CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/777,806, filed Dec. 31, 1996 and entitled “Stress Resistant Connector and Method for Reducing Stress in Housing Thereof”; and U.S. patent application Ser. No. 08/728,194 filed Oct. 10, 1996 entitled “High Density Connector”, now U.S. Pat. No. 6,024,584. 
     This application claims the priority of U.S. Provisional patent application Ser. No.60/027,611, filed Oct. 10, 1996 entitled “Low Profile Array Connector”. 
    
    
     This application is also related to U.S. patent application Ser. No. 08/777,579, entitled “High Density Connector”; Ser. No. 08/778,380, entitled “Method for Manufacturing High Density Connector”; and U.S. patent application Ser. No. 08/778,398, entitled “Contact for Use in an Electrical Connector”, all filed on December 31, 1996. The disclosures of the above identified applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrical connectors and more particularly to high I/O density connectors, having a low mated height. 
     2. Brief Description of Prior Developments 
     The drive to reduce the size of electronic equipment, particularly personal portable devices, and to add additional functions to such equipment, has resulted in an ongoing drive for miniaturization of all components, especially electrical connectors. Efforts to miniaturize connectors have included reducing the pitch between terminals in single or double row linear connectors, so that a relatively high number of I/O or other lines can be interconnected by connectors that fit within tightly circumscribed areas on the circuit substrates allotted for receiving connectors. The drive for miniaturization has also been accompanied by a shift in preference to surface mount techniques (SMT) for mounting components on circuit boards. The confluence of the increasing use of SMT and the required fine pitch of linear connectors has resulted in approaching the limits of SMT for high volume, low cost operations. Reducing the pitch of the terminals increases the risk of bridging adjacent solder pads or terminals during reflow of the solder paste. 
     To satisfy the need for increased I/O density, array connectors have been proposed. Such connectors have a two dimensional array of terminals mounted on an insulative substrate and can provide improved density. However, these connectors present certain difficulties with respect to attachment to the circuit substrates by SMT techniques because the surface mount tails of most, if not all, of the terminals must be beneath the connector body. As a result, the mounting techniques used must be highly reliable because it is difficult to visually inspect the solder connections or repair them, if faulty. 
     In the mounting of an integrated circuit (IC) on a plastic or ceramic substrate the use of ball grid array (BGA) and other similar packages has become common. In a BGA package, spherical solder balls attached to the IC package are positioned on electrical contact pads of a circuit substrate to which a layer of solder paste has been applied, typically by use of a screen or mask. The unit is then heated to a temperature at which the solder paste and at least a portion or all of the solder ball melt and fuse to an underlying conductive pad formed on the circuit substrate. The IC is thereby connected to the substrate without need of external leads on the IC. 
     While the use of BGA and similar systems in connecting an IC to a substrate has many advantages, a corresponding means for mounting an electrical connector or similar component on a printed wiring board (PWB) or other substrate has become desirable. It is important for most situations that the substrate-engaging surfaces of the solder balls are coplanar to form a substantially flat mounting surface, so that in the final application the balls will reflow and solder evenly to a planar printed circuit board substrate. Any significant differences in solder coplanarity on a given substrate can cause poor soldering performance when the connector is reflowed onto a printed circuit board. To achieve high soldering reliability, users specify very tight coplanarity requirements, usually on the order of 0.004 to 0.008 inches (or 0.1016 mm to 0.2032 mm). Coplanarity of the solder balls is influenced by the size of the solder ball and its positioning on the connector. The final size of the ball is dependent on the total volume of solder initially available in both the solder paste and the solder balls. In applying solder balls to a connector contact, this consideration presents particular challenges because variations in the volume of the connector contact received within the solder mass affect the potential variability of the size of the solder mass and therefore the coplanarity of the solder balls on the connector along the mounting surface. 
