Abstract:
An electrical connector mountable to a substrate. The electrical connector comprises a housing, and a surface mount contact and hold down both secured to the housing. The hold down may be a surface mount hold down. A ball grid array connector comprising a housing, a plurality of contacts secured to the housing, a plurality of fusible elements secured to the contacts for mounting the connector to a substrate, and a hold down secured to the housing. The hold down may be a surface mount hold down. A method of mounting an electrical connector to a substrate. The method comprises providing a substrate and an electrical connector having a contact and a hold down, and securing the contact and the hold down to the substrate. The contact may be secured to a surface of the substrate. A method of securing an electrical connector to a substrate. The method comprises providing a substrate and an electrical connector having a contact and a hold down, and mounting the contact to the substrate. The method further comprises balancing the electrical connector on the substrate such that the electrical connector remains substantially parallel to the substrate during the forming of the first and second solder joints.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Applications Ser. No. 60/160,482, which was filed on Oct. 19, 1999. In addition, the subject matter disclosed herein is related to the subject matter disclosed in copending application Ser. No. 09/691,811, filed on Oct. 19, 2000. Both applications are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to electrical connectors. More specifically, the invention relates to electrical connectors with strain relief features. 
     2. Brief Description of Earlier Developments 
     Various types of electrical connectors rely upon surface mount technology (SMT) to secure the connector&#39;s contacts to an underlying substrate. SMT connectors provide numerous benefits over earlier connectors, such as simplified manufacturing and lower costs. 
     While providing such advantages, the use of SMT may raise other issues. One concern, for example, involves the ability of the solder joint between the contact and the underlying substrate to absorb forces caused by, for example, shipping, handling, mating and thermal cycling. Should one solder joint become unusable as a result of damage from any of these events, the entire connector adversely may be affected. 
     Ball grid array (BGA) technology is one type of SMT. Generally, an electrical connector using a BGA has a housing with a contact therein. A fusible element, typically a solder ball, secures to each contact. The solder balls serve as the primary connection between the contact and the surface of the substrate. A reflow process fuses the solder ball to the substrate. During the reflow process, a beneficial “self-centering” feature of the BGA technology occurs. Specifically, as the solder reflows, the surface tension of the solder helps to align the connector properly with the conductive pads on the underlying substrate. 
     As with SMT connectors, forces on the solder joint in a BGA connector also poses a concern. Because of the self-centering ability of BGA connectors, however, many of the solutions used in SMT connectors cannot be used on BGA connectors. Therefore, a need exists to develop techniques for providing strain relief to BGA connectors. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an electrical connector with strain relief features. 
     It is a further object of the invention to provide a surface mounted electrical connector with strain relief features. 
     It is a further object of the invention to provide a ball grid array electrical connector with strain relief features. 
     It is a further object of the invention to provide strain relief features to a ball grid array electrical connector compatible with the self-centering capability of the connector. 
     It is a further object of the invention to provide an electrical connector made with simplified manufacturing steps. 
     These and other objects of the invention are achieved in one aspect of the invention by an electrical connector mountable to a substrate. The electrical connector comprises a housing, and a surface mount contact and hold down both secured to the housing. The hold down may be a surface mount hold down. The connector may further comprise a shield generally surrounding the housing, wherein the hold down is part of the shield. The surface mount contact may include a fusible element in the form of a solder ball. The surface mount contacts may form a matrix array. Also, the electrical connector may be constructed such that it remains substantially parallel when mounted to the substrate. The electrical connector may include a standoff secured to the housing, wherein the standoff is adapted to retain the housing a distance from a surface of the substrate. The standoff may be a part of the shield. 
