Patent Publication Number: US-2009239395-A1

Title: Electrical connector lead frame

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 12/062,581, filed Apr. 4, 2008 which claims priority to U.S. Provisional Application 60/921,741, filed Apr. 4, 2007 and both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     This invention relates generally to electrical interconnection systems and more specifically to improved signal integrity in interconnection systems, particularly in high speed electrical connectors. 
     2. Discussion of Related Art 
     Electrical connectors are used in many electronic systems. It is generally easier and more cost effective to manufacture a system on several printed circuit boards (“PCBs”) that are connected to one another by electrical connectors than to manufacture a system as a single assembly. A traditional arrangement for interconnecting several PCBs is to have one PCB serve as a backplane. Other PCBs, which are called daughter boards or daughter cards, are then connected through the backplane by electrical connectors. 
     Electronic systems have generally become smaller, faster and functionally more complex. These changes mean that the number of circuits in a given area of an electronic system, along with the frequencies at which the circuits operate, have increased significantly in recent years. Current systems pass more data between printed circuit boards and require electrical connectors that are electrically capable of handling more data at higher speeds than connectors of even a few years ago. 
     One of the difficulties in making a high density, high speed connector is that electrical conductors in the connector can be so close that there can be electrical interference between adjacent signal conductors. To reduce interference, and to otherwise provide desirable electrical properties, shield members are often placed between or around adjacent signal conductors. The shields prevent signals carried on one conductor from creating “crosstalk” on another conductor. The shield also impacts the impedance of each conductor, which can further contribute to desirable electrical properties. 
     Other techniques may be used to control the performance of a connector. Transmitting signals differentially can also reduce crosstalk. Differential signals are carried on a pair of conducting paths, called a “differential pair.” The voltage difference between the conductive paths represents the signal. In general, a differential pair is designed with preferential coupling between the conducting paths of the pair. For example, the two conducting paths of a differential pair may be arranged to run closer to each other than to adjacent signal paths in the connector. No shielding is desired between the conducting paths of the pair, but shielding may be used between differential pairs. Electrical connectors can be designed for differential signals as well as for single-ended signals. 
     Examples of differential electrical connectors are shown in U.S. Pat. No. 6,293,827, U.S. Pat. No. 6,503,103, U.S. Pat. No. 6,776,659, and U.S. Pat. No. 7,163,421, all of which are assigned to the assignee of the present application and are hereby incorporated by reference in their entireties. 
     SUMMARY 
     In one aspect the, invention relates to a lead frame for an electrical connector. The lead frame includes a plurality of conductors disposed along a column that defines a column direction. At least one of the conductors has a dual beam structure defining first and second contact points to make contact with a complementary connector. The first and second contact points are offset in a direction perpendicular to the column direction. 
     In another aspect, the invention relates to an electrical connector with a support member and a plurality of wafer assemblies supported by the support member. Each of the wafer assemblies including a plurality of conductors disposed along a column that defines a column direction. At least one of the conductors has a multi-beam structure defining at least first and second contact points to make contact with a complementary conductor in a complementary connector. The first and second contact points are offset in a direction perpendicular to the column direction. 
     In yet another aspect, the invention relates to a wafer subassembly for an electrical connector. The wafer subassembly includes an insulative housing with a plurality of conductors held within the insulative housing. The plurality of conductors is disposed along a column that defines a column direction. At least one of the conductors has a multi-beam beam structure including at least first and second beams defining first and second contact points, respectively, to make contact with a complementary connector. The first and second contact points are offset in a direction perpendicular to the column direction and being offset in a direction parallel to the column direction. 
     The foregoing is a non-limiting summary of the invention, which is defined by the attached claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
         FIG. 1  is a perspective view of an electrical interconnection system according to an embodiment of the present invention; 
         FIGS. 2A and 2B  are views of a first and second side of a wafer forming a portion of the electrical connector of  FIG. 1 ; 
         FIG. 2C  is a cross-sectional representation of the wafer illustrated in  FIG. 2B  taken along the line  2 C- 2 C; 
         FIG. 3  is a cross-sectional representation of a plurality of wafers stacked together according to an embodiment of the present invention; 
         FIG. 4A  is a plan view of a lead frame used in the manufacture of a connector according to an embodiment of the invention; 
         FIG. 4B  is an enlarged detail view of the area encircled by arrow  4 B- 4 B in  FIG. 4A ; 
         FIG. 5A  is a cross-sectional representation of a backplane connector according to an embodiment of the present invention; 
         FIG. 5B  is a cross-sectional representation of the backplane connector illustrated in  FIG. 5A  taken along the line  5 B- 5 B; 
         FIGS. 6A-6C  are enlarged detail views of conductors used in the manufacture of a backplane connector according to an embodiment of the present invention; 
         FIG. 7A  is a sketch of the mating portions of lead frames in two mating connectors; 
         FIG. 7B  is a sketch of the mating contacts of a portion of a lead frame in a connector according to an alternative embodiment of the invention; 
         FIG. 7C  is a sketch of the mating contact portions of the lead frames of two mating connectors according to a further alternative embodiment of the invention; 
         FIG. 8A  is a sketch illustrating positioning of mating contact portions in a connector according to an embodiment of the invention; 
         FIG. 8B  is a cross-section through the mating contact portions of an electrical connector system with mating contact portions positioned as shown in  FIG. 8A ; 
         FIG. 9A  is a sketch illustrating positioning of mating contact portions of an electrical connector according to an embodiment of the invention; 
         FIGS. 9B ,  9 C and  9 D are cross-sections through the mating contact portions in alternative embodiments of an electrical connector having mating contact portions positioned as illustrated in  FIG. 9A ; 
         FIG. 10A  is a sketch illustrating a connector footprint according to an embodiment of the invention; 
         FIG. 10B  is a sketch of a connector footprint according to an embodiment of the invention; 
         FIG. 11A  is a schematic representation of a pair of conductive elements for an electrical connector according to an embodiment of the invention; 
         FIGS. 11B ,  11 C and  11 D are side views of the pair of conductive elements of  FIG. 11A  according to alternative embodiments of the invention; 
         FIG. 12A  is a sketch of antipads in a connector footprint according to the prior art; and 
         FIGS. 12B and 12C  are sketches of alternative embodiments of antipads in footprints for connectors according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     This invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” “having,” “containing,” or “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     Referring to  FIG. 1 , an electrical interconnection system  100  with two connectors is shown. The electrical interconnection system  100  includes a daughter card connector  120  and a backplane connector  150 . 
     Daughter card connector  120  is designed to mate with backplane connector  150 , creating electronically conducting paths between backplane  160  and daughter card  140 . Though not expressly shown, interconnection system  100  may interconnect multiple daughter cards having similar daughter card connectors that mate to similar backplane connections on backplane  160 . Accordingly, the number and type of subassemblies connected through an interconnection system is not a limitation on the invention. 
       FIG. 1  shows an interconnection system using a right-angle, backplane connector. It should be appreciated that in other embodiments, the electrical interconnection system  100  may include other types and combinations of connectors, as the invention may be broadly applied in many types of electrical connectors, such as right angle connectors, mezzanine connectors, card edge connectors and chip sockets. 
     Backplane connector  150  and daughter connector  120  each contains conductive elements. The conductive elements of daughter card connector  120  are coupled to traces, of which trace  142  is numbered, ground planes or other conductive elements within daughter card  140 . The traces carry electrical signals and the ground planes provide reference levels for components on daughter card  140 . Ground planes may have voltages that are at earth ground or positive or negative with respect to earth ground, as any voltage level may act as a reference level. 
     Similarly, conductive elements in backplane connector  150  are coupled to traces, of which trace  162  is numbered, ground planes or other conductive elements within backplane  160 . When daughter card connector  120  and backplane connector  150  mate, conductive elements in the two connectors mate to complete electrically conductive paths between the conductive elements within backplane  160  and daughter card  140 . 
     Backplane connector  150  includes a backplane shroud  158  and a plurality conductive elements (see  FIGS. 6A-6C ). The conductive elements of backplane connector  150  extend through floor  514  of the backplane shroud  158  with portions both above and below floor  514 . Here, the portions of the conductive elements that extend above floor  514  form mating contacts, shown collectively as mating contact portions  154 , which are adapted to mate to corresponding conductive elements of daughter card connector  120 . In the illustrated embodiment, mating contacts  154  are in the form of blades, although other suitable contact configurations may be employed, as the present invention is not limited in this regard. 
     Tail portions, shown collectively as contact tails  156 , of the conductive elements extend below the shroud floor  514  and are adapted to be attached to backplane  160 . Here, the tail portions are in the form of a press fit, “eye of the needle” compliant sections that fit within via holes, shown collectively as via holes  164 , on backplane  160 . However, other configurations are also suitable, such as surface mount elements, spring contacts, solderable pins, etc., as the present invention is not limited in this regard. 
     In the embodiment illustrated, backplane shroud  158  is molded from a dielectric material such as plastic or nylon. Examples of suitable materials are liquid crystal polymer (LCP), polyphenyline sulfide (PPS), high temperature nylon or polypropylene (PPO). Other suitable materials may be employed, as the present invention is not limited in this regard. All of these are suitable for use as binder materials in manufacturing connectors according to the invention. One or more fillers may be included in some or all of the binder material used to form backplane shroud  158  to control the electrical or mechanical properties of backplane shroud  150 . For example, thermoplastic PPS filled to 30% by volume with glass fiber may be used to form shroud  158 . 
     In the embodiment illustrated, backplane connector  150  is manufactured by molding backplane shroud  158  with openings to receive conductive elements. The conductive elements may be shaped with barbs or other retention features that hold the conductive elements in place when inserted in the opening of backplane shroud  158 . 
     As shown in  FIG. 1  and  FIG. 5A , the backplane shroud  158  further includes side walls  512  that extend along the length of opposing sides of the backplane shroud  158 . The side walls  512  include grooves  172 , which run vertically along an inner  10  surface of the side walls  512 . Grooves  172  serve to guide front housing  130  of daughter card connector  120  via mating projections  132  into the appropriate position in shroud  158 . 
     Daughter card connector  120  includes a plurality of wafers  122   1  . . .  122   6  coupled together, with each of the plurality of wafers  122   1  . . .  122   6  having a housing  260  (see  FIGS. 2A-2C ) and a column of conductive elements. In the illustrated embodiment, each column has a plurality of signal conductors  420  (see  FIG. 4A ) and a plurality of ground conductors  430  (see  FIG. 4A ). The ground conductors may be employed within each wafer  122   1  . . .  122   6  to minimize crosstalk between signal conductors or to otherwise control the electrical properties of the connector. 
     Wafers  122   1  . . .  122   6  may be formed by molding housing  260  around conductive elements that form signal and ground conductors. As with shroud  158  of backplane connector  150 , housing  260  may be formed of any suitable material and may include portions that have conductive filler or are otherwise made lossy. 
     In the illustrated embodiment, daughter card connector  120  is a right angle connector and has conductive elements that traverse a right angle. As a result, opposing ends of the conductive elements extend from perpendicular edges of the wafers  122   1  . . .  122   6 . 
