Abstract:
An electrical connector is disclosed. The electrical connector comprises a plurality of contacts that are adapted to be electrically connected to a substrate. A first body of reflowable, electrically conductive material is placed on a contact in order to provide an electrical path between the connector and the substrate. In addition, a second body of reflowable, electrically conductive material is placed on another contact. This second body provides mechanical strain relief between the connector and the substrate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electrical connector. More specifically, the present invention relates to high input/output density connectors, such as surface mount connectors using ball grid array (BGA) technology. 
     BACKGROUND OF THE INVENTION 
     The recent drive for smaller, more functional electronic equipment, particularly personal portable devices, has created an ongoing need 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. This permits a relatively larger number of connections in the ever decreasing space allotted for connectors on circuit substrates. The drive for smaller electronic equipment has also been accompanied by a recent preference for surface mount techniques (SMT) for mounting components on circuit boards. However, because reducing the pitch between terminals increases the risk of bridging adjacent solder pads during the reflow of solder paste, SMT has been pushed to its limits for high volume, low cost operations. 
     To satisfy the need for increased terminal density in SMT, array connectors have been proposed. In particular, as described in PCT Application No. PCT/US97/18066, filed Oct. 7, 1997, entitled High Density Connector and Method of Manufacture, incorporated herein by reference, ball grid array (BGA) connectors have become a reliable and efficient technique for mounting high density electrical connectors on substrates using SMT. BGA connectors have an insulative connector housing. One side of the connector housing has a matrix of spherical solder balls, positioned to engage the conductive paths of a circuit substrate. The opposite side of the connector housing has a corresponding matrix of contact terminals, which extend through the connector housing and connect electrically to the solder balls. These contact terminals are designed to engage another BGA connector, similarly connected to another substrate, thus permitting board-to-board interconnection. BGA connectors may be used to interconnect a number of various types of circuit substrates, including flexible circuits. 
     A flexible circuit is a pliable electrical conductor device in which conductive tracings are photolithographed on a base sheet of polyimide or polyester film, such as manufactured and sold by E. I. du Pont de Nemours &amp; Co. under the trademarks “Kapton” (U.S. Pat. No. 3,781,596) or “Mylar.” Because of their light weight and ability to bend and adapt to confined locations, flexible circuits are used in a variety of applications, including portable computers and portable communication devices. In addition, because of their ability to flex resiliently, flexible circuits are used on moving devices, like hard disk drives and compact disk pick-ups. Flexible circuits may be interconnected either to conventional circuit board substrates or to other flexible circuit substrates. There are a number of conventional approaches to accomplish interconnections involving flexible circuits. One conventional approach is to simply solder the flexible circuit to the substrate. This approach, however, makes assembly and disassembly impractical. A second conventional approach is to solder a connector to the flexible circuit for connection with another connector soldered onto the substrate. Presently, however, this approach limits terminal density and requires a stiffener device to be attached to the flexible circuit to withstand the mechanical forces imposed on the connector. 
     The recent advent of the BGA connector has overcome the problem associated with low terminal density. As a result, the BGA connector has been used in flexible circuit applications. However, because of the dynamic environment of a flexible circuit, the BGA connector was also used in conjunction with a stiffener to be able to withstand the significant mechanical forces between it and the flexible substrate. Notably, these mechanical forces may be of concern when using BGA connectors to interconnect non-flexible circuit substrates as well. 
     Therefore, a need exists for providing mechanical strain relief to a BGA connector system, without requiring an additional stiffener device. 
     SUMMARY OF THE INVENTION 
     The disadvantages of prior connector systems are overcome and significant advantages achieved in an electrical connector having a plurality of contacts that are adapted to be electrically connected to a substrate. A first body of reflowable, electrically conductive material is placed on a contact, in order to provide an electrical path between the connector and the substrate. In addition, a second body of reflowable, electrically conductive material is placed on another contact. This second body provides mechanical strain relief between the connector and the substrate. In an alternative embodiment, the contacts of the electrical connector may be arranged in a matrix array. In this alternative embodiment, the second body may be disposed on adjacent contacts forming one or more rows or columns in the array. In addition, the second body may be disposed on adjacent contacts located in one or more corners of the array. A method for providing mechanical strain relief to a connector system is achieved by providing an electrical connector such as described above, and mounting the electrical connector to a substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood, and its numerous objects and advantages will become apparent by reference to the following detailed description of the invention, when taken in conjunction with the following drawings where: 
     FIG. 1 is a perspective view of one embodiment of the present invention in which a plurality of contacts form a matrix array; 
     FIG. 2 is a perspective view of the embodiment depicted in FIG. 1 in a mated, closed condition; 
     FIG. 3 is a side view of the embodiment depicted in FIG. 2; 
     FIG. 4 is a side cross-sectional view in fragment of the alternative embodiment; 
     FIG. 5 is a plan view of a preferred embodiment according to the invention; 
     FIG. 6 is a plan view of another preferred embodiment according to the invention; 
     FIG. 7 is a plan view of another preferred embodiment according to the invention; 
     FIG. 8 is a plan view of another preferred embodiment according to the invention; and 
     FIG. 9 is a block diagram of a detection device coupled to at least one contact in a preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In general, the present invention is a board-to-board electrical connector system, wherein an electrical connector is attached to a corresponding board by a plurality of solder balls, some of which provide mechanical strain relief to the connector system. FIGS. 1-3 show various views of one embodiment of the invention. 