     These and other objects of the invention are achieved in one aspect of the invention by a ball grid array connector comprising a housing, a plurality of contacts within the housing, a plurality of fusible elements secured to the contacts for mounting the connector to a substrate, and a hold down secured to the housing. The hold down may be a surface mount hold down. The ball grid array connector may further comprise a shield generally surrounding the housing, wherein the hold down is part of the shield. Also, the ball grid array connector may include a standoff extending from the housing and adapted to retain the housing a distance from a surface of the substrate. The standoff may be a part of the shield. The ball grid array connector may be constructed such that it remains substantially parallel when mounted to the substrate. 
     These and other objects of the invention are achieved in one aspect of the invention by a method of mounting an electrical connector to a substrate. The method comprises providing a substrate and an electrical connector having a contact and a hold down, and securing the contact and the hold down to the substrate. The contact may be secured to a surface of the substrate. The contact may be secured before the hold down is secured, using a soldering technique, for example, to permit the connector to self-center on the substrate. The electrical connector may include a shield that is secured to the substrate. The method may further comprise constructing the electrical connector such that it remains substantially parallel when mounted to the substrate. Also, the method may comprise balancing the electrical connector on the substrate such that the electrical connector remains substantially parallel to the substrate during they are attached. The electrical connector may be a ball grid array connector. 
     These and other objects of the invention are achieved in one aspect of the invention by a method of securing an electrical connector to a substrate. The method comprises providing a substrate and an electrical connector having a contact and a hold down, and mounting the contact to the substrate. The method further comprises balancing the electrical connector on the substrate such that the electrical connector remains substantially parallel to the substrate during the forming of the first and second solder joints. Such balancing may be accomplished by removing material from and/or adding material to the electrical connector. Alternatively, the balancing may be accomplished by exerting an external force on the electrical connector and/or the substrate during the forming of the first and second solder joints. 
     These and other objects of the invention are achieved in one aspect of the invention by an improved array connector having a plurality of fusible elements that are able to support a nominal mass without an undesired flattening of the fusible elements. The nominal mass is less than a mass of the connector. The improvement comprises a feature on the connector that prevents the undesired flattening of the fusible elements. The feature may comprise a standoff. Alternatively, the feature may comprise an area of material removed from a housing of the connector. 
     These and other objects of the invention are achieved in one aspect of the invention by an improved electrical connector mountable to a substrate, having an array of fusible elements, and exhibiting an unbalance relative to the fusible elements. The improvement comprises a feature on the connector that prevents the unbalance from causing an undesired skewing of the connector when mounting the connector to the substrate. The feature may comprise a standoff. Alternatively, the feature may comprise an area of material removed from a housing of the connector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other uses and advantages of the invention will become apparent to those skilled in the art upon reference to the specification and the drawings, in which: 
     FIG. 1 is a top perspective view of a first alternative embodiment of the invention; 
     FIG. 2A is a bottom perspective view of the electrical connector in FIG. 1; 
     FIG. 2B is a side view of the electrical connector in FIG. 1; 
     FIG. 3 is a top perspective view of a second alternative embodiment of the invention; 
     FIG. 4A is a bottom perspective view of the electrical connector in FIG. 3; 
     FIG. 4B is a bottom perspective view of the electrical connector in FIG. 3 with an alternative housing embodiment; and 
     FIG. 5 is a perspective view of the electrical connector in FIG. 3 modified to ensure that the connector remains substantially parallel to the substrate during a reflow process, according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Each of the alternative embodiments described herein relates to surface mounted electrical connectors having strain relief features. Preferably, fusible elements, such as solder balls, secure the contacts to conductive elements on the substrate using ball grid array (BGA) technology. Because during reflow BGA connectors tend to precisely align relative to the conductive pads on the substrate (known as “self-centering”), the strain relief features discussed herein preferably do not interfere with this desirable characteristic. Each alternative embodiment will now be described in more detail. 
     FIGS. 1,  2 A and  2 B display electrical connector  100 . Receptacle backplane connector  100  uses many of the features described in U.S. Pat. No. 6,116,926, herein incorporated by reference. Because a detailed discussion of the interior features of connector  100  are unnecessary for an understanding of the invention, only a brief summary of the interior features follows. 