     Another problem presented in soldering connectors to a substrate is that connectors often have insulative housings which have relatively complex shapes, for example, ones having numerous cavities. Residual stresses in such thermoplastic housings can result from the molding process, from the build up of stress as a result of contact insertion, or a combination of both. These housings may become warped or twisted either initially or upon heating to temperatures necessary in SMT processes, such as temperatures necessary to reflow the solder balls. Such warping or twisting of the housing can cause a dimensional mismatch between the connector assembly and the PWB, resulting in unreliable soldering because the surface mounting elements, such as solder balls, are not sufficiently in contact with the solder paste or close to the PWB prior to soldering. The parent and related applications previously identified are directed to solutions to these design challenges. The drive for reduced connector size relates not only to footprint dimensions but also to mated connector height. As electrical equipment shrinks in size, the necessity arises for stacking circuit boards closer together. This invention concerns high density connectors, particularly low profile connectors for reducing the spacing between stacked circuit boards, and more particularly connectors utilizing ball grid array attachment techniques. 
     SUMMARY OF THE INVENTION 
     Electrical connectors according to the present invention provide high I/O density and reduced stacking height. 
     Mated connector height is reduced by utilization of recessed areas in the mating interface of one connector body for receiving the distal portion of a terminal associated with a mating connector. Reduced mated connector height is also achieved by providing a relief area in the connector body to allow flexure of the lower sections of the contact arms of the contact terminal. 
     Overall contact length is reduced by extending cantilevered receptacle contact arms beyond a bight in the terminal toward a plug contact having a relatively short retention base. Both the plug and receptacle contact terminals are received in a passage having a retention feature that engages the contact terminal centrally, thereby allowing a maximization of beam length and the achievement of acceptable performance characteristics. Contact terminal retention features may be located at an intermediate location along the length of one or more of the contact arms. 
     Thermal breaks may be placed in the retention section of the contact terminal. The breaks control solder wicking along the terminal from a mounting surface, where a body of fusible material is formed on the terminal. 
     Contact terminals may be retained in the connector body by a projection or projections in the terminal retention passage that engage the retention section of the terminal or an opening formed in the retention section of the terminal. This terminal mounting arrangement minimizes the accumulation of stress in the connector body, thereby reducing the tendency of the molded connector body to bow or warp. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The method and connector of the present invention is further described with reference to the accompanying drawings in which: 
     FIG. 1 is a top plan view of a plug connector embodying the present invention; 
     FIG. 2 is an enlarged view of the area A of the plug shown in FIG. 1; 
     FIG. 3 is a cross section of the area shown in FIG. 2 taken in the direction of line  3 — 3 ; 
     FIG. 4 is a partially cut away cross sectional view of the plug element shown in FIGS. 1-3 mated with a receptacle; 
     FIG. 5 is a partially cut away cross sectional view of the receptacle and plug shown in FIG. 4 in an orientation normal to that shown in FIG.  4  and mounted between stacked circuit substrates; 
     FIG. 6 is an elevational view of the receptacle contact terminal shown in FIGS. 4 and 5; 
     FIG. 7 is a side view of the receptacle contact terminal shown in FIG. 6; 
     FIG. 8 is a top view of the receptacle contact terminal shown in FIGS. 6 and 7; 
     FIG. 9 is an elevational view of a second embodiment of the receptacle contact terminal; and 
     FIG. 10 is a cut away cross sectional view along line C—C of FIG. 9 of the retention section of the contact terminal retained in a passage. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a plug connector  20  having a connector body or housing comprising a substantially planar base member  22  and a surrounding peripheral wall  24 . On each end wall there are polarizing/alignment tabs  26  upstanding from the wall  24  to assure proper mating of the plug connector  20  with its companion receptacle connector  52 , described later. Preferably the connector body is formed as an integral one piece part by molding an insulative polymer. Preferably, the polymer is one capable of withstanding SMT (Surface Mount Technology) reflow temperatures, for example, a liquid crystal polymer. 
     The plug connector  20  includes an array of plug contact terminals  28  that are retained in a desired pattern, such as a two dimensional matrix, on the connector body. For purposes of simplicity of the drawing, only a few of the terminal sites are shown. 
     Referring to FIG. 3, each plug terminal  28  comprises a substantially planar contact terminal having a mating section  30  for mating with a receptacle contact terminal  72 , to be later described. Plug terminal  28  also includes a retention section  32  adapted to be retained in the connector body  22  in a manner that will hereinafter be described. The retention section  32  includes a pair of opposed shoulders  34  against which an insertion tool is applied to insert the terminal  28  into a terminal passage  38  formed in the connector body  22 . Burrs or barbs can also be formed at shoulders  34  to aid in retention of the terminal in the passage  38 . A solder tab  36  extends from the retention section  32  through slot-shaped opening  53  at the bottom of the passage  38  and is adapted to have a fusible substrate contact mass or body, such as solder ball  35 , fused thereon. Preferably, the lead edge of the solder tab is beveled toward its tip on one or both sides of the terminal, as by chamfer or bevel  37 . Solder balls  35  are fused onto plug terminals  28  and receptacle terminals  72  (described later) by techniques described in parent applications Ser. Nos. 08/778,806 and 08/728,194. 