     An electrical connector  100  includes a plug  104  and a receptacle  103 ; Plug  104  and receptacle  103  are removably engageable. Plug  104  and receptacle  103  each may have generally planar insulative housings  105  and  106 , respectively. These housings.  105 ,  106  are preferably manufactured from an electrically non-conductive plastic, such as liquid crystal polymer (LCP). Plug  104  has a first edge  115 , a second edge  117 , a first side  111 , and a second side  113 . Similarly, receptacle  103  has a first edge  116 , a second edge  118 , a first side  112 , and a second side  114 . Connector  100  has an overall length l and an overall width w. Receptacle  103  and plug  104  typically have a peripheral wall to protect the contacts and to provide rough alignment of receptacle  103  and plug  104  during mating. Connector  100  preferably interconnects two substrates  301 ,  302  (as shown in FIG.  3 ), such as flexible printed circuit boards. Housing  105  has a mounting end  107  facing substrate  301  and a mating end  108  facing receptacle  103 . Similarly, receptacle  103  has a housing  106  that has a mounting end  110  facing substrate  302  and a mating end  109  facing plug  104 . 
     A plurality of contacts  102  extend generally perpendicular from mating end  109  of receptacle  103 . Similarly, a plurality of contacts  401  (shown in FIG. 4) extend generally perpendicular from mating end  108  of plug  104 . Contacts  102  which mate with contacts  401 , may be any one of a number of different types, including blade-type and round pin contacts. In addition, contacts  102 ,  401  may include several different types of contacts in a single plug or receptacle. 
     Plurality of contacts  102  on receptacle  103  extend through housing  106  from mating end  109  to mounting end  110 . Fusible elements, such as solder balls  303 , attach to contacts  102  on mounting end  110  of housing  106 . Similarly for plug  104 , plurality of contacts  401  (shown in FIG. 4) extend through housing  105  from mating end  108  to mounting end  107 . Fusible elements, such as solder balls  101 , attach to contacts  401  on mounting end  107  of housing  105 . Preferably, contacts  102 , 401  form a matrix array of contacts, as shown in FIGS. 1 and 2. Consequently, solder balls  101 ,  303  also preferably form a matrix array. As used throughout, a column refers to a group of solder balls  101 , 303  that extend along length l of connector  100 . A row refers to a group of solder balls  101 ,  303  that extend along width w of connector  100 . 
     As shown in FIG. 3, plug  104  mounts to substrate  301  via solder balls  101 , preferably with present reflow techniques used in Ball Grid Array (BGA) technology. Similarly, receptacle  103  mounts to substrate  302  via solder balls  303 . Because contacts  102  and  401  are electrically connected to solder balls  303  and  101 , respectively, when connector  100  is in a closed, mated position (as shown in FIGS.  2 - 4 ), substrate  301  is electrically interconnected to substrate  302 . Preferably, either one or both of substrates  301  and  302  are flexible circuit substrates. 
     Referring now to FIG. 4, contacts  102  reside within apertures  404  of housing  106 . Contacts  102  are held within apertures  404  by interference fit and extend generally perpendicular to mating end  108  of housing  105 . Similarly, contacts  401  reside within apertures  403  of housing  105 , and are held within apertures  403  by interference fit. Contacts  401  extend generally perpendicular to mating end  109  of housing  106 . Contacts  102  and contacts  401  have tail portions  405  and  406 , respectively. Tail portions  405  and  406  extend into recesses  415  and  416 , respectively. Tail portions  405  and  406  provide an electrical connection point for solder balls  301  and  101 , respectively, which may be placed in recesses  414  and  415  of housings  105  and  106 , respectively. PCT Application No. PCT/US97/18066, filed Oct. 7, 1997, entitled High Density Connector and Method of Manufacture, discloses methods of securing a solder ball to a contact and of securing a solder ball to a substrate. 
     Contacts  102  also have upper arms  407  and  411 , flexibly connected to tail portions  405  of contacts  102 . Upper arms  407  and  411  have a converging section  412  and an outwardly diverging lead-in section  413 . As plug  104  and receptacle  103  mate, upper portions  408  of contacts  401  flexibly engage upper arms  407  and  411  of contacts  102 . Recess  416  permits upper arms  407  and  411  to outwardly flex and accept upper portions  408  of contacts  401  into outwardly diverging lead-in section  413 . As a result, contacts  102  are electrically connected to contacts  401 , and thus solder balls  303  are electrically connected to solder balls  101 . Although FIG. 4 shows contacts  401  as blade-type contacts and contacts  102  as scissor-type contacts, it should be appreciated that they may be any one of a number of different type contacts, including round pin contacts. In addition, contacts  102 , 401  may include several different types of contacts in a single plug or receptacle. 