     Each conductive element of wafers  122   1  . . .  122   6  has at least one contact tail, shown collectively as contact tails  126  that can be connected to daughter card  140 . Each conductive element in daughter card connector  120  also has a mating contact portion, shown collectively as mating contacts  124 , which can be connected to a corresponding conductive element in backplane connector  150 . Each conductive element also has an intermediate portion between the mating contact portion and the contact tail, which may be enclosed by or embedded within a wafer housing  260  (see  FIG. 2 ). 
     The contact tails  126  electrically connect the conductive elements within daughter card and connector  120  to conductive elements, such as traces  142  in daughter card  140 . In the embodiment illustrated, contact tails  126  are press fit “eye of the needle” contacts that make an electrical connection through via holes in daughter card  140 . However, any suitable attachment mechanism may be used instead of or in addition to via holes and press fit contact tails. 
     In the illustrated embodiment, each of the mating contacts  124  has a dual beam structure configured to mate to a corresponding mating contact  154  of backplane connector  150 . The conductive elements acting as signal conductors may be grouped in pairs, separated by ground conductors in a configuration suitable for use as a differential electrical connector. However, embodiments are possible for single-ended use in which the conductive elements are evenly spaced without designated ground conductors separating signal conductors or with a ground conductor between each signal conductor. 
     In the embodiments illustrated, some conductive elements are designated as forming a differential pair of conductors and some conductive elements are designated as ground conductors. These designations refer to the intended use of the conductive elements in an interconnection system as they would be understood by one of skill in the art. For example, though other uses of the conductive elements may be possible, differential pairs may be identified based on preferential coupling between the conductive elements that make up the pair. Electrical characteristics of the pair, such as its impedance, that make it suitable for carrying a differential signal may provide an alternative or additional method of identifying a differential pair. As another example, in a connector with differential pairs, ground conductors may be identified by their positioning relative to the differential pairs. In other instances, ground conductors may be identified by their shape or electrical characteristics. For example, ground conductors may be relatively wide to provide low inductance, which is desirable for providing a stable reference potential, but provides an impedance that is undesirable for carrying a high speed signal. 
     For exemplary purposes only, daughter card connector  120  is illustrated with six wafers  122   1  . . .  122   6 , with each wafer having a plurality of pairs of signal conductors and adjacent ground conductors. As pictured, each of the wafers  122   1  . . .  122   6  includes one column of conductive elements. However, the present invention is not limited in this regard, as the number of wafers and the number of signal conductors and ground conductors in each wafer may be varied as desired. 
     As shown, each wafer  122   1  . . .  122   6  is inserted into front housing  130  such that mating contacts  124  are inserted into and held within openings in front housing  130 . The openings in front housing  130  are positioned so as to allow mating contacts  154  of the backplane connector  150  to enter the openings in front housing  130  and allow electrical connection with mating contacts  124  when daughter card connector  120  is mated to backplane connector  150 . 
     Daughter card connector  120  may include a support member instead of or in addition to front housing  130  to hold wafers  122   1  . . .  122   6 . In the pictured embodiment, stiffener  128  supports the plurality of wafers  122   1  . . .  122   6 . Stiffener  128  is, in the embodiment illustrated, a stamped metal member. Though, stiffener  128  may be formed from any suitable material. Stiffener  128  may be stamped with slots, holes, grooves or other features that can engage a wafer. 
     Each wafer  122   1  . . .  122   6  may include attachment features  242 ,  244  (see  FIGS. 2A-2B ) that engage stiffener  128  to locate each wafer  122  with respect to another and further to prevent rotation of the wafer  122 . Of course, the present invention is not limited in this regard, and no stiffener need be employed. Further, although the stiffener is shown attached to an upper and side portion of the plurality of wafers, the present invention is not limited in this respect, as other suitable locations may be employed. 
       FIGS. 2A-2B  illustrate opposing side views of an exemplary wafer  220 A. Wafer  220 A may be formed in whole or in part by injection molding of material to form housing  260  around a wafer strip assembly such as  410 A or  410 B ( FIG. 4 ). In the pictured embodiment, wafer  220 A is formed with a two shot molding operation, allowing housing  260  to be formed of two types of material having different material properties. Insulative portion  240  is formed in a first shot and lossy portion  250  is formed in a second shot. However, any suitable number and types of material may be used in housing  260 . In one embodiment, the housing  260  is formed around a column of conductive elements by injection molding plastic. 
     In some embodiments, housing  260  may be provided with openings, such as windows or slots  264   1  . . .  264   6 , and holes, of which hole  262  is numbered, adjacent the signal conductors  420 . These openings may serve multiple purposes, including to: (i) ensure during an injection molding process that the conductive elements are properly positioned, and (ii) facilitate insertion of materials that have different electrical properties, if so desired. 
     To obtain the desired performance characteristics, one embodiment of the present invention may employ regions of different dielectric constant selectively located adjacent signal conductors  310   1 B,  310   2 B . . .  310   4 B of a wafer. For example, in the embodiment illustrated in  FIGS. 2A-2C , the housing  260  includes slots  264   1  . . .  264   6  in housing  260  that position air adjacent signal conductors  310   1 B,  310   2 B . . .  310   4 B. 
     The ability to place air, or other material that has a dielectric constant lower than the dielectric constant of material used to form other portions of housing  260 , in close proximity to one half of a differential pair provides a mechanism to de-skew a differential pair of signal conductors. The time it takes an electrical signal to propagate from one end of the signal connector to the other end is known as the propagation delay. In some embodiments, it is desirable that each signal within a pair have the same propagation delay, which is commonly referred to as having zero skew within the pair. The propagation delay within a conductor is influenced by the dielectric constant of material near the conductor, where a lower dielectric constant means a lower propagation delay. The dielectric constant is also sometimes referred to as the relative permittivity. A vacuum has the lowest possible dielectric constant with a value of 1. Air has a similarly low dielectric constant, whereas dielectric materials, such as LCP, have higher dielectric constants. For example, LCP has a dielectric constant of between about 2.5 and about 4.5. 
     Each signal conductor of the signal pair may have a different physical length, particularly in a right-angle connector. According to one aspect of the invention, to equalize the propagation delay in the signal conductors of a differential pair even though they have physically different lengths, the relative proportion of materials of different dielectric constants around the conductors may be adjusted. In some embodiments, more air is positioned in close proximity to the physically longer signal conductor of the pair than for the shorter signal conductor of the pair, thus lowering the effective dielectric constant around the signal conductor and decreasing its propagation delay. 
     However, as the dielectric constant is lowered, the impedance of the signal conductor rises. To maintain balanced impedance within the pair, the size of the signal conductor in closer proximity to the air may be increased in thickness or width. This results in two signal conductors with different physical geometry, but a more equal propagation delay and more inform impedance profile along the pair. 
       FIG. 2C  shows a wafer  220  in cross section taken along the line  2 C- 2 C in  FIG. 2B . As shown, a plurality of differential pairs  340   1  . . .  340   4  are held in an array within insulative portion  240  of housing  260 . In the illustrated embodiment, the array, in cross-section, is a linear array, forming a column of conductive elements. 
     Slots  264   1  . . .  264   4  are intersected by the cross section and are therefore visible in  FIG. 2C . As can be seen, slots  264   1  . . .  264   4  create regions of air adjacent the longer conductor in each differential pair  340   1 ,  340   2  . . .  340   4 . Though, air is only one example of a material with a low dielectric constant that may be used for de-skewing a connector. Regions comparable to those occupied by slots  264   1  . . .  264   4  as shown in  FIG. 2C  could be formed with a plastic with a lower dielectric constant than the plastic used to form other portions of housing  260 . As another example, regions of lower dielectric constant could be formed using different types or amounts of fillers. For example, lower dielectric constant regions could be molded from plastic having less glass fiber reinforcement than in other regions. 
       FIG. 2C  also illustrates positioning and relative dimensions of signal and ground conductors that may be used in some embodiments. As shown in  FIG. 2C , intermediate portions of the signal conductors  310   1 A . . .  310   4 A and  310   1 B . . .  310   4 B are embedded within housing  260  to form a column. Intermediate portions of ground conductors  330   1  . . .  330   4  may also be held within housing  260  in the same column. 
     Ground conductors  330   1 ,  330   2  and  330   3  are positioned between two adjacent differential pairs  340   1 ,  340   2  . . .  340   4  within the column. Additional ground conductors may be included at either or both ends of the column. In wafer  220 A, as illustrated in  FIG. 2C , a ground conductor  330   4  is positioned at one end of the column. As shown in  FIG. 2C , in some embodiments, each ground conductor  330   1  . . .  330   4  is preferably wider than the signal conductors of differential pairs  340   1  . . .  340   4 . In the cross-section illustrated, the intermediate portion of each ground conductor has a width that is equal to or greater than three times the width of the intermediate portion of a signal conductor. In the pictured embodiment, the width of each ground conductor is sufficient to span at least the same distance along the column as a differential pair. 
     In the pictured embodiment, each ground conductor has a width approximately five times the width of a signal conductor such that in excess of 50% of the column width occupied by the conductive elements is occupied by the ground conductors. In the illustrated embodiment, approximately 70% of the column width occupied by conductive elements is occupied by the ground conductors  330   1  . . .  330   4 . Increasing the percentage of each column occupied by a ground conductor can decrease cross talk within the connector. 
     Other techniques can also be used to manufacture wafer  220 A to reduce crosstalk or otherwise have desirable electrical properties. In some embodiments, one or more portions of the housing  260  are formed from a material that selectively alters the electrical and/or electromagnetic properties of that portion of the housing, thereby suppressing noise and/or crosstalk, altering the impedance of the signal conductors or otherwise imparting desirable electrical properties to the signal conductors of the wafer. 
     In the embodiment illustrated in  FIGS. 2A-2C , housing  260  includes an insulative portion  240  and a lossy portion  250 . In one embodiment, the lossy portion  250  may include a thermoplastic material filled with conducting particles. The fillers make the portion “electrically lossy.” In one embodiment, the lossy regions of the housing are configured to reduce crosstalk between at least two adjacent differential pairs  340   1  . . .  340   4 . The insulative regions of the housing may be configured so that the lossy regions do not attenuate signals carried by the differential pairs  340   1  . . .  340   4  an undesirable amount. 
     Materials that conduct, but with some loss, over the frequency range of interest are referred to herein generally as “lossy” materials. Electrically lossy materials can be formed from lossy dielectric and/or lossy conductive materials. The frequency range of interest depends on the operating parameters of the system in which such a connector is used, but will generally be between about 1 GHz and 25 GHz, though higher frequencies or lower frequencies may be of interest in some applications. Some connector designs may have frequency ranges of interest that span only a portion of this range, such as 1 to 10 GHz or 3 to 15 GHz or 3 to 6 GHz. 
     Electrically lossy material can be formed from material traditionally regarded as dielectric materials, such as those that have an electric loss tangent greater than approximately 0.003 in the frequency range of interest. The “electric loss tangent” is the ratio of the imaginary part to the real part of the complex electrical permittivity of the material. 