     As illustrated in FIG. 3, the contact terminal  28  is retained in the terminal passage  38  formed in the connector body  22 . The passage  38  extends from a mating interface or surface  40  toward a mounting surface  42 . A recess such as a well or pocket  50  is formed in the mounting surface  42  in alignment and communication with each passage  38  through slot opening  53 . The mating contact section  30  extends from the mating interface  40  and the solder tab  36  extends into the pocket  50 . The terminal  28  is positioned substantially in alignment with a medial plane MP (FIG. 2) of the passage  38 . 
     The terminal contacts  28  are secured in the body  22  in a manner to avoid the inducing of stress into the molded plastic body upon insertion of the terminals. This objective is achieved in the preferred embodiment by the utilization of the opposed projections  48 . A lead-in surface  49  is formed at the top of each projection  48 . The distance between the distal portions of the projections  48  is less than the thickness of the metal terminal  28  thereby creating an interference fit. Thus the distal portion of each projection  48  is engaged and deformed by the contact terminal as the terminal  28  is inserted into the passage  38  and slot  53 . Preferably, the distal positions of projections  48  are spaced equidistant from the medial plane MP, so that there is substantially equal amounts of deformation of each projection upon insertion of the terminal. As a result, the normal forces against terminal retention section  32  are substantially balanced, thereby aiding in alignment along medial plane MP. The contact terminal is securely held in the passage  38  and slot  53  by the normal force exerted on the contact terminal by the deformed projections. The lead-in surfaces  49  and beveled tips  37  reduce the likelihood of skiving of the projection  48  during insertion, thereby minimizing the removal of material from the projection  48 . The distal portion of each projection deforms and develops a retention force, but one that is localized, so that accumulation of stresses in the housing is avoided. The provision of a pair of opposed, substantially identical projections  48 , equidistant from medial plane MP aids in close tolerance positioning of the contact terminal  28  along the medial plane MP. 
     One of the advantages of the terminal retention structure illustrated in FIG.  3  and disclosed in the above noted parent applications is believed to arise from the situation that after reflow to attach the solder ball  35  to the terminal  28 , the terminal is secured in housing  22  in a locked condition under close to “zero clearance” conditions. This results from the following conditions. The terminal  28  is “bottomed” in passage  38  by inserting the terminal until bottom shoulders  33  engage passage bottom surfaces  39 . This locates the terminal  28  in a vertical downward position, with respect to the view of FIG.  3 . After reflow to attach the solder mass  35  onto tab  36 , by techniques described, for example, in parent application Ser. Nos. 08/728,194 and 08/778,806, the solder ball and/or solder paste disposed in pocket  50  form a mass that fills and conforms to the shape of the pocket  50 . As a result the solder mass  35  engages the bottom  51  of the well  50 . Thus, the reflowed solder mass  35  serves to prevent movement of the terminal  28  upward (in the FIG. 3 sense) out of passage  38 . 
     The terminal  28  is located in side to side directions by engagement of side edges  43  of the retention section  32  against the lateral side walls  41  of the passage  38 . Preferably side walls  41  and side edges  43  have a matching taper, as shown, to aid in true positioning of terminal  28 . Turning to FIG. 2, the terminal  28  is held centrally positioned within passage  38  (in the left to right directions in FIG. 2) by the opposed projections  48 . This results in the location of terminal  28  in housing  22  under tolerance conditions that approach tolerances achieved in insert molding. The improved overall, achievable tolerance levels result from minimization of clearances that are normally present when metal terminals are post-inserted into a plastic housing. That is, positional tolerances are lessened, leaving fit tolerances (the tolerances between mating connectors) as the principal tolerance to be accommodated in the parts. The terminal pitch is maintained during insertion as if the terminals are still mounted on a carrier strip. The close pitch tolerance achieved during the terminal blanking operations is substantially maintained after terminal insertion, by employment of the contact retention system disclosed above. 