     Solder balls  101  and  303  may provide a variety of electrical functions, including carrying a signal, ground or power. In the present invention certain of solder balls  101  and  303  have been added to provide mechanical strain relief to the connection between substrates  301 , 302  and connector  100 , at the same time the remainder of solder balls  101  and  303  function to provide electrical interconnection. 
     Although the need for such mechanical strain relief always prevails, it is of particular concern where solder balls  101 ,  303  are attached to substrates  301 , 302  that are flexible circuits. This is so because flexible circuits are bendable, and thus are used in applications where high mechanic strain is often induced on the connection between substrates  301 ,  302  and solder balls  101 ,  303 . The amount and direction of the strain depend upon the particular application. The reduction of strain is unique to each application as well (depending on the operating environment and operational life requirements). 
     FIGS. 5-8 show plan views of various preferred arrangements of solder balls  101  on housing  105  of plug  104  designated as mechanical strain relief solder balls  501  with respect to solder balls  101  designated as electrical signaling solder balls  502 . Although FIGS. 5-8 show the arrangement of solder balls  101  on plug  104 , it should be appreciated that the same configuration may be present for solder balls  303  on housing  106  of receptacle  103 . In each of the figures, mechanical strain relief solder balls  501  located on plug  104  are shown filled-in or solid, while electrical signaling solder balls  502  are shown without fill or hollow. Although FIGS. 5-8 designate solder balls  101  as either mechanical strain relief solder balls  501  or electrical signaling solder balls  502  it should be appreciated that a portion of solder balls  101  may have no function whatsoever. Moreover, while FIG. 4 shows solder balls  101  and  303  attached to corresponding contacts  102  and  401 , respectively, it should be appreciated that mechanical strain relief solder balls  501  may not be required to be attached to contacts  102  and  401 , but may be secured to housings  105 ,  106  by other mounting techniques. 
     FIG. 5 shows mechanical strain relief solder balls  501  arranged along two rows nearest second side  113  of plug  104 . FIG. 6 shows mechanical strain relief solder balls  501  arranged along two columns nearest first edge  115  of plug  104  and receptacle  103 . FIG. 7 shows mechanical strain relief solder balls  501  grouped in corners of plug  104  and receptacle  103 . Although FIG. 7 shows mechanical strain relief solder balls  501  located in every corner of plug  104 , it should be appreciated that mechanical strain relief solder balls  501  may be grouped in any one or more of the corners, depending on the specific application of the connector system (i.e., depending on the particular strain relief desired). Finally, FIG. 8 shows mechanical strain relief solder balls  501  located around a periphery of electrical signal solder balls  502 . The arrangement of mechanical strain relief solder balls  501  shown in FIG. 8 permits mechanical strain relief for the connector system in all directions. Deciding which arrangement is most preferred is determined by considering which part of the plug will undergo mechanical strain first, and placing mechanical strain relief solder balls  501  thereon. 
     FIG. 9 is a block diagram of a detection device coupled to the connector system. In particular, a detection device  901  is coupled to mechanical strain relief solder ball  501  by a first conductor  902 . Detection device  901  also is coupled to substrate  301  by a second conductor  903 , at connection point  904 , which is located between substrate  301  and mechanical strain relief solder ball  501 . When mechanical strain relief solder ball  501  separates from substrate  301 , detection device  901  detects an open circuit. Detection device  901  may then send a signal  905  over a third conductor  908  to a warning device  906 , so as to notify a user that mechanical strain soon may cause electrical signal solder balls  502  to separate from substrate  301  as well, thus allowing the user to prevent a board-to-board electrical disconnection. 
     Although FIG. 9 shows detection device  901  external to substrate  301 , it should be appreciated that detection device  901  may be integrated on substrate  301 . In addition, although detection device  901  is shown coupled to substrate  301  and mechanical strain relief solder ball  501  only, it should be appreciated that detection device  901  may be coupled to substrate  301  and other mechanical strain relief solder balls attached thereto (not shown). Finally, although FIG. 9 shows detection device  901  coupled to one mechanical strain relief solder ball  501 , it should be appreciated that detection device  901  may be coupled to a plurality of mechanical strain relief solder balls  501 , as required by the specific application of the connector system. 
     Those skilled in the art will appreciate that numerous changes and modifications may be made to the preferred embodiments of the invention and that such changes and modifications may be made without departing from the spirit of the invention. For example, it should be understood that solder balls  101 ,  303  may be placed in numerous arrangements, including the demonstrated matrix array. It should also be understood that FIGS. 5-8 demonstrate just a few of the many possible configurations of mechanical strain relief solder balls  501 . It is therefore intended that the appended claims cover all such equivalent variations as fall within the true spirit and scope of the invention.