a deterioration in workability.  To avoid this, a technique for improving the workability is required. 
FIG.  11 shows a known cable plug-style connector I in a condition prior to connection to to a mating connector, taking the form of a receptacle connector 302 which mates with the plug connector 300.  In order to connect the two connectors 300, 302 together, the plug connector 300 is moved towards the receptacle connector 302 in parallel to a mounting substrate P. This is commonly referred to as a horizontal type connector device.  The coaxial cables C are respectively connected to terminals of the plug connector 300 (the terminals of the plug connector are referred to as plug-side terminals).  When the plug connector 300 and the receptacle connector 302 are connected with each other, terminals of the receptacle connector 302 (“receptacle-side terminals”) come into contact with the plug-side terminals, allowing electrical signals from the coaxial cables C to flow into electric components (not shown) on the substrate P, on which the receptacle connector 302 is provided. 
FIG.  12 is a longitudinal sectional view showing the plug connector 300 as connected to the receptacle connector 302.  As shown in FIG.  12, the two connectors are connected with each other while being in close proximity to the substrate P. A blade portion of the plug style connector 300 is received in the receptacle of the receptacle connector 302. 
As opposed to the horizontal type connector device as described above, as shown in FIGS.  13 and 14, a connector device of a type in which a plug connector 400 is moved from above towards a receptacle connector 402 mounted on the substrate P to thereby connect the two connectors is referred to as a vertical type connector device.  In this vertical type connector device, the plug connector 400 is placed on top of the receptacle connector 402 (it should be noted that the words “upper (top)” and “lower (bottom)” as used herein are taken to mean respectively, the top and bottom sides of the connector as viewed in the drawings. 
In the known horizontal-type connector device, the receptacle connector 302 is connected to the plug connector 300, which is flat on the substrate P, while being in close proximity to the substrate P. Accordingly, the horizontal type connector device can be reduced in height as compared with the vertical type connector device in which the plug connector is placed on the receptacle connector.  However, the fingers of the operator may touch the substrate P while inserting the plug connector into the receptacle connector, thus hindering the operation or damaging the wiring. 
In contrast, in the case of the vertical type connector device, the plug connector 400 is connected from above the receptacle connector 402, so the substrate P does not interfere with the connection and hence good workability is ensured.  However, the plug connector 400 is stacked on top of the receptacle connector 402, with the result that the height dimension of the connector device upon connecting the two connectors becomes large, which poses a problem in terms of height reduction. 
In view of this, there has been proposed a vertical type connector which not only excels in workability but also enables height reduction.  Such a structure is shown in FIGS.  15 and 16.  FIG.  15 shows aa plug connector 500 ready for mating to a receptacle connector 502, and FIG.  16 shows the receptacle connector 502 and the plug connector 500 mated together.  Formed in the receptacle connector 502 is an upwardly open fitting recess 503 into which the plug connector 500 is fitted from above.  Since the plug connector 500 is thus fitted into the receptacle connector 502, it is possible to achieve height reduction while maintaining the advantage of the vertical type connector device, that is, while ensuring good workability in connecting the plug connector and the receptacle connector with each other. 
Further, in the receptacle connector 502, there is formed a release hole 510 in the form of a cutout that communicates with the fitting recess 503 so as not to prevent the coaxial cables C from projecting outwards upon connecting the plug connector 500 and the receptacle connector 502 with each other (see FIGS.  15 & 16).  The fitting recess 503 is thus open at the top and at the rear.  It should be noted that for the sake of convenience, the portion of the receptacle connector 502 in which the release hole 510 is formed is referred to as a cable insertion portion and denoted by reference symbol 502b.  
In order to ensure reliable connection between the receptacle-side terminals 505 and the plug-side terminals 506, the receptacle-side terminal 505 is shaped so as to generate a contact pressure between the receptacle-side terminal 505 and the plug-side terminal 506 upon abutting against the plug-side terminal 506.  The contact pressure also acts as a force for retaining the connection between the plug connector 500 and the receptacle connector 502 (hereinafter refereed to as the connector connection retaining force). 
The connector connection retaining force acts to press the plug connector 500 towards the cable insertion portion 502b of the receptacle connector 502 upon fitting the plug connector 500 into the fitting recess 503 of the receptacle connector 502.  In this regard, the receptacle connector 502 has, at the cable insertion portion 502b, the release hole 510 open at the rear as described above.  Since the release hole 510 communicates with the fitting recess 503 of the receptacle connector 502, the receptacle connector 502 has the fitting recess 503 that is open at the top and at the rear, so the mechanical strength of the receptacle connector 502 is accordingly weak. 
Upon connecting the plug connector 500 and the receptacle connector 502 together, stress concentration resulting from the contact pressure will occur, in particular, at a location of the cable insertion portion 502b where the release hole 510 in the form of a cutout is formed and in the vicinity of the location, causing distortion.  As a result, there is a fear of the receptacle connector 502 undergoing damaging deformation. 
Further, when the coaxial cables C are pried upwards so a force for releasing the connection between the plug connector 500 and the receptacle connector 502 acts on the plug connector 500 and the receptacle connector 502 that have been connected together, the plug connector 500 is detached from the receptacle connector 502, which may hinder the electrical connection of the connector device. 
The present invention is directed to a reduced height wire to board connector of the plug style which avoids the shortcomings of the prior art described above.