     While the cross sectional shape of the projections  48  shown in FIGS. 2 and 3 is preferred, projections or ribs of somewhat different shape and size may be employed. An explanation of the mechanism of this retention system is described in parent application Ser. Nos. 08/728,194 and 08/778,806. The deformation of the projections  48  by the terminals  28  create frictional forces sufficient to hold the position of the terminals in the housing prior to reflow of the solder balls  35 . 
     Adjacent each of the passages  38  are one or more tip receiving recesses  44 ,  46  that are adapted to receive the distal portions of mating receptacle contact terminals  72 . As shown, the recesses  44 ,  46  are formed with one side contiguous with the passages  38 . In the embodiment shown in FIGS. 2 and 3, the recesses are on opposite sides of the medial plane MP. These recesses are also laterally offset from each other, that is, they are on opposite sides of a central plane C that is orthogonal to the medial plane MP. FIGS. 4 and 5, show the distal portions of contact arms of the receptacle contact terminal  72  received in recesses  44 ,  46 . 
     Referring to FIGS. 4 and 5, a receptacle connector  52  for mating with the plug connector  20  is illustrated. The receptacle connector  52  includes a body  54 , preferably formed of the same insulative molded polymer as plug connector  20 . Surrounding the body  54  is a peripheral wall  56 , that includes cut out regions (not shown) for receiving the polarizing/locating tabs  26  of the plug connector. The base or body member  54  includes receptacle passages  62  for receiving of receptacle terminals  72 . When utilizing receptacle terminals of the type illustrated in FIGS. 6,  7  and  8 , the passages  62  preferably include opposed relief areas  64  for accommodating receipt of plug terminal  28  in the formed contact arms  78   a,    78   b  (FIGS.  4  and  5 ). The relief areas  64  are preferably formed with lead-in surfaces  65  that extend and include the top portions of the projections  68 . The passages  62  also include side walls  66 . Opposed terminal retention projections  68  extend from the side walls  66  toward base sections  76  (FIGS. 6 and 7) of the receptacle terminals  72 . The projections  68  are deformed upon insertion of the receptacle terminals  72  in the same manner as described above with respect to the projections  48  in the plug connector  20 . The chamfer  87  of tips  88  and lead-in surfaces  65  aid in achieving deformation rather than removal of the distal portions of the projections  68 , as previously described in connection with FIG.  3 . 
     Each receptacle passage  62  extends from the mating interface  58  of body  54  to a well or pocket  70  formed in the mounting interface or surface  60 . As shown in FIG. 4, the pocket  70  are adapted to receive a substrate contact mass, such as solder balls  74  that are fused to the terminals  72  and substantially fill and conform to the shape of the pocket  70 . Thus the receptacle terminals are retained and located substantially in the same manner as plug terminals  28 . 
     As illustrated in FIG. 5, the configurations of the plug and connector bodies  22  and  54  and the configurations of the plug contact terminals  28  and receptacle contact terminals  72  allow minimization of the height of the mated connectors. This in turn allows the stacking height T between stacked circuit substrates S to be minimized after a second reflow of the solder balls  35   a  and  74   a.    
     Turning now to FIGS. 6-8, a preferred form of receptacle terminal  72  is described in further detail. Each receptacle contact terminal includes a base portion  76  and a pair of cantilevered spring contact arms  78   a,    78   b.  As shown in FIG. 7, the base portion  76  is substantially planar and can be considered as defining a longitudinally extending central plane P of the contact. As shown in FIG. 7, each of the contact arms  78   a,    78   b  diverges oppositely from the plane P in the central region of the contact arms to form between them a bight  79 , which is spaced from the bottom  86  of the gap located between the two contact arms. 
     The distal portions of the arms  78   a,    78   b  then converge toward the plane P to form contact sections  80  for engaging the plug terminals. Lead-in portions  82  are formed at the ends of the arms  78   a,    78   b  to aid in mating with the plug contact  28 . A sharp shoulder  84  is formed intermediate the ends of each of the arms  78   a,    78   b.  The sharp shoulder acts as a barb to aid in retention of the terminal within the passage  62 . These shoulders, as well as the shoulders  34  of plug contacts  28  are engaged by tooling to insert the metal contacts into the respective plastic bodies. The sharp corners aid in retaining the terminals in the respective passages. 