     Electrically lossy materials can also be formed from materials that are generally thought of as conductors, but are either relatively poor conductors over the frequency range of interest, contain particles or regions that are sufficiently dispersed that they do not provide high conductivity or otherwise are prepared with properties that lead to a relatively weak bulk conductivity over the frequency range of interest. Electrically lossy materials typically have a conductivity of about 1 siemans/meter to about 6.1×10 7  siemans/meter, preferably about 1 siemans/meter to about 1×10 7  siemans/meter and most preferably about 1 siemans/meter to about 30,000 siemans/meter. 
     Electrically lossy materials may be partially conductive materials, such as those that have a surface resistivity between 1 Ω/square and 10 6  Ω/square. In some embodiments, the electrically lossy material has a surface resistivity between 1 Ω/square and 10 3  Ω/square. In some embodiments, the electrically lossy material has a surface resistivity between 10 Ω/square and 100 Ω/square. As a specific example, the material may have a surface resistivity of between about 20 Ω/square and 40 Ω/square. 
     In some embodiments, electrically lossy material is formed by adding to a binder a filler that contains conductive particles. Examples of conductive particles that may be used as a filler to form an electrically lossy material include carbon or graphite formed as fibers, flakes or other particles. Metal in the form of powder, flakes, fibers or other particles may also be used to provide suitable electrically lossy properties. Alternatively, combinations of fillers may be used. For example, metal plated carbon particles may be used. Silver and nickel are suitable metal plating for fibers. Coated particles may be used alone or in combination with other fillers, such as carbon flake. In some embodiments, the conductive particles disposed in the lossy portion  250  of the housing may be disposed generally evenly throughout, rendering a conductivity of the lossy portion generally constant. An other embodiments, a first region of the lossy portion  250  may be more conductive than a second region of the lossy portion  250  so that the conductivity, and therefore amount of loss within the lossy portion  250  may vary. 
     The binder or matrix may be any material that will set, cure or can otherwise be used to position the filler material. In some embodiments, the binder may be a thermoplastic material such as is traditionally used in the manufacture of electrical connectors to facilitate the molding of the electrically lossy material into the desired shapes and locations as part of the manufacture of the electrical connector. However, many alternative forms of binder materials may be used. Curable materials, such as epoxies, can serve as a binder. Alternatively, materials such as thermosetting resins or adhesives may be used. Also, while the above described binder materials may be used to create an electrically lossy material by forming a binder around conducting particle fillers, the invention is not so limited. For example, conducting particles may be impregnated into a formed matrix material or may be coated onto a formed matrix material, such as by applying a conductive coating to a plastic housing. As used herein, the term “binder” encompasses a material that encapsulates the filler, is impregnated with the filler or otherwise serves as a substrate to hold the filler. 
     Preferably, the fillers will be present in a sufficient volume percentage to allow conducting paths to be created from particle to particle. For example, when metal fiber is used, the fiber may be present in about 3% to 40% by volume. The amount of filler may impact the conducting properties of the material. 
     Filled materials may be purchased commercially, such as materials sold under the trade name Celestran® by Ticona. A lossy material, such as lossy conductive carbon filled adhesive preform, such as those sold by Techfilm of Billerica, Mass., US may also be used. This preform can include an epoxy binder filled with carbon particles. The binder surrounds carbon particles, which acts as a reinforcement for the preform. Such a preform may be inserted in a wafer  220 A to form all or part of the housing and may be positioned to adhere to ground conductors in the wafer. In some embodiments, the preform may adhere through the adhesive in the preform, which may be cured in a heat treating process. Various forms of reinforcing fiber, in woven or non-woven form, coated or non-coated may be used. Non-woven carbon fiber is one suitable material. Other suitable materials, such as custom blends as sold by RTP Company, can be employed, as the present invention is not limited in this respect. 
     In the embodiment illustrated in  FIG. 2C , the wafer housing  260  is molded with two types of material. In the pictured embodiment, lossy portion  250  is formed of a material having a conductive filler, whereas the insulative portion  240  is formed from an insulative material having little or no conductive fillers, though insulative portions may have fillers, such as glass fiber, that alter mechanical properties of the binder material or impacts other electrical properties, such as dielectric constant, of the binder. In one embodiment, the insulative portion  240  is formed of molded plastic and the lossy portion is formed of molded plastic with conductive fillers. In some embodiments, the lossy portion  250  is sufficiently lossy that it attenuates radiation between differential pairs to a sufficient amount that crosstalk is reduced to a level that a separate metal plate is not required. 
     To prevent signal conductors  310   1 A,  310   1 B . . .  310   4 A, and  310   4 B from being shorted together and/or from being shorted to ground by lossy portion  250 , insulative portion  240 , formed of a suitable dielectric material, may be used to insulate the signal conductors. The insulative materials may be, for example, a thermoplastic binder into which non-conducting fibers are introduced for added strength, dimensional stability and to reduce the amount of higher priced binder used. Glass fibers, as in a conventional electrical connector, may have a loading of about 30% by volume. It should be appreciated that in other embodiments, other materials may be used, as the invention is not so limited. 
     In the embodiment of  FIG. 2C , the lossy portion  250  includes a parallel region  336  and perpendicular regions  334   1  . . .  334   4 . In one embodiment, perpendicular regions  334   1  . . .  334   4  are disposed between adjacent conductive elements that form separate differential pairs  340   1  . . .  340   4 . 
     In some embodiments, the lossy regions  336  and  334   1  . . .  334   4  of the housing  260  and the ground conductors  330   1  . . .  330   4  cooperate to shield the differential pairs  340   1  . . .  340   4  to reduce crosstalk. The lossy regions  336  and  334   1  . . .  334   4  may be grounded by being electrically connected to one or more ground conductors. This configuration of lossy material in combination with ground conductors  330   1  . . .  330   4  reduces crosstalk between differential pairs within a column. 
     As shown in  FIG. 2C , portions of the ground conductors  330   1  . . .  330   4 , may be electrically connected to regions  336  and  334   1  . . .  334   4  by molding portion  250  around ground conductors  340   1  . . .  340   4 . In some embodiments, ground conductors may include openings through which the material forming the housing can flow during molding. For example, the cross section illustrated in  FIG. 2C  is taken through an opening  332  in ground conductor  330   1 . Though not visible in the cross section of  FIG. 2C , other openings in other ground conductors such as  330   2  . . .  330   4  may be included. 
     Material that flows through openings in the ground conductors allows perpendicular portions  334   1  . . .  334   4  to extend through ground conductors even though a mold cavity used to form a wafer  220 A has inlets on only one side of the ground conductors. Additionally, flowing material through openings in ground conductors as part of a molding operation may aid in securing the ground conductors in housing  260  and may enhance the electrical connection between the lossy portion  250  and the ground conductors. However, other suitable methods of forming perpendicular portions  334   1  . . .  334   4  may also be used, including molding wafer  320 A in a cavity that has inlets on two sides of ground conductors  330   1  . . .  330   4 . Likewise, other suitable methods for securing the ground contacts  330  may be employed, as the present invention is not limited in this respect. 
     Forming the lossy portion  250  of the housing from a moldable material can provide additional benefits. For example, the lossy material at one or more locations can be configured to set the performance of the connector at that location. For example, changing the thickness of a lossy portion to space signal conductors closer to or further away from the lossy portion  250  can alter the performance of the connector. As such, electromagnetic coupling between one differential pair and ground and another differential pair and ground can be altered, thereby configuring the amount of loss for radiation between adjacent differential pairs and the amount of loss to signals carried by those differential pairs. As a result, a connector according to embodiments of the invention may be capable of use at higher frequencies than conventional connectors, such as for example at frequencies between 10-15 GHz. 
     As shown in the embodiment of  FIG. 2C , wafer  220 A is designed to carry differential signals. Thus, each signal is carried by a pair of signal conductors  310   1 A and  310   1 B, . . .  310   4 A, and  310   4 B. Preferably, each signal conductor is closer to the other conductor in its pair than it is to a conductor in an adjacent pair. For example, a pair  340   1  carries one differential signal, and pair  340   2  carries another differential signal. As can be seen in the cross section of  FIG. 2C , signal conductor  310   1 B is closer to signal conductor  310   1 A than to signal conductor  310   2 A. Perpendicular lossy regions  334   1  . . .  334   4  may be positioned between pairs to provide shielding between the adjacent differential pairs in the same column. 
     Lossy material may also be positioned to reduce the crosstalk between adjacent pairs in different columns.  FIG. 3  illustrates a cross-sectional view similar to  FIG. 2C  but with a plurality of subassemblies or wafers  320 A,  320 B aligned side to side to form multiple parallel columns. 
     As illustrated in  FIG. 3 , the plurality of signal conductors  340  may be arranged in differential pairs in a plurality of columns formed by positioning wafers side by side. It is not necessary that each wafer be the same and different types of wafers may be used. 
     It may be desirable for all types of wafers used to construct a daughter card connector to have an outer envelope of approximately the same dimensions so that all wafers fit within the same enclosure or can be attached to the same support member, such as stiffener  128  ( FIG. 1 ). However, by providing different placement of the signal conductors, ground conductors and lossy portions in different wafers, the amount that the lossy material reduces crosstalk relative for the amount that it attenuates signals may be more readily configured. In one embodiment, two types of wafers are used, which are illustrated in  FIG. 3  as subassemblies or wafers  320 A and  320 B. 
     Each of the wafers  320 B may include structures similar to those in wafer  320 A as illustrated in  FIGS. 2A ,  2 B and  2 C. As shown in  FIG. 3 , wafers  320 B include multiple differential pairs, such as pairs  340   5 ,  340   6 ,  340   7  and  340   8 . The signal pairs may be held within an insulative portion, such as  240 B of a housing. Slots or other structures, not numbered) may be formed within the housing for skew equalization in the same way that slots  264   1  . . .  264   6  are formed in a wafer  220 A. 
     The housing for a wafer  320 B may also include lossy portions, such as lossy portions  250 B. As with lossy portions  250  described in connection with wafer  320 A in  FIG. 2C , lossy portions  250 B may be positioned to reduce crosstalk between adjacent differential pairs. The lossy portions  250 B may be shaped to provide a desirable level of crosstalk suppression without causing an undesired amount of signal attenuation. 
     In the embodiment illustrated, lossy portion  250 B may have a substantially parallel region  336 B that is parallel to the columns of differential pairs  340   5  . . .  340   8 . Each lossy portion  250 B may further include a plurality of perpendicular regions  334   1 B . . .  334   5 B, which extend from the parallel region  336 B. The perpendicular regions  334   1 B . . .  334   5 B may be spaced apart and disposed between adjacent differential pairs within a column. 
     Wafers  320 B also include ground conductors, such as ground conductors  330   5  . . .  330   9 . As with wafers  320 A, the ground conductors are positioned adjacent differential pairs  340   5  . . .  340   8 . Also, as in wafers  320 A, the ground conductors generally have a width greater than the width of the signal conductors. In the embodiment pictured in  FIG. 3 , ground conductors  330   5  . . .  330   8  have generally the same shape as ground conductors  330   1  . . .  330   4  in a wafer  320 A. However, in the embodiment illustrated, ground conductor  330   9  has a width that is less than the ground conductors  330   5  . . .  330   8  in wafer  320 B. 