     Connector  100  is modular, formed by a series of sub-assemblies  101 . Rear insulative housing  103  and front insulative housing  105  can latch together and surround sub-assemblies  101  to form connector  100 . Front housing  105  includes lead-in openings  107  that accept conductive pins from a mating connector (as shown in FIGS. 3,  4 A and  4 B). Openings  107  form a differential pair arrangement, with two rows of openings  107   s  that receive signal pins flanked by a row of openings  107   g  that receive ground pins. 
     An external shield  109  can surround at least rear housing  103 . Shield  109  is preferably made from a suitable solderable material such as a copper alloy. Shield  109  preferably extends along at least three sides of connector  100 , the rear wall and the two side walls. For retention on connector  100 , shield  109  also includes bent tabs  111  that extend along portions of the bottom wall of connector  100  (i.e., the wall that faces substrate S). 
     Sub-assemblies  101  contain the ground contacts  113  and signal contacts  115  of connector  100 . Ground and signal contacts  113 ,  115  mate with corresponding ground and signal pins of the mating connector. Differently than shown in U.S. Pat. No. 6,116,926, the signal and ground contacts of connector  100  surface mount to a substrate S, typically a multi-layer circuit board (MLB). 
     The preferred method of surface mounting connector  100  utilizes BGA technology. International Publication number WO 98/15991 (International Patent Application number PCT/US97/18354), herein incorporated by reference, describes various connectors and methods of making connectors using BGA technology. 
     Fusible elements  117  secure to a distal region of contacts  113 ,  115 . Preferably, fusible elements  117  are solder balls fused to contacts  113 ,  115  during a reflow process. Contacts  113 ,  115  can have a tab at a distal end region to which solder balls  117  fuse. However, any other manner of securing flusible elements  117  to contacts  113 ,  115  could be used, such as placing the distal ends of contacts  113 ,  115  within a pocket (in FIG. 4B) of the connector housing. 
     As shown in FIG. 1, substrate S has an array of conductive pads C connected to suitable traces (not shown) to transmit signals or for grounding purposes, for example. Pads C correspond to the array of fusible elements  117  secured to contacts  113 ,  115  on connector  100 . 
     A reflow process, typically subsequent to the reflow process that fused solder balls  117  to contacts  113 ,  115 , fuses solder balls  117  to pads C. Typically, pads C have solder paste (not shown) thereon to accept and to temporarily secure solder balls  117  to substrate S. Typically a squeegee drawn across a stencil (not shown) placed on substrate S provides suitable amounts of solder paste at desired locations. The reflow process fuses solder ball  117  to pad C on substrate S, thus creating an electrical path between contacts  113 ,  115  and substrate S. 
     Due to the mechanical loading requirements and the durability of these types of connectors, backplane connectors such as right angle connector  100  may require strain relief features to protect the solder joints formed by solder balls  117 . Accordingly, substrate S includes additional conductive pads H at suitable locations, such as surrounding conductive pads C. Conductive pads H preferably match the locations of tabs  111  of external shield  109 . In this way, tabs  111  act as hold downs to secure housing  109  to the surface of substrate S. 
     The reflow process used to secure solder balls  117  to substrate S preferably also secures conductive shield  109  to substrate S. As with conductive pads C, conductive pads H receive solder paste during the squeegee operation. The reflow process fuses shield  109  to substrate S. 
     Securing conductive shield  109  to substrate S serves as the strain relief for connector  100 . Because shield  109  surface mounts to substrate S, the strain relief feature of connector  100  does not interfere with the self-centering characteristic of fusible elements  117  during reflow. 