     The use of the laterally offset contact arms  78   a,    78   b  provides numerous advantages including minimization of the front-to-back dimension of the terminal, even when deflected to the phantom line position shown in FIG. 7 by the entry of the plug contact  28  between the two arms  78   a,    78   b.  Further, the utilization of the terminal retention projections  68  as shown in FIGS. 4 and 5 allows a maximization of the length of the contact arm  78   a,    78   b  thereby allowing the development of suitable amounts of deflection to generate appropriate contact normal forces and sufficient contact wipe. 
     As shown in FIG. 6, a solder tab  88  projects from the base section  76 . In a preferred form, the solder tab  88  is adapted to have a solder ball fused onto it. As previously discussed in connection with plug terminal  28 , the leading edge of the terminal  72  is provided with appropriate lead-in structure, such as chamfered surfaces  87 . The base section may be provided with thermal break structure to minimize solder wicking from the pocket  70  onto the terminal. As shown in FIG. 6, the thermal break structure can comprise a pair of openings  89 . This structure may be used in conjunction with the formation of a passivated surface on base section  76  or the application of other appropriate anti-solder wicking coatings, such as organo-fluoro polymers known the art. The thermal breaks, with or without passivation and/or anti-wicking coatings, retard the flow of solder along the contact, when solder paste in pocket  70  is reflowed to secure the solder ball  74  on the solder tab  88 . The plug terminal  28  may also include such anti-solder wicking adjuncts as thermal breaks, passivation, coatings or a combination thereof. Referring to FIGS. 9 and 10, an alternative structure is shown for retaining terminals, such as the receptacle contact terminals  90  in a connector housing. In this embodiment, passages  91  are formed to receive the terminals  90 . Within each of the passages  91 , one or more projections  94  are formed to extend from the side walls of the passage. Each terminal has an opening  96  that is sized and shaped to receive at least a portion of one or both of the projections  94 . Ideally, the shape of the opening  96  corresponds to the shape of the projections  94 , so that the terminal is constrained by the projections against sideways and longitudinal movement, as well as front to back movement. The distal portions of the projections  94  are spaced apart a distance less than the thickness of the material from which the terminal  90  is formed and preferably equidistant from the medial plane MP. 
     Upon insertion of the terminal  90  into the passage  91 , the projections  94  are deformed or spread slightly by the terminal tip or solder tab  98 . The beveled or chamfered surface  95  reduces the tendency of the solder tab  98  to skive the distal portions of the projections  94 . When the terminals are in a fully inserted position, the projections  94  are aligned with the opening  96  and the distal portions thereof enter the opening  96 . As a result, any stress imparted on the connector body is localized to the distal regions of the projections  94 . Because a significant portion of the stress is relieved when the projections  94  enter opening  96 , there is avoidance of stress build up that could cause warpage or bowing of the connector body. Preferably, the longitudinal cross section of retention section  92  is substantially symmetrical about a central longitudinal plane, so that there is a self-centering action imposed on the contact terminal  90  as the base  92  is inserted into the passage  91 . The opening  96  also can function as a thermal break to retard solder wicking, in the same manner as openings  89  in the FIG. 6 embodiment. The terminal  90  may also include passivation or anti-wicking coatings to prevent solder flow toward the contact sections. 
     While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Further, the arrangements described can be used with respect to components other than connectors, that comprise housings formed of insulative materials which carry elements to be fused onto a PWB or other electrical substrate. 
     Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.
An electrical connector having a connector body with a passage extending from an interface portion toward a mounting portion; a conductive terminal having a base section secured in the passage and a tab adjacent the mounting interface; and an opening through the terminal at a location within the body to provide a thermal break for retarding the flow of liquid solder along said terminal. A method of making an electrical connector, comprising the steps of: inserting a contact in a passage of a connector body to a generally fixed position, a void existing between the contact and a wall defining the passage and adjacent a mounting interface; and reflowing a fusible element to attach to the contact and to fill the void. A method of retaining a contact within a body of an electrical connector, comprising the steps of: inserting a contact into an aperture in a connector body; limiting entry of the contact into the aperture; and reflowing a fusible element to attach to the contact. The fusible element prevents removal of the contact from the body.