     Ground conductor  330   9  is narrower to provide desired electrical properties without requiring the wafer  320 B to be undesirably wide. Ground conductor  330   9  has an edge facing differential pair  340   8 . Accordingly, differential pair  340   8  is positioned relative to a ground conductor similarly to adjacent differential pairs, such as differential pair  330   8  in wafer  320 B or pair  340   4  in a wafer  320 A. As a result, the electrical properties of differential pair  340   8  are similar to those of other differential pairs. By making ground conductor  330   9  narrower than ground conductors  330   8  or  330   4 , wafer  320 B may be made with a smaller size. 
     A similar small ground conductor could be included in wafer  320 A adjacent pair  340   1 . However, in the embodiment illustrated, pair  340   1  is the shortest of all differential pairs within daughter card connector  120 . Though including a narrow ground conductor in wafer  320 A could make the ground configuration of differential pair  340   1  more similar to the configuration of adjacent differential pairs in wafers  320 A and  320 B, the net effect of differences in ground configuration may be proportional to the length of the conductor over which those differences exist. Because differential pair  340   1  is relatively short, in the embodiment of  FIG. 3 , a second ground conductor adjacent to differential pair  340   1 , though it would change the electrical characteristics of that pair, may have relatively little net effect. However, in other embodiments, a further ground conductor may be included in wafers  320 A. 
       FIG. 3  illustrates a further feature possible when using multiple types of wafers to form a daughter card connector. Because the columns of contacts in wafers  320 A and  320 B have different configurations, when wafer  320 A is placed side by side with wafer  320 B, the differential pairs in wafer  320 A are more closely aligned with ground conductors in wafer  320 B than with adjacent pairs of signal conductors in wafer  320 B. Conversely, the differential pairs of wafer  320 B are more closely aligned with ground conductors than adjacent differential pairs in the wafer  320 A. 
     For example, differential pair  340   6  is proximate ground conductor  330   2  in wafer  320 A. Similarly, differential pair  340   3  in wafer  320 A is proximate ground conductor  330   7  in wafer  320 B. In this way, radiation from a differential pair in one column couples more strongly to a ground conductor in an adjacent column than to a signal conductor in that column. This configuration reduces crosstalk between differential pairs in adjacent columns. 
     Wafers with different configurations may be formed in any suitable way.  FIG. 4A  illustrates a step in the manufacture of wafers  320 A and  320 B according to one embodiment. In the illustrated embodiment, wafer strip assemblies, each containing conductive elements in a configuration desired for one column of a daughter card connector, are formed. A housing is then molded around the conductive elements in each wafer strip assembly in an insert molding operation to form a wafer. 
     To facilitate the manufacture of wafers, signal conductors, of which signal conductor  420  is numbered and ground conductors, of which ground conductor  430  is numbered, may be held together on a lead frame  400  as shown in  FIG. 4A . As shown, the signal conductors  420  and the ground conductors  430  are attached to one or more carrier strips  402 . In one embodiment, the signal conductors and ground conductors are stamped for many wafers on a single sheet. The sheet may be metal or may be any other material that is conductive and provides suitable mechanical properties for making a conductive element in an electrical connector. Phosphor-bronze, beryllium copper and other copper alloys are example of materials that may be used. 
       FIG. 4A  illustrates a portion of a sheet of metal in which wafer strip assemblies  410 A,  410 B have been stamped. Wafer strip assemblies  410 A,  410 B may be used to form wafers  320 A and  320 B, respectively. Conductive elements may be retained in a desired position on carrier strips  402 . The conductive elements may then be more readily handled during manufacture of wafers. Once material is molded around the conductive elements, the carrier strips may be severed to separate the conductive elements. The wafers may then be assembled into daughter board connectors of any suitable size. 
       FIG. 4A  also provides a more detailed view of features of the conductive elements of the daughter card wafers. The width of a ground conductor, such as ground conductor  430 , relative to a signal conductor, such as signal conductor  420 , is apparent. Also, openings in ground conductors, such as opening  332 , are visible. 
     The wafer strip assemblies shown in  FIG. 4A  provide just one example of a component that may be used in the manufacture of wafers. For example, in the embodiment illustrated in  FIG. 4A , the lead frame  400  includes tie bars  452 ,  454  and  456  that connect various portions of the signal conductors  420  and/or ground strips  430  to the lead frame  400 . These tie bars may be severed during subsequent manufacturing processes to provide electronically separate conductive elements. A sheet of metal may be stamped such that one or more additional carrier strips are formed at other locations and/or bridging members between conductive elements may be employed for positioning and support of the conductive elements during manufacture. Accordingly, the details shown in  FIG. 4A  are illustrative and not a limitation on the invention. 
     Although the lead frame  400  is shown as including both ground conductors  430  and the signal conductors  420 , the present invention is not limited in this respect. For example, the respective conductors may be formed in two separate lead frames. Indeed, no lead frame need be used and individual conductive elements may be employed during manufacture. It should be appreciated that molding over one or both lead frames or the individual conductive elements need not be performed at all, as the wafer may be assembled by inserting ground conductors and signal conductors into preformed housing portions, which may then be secured together with various features including snap fit features. 
       FIG. 4B  illustrates a detailed view of the mating contact end of a differential pair  424   1  positioned between two ground mating contacts  434   1  and  434   2 . As illustrated, the ground conductors may include mating contacts of different sizes. The embodiment pictured has a large mating contact  434   2  and a small mating contact  434   1 . To reduce the size of each wafer, small mating contacts  434   1  may be positioned on one or both ends of the wafer. 
       FIG. 4B  illustrates features of the mating contact portions of the conductive elements within the wafers forming daughter board connector  120 .  FIG. 4B  illustrates a portion of the mating contacts of a wafer configured as wafer  320 B. The portion shown illustrates a mating contact  434   1  such as may be used at the end of a ground conductor  330   9  ( FIG. 3 ). Mating contacts  424   1  may form the mating contact portions of signal conductors, such as those in differential pair  340   8  ( FIG. 3 ). Likewise, mating contact  434   2  may form the mating contact portion of a ground conductor, such as ground conductor  330   8  ( FIG. 3 ). 
     In the embodiment illustrated in  FIG. 4B , each of the mating contacts on a conductive element in a daughter card wafer is a dual beam contact. Mating contact  434   1  includes beams  460   1  and  460   2 . Mating contacts  424   1  includes four beams, two for each of the signal conductors of the differential pair terminated by mating contact  424   1 . In the illustration of  FIG. 4B , beams  460   3  and  460   4  provide two beams for a contact for one signal conductor of the pair and beams  460   5  and  460   6  provide two beams for a contact for a second signal conductor of the pair. Likewise, mating contact  434   2  includes two beams  460   7  and  460   8 . 
     Each of the beams includes a mating surface, of which mating surface  462  on beam  460   1  is numbered. To form a reliable electrical connection between a conductive element in the daughter card connector  120  and a corresponding conductive element in backplane connector  150 , each of the beams  460   1  . . .  460   8  may be shaped to press against a corresponding mating contact in the backplane connector  150  with sufficient mechanical force to create a reliable electrical connection. Having two beams per contact increases the likelihood that an electrical connection will be formed even if one beam is damaged, contaminated or otherwise precluded from making an effective connection. 
     Each of beams  460   1  . . .  460   8  has a shape that generates mechanical force for making an electrical connection to a corresponding contact. In the embodiment of  FIG. 4B , the signal conductors terminating at mating contact  424   1  may have relatively narrow intermediate portions  484   1  and  484   2  within the housing of wafer  320 D. However, to form an effective electrical connection, the mating contact portions  424   1  for the signal conductors may be wider than the intermediate portions  484   1  and  484   2 . Accordingly,  FIG. 4B  shows broadening portions  480   1  and  480   2  associated with each of the signal conductors. 
     In the illustrated embodiment, the ground conductors adjacent broadening portions  480   1  and  480   2  are shaped to conform to the adjacent edge of the signal conductors. Accordingly, mating contact  434   1  for a ground conductor has a complementary portion  482   1  with a shape that conforms to broadening portion  480   1 . Likewise, mating contact  434   2  has a complementary portion  482   2  that conforms to broadening portion  480   2 . By incorporating complementary portions in the ground conductors, the edge-to-edge spacing between the signal conductors and adjacent ground conductors remains relatively constant, even as the width of the signal conductors change at the mating contact region to provide desired mechanical properties to the beams. Maintaining a uniform spacing may further contribute to desirable electrical properties for an interconnection system according to an embodiment of the invention. 
     Some or all of the construction techniques employed within daughter card connector  120  for providing desirable characteristics may be employed in backplane connector  150 . In the illustrated embodiment, backplane connector  150 , like daughter card connector  120 , includes features for providing desirable signal transmission properties. Signal conductors in backplane connector  150  are arranged in columns, each containing differential pairs interspersed with ground conductors. The ground conductors are wide relative to the signal conductors. Also, adjacent columns have different configurations. Some of the columns may have narrow ground conductors at the end to save space while providing a desired ground configuration around signal conductors at the ends of the columns. Additionally, ground conductors in one column may be positioned adjacent to differential pairs in an adjacent column as a way to reduce crosstalk from one column to the next. Further, lossy material may be selectively placed within the shroud of backplane connector  150  to reduce crosstalk, without providing an undesirable level attenuation for signals. Further, adjacent signals and grounds may have conforming portions so that in locations where the profile of either a signal conductor or a ground conductor changes, the signal-to-ground spacing may be maintained. 
       FIGS. 5A-5B  illustrate an embodiment of a backplane connector  150  in greater detail. In the illustrated embodiment, backplane connector  150  includes a shroud  510  with walls  512  and floor  514 . Conductive elements are inserted into shroud  510 . In the embodiment shown, each conductive element has a portion extending above floor  514 . These portions form the mating contact portions of the conductive elements, collectively numbered  154 . Each conductive element has a portion extending below floor  514 . These portions form the contact tails and are collectively numbered  156 . 
     The conductive elements of backplane connector  150  are positioned to align with the conductive elements in daughter card connector  120 . Accordingly,  FIG. 5A  shows conductive elements in backplane connector  150  arranged in multiple parallel columns. In the embodiment illustrated, each of the parallel columns includes multiple differential pairs of signal conductors, of which differential pairs  540   1 ,  540   2  . . .  540   4  are numbered. Each column also includes multiple ground conductors. In the embodiment illustrated in  FIG. 5A , ground conductors  530   1 ,  530   2  . . .  530   5  are numbered. 
     Ground conductors  530   1  . . .  530   5  and differential pairs  540   1  . . .  540   4  are positioned to form one column of conductive elements within backplane connector  150 . That column has conductive elements positioned to align with a column of conductive elements as in a wafer  320 B ( FIG. 3 ). An adjacent column of conductive elements within backplane connector  150  may have conductive elements positioned to align with mating contact portions of a wafer  320 A. The columns in backplane connector  150  may alternate configurations from column to column to match the alternating pattern of wafers  320 A,  320 B shown in  FIG. 3 . 