     The invention may protect the solder balls from being flattened during reflow because of the weight of the connector. Such flattening may cause unwanted bridging between adjacent solder balls. This may be accomplished through a number of techniques. For example, the connector may include a standoff. The standoff limits the ability of the connector to approach the substrate when the solder balls liquefy. In other words, the standoff prevents the solder balls from being flattened by the weight of the connector and from possibly bridging with adjacent solder balls. The standoffs may be made from any suitable material. Alternatively, as shown in FIG. 2B, shield  109  acts as the standoff. Shield  109  allows only a portion of solder balls  117  to extend a distance d beyond shield  109 . Distance d is selected so as to limit the flattening of solder balls during the reflow process. As an example, the stand-off could allow up to 40 percent, preferably 30 percent, flattening of the solder balls. Distance d also is selected to limit the bridging of adjacent solder balls during the reflow process. In this way, shield  209  acts like a standoff by preventing connector  100  from skewing on the PCB during reflow. 
     FIGS. 3,  4 A and  4 B display electrical connector  200 . Backplane header connector  200  preferably mates with backplane receptacle connector  100 . Clearly, connectors  100 ,  200  are used in a backplane system, for example, to connect a daughtercard to a motherboard. 
     Connector  200  uses many of the features described in U.S. patent application Ser. No. 09/302,027, herein incorporated by reference. Because a detailed discussion of certain features of connector  200  are unnecessary for an understanding of the invention, only a brief summary of these features follows. 
     Connector  200  includes an insulative housing  201  with apertures therethrough that accept signal pins  203 , ground pins  205  and ground shields  207 . Signal and ground pins  203 ,  205  extend from housing  201  and correspond to the arrangement of lead-in apertures  107  in connector  100  for mating with signal and ground contacts  113 ,  115  (as shown in FIGS. 1,  2 A and  2 B). Ground shields  207  remain within housing  201 , engage ground pins  205  and act to surround signal pins  203 . 
     An external shield  209  can surround housing  201 . Shield  209  preferably is made from a suitable solderable material such as a copper alloy. As shown in FIGS. 3,  4 A, and  4 B, shield  209  can extend along the side walls of connector  200 . For retention on connector  200 , shield  209  also includes bent tabs  211  that extend along portions of the bottom wall of connector  200  (i.e., the wall that faces substrate S). 
     As with connector  100  (as shown in FIGS. 1,  2 A and  2 B), connector  200  surface mounts to substrate S, preferably using the BGA technology discussed in International Publication number WO 98/15991. Fusible elements  213  secure to a distal region of contacts  203 ,  205 . Preferably, fusible elements  213  are solder balls fused to contacts  203 ,  205  during a reflow process. Contacts  203 ,  205  can have a tab at the distal region to which solder balls  213  fuse. However, any other manner of securing fusible elements  213  to contacts  203 ,  205  may be used, such as a pocket  215  in the bottom surface of connector housing  201  (as shown in FIG.  4 B). 
     As shown in FIG. 3, substrate S has an array of conductive pads C connected to suitable traces (not shown) to transmit signals or for grounding purposes, for example. Pads C correspond to the array of fusible elements  213  secured to contacts  203 ,  205  on connector  200 . 
     A reflow process, typically subsequent to the reflow process that fused solder balls  213  to contacts  203 ,  205 , fuses solder balls  213  to pads C. Typically, pads C have solder paste (not shown) thereon to accept and to temporarily secure solder balls  213  to substrate S. Typically a squeegee drawn across a stencil (not shown) placed on substrate S provides suitable amounts of solder paste at desired locations. The reflow process fuses solder ball  213  to pad C on substrate S, thus creating an electrical path between contacts  203 ,  205  and substrate S. 
     As with connector  100  (as shown in FIGS. 1,  2 A and  2 B), connector  200  may require strain relief features to protect the solder joints formed by solder balls  213 . Accordingly, substrate S includes additional conductive pads H at suitable locations, such as surrounding conductive pads C. Conductive pads H preferably match the locations of tabs  211  of external shield  209 . In this way, tabs  211  act as hold downs to secure housing  209  to the surface of substrate S. 
