Patent Publication Number: US-7722401-B2

Title: Differential electrical connector with skew control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to U.S. Provisional Application 60/921,696, filed Apr. 4, 2007 and 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 OF INVENTION 
   An improved differential electrical connector is provided with selective positioning of regions of relatively higher and relatively lower dielectric constant material adjacent signal conductors of a differential pair. The material of relatively lower dielectric constant may be placed in regions between a longer signal conductor of a differential and an adjacent ground conductor. The lower dielectric constant material also may be selectively placed adjacent to curved segments of the differential pair. 
   Accordingly, in one aspect, the invention relates to an electrical connector with a housing and a plurality of conductors disposed at least in part within the housing. The plurality of conductors are disposed in a plane and include a first signal conductor and a second signal conductor, longer than the first signal conductor. A ground conductor is adjacent the second conductor. The housing comprises at least one first region of a first dielectric constant. That region is disposed along at least a portion of a length of the first signal conductor. At least one second region of the housing has a second dielectric constant, lower than the first dielectric constant. That region is disposed along at least a portion of a length of the second signal conductor between the second signal conductor and the ground conductor. 
   In another aspect, the invention relates to an electrical connector that has a plurality of signal conductors disposed at least in part within the housing. The signal conductors comprise a plurality of differential signal pairs with a first conductor and a second conductor. Each differential pair has at least one curved portion at which the second conductor has a larger radius of curvature than the first conductor. A housing for the connector comprises at least one first region of a first dielectric constant, the at least one first region being disposed along at least portions of lengths of the first conductors of the plurality of differential pairs. A plurality of second regions of the housing has a second dielectric constant. The plurality of second regions is disposed along at least portions of lengths of the second conductors of the plurality of differential pairs adjacent the curved portions of the second conductors. 
   In another aspect, the invention relates to an electrical connector comprising a plurality of subassemblies. Each subassembly comprises a plurality of conductors disposed in a plane. The plurality of conductors comprises a plurality of pairs, each pair comprising a first conductor and a second conductor. A plurality of the conductors are wide conductors, which are positioned adjacent a second conductor of a pair of the plurality of pairs. The plurality of wide conductors have a width greater than a width of the first conductors and the second conductors of the plurality of the pairs. A housing for the connector comprises insulative material of a first dielectric constant holding at least a portion of the first conductor of each of the plurality of pairs and a plurality of regions of a second dielectric constant. The second dielectric constant is lower than the first dielectric constant. Each of the plurality of regions is disposed along at least a portion of a length of a second conductor of the plurality of pairs between the second conductor and a wide conductor adjacent the second conductor. A support member holds the plurality of subassemblies side-by-side. 

   
     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 cross-sectional representation of a portion of a wafer according to an embodiment of the present invention; 
       FIG. 7B  is a sketch of a curved portion of conductive elements in the wafer of  FIG. 7A ; 
       FIG. 8  is a sketch of a wafer strip assembly according to an embodiment of the present invention; and 
       FIG. 9  is a cross-sectional representation of a wafer according to an alternative embodiment 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 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  FIG. 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  illustrates additional details of construction techniques that may used to improve electrical properties of a differential connector.  FIG. 7A  shows a cross-section of a wafer  720 . As with wafer  220 A shown in  FIG. 2C , wafer  720  includes a housing with an insulative portion  740  and a lossy portion  750 . 
   A column of conductive elements is held within the housing of wafer  720 .  FIG. 7  shows two pairs,  742   2  and  742   3 , of the signal conductors in the column. Three ground conductors,  730   1 ,  730   2  and  730   3  are also shown. Wafer  720  may have more or fewer conductive elements. Two signal pairs and three ground conductors are shown for simplicity of illustration, but the number of conductive elements in a column is not a limitation on the invention. 
   In the example of  FIG. 7A , wafer  720  is configured for use in a right angle connector, which causes each differential pair to have at least one curved portion to enable the pairs to carry signals between orthogonal edges of the connector. Such a configuration results in the signal conductors of the pairs having different lengths, at least in the curved portions. These differences in the lengths of the conductors of a differential pair can cause skew. More generally, skew can occur within any differential pair configured so that a conductor of the differential pair is longer than the other and the specific configuration of the connector is not a limitation of the invention. 