     Ground conductors  530   2 ,  530   3  and  530   4  are shown to be wide relative to the signal conductors that make up the differential pairs by  540   1  . . .  540   4 . Narrower ground conductive elements, which are narrower relative to ground conductors  530   2 ,  530   3  and  530   4 , are included at each end of the column. In the embodiment illustrated in  FIG. 5A , narrower ground conductors  530   1  and  530   5  are including at the ends of the column containing differential pairs  540   1  . . .  540   4  and may, for example, mate with a ground conductor from daughter card  120  with a mating contact portion shaped as mating contact  434   1  ( FIG. 4B ). 
       FIG. 5B  shows a view of backplane connector  150  taken along the line labeled B-B in  FIG. 5A . In the illustration of  FIG. 5B , an alternating pattern of columns of  560 A- 560 B is visible. A column containing differential pairs  540   1  . . .  540   4  is shown as column  560 B. 
       FIG. 5B  shows that shroud  510  may contain both insulative and lossy regions. In the illustrated embodiment, each of the conductive elements of a differential pair, such as differential pairs  540   1  . . .  540   4 , is held within an insulative region  522 . Lossy regions  520  may be positioned between adjacent differential pairs within the same column and between adjacent differential pairs in adjacent columns. Lossy regions  520  may connect to the ground contacts such as  530   1  . . .  530   5 . Sidewalls  512  may be made of either insulative or lossy material. 
       FIGS. 6A ,  6 B and  6 C illustrate in greater detail conductive elements that may be used in forming backplane connector  150 .  FIG. 6A  shows multiple wide ground contacts  530   2 ,  530   3  and  530   4 . In the configuration shown in  FIG. 6A , the ground contacts are attached to a carrier strip  620 . The ground contacts may be stamped from a long sheet of metal or other conductive material, including a carrier strip  620 . The individual contacts may be severed from carrier strip  620  at any suitable time during the manufacturing operation. 
     As can be seen, each of the ground contacts has a mating contact portion shaped as a blade. For additional stiffness, one or more stiffening structures may be formed in each contact. In the embodiment of  FIG. 6A , a rib, such as  610  is formed in each of the wide ground conductors. 
     Each of the wide ground conductors, such as  530   2  . . .  530   4  includes two contact tails. For ground conductor  530   2  contact tails  656   1  and  656   2  are numbered. Providing two contact tails per wide ground conductor provides for a more even distribution of grounding structures throughout the entire interconnection system, including within backplane  160  because each of contact tails  656   1  and  656   2  will engage a ground via within backplane  160  that will be parallel and adjacent a via carrying a signal.  FIG. 4A  illustrates that two ground contact tails may also be used for each ground conductor in daughter card connector. 
       FIG. 6B  shows a stamping containing narrower ground conductors, such as ground conductors  530   1  and  530   5 . As with the wider ground conductors shown in  FIG. 6A , the narrower ground conductors of  FIG. 6B  have a mating contact portion shaped like a blade. 
     As with the stamping of  FIG. 6A , the stamping of  FIG. 6B  containing narrower grounds includes a carrier strip  630  to facilitate handling of the conductive elements. The individual ground conductors may be severed from carrier strip  630  at any suitable time, either before or after insertion into backplane connector shroud  510 . 
     In the embodiment illustrated, each of the narrower ground conductors, such as  530   1  and  530   2 , contains a single contact tail such as  656   3  on ground conductor  530   1  or contact tail  656   4  on ground conductor  530   5 . Even though only one ground contact tail is included, the relationship between number of signal contacts is maintained because narrow ground conductors as shown in  FIG. 6B  are used at the ends of columns where they are adjacent a single signal conductor. As can be seen from the illustration in  FIG. 6B , each of the contact tails for a narrower ground conductor is offset from the center line of the mating contact in the same way that contact tails  656   1  and  656   2  are displaced from the center line of wide contacts. This configuration may be used to preserve the spacing between a ground contact tail and an adjacent signal contact tail. 
     As can be seen in  FIG. 5A , in the pictured embodiment of backplane connector  150 , the narrower ground conductors, such as  530   1  and  530   5 , are also shorter than the wider ground conductors such as  530   2  . . .  530   4 . The narrower ground conductors shown in  FIG. 6B  do not include a stiffening structure, such as ribs  610  ( FIG. 6A ). However, embodiments of narrower ground conductors may be formed with stiffening structures. 
       FIG. 6C  shows signal conductors that may be used to form backplane connector  150 . The signal conductors in  FIG. 6C , like the ground conductors of  FIGS. 6A and 6B , may be stamped from a sheet of metal. In the embodiment of  FIG. 6C , the signal conductors are stamped in pairs, such as pairs  540   1  and  540   2 . The stamping of  FIG. 6C  includes a carrier strip  640  to facilitate handling of the conductive elements. The pairs, such as  540   1  and  540   2 , may be severed from carrier strip  640  at any suitable point during manufacture. 
     As can be seen from  FIGS. 5A ,  6 A,  6 B and  6 C, the signal conductors and ground conductors for backplane connector  150  may be shaped to conform to each other to maintain a consistent spacing between the signal conductors and ground conductors. For example, ground conductors have projections, such as projection  660 , that position the ground conductor relative to floor  514  of shroud  510 . The signal conductors have complimentary portions, such as complimentary portion  662  ( FIG. 6C ) so that when a signal conductor is inserted into shroud  510  next to a ground conductor, the spacing between the edges of the signal conductor and the ground conductor stays relatively uniform, even in the vicinity of projections  660 . 
     Likewise, signal conductors have projections, such as projections  664  ( FIG. 6C ). Projection  664  may act as a retention feature that holds the signal conductor within the floor  514  of backplane connector shroud  510  ( FIG. 5A ). Ground conductors may have complimentary portions, such as complementary portion  666  ( FIG. 6A ). When a signal conductor is placed adjacent a ground conductor, complimentary portion  666  maintains a relatively uniform spacing between the edges of the signal conductor and the ground conductor, even in the vicinity of projection  664 . 
       FIGS. 6A ,  6 B and  6 C illustrate examples of projections in the edges of signal and ground conductors and corresponding complimentary portions formed in an adjacent signal or ground conductor. Other types of projections may be formed and other shapes of complementary portions may likewise be formed. 
     To facilitate use of signal and ground conductors with complementary portions, backplane connector  150  may be manufactured by inserting signal conductors and ground conductors into shroud  510  from opposite sides. As can be seen in  FIG. 5A , projections such as  660  ( FIG. 6A ) of ground conductors press against the bottom surface of floor  514 . Backplane connector  150  may be assembled by inserting the ground conductors into shroud  510  from the bottom until projections  660  engage the underside of floor  514 . Because signal conductors in backplane connector  150  are generally complementary to the ground conductors, the signal conductors have narrow portions adjacent the lower surface of floor  514 . The wider portions of the signal conductors are adjacent the top surface of floor  514 . Because manufacture of a backplane connector may be simplified if the conductive elements are inserted into shroud  510  narrow end first, backplane connector  150  may be assembled by inserting signal conductors into shroud  510  from the upper surface of floor  514 . The signal conductors may be inserted until projections, such as projection  664 , engage the upper surface of the floor. Two-sided insertion of conductive elements into shroud  510  facilitates manufacture of connector portions with conforming signal and ground conductors. 
       FIG. 7A  is a sketch of a portion of a lead frame such as may be used in a daughter card connector according to an embodiment of the invention.  FIG. 7A  shows mating contacts  424   1 , which may be the mating contact portions of a pair of signal conductors in a daughter card wafer. As shown, mating contacts  424   1  are aligned to fall in a column C of mating contact portions in a daughter card connector. 
     Also aligned with mating contacts  424   1  in column C are mating contacts  434   1  and  434   2 , which may form the mating contact portions of ground conductors within the daughter card connector. The illustrated configuration positions a ground conductor in the column on both sides of mating contacts  424   1 . Mating contact  434   1  is, in the embodiment illustrated, narrower than mating contact  434   2 . 
     As described above, it is desirable in some embodiments to have ground conductors within a column to be wider than the signal conductors. However, expanding the width of the ground conductors can increase the size of the electrical connector in a dimension along the column. In some embodiments, it may be desirable to limit the dimension of the electrical connector in a dimension along the columns of signal conductors. One approach to limiting the width of the connector is, as shown in  FIG. 7A , to make mating contacts at an end of a column, such as mating contact  434   1 , narrower than other mating contacts in the column, such as mating contact  434   2 . The narrower mating contact  434   1  may otherwise be formed with the same shape as mating contact  434   2 . 
     An alternative approach for reducing the size of the connector in a dimension along the columns of mating contacts is to offset the points of contacts for the dual beam mating contact portions. In the embodiment of  FIG. 7A , the contact points are not offset. As shown, mating contact  434   2  has two beams  460   7  and  460   8 . Each of these beams has a mating surface  722   1  and  722   2 , respectively. When an electrical connector containing mating surfaces  722   1  and  722   2  is mated with a complementary connector, mating contact  434   2  will make contact with a mating contact in the complementary connector at mating surfaces  722   1  and  722   2 . In the embodiment illustrated, the mating contact in the complementary connector is shown as ground conductor  530   2 . In this embodiment, ground conductor  530   2  is shown as a blade, such as may be used in a backplane connector as described above in connection with  FIG. 5 . However, the shape of the mating contact is not a limitation on the invention. 
     As shown, mating surfaces  722   1  and  722   2  contact ground conductor  530   2  at contact points  710   1  and  710   2 , respectively. For the contact configuration shown in  FIG. 7A , contact points  710   1  and  710   2  are aligned in the direction of column C. To ensure that mating contact  434   2  makes reliable contact with ground conductor  530   2 , ground conductor  530   2  may be constructed to have a width W 1  along the column. W 1  is larger than the width of mating contact  434   2  at the mating interface. This additional width ensures that, even with misalignment between a connector holding mating contact  434   2  and a connector holding ground conductor  530   2 , both mating surfaces  722   1  and  722   2  will contact ground conductor  530   2 . 
     In some embodiments, a mating contact having a width less than W 1  may be desired.  FIGS. 7B and 7C  illustrate alternative embodiments of a ground contact  434   2  that may be used with a mating ground conductor shaped as a blade like ground conductor  530   2  but having a width less than W 1 .  FIG. 7B  shows a mating contact  750  that may be used in place of mating contact  434   2 . In such an embodiment, mating contact  750  may form the mating contact portion of a wide ground conductor positioned between adjacent pairs of signal conductors in a daughter card wafer. However, the contact configuration illustrated in  FIG. 7B  may be used in connection with any suitable conductive element. 
     As with mating contact  434   2 , mating contact  750  contains two beams  752   1  and  752   2 , each providing a mating surface,  732   1  and  732   2 , respectively. However, beams  752   1  and  752   2  are configured such that mating surface  732   2  is offset relative to mating surface  732   1  in a direction perpendicular to column C. When mating contact  750  engages ground conductor  730 , mating surfaces  732   1  and  732   2  engage ground conductor  730  at contact points  734   1  and  734   2 . Contact point  734   2  is offset in the direction O from contact point  734   1 . As illustrated, the direction O is perpendicular to column C. Because of this offset in contact point  734   1  and  734   2 , ground contact  730  may have a width W 1B  that is less than width W 1  of ground conductor  530   2 . 