     The reflow process used to secure solder balls  213  to substrate S preferably also secures conductive shield  209  to substrate S. As with conductive pads C, conductive pads H receive solder paste during the squeegee operation. The reflow process fuses shield  209  to substrate S. 
     Securing conductive shield  209  to substrate S serves as the strain relief for connector  200 . Because shield  209  surface mounts to substrate S, the strain relief feature of connector  200  does not interfere with the self-centering characteristic of fusible elements  213  during reflow. 
     FIG. 4B shows connector  200  with an alternative housing embodiment. In particular, FIG. 4B shows connector  200  with a one-piece continuous housing structure  216 . In addition, housing  216  includes ball pockets  215  for receiving solder paste and solder balls  213 . The pockets also receive the mounting portion of contacts  203 ,  205 . 
     As previously discussed with reference to connector  100  (as shown in FIGS. 1,  2 A, and  2 B), the invention may protect the solder balls from being flattened during reflow because of the weight of the connector. Such flattening may cause unwanted bridging between adjacent solder balls. This may be accomplished through a number of techniques. For example, the connector may include a standoff. The standoff limits the ability of the connector to approach the substrate when the solder balls liquefy. In other words, the standoff prevents the solder balls from being flattened by the weight of the connector and from bridging with adjacent solder balls. The standoffs may be made from any other suitable material. As shown in FIG. 2B, shield  209  allows only a portion of the solder balls to extend a distance beyond shield  209 . The distance is selected so as to limit the flattening of solder balls during the reflow process. The distance also is selected to limit the bridging of adjacent solder balls during the reflow process. In this way, shield  209  acts like a standoff by preventing connector  200  from skewing on the PCB during reflow. 
     FIG. 5 is another example of how the invention ensures that the BGA connector remains substantially parallel to the substrate during reflow. As discussed with reference to connectors  100  and  200 , the BGA connector is attached to the substrate by heating the solder balls until the solder melts and becomes fused to the conductive pads on the substrate. The surface tension of the solder centers the connector on the traces of the substrate. However, in applications where the connector must be manufactured in an unbalanced state (i.e., the connector has more mass on one side of the solder then the other side), the connector may become skewed with respect to the substrate during the reflow process. As a result, certain of the solder joints may fail under a less than nominal mechanical force. Also, the skew may cause undesired bridging of adjacent solder balls. The invention ensures that the BGA connector remains substantially parallel to the substrate during reflow. 
     As shown in FIG. 5, portions of housing  201  on connector  200  (shown dashed for purposes of clarity) may be added and/or removed to allow the mass of connector  200  to be balanced evenly over the ball grid array. In particular, portions  502  and  505  may be removed from heavier sections of housing  201 . Portions  503  and  504  may be added to lighter sections of housing  201 . Although FIG. 5 shows portions  502 - 505  in certain locations, it should be appreciated that the location, as well as the size and weight of portions  502 - 505  will vary depending upon the physical characteristics of connector  200 . 
     Although FIG. 5 illustrates balancing connector  200  on the substrate by modifying the physical characteristics of the connector, it should be appreciated that the invention is not so limited. The invention may accomplish such balancing using a number of techniques. For example, an external force may be applied to certain areas of connector  200  during the reflow process. The magnitude of such a force would be determined so as to overcome the skewed relation of connector  200  and substrate S, caused by the imbalance of the housing over the ball grid array. In another embodiment, a similar force may be applied to substrate S, in addition to or instead of the connector. Therefore, the invention includes any technique that overcomes the inherent imbalance of the connector over its ball grid array, and allows the connector to be substantially parallel with the attached substrate after reflow. Although the balancing aspect of the invention was discussed with reference to connector  200 , it should be appreciated that such balancing may be applied to any connector, including receptacle connector  100  (as shown in FIGS. 1,  2 A and  2 B), for example. 
     While the 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 invention without deviating therefrom. Therefore, the 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.