   In the embodiment illustrated, signal conductor  744   2 B is longer than signal conductor  744   2 A in pair  742   2 . Likewise, signal conductor  744   3 B is longer than signal conductor  744   3 A in pair  742   3 . To reduce skew, the propagation speed of signals through the longer signal conductor may be increased relative to the propagation speed in the shorter signal conductor of the pair. Selective placement of regions of material with different dielectric constant may provide the desired relative propagation speed. 
   In the embodiment illustrated, for each of the pairs  742   2  and  742   3 , a region of relatively low dielectric material may be incorporated into wafer  720  in the vicinity of each of the longer signal conductors. In the embodiment illustrated, regions  710   2  and  710   3  are incorporated into wafer  720 . In contrast, the housing of wafer  720  in the vicinity of the shorter signal conductor of each pair creates regions of relatively higher dielectric constant material. In the embodiment of  FIG. 7A , regions  712   2  and  712   3  of higher dielectric constant material are shown adjacent signal conductors  744   2 A and  744   3 A. 
   Regions of lower dielectric constant material and higher dielectric constant material may be formed in any suitable way. In embodiments in which the insulative portions of the housing for wafer  720  are molded from plastic filled with glass fiber loaded to approximately 30% by volume, regions  712   2  and  712   3  of higher dielectric constant material may be formed as part of forming the insulative portion of the housing for wafer  720 . Regions  710   2  and  710   3  of lower dielectric constant material may be formed by voids in the insulative material used to make the housing for wafer  720 . An example of a connector with lower dielectric constant regions formed by voids in an insulative housing is shown in  FIG. 2B . 
   However, regions of lower dielectric constant material may be formed in any suitable way. For example, the regions may be formed by adding or removing material from region  710   2  and  710   3  to produce regions of desired dielectric constant. For example, region  710   2  and  710   3  may be molded of material with less or different fillers than the material used to form region  712   2  and  712   3 . 
   Regardless of the specific method used to form regions of lower dielectric constant, in some embodiments, those regions are positioned generally between the longer signal conductor and an adjacent ground conductor. For example, region  710   2  is positioned between signal conductor  744   2 B and ground conductor  730   2 . Likewise, region  710   3  is positioned between signal conductor  744   3 B and ground conductor  730   3 . 
   The inventors have appreciated that positioning regions of higher dielectric constant material between the longer signal conductor of a differential pair and an adjacent ground is desirable for reducing skew. While not being bound by any particular theory of operation, the inventors theorize that the common mode components of the signal carried by a differential pair may be heavily influenced by differences in the length of the conductors of the pair caused by curves in the differential pair. In the example of  FIG. 7A , common mode components of a signal carried on pair  742   2  propagate predominately in the regions of wafer  720  between signal conductor  744   2 A and ground  730   1  and between signal conductor  744   2 B and ground conductor  730   2 . In contrast, the differential mode components of the signal propagate generally in the region between signal conductors  744   2 A and  744   2 B. 
   The reasons why common mode components of a signal are most heavily influenced by skew are illustrated in  FIG. 7B , which shows a curved portion of differential pair  742   2 . Common mode components of the signals propagate on differential pair  742   2  in regions  760   1  and  760   3 . Differential mode components of the signal propagate in region  760   2 . The differences in the length of a path through regions  760   1  and  760   3  that common mode components may travel is greater than the differences in lengths of paths differential mode signals may travel through region  760   2 . 
   As can be seen in  FIG. 7B , the difference in length of each of the conductive elements in a curved portion depends on the radii of curvature of the conductive elements. In the example illustrated, ground conductor  730   1  has an edge with a radius of curvature of R 1 . Signal conductor  744   2 A has an radius of curvature of R 2 . Likewise, signal conductor  744   2 B and ground conductor  730   2  have radii of curvature of R 3  and R 4 , respectfully. 
   Common mode components propagating in region  760   3  must cover a distance that is generally proportional to the radius of curvature R 4 . The distance that a common mode component travels through region  760   1  is proportional to the radius of curvature R 1 . Therefore, skew in the common mode components will be proportional to the difference (R 4 -R 1 ). 
   In contrast, the difference in path lengths traveled by the differential mode components traveling through region  760   2  is proportional to the difference in the radii of curvature defining the boundaries of region  760   2 . In the configuration of  FIG. 7B , that distance, and therefore differential mode skew, is proportional to (R 3 -R 2 ). As can be seen, (R 4 -R 1 ) is longer than (R 3 -R 2 ), which indicates the common mode skew is potentially larger than the differential mode skew. To reduce skew, particularly common mode skew, it may desirable for common mode components in region  760   3  to propagate faster than the common mode components in region  760   1 . Accordingly, the material forming the housing of wafer  720  in region  760   3  may have a lower dielectric constant than the material in region  760   1 . 