     In the embodiment of  FIG. 7B , mating surface  732   2  is offset from mating surface  732   1  by forming beam  752   2  within beam  752   1 . When a lead frame having a mating contact with a beam is incorporated into an electrical connector, the leading edge of the beam may be held within the connector housing in a way that the distal end of the beam is blocked from coming into contact with a conductive element in a mating conductor. Such a construction may avoid “stubbing” of the conductive element in the mating conductor on the beam, which can both prevent proper mating and damage the connector. With a mating contact as illustrated in  FIG. 7B , the distal end of beam  752   1  may be mounted in a housing to prevent stubbing. The distal end of beam  752   2  may not be guarded by the housing. However, the configuration as shown positions the distal end of beam  752   2  behind distal portion  736  of beam  752   1 , which prevents “stubbing” of ground conductor  730  on beam  752   2 . 
     The embodiment of  FIG. 7B  is just one example of a configuration that may be used to form offset contact points.  FIG. 7C  shows an alternative embodiment. Mating contact  760  contains beams  762   1  and  762   2 . The two beams provide two mating surface,  742   1  and  742   2 . Beam  762   2  is shorter than beam  762   1 , causing mating surface  742   2  to be offset from contact point  742   1 . Accordingly, when mating contact  760  engages a mating contact in another connector, such as ground conductor  740 , mating surfaces  742   1  and  742   2  engage ground conductor  740  at offset contact points  744   1  and  744   2 . As shown, contact point  744   2  is offset from contact point  744   2  in direction O. As a result, ground conductor  740  may have a width W 1C  that is narrower than width W 1  of ground conductor  530   2  ( FIG. 7A ). Furthermore, because beam  762   2  is not fully contained within beam  762   1  as in the configuration of  FIG. 7B , the distal end of beam  762   1  in the vicinity of mating surface  742   1  may be narrower than the distal end of beam  752   1  in the vicinity of mating surface  732   1  ( FIG. 7B ). Accordingly, width W 1C  of ground conductor  740 , in some embodiments, may be narrower than width W 1B  of ground conductor  730  ( FIG. 7B ). The embodiments of  FIG. 7C  may also be used in a manner that reduces stubbing. The distal end of beam  762   1  may be guarded in a housing. The distal end of beam  742   2  is guarded by portion  746 , thereby preventing stubbing of ground conductor  740  on beam  742   2 . 
     In the embodiment illustrated in  FIG. 7A , adjacent pairs of signal conductors along a column are separated by wide ground conductors that terminate in mating contacts, such as mating contact  434   2 . However, offset contact points as in the embodiments of  FIGS. 7B and 7C  may be used with other conductive elements. For example, some wafers, such as wafers  320 B ( FIG. 3 ) may have ground conductors at the end of a column that terminate in a narrower mating contact, such as mating contact  434   1 . These narrower grounds may have mating contacts with offset contact points. Likewise, the signal conductors in a pair may have mating contacts that also use multiple beams with offset contact points. Such an arrangement may allow narrower conductive elements for the signal conductors and/or narrow grounds in a mating connector. Accordingly, though  FIGS. 7B and 7C  illustrate offset points of contact only in connection with a wide ground conductor, similar approaches may be used in connection with mating contacts for conductive elements carrying signals or for narrow mating contacts for ground conductors. 
     Though electrical interconnection system  100  as described above provides a high speed, high density interconnection system with desirable electrical properties, other features may be incorporated to provide even lower crosstalk or otherwise provide performance characteristics that are desirable in some embodiments.  FIGS. 9A ,  9 B,  9 C and  9 D illustrate features that may be incorporated in some embodiments. For comparison,  FIGS. 8A and 8B  illustrate the mating interface portion of electrical interconnection system  100  ( FIG. 1 ). As described above, both a daughter card connector  120  and backplane connector  150  contain conductive elements positioned in columns. In the embodiment illustrated, each column contains pairs of signal conductors separated by ground conductors.  FIG. 8A  schematically illustrates this configuration. 
       FIG. 8A  shows only a portion of a mating interface of an electrical interconnection system. The portion shown contains two columns, C 1  and C 2 . Two pairs of signal conductors are shown in each of columns C 1  and C 2 . Signal conductors  812   1 A and  812   1 B form one pair in column C 1 . A second pair in column C 1  is formed by signal conductors  812   2 A and  812   2 B. The portion of column C 2  illustrated also contains two pairs of signal conductors, formed by signal conductors  812   3 A and  812   3 B, with a second pair formed by signal conductors  812   4 A and  812   4 B. Each column contains ground conductors adjacent the pairs of signal conductors. Accordingly, column C 1  contains ground conductors  810   1 ,  810   2  and  810   3 . Likewise, column C 2  contains ground conductors  810   4 ,  810   5  and  810   6 . 
     The portions shown in  FIG. 8A  contain two pairs of signal conductors with adjacent ground conductors. However, in many embodiments a connector will have more than two pairs per column. The alternating pattern of signal pairs and ground conductors may be extended to provide any number of pairs of signal conductors within each column. The repeating pattern may end with a signal conductor at the end of the column. Though in other embodiments, a column may end with a ground conductor of the same width as other ground conductors in the column or with a narrower ground conductor. 
       FIG. 8B  illustrates in cross section the configuration of conductive elements in mating connectors that form the pattern shown in  FIG. 8A . In the embodiment shown, the conductive elements forming the mating interface are contained within housing  820 . Housing  820  may be formed by a front housing  130  ( FIG. 1 ) or other suitable component. For simplicity,  FIG. 8B  shows an even smaller portion of the two columns than is illustrated in  FIG. 8A , showing only one pair of signal conductors in each column. For example, the portion shown may correspond to ground conductors  810   1 ,  810   2  and  810   4 , with signal conductors  812   1 A,  812   1 B,  812   3 A and  812   3 B. However, as observed above in connection with  FIG. 8A , the alternating pattern of signal pairs and ground conductors may be repeated to provide any suitable number of pairs of signal conductors in each column. 
     In electrical interconnection system  100  ( FIG. 1 ), blade-shaped mating contacts from backplane connector  150  enter front housing portion  130  of daughter card connector  120  where they mate with beam-shaped contacts at the ends of conductive elements within daughter card connector  120 . Thus, the mating interface between daughter card connector  120  and backplane connector  150  is formed by beams pressing against blades. 
       FIG. 8A  shows both blades and beams for both signal and ground conductors positioned so that the conductive elements at the mating interface are aligned in columns. The configuration illustrated in  FIG. 8A  may be achieved by positioning the blades of the signal conductors and ground conductors in parallel columns and positioning beam-shaped contacts to mate on the same sides of the blades. As illustrated in  FIG. 8B , ground blades  830   1 , signal blades  832   1 A and  832   1 B and ground blade  830   2  are positioned in parallel in column C 1  Similarly, ground blade  830   3  is positioned in parallel with signal blades  832   2 A and  832   2 B in column C 2 . As a result, when the ground blades in a first conductor mate with beams in a second conductor, the conductive elements mate in a plane containing mating surface of the ground blades. This configuration is illustrated in  FIG. 8B , which shows the beams  840   1 A and  840   1 B of a ground conductor engaging blade  830   1 . Beams  834   1 A and  836   1 A likewise engage a signal blade  832   1 A and beams  834   1 B and  836   1 B likewise engage a blade  832   1 B. Continuing along column C 1 , beams  840   2 A and  840   2 B of a ground conductor engage blade  830   2 . 
     The same pattern appears in column C 2 . Blades  830   3 ,  832   2 A and  832   2 B are positioned in parallel along column C 2 . Beams  840   3 A,  840   3 B,  834   2 A,  836   2 A,  834   2 B and  836   2 B engage these blades in a plane along the column. 
       FIG. 9A  illustrates an alternative configuration that may reduce crosstalk between pairs of signal conductors in adjacent columns, particularly crosstalk that may be generated at the mating interface.  FIG. 9A  illustrates portions of two columns of conductive elements within a connector according to an alternative embodiment of the invention.  FIG. 9A  shows portions of columns C 3  and C 4 . As with columns C 1  and C 2  illustrated in  FIG. 8A , portions of each of columns C 3  and C 4  containing two pairs of signal conductors are shown. In column C 3 , signal conductors  912   1 A and  912   1 B form one pair of signal conductors. Signal conductors  912   2 A and  912   2 B form a second pair. In column C 4 , signal conductors  912   3 A and  912   3 B form one pair and signal conductors  912   4 A and  912   4 B form a second pair. A ground conductor is positioned adjacent each pair of signal conductors. Accordingly, column C 3  contains ground conductors  910   1 ,  910   2  and  910   3 . Column C 4  contains ground conductors  910   4 ,  910   5  and  910   6 . 
     The configuration of  FIG. 9A  differs from the configuration of  FIG. 8A  in that the signal conductors within each pair are rotated relative to the column. In the embodiment illustrated, the rotation of each pair is achieved by separating the center lines of the signal conductors forming the pair by a distance S 1  in a direction perpendicular to the column. Accordingly,  FIG. 9A  shows that signal conductor  912   1 A is centered below the center line of column C 3 . Signal conductor  912   1 B is centered above the center line of column C 3 , with a resulting separation of S 1  between the centers of signal conductors  912   1 A and  912   1 B. With this separation, a line through the signal conductors  912   1 A and  912   1 B is skewed by an angle α relative to column C 3 . 
     In the embodiment illustrated, each of the pairs of signal conductors within column C 3  may be similarly rotated by the angle α. Accordingly, signal conductor  912   2 A is offset from the center line of column C 3  by the same amount as signal conductor  912   1 A. Signal conductor  912   2 B is offset from the center line of column C 3  by the same amount as signal conductor  912   1 B. Though it is not necessary that all pairs in a column be rotated the same amount. 
     The pairs of signal conductors in other columns may be rotated similarly by an angle α. However, pairs in each column may be rotated by different amounts. For example, in the embodiment of  FIG. 9A , the signal conductors that make up pairs in column C 4  are rotated by an angle β. The angles α and β may have any suitable magnitude and direction. In the embodiment illustrated, the angles α and β have magnitudes that are approximately equal but are of opposite direction. 
       FIG. 9B  illustrates a manner in which the offsets illustrated in  FIG. 9A  may be achieved.  FIG. 9B , like  FIG. 8B , illustrates a cross section through the mating interface of a connector. In the embodiment of  FIG. 9B , rotation is achieved by positioning signal conductor blades offset from the center line of each column. Accordingly,  FIG. 9B  shows ground blades  930   1 A and  930   2  positioned along a line defining column C 3 . In contrast, signal blade  932   1 A is offset from the center line of column C 3 . Signal blade  932   1 B is offset on the other side of the center line of column C 3 . In column C 4 , ground blade  930   3  is positioned along the center line of the column. Signal blade  932   2 A is offset in one direction relative to the center line and signal blade  932   2 B is offset in an opposite direction. In a connector system such as system  100  ( FIG. 1 ), such offsets may be formed by positioning blades in a backplane connector in columns with offsets as illustrated. However, any suitable mechanism may be used to form the offsets. 
     To achieve mating, the signal beams may be offset by a corresponding amount. Accordingly, signal beams  934   1 A and  936   1 A are shown with an offset that matches that of signal blade  932   1 A. Signal beams  934   1 B and  936   1 B are similarly shown with an offset that matches that of signal blade  932   1 B. Signal beams  934   2 A and  936   2 A have an offset matching that of signal blade  932   2 A and signal beams  934   2 B and  936   2 B have an offset matching that of  932   2 B. In a connector system such as system  100  ( FIG. 1 ), such offsets may be obtained by forming lead frames, such as lead frame  400  ( FIG. 4A ) used to construct a daughter card, in a forming die to create the desired relative position of conductive elements prior to incorporating the conductive elements in a housing. However, other mechanisms are possible, such as using two lead frames, each holding one signal conductor of a pair. Accordingly, any suitable mechanism may be used to create the offsets. 
       FIG. 9C  illustrates an alternative construction technique that may be used to provide rotation as illustrated in  FIG. 9A . In the embodiment of  FIG. 9C , each of the blades of a column is centered along the center line of the column. However, rotation is achieved by positioning the signal beams of a pair to contact the signal blades on opposite sides. For example, in column C 3 , ground blades  940   1  and  940   2  are centered along the center line of column C 3 . Likewise, signal blades  942   1 A and  942   1 B are centered along the center line of the column. However, offset is achieved by positioning signal beams  944   1 A and  946   1 A to mate with a surface of signal blade  942   1 A that is on one side of the center line of column C 3 . Signal beams  944   1 B and  946   1 B are positioned to mate on a surface of signal blade  942   1 B that is on the opposite side of the center line of column C 3 . 
     A similar configuration is used in column C 4  to offset of the conductive elements of a pair in a direction perpendicular to a column. As shown, ground blade  940   3  and signal blades  942   2 A and  942   2 B are centered along the center line of column C 4 . However, rotation of the signal pairs is achieved by positioning signal beams  944   2 A and  946   2 A to contact signal blade  942   2 A on one side of the center line of column C 4  while signal beams  944   2 B and  946   2 B are positioned to contact signal blade  942   2 B on an opposite side of the center line. 
     Yet a further embodiment is shown in  FIG. 9D , which illustrates that rotation may be introduced by both offsetting the position of the signal conductors relative to the center line of the column and alternating the sides of the signal blades where the signal beams contact. Accordingly,  FIG. 9D  shows that ground blades  950   1  and  950   2  are positioned along the center line of column C 3 . Signal blade  952   1 A is offset relative to the center line in one direction and signal blade  952   2 B is offset in an opposite direction. Likewise, in column C 4 , ground blade  9503  is positioned along the center line of the column but signal blade  952   2 B is offset in one direction relative to the center line and signal blade  952   2 A is offset in the opposite direction. To provide greater rotation, signal beams  954   1 A and  956   1 A contact signal blade  952   1 A on one side while signal beams  954   1 B and  956   1 B contact signal blade  952   2 B on an opposite side. Similarly, signal beams  954   2 B and  956   2 B contact signal blade  952   2 B on one side and signal beams  954   2 A and  956   2 A contact signal blade  952   2 A on the opposite side. 
     Rotating the conductors within a pair relative to a column is believed to reduce the pair-to-pair crosstalk between pairs in adjacent columns.  FIGS. 9A ,  9 B,  9 C and  9 D illustrate how rotation can be introduced into the mating contact portion of an electrical interconnection system. Rotation can similarly be introduced in the electrically conductive elements that form signal pairs in other portions of the interconnection system. For example,  FIG. 10A  shows a portion of a connector footprint in backplane  160  ( FIG. 1 ). For a connector such as is illustrated in  FIG. 1  in which conductive elements have press fit contact tails, a connector footprint may be formed with vias in backplane  160 . However, depending on the form of contact tail, a connector footprint may be formed with other conductive elements, such as pads, in a printed circuit board. 
       FIG. 10A  shows a portion of a connector footprint containing columns C 5  and C 6  of vias. A connector footprint may be formed with any suitable number of columns and two are shown for simplicity. The portion of the connector footprint illustrated by  FIG. 10A  provides vias for attaching two pairs of conductive elements for two differential signals in each column with ground conductors adjacent each pair. However, each column may have any suitable number of signal conductors and ground conductors. 
     Vias  1010   1 A and  1010   1 B are positioned to receive contact tails from a ground conductor, such as contact tails  656   1  and  656   2  from ground conductor  530   2  ( FIG. 6A ). Vias  1010   2 A and  1010   2 B are similarly positioned to receive contact tails from a second ground conductor. Vias  1010   3 A and  1010   3 B are positioned to receive contact tails from yet a third ground conductor. In column C 6 , vias  1010   4 A and  1010   4 B are positioned to receive contact tails from a fourth ground conductor. Vias  1010   5 A and  1010   5 B are positioned to receive contact tails from a fifth ground conductor and vias  1010   6 A and  1010   6 B are likewise positioned to receive contact tails from a sixth ground conductor. 
     Vias  1012   1 A and  1012   1 B are positioned to receive contact tails, such as contact tails  656   6  and  656   7  associated with a pair  540   2  of signal conductors ( FIG. 6C ). Vias  1012   2 A and  1012   2 B are similarly positioned to receive contact tails from a second pair of signal conductors. Within column C 6 , vias  1012   3 A and  1012   3 B are positioned to receive contact tails from a third pair of signal conductors and vias  1012   4 A and  1012   4 B are positioned to receive contact tails from a fourth pair of signal conductors. In the embodiment of  FIG. 10A , all of the vias in column C 5  are positioned along the center line of the column. Similarly, the vias in column C 6  are also positioned along the center line. 
       FIG. 10B  shows an alternative embodiment of a connector footprint formed in backplane  160 ′. The configuration of vias in  FIG. 10B  differs from that in  FIG. 10A  in that the vias associated with each pair of signal conductors are rotated. As with the connector footprint in  FIG. 10A , vias are disposed in columns, of which columns C 7  and C 8  are shown. Column C 7  contains vias for receiving contact tails from two pairs of signal conductors and adjacent ground conductors. Vias  1022   1 A and  1022   1 B receive contact tails from a first pair of signal conductors. Vias  1022   2 A and  1022   2 B are positioned to receive contact tails from a second pair of signal conductors. Vias  1020   1 A and  1020   1 B are positioned to receive contact tails from a ground conductor. Vias  1020   2 A and  1020   2 B are positioned to receive contact tails from a second ground conductor. Vias  1020   3 A and  1020   3 B are positioned to receive contact tails from a third ground conductor. In column C 7 , via  1022   1 B is offset from the center line of column C 7  in a first direction and via  1022   1 A is offset from the center line in the opposite direction. As a result, the vias  1022   1 A and  1022   1 B are separated by a distance S 2  creating rotation at an angle α. Vias  1022   2 A and  1022   2 B are similarly offset from the center line of column C 7 . 
     Within column C 8 , vias  1022   3 A and  1022   3 B are positioned to receive contact tails from a third pair of signal conductors. Vias  1022   4 A and  1022   4 B are positioned to receive contact tails from a fourth pair of signal conductors. Vias  1020   4 A and  1020   4 B are positioned to receive contact tails from a fourth ground conductor. Likewise, vias  1020   5 A and  1020   5 B are positioned to receive contact tails from a fifth ground conductor and vias  1020   6 A and  1020   6 B are positioned to receive contact tails from a sixth ground conductor. As with the vias for receiving contact tails from signal conductors in column C 7 , vias  1022   3 A and  1022   3 B are rotated at an angle β, as illustrated. Vias  1022   4 A and  1022   4 B are similarly rotated. 
     In the embodiment illustrated, all of the pairs of signal conductors within each column are offset by approximately the same amount. Though, pairs of signal conductors in adjacent columns are rotated in opposite directions. Accordingly, in  FIG. 10B , angle α and angle β are shown to be of approximately equal magnitude but in opposite directions. However, neither the amount nor direction of the rotation is a limitation of the invention. Different signal conductors within the same column may be rotated by the same or different amounts. Likewise, the rotation angles in all columns may be the same or different. 
     Turning to  FIGS. 11A ,  11 B and  11 C, embodiments of signal conductors that may be used with the footprint of  FIG. 10B  and the mating interfaces depicted in  FIGS. 9B ,  9 C and  9 D are illustrated.  FIG. 11A  shows a pair  1124  of signal conductors, containing signal conductors  1124 A and  1124 B. The pair has a contact tail portion  1130 , containing contact tails  1130 A and  1130 B, respectively. 
       FIG. 11A  shows the mating contact surfaces of the signal conductors  1124 A and  1124 B. The signal conductors are here shaped as blades, such as may be used in a backplane connector, like backplane connector  120  (See  FIG. 1 ). However, the specific configuration of the signal conductors is not a limitation on the invention and rotation may be introduced into pairs of conductive elements shaped as beams or in any other configuration. 
       FIG. 11B  shows a side view of pair  1124 , taken from the perspective of line B-B ( FIG. 11A ). In the embodiment of  FIG. 11B , both signal conductors  1124 A and  1124 B are generally planar and parallel along their length. The signal conductors are spaced by a distance S 2 . Though  FIG. 11B  does not show structural members of a connector holding signal conductors  1124 A and  1124 B in the position illustrated, a connector may be formed with a housing or other support structure fixing the signal conductors  1124 A and  1124 B as shown so that the center line of the column in which signal conductors  1124 A and  1124 B are mounted passes between the signal conductors. Accordingly, a connector containing signal conductors positioned as shown in  FIG. 11B  may be used with a footprint as shown in  FIG. 10B  in which signal conductors in a pair are offset by an amount S 2 . 
     In the embodiment of  FIG. 11B , because the signal conductors  1124 A and  1124 B are generally parallel and planar, mating contact portions  1132  are likewise separated by a distance S 2 . Accordingly, signal conductors configured as shown in  FIG. 11B  are also suitable for use in connector configurations as illustrated in  FIGS. 9B and 9D  in which the mating contact portions of the signal conductors are also offset from a counterline of a column. 
     An alternative embodiment is illustrated in  FIG. 11C . In the embodiment of  FIG. 11C , the contact tails  1130 A′ and  1130 B′ are offset by a distance S 2 . Like the embodiment illustrated in  FIG. 11B , the signal conductors as illustrated in  FIG. 11C  may be fixed in a housing and used with a connector footprint such as is illustrated in  FIG. 10B . However, the embodiment of  FIG. 11C  differs from the embodiment of  FIG. 11B  in that the mating contact portions  1132  are aligned. Accordingly, signal conductors in the configuration shown in  FIG. 11C  may be used in embodiments such as are illustrated in  FIGS. 8B and 9C  in which the mating contact portions of the signal conductors are aligned with the center line of a column. 
       FIG. 11D  shows yet a further embodiment for the construction of a pair of signal conductors. In the embodiment of  FIG. 11D , the contact tails  1130 A″ and  1130 B″ of the signal conductors are aligned. Accordingly, signal conductors configured as illustrated in  FIG. 11D  are useful for a connector footprint as illustrated in  FIG. 10A . However, the mating contact portions of signal conductors  1124 A and  1124 B are, in the embodiment illustrated in  FIG. 11D , offset by a distance S 2 . Accordingly, signal conductors formed as shown in  FIG. 11D  may be used in embodiments such as are illustrated in  FIGS. 9B and 9D . 
     The alternatives illustrated in  FIGS. 11A ,  11 B,  11 C and  11 D demonstrate that different portions of the conductive elements forming signal pairs within an electrical interconnection system may be offset by different amounts in different portions of the electrical interconnection system. Rotation of the signal pairs may be provided at the mating interface and/or at the connector footprint where contact tails of the signal conductors connect to conductive elements within a printed circuit board. Similarly, conductive elements forming a pair within a wafer or other portion of an electrical connector may likewise be rotated. 
     Turning to  FIGS. 12A and 12B , a further technique for altering the electrical properties of an interconnection system is illustrated. The inventors have appreciated that altering the configuration of apertures in a ground plane around pairs of signal conductors may improve both the insertion loss and the linearity of the insertion loss associated with a pair of signal conductors.  FIGS. 12B and 12C  illustrate improved printed circuit board configurations according to embodiments of the invention. In contrast,  FIG. 12A  illustrates a board configuration as is known in the art. 
       FIG. 12A  illustrates a portion of a printed circuit board  1260 . As is known in the art, a printed circuit board may be constructed with multiple layers of conductive elements separated by insulative members. Frequently, layers containing signal traces alternate with layers supplying reference potentials, sometimes called “ground planes.”  FIG. 12A  illustrates a ground plane  1210 . The layer as shown may appear at any level of the printed circuit board  1260 , including at a surface or embedded within the board. Further, a printed circuit board may contain multiple ground planes. Thus, the structure shown in  FIG. 12A  may be repeated at one or more layers of a printed circuit board. 
     Frequently, vias carrying a ground potential are electrically coupled to the ground plane  1210 . A connection may be formed by forming ground vias, such as  1240   1 ,  1240   2  or  1240   3  through the ground plane and plating the walls of the via with metal or other conductor. That electrical coupling is facilitated by ensuring that, regardless of any patterning of ground plane  120   1 , a region of ground plane  1210  remains as a “pad”  1242   1  around each via. A similar technique is also used around signal vias. Even though ground plane  1210  may be patterned, a portion of the conductive layer used to form ground plane  1210  is left as a pad, such as pad  1242   2 , in the vicinity of each signal via. 
     The pads around the signal vias are separated from the rest of ground plane  1210 . Openings may be patterned in ground plane  1210  where vias carrying signals pass through the layer. Without the openings, the signals in the vias could be shorted to the ground plane  1210 . Openings in the ground plane to avoid making electrical contact to ground plan  120  are sometimes call “antipads.” 
     In  FIG. 12A , vias  1230   1 A and  1230   1 B are shown positioned to carry a differential signal. Likewise, vias  1230   2 A and  1230   2 B are positioned to carry a second differential signal. Vias  1230   3 A and  1230   3 B are positioned to carry a third differential signal. Antipad  1220   1  is formed around vias  1230   1 A and  1230   1 B to prevent the vias from being shorted to ground plane  1210 . Likewise, antipad  1220   2  is formed around vias  1230   2 A and  1230   2 B and antipad  1220   3  s vias  1230   3 A and  1230   3 B. 
     As described in U.S. Pat. No. 6,607,402, which is hereby incorporated by reference, forming antipads around pairs of signal conductors may impart desirable electrical characteristics to the pair. 
     The inventors have appreciated that by extending the antipads closer to ground vias than is illustrated in  FIG. 12A , the electrical characteristics of signal pairs enclosed within the antipads may be improved.  FIG. 12B  shows an embodiment of a printed circuit board  1262  in which antipads  1222   1 ,  1222   2  and  1222   3  are extended towards adjacent ground vias. In the embodiment illustrated in  FIG. 12B , ground plane  1212 , forming one layer of printed circuit board  1262 , is coupled to ground vias  1240   1 A,  1240   1 B,  1240   2 A and  1240   2 B. In the embodiment illustrated, ground vias  1240   1 A and  1240   1 B are, like ground vias  1010   1 A and  1010   1 B ( FIG. 10A ) positioned to receive contact tails from a ground conductor. Likewise ground vias  1240   2 A and  1240   2 B are positioned to receive contact tails from a second similar ground conductor. 
     In the embodiment of  FIG. 12B , the vias associated with each pair of signal conductors are likewise enclosed within an antipad. As shown, pair of vias  1230   1 A and  1230   1 B is enclosed within antipad  1222   1 . A pair of vias  1230   2 A and  1230   2 B is enclosed within antipad  1222   2  and a pair of vias  1230   3 A and  1230   3 B is enclosed within antipad  1222   3 . In comparison to the antipads, such as  1220   1 ,  1220   2  and  1220   3  shown in  FIG. 12A , antipads  1222   1 ,  1222   2  and  1222   3  are elongated along the dimension of column C 10 . As a result of the elongation, each of the antipads has a portion extending towards an adjacent ground via. In contrast to the configuration shown in  FIG. 12A  in which the edges of the antipads are approximately equidistant between adjacent signal and ground vias, the antipads illustrated in  FIG. 12B  have an edge that is closer to the ground via than the signal via. In the embodiment illustrated, the edges of the antipads extend to or beyond a line tangent to the ground via and perpendicular to column C 10 . For example, antipad  1222   1  extends towards via  1240   1 A past tangent line  1270   1 . Likewise, antipad  1222   2  extends towards ground via  1240   1 B past tangent line  1270   2 . In the opposite direction, antipad  1222   2  extends towards ground via  1240   2 A past tangent line  1270   3 . Similarly, antipad  1222   3  extends towards ground via  1240   2 B past tangent line  1270   4 . 
     The specific dimensions of the vias and antipads is not a limitation. However, in some embodiments, the vias may be formed by drilling a hole with a drill having a diameter between 0.3 and 0.7 millimeters. In some embodiments, a 0.55 millimeter drill may be used. Pads around the vias may extend the diameter of the via to a diameter between 0.7 millimeters and 1 millimeter. In some embodiments, the pads may have a diameter of 0.828 millimeters. The signal vias may be spaced on center by approximately 1.6 millimeters. The ground vias may be spaced on center by approximately 1.56 millimeters. The spacing between adjacent signal vias and ground vias may be approximately 1.1 millimeter. With such dimension, antipads configured as in  FIG. 12A  may extend from the center of a signal via approximately 0.539 mm. In the embodiment illustrated in  FIG. 12B , the antipads may be elongated by approximately 0.35 millimeters in each direction along column C 10 . With the dimensions, the tangent of the antipad extends to a point that is approximately 80% of the distance between the signal via and adjacent ground via. These diameters may result in the configuration illustrated in  FIG. 12B . However, enlarged antipads may be used with vias of any suitable diameter and spacing. 
       FIG. 12B  illustrates extended oval antipads. However, the shape of the antipads is not a limitation on the invention. For example,  FIG. 12C  illustrates an alternative embodiment using rectangular antipads. In each case, even if the outline of the antipad extends into the pad around a ground via, the pad is not removed, creating in some embodiments an antipad perimeter that is not a perfect rectangle or a perfect oval.  FIG. 12C  shows antipads  1224   1 ,  1224   2  and  1224   3  formed in ground layer  1214  of printed circuit board  1264 . As shown, antipad  1224   1  encloses a pair of signal vias  1230   1 A and  1230   1 B. Antipad  1224   2  encloses a pair of signal vias  1230   2 A and  1230   2 B and antipad  1224   3  enclosed signal vias  1230   3 A and  1230   3 B. The pairs of signal vias are separated along column C 11  by ground vias. In the embodiment of  FIG. 12C , the ground vias are shown in pairs, including ground vias  1240   1 A and  1240   1 B and the pair of ground vias  1240   2 A and  1240   2 B. 
     Though the antipads illustrated in  FIG. 12C  have a different shape than those of  FIG. 12B , they similarly extend past the halfway point between a signal via and an adjacent ground via. In the embodiment shown, the antipads extend past the tangent of an adjacent ground via. For example, antipad  1224   1  extends towards ground via  1240   1 A past tangent  1270   1 . Similarly, antipad  1224   2  extends towards ground via  1240   1 B past tangent  1270   2 . At the opposing end, antipad  1224   2  extends towards ground via  1240   2 A past tangent  1270   3 . Similarly, antipad  1224   3  extends towards ground via  1240   2 B past tangent  1270   4 . 
     In the embodiment illustrated in  FIG. 12C , the antipads extend past tangent lines  1270   1 ,  1270   2 ,  1270   3  and  1270   4  to approximately the center of the adjacent ground vias. Using pairs of ground vias facilitates such an arrangement. If a single ground via were used adjacent pairs of signal conductors, antipads surrounding pairs on opposite sides could not both extend to the center line of the ground via without completely surrounding the ground via by antipads. If the ground via were completely surrounded by antipads, it would not be connected to ground plane  1214  and could therefore exhibit undesirable electrical properties. However, by using pairs of ground conductors, a region of ground plane  1214  remains between the pairs. For example, even though antipads  1224   1  and  1224   2  on opposing sides of the pair of ground vias  1240   1 A and  1240   1 B each extend approximately to the center line of their adjacent ground vias, region  1280   1  of ground plane  1214  remains. Region  1280   1  connects both ground vias  1240   1 A and  1240   1 B to ground plane  1214  and may therefore promote desirable electrical properties of the ground vias. Additionally, region  1280   1  may aid in isolating signals passing through vias  1230   1 A and  1230   1 B from signals passing through vias  1230   2 A and  1230   2 B. Likewise, region  1280   2  between ground vias  1240   2 A and  1240   2 B may aid in connecting vias  1240   2 A and  1240   2 B to ground plane  1214  and may also isolate the signal carried on vias  1230   2 A and  1230   2 B from a signal carried on vias  1230   3 A and  1230   3 B. 
     Accordingly, in some embodiments it may be desirable to employ elongated antipads in columns in which pairs of signal vias are separated by pairs of ground vias. However, the specific configuration of vias is not a limitation on the invention and elongated antipads may be used within a suitable pattern of signal and ground vias. 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. 
     For example, examples of techniques from modifying characteristics of an electrical connector were described. These techniques may be used alone or in any suitable combination. 
     As one specific example,  FIGS. 12B and 12C  illustrate elongated antipads used in a printed circuit board in which pairs of signal conductors are aligned with columns. Elongated antipads may also be used with rotated pairs as illustrated in  FIG. 10B . 
     Further, although many inventive aspects are shown and described with reference to a daughter board connector, it should be appreciated that the present invention is not limited in this regard, as the inventive concepts may be included in other types of electrical connectors, such as backplane connectors, cable connectors, stacking connectors, mezzanine connectors, or chip sockets. 
     As a further example, connectors with four differential signal pairs in a column were used to illustrate the inventive concepts. However, the connectors with any desired number of signal conductors may be used. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.