   As can be seen by comparing  FIGS. 7A and 7B , region  760   3  ( FIG. 7B ) overlaps region  710   2  ( FIG. 7A ). Region  760   1  ( FIG. 7B ) overlaps region  712   2 . Accordingly, positioning material of a lower dielectric constant in regions  710   2  and  710   3  as shown in  FIG. 7A  may reduce skew. 
   More generally, material of a lower dielectric constant positioned in region R ( FIG. 7A ), which extends outward from the center of a differential pair towards a distal edge  732  of an adjacent ground conductor  730   2 , may reduce skew. 
   It is not necessary that the entire region R be occupied by material of a lower dielectric constant. In some embodiments, the region of lower dielectric constant material, such as region  710   2 , does not extend to the distal edge  732  of an adjacent ground conductor. Rather, the region of lower dielectric constant material extends no farther the midpoint of the ground conductor. 
   A comparison of  FIG. 7A  and  FIG. 7B  also illustrates that it is not necessary to alter the dielectric constant of all the material adjacent a signal conductor. Altering the average, or effective, dielectric constant adjacent a signal conductor may be adequate to reduce skew. Thus, even if the entire region R is not completely filled with a lower dielectric constant material, the average dielectric constant may be adequately lowered to de-skew a differential pair. 
   For example, region  760   3  ( FIG. 7B ) extends above and below the plane containing the conductive elements. However, region  710   2  extends generally from a surface  722  of wafer  720  to the plane containing the signal conductors of differential pair  742   2 . Region  714   2  ( FIG. 7A ) extends below the plane of the signal conductors and contains material of a higher dielectric constant similar to region  712   2 . Nonetheless, incorporation of region  710   2  changes the average or effective dielectric constant of the material adjacent signal conductor  744   2 B, which is sufficient to alter the speed of propagation of signals through signal conductor  744   2 B. Thus, extending a region of lower dielectric constant material from surface  722  to approximately a plane containing the signal conductors as shown in  FIG. 7A  may be sufficient to improve the skew characteristics of differential pair  742   2  and is easy to manufacture using an insert molding operation. However, in other embodiments, region  710   2  could extend from surface  722  to below the plane containing a differential pair  742   2 . Such an embodiment could be formed, for example, by inserting material into wafer  720  from both surfaces  722  and  724 . Alternatively, differential pair  742   2  can be de-skewed even if region  710   2  of material of a lower dielectric constant does not extend all the way to the plane containing the signal conductors of pair  742   2 . Accordingly, the specific size and shape of a region of lower dielectric constant material is not limited to the configurations pictured, and any suitable configuration may be used. 
   Incorporating regions of lower dielectric constant material may alter other properties of the differential pairs in wafer  720 . For example, the impedance of signal conductor  744   2 B may be increased by a region of lower dielectric constant material  710   2 . To compensate for an increase of impedance, the width of a signal conductor adjacent a region of lower dielectric constant may be wider than the corresponding signal conductor of the pair. For example,  FIG. 7A  shows signal conductor  744   2 B having a width W 2  that is greater than width W 1  of signal conductor  744   2 A. Known relationships between the impedance of a signal conductor and the dielectric constant of the material surrounding it may be used to compute a width W 2  and W 1  to provide signal conductors with similar impedances. 
     FIG. 7B  illustrates a further characteristic of the placement of region of material of lower dielectric constant. As described above, differences in the length of the conductors associated with a differential pair occur where the differential pair curves. To keep the signals propagating through the conductors of a differential pair in unison, it may be desirable to alter the speed of propagation only or predominantly in curved segments of the differential pair. 
     FIG. 8  is a sketch of a wafer strip assembly  410 A, showing the entire length of each differential pair within a daughter card wafer. As can be seen in  FIG. 8 , the differential pairs have curved segments, such as curved segments  810   1 ,  810   2 ,  810   3  . . .  810   7 . In some embodiments, regions of material of relatively lower dielectric constant may be placed adjacent a longer signal conductor of each differential pair only in a curved region  810   1 ,  810   2  . . .  810   7 . The length along the signal conductors of each of the regions of material of relatively lower dielectric constant may be proportionate to the difference in length between the shorter signal conductor of the differential pair and the longer signal conductor of the differential pair traversing that curved region. 
   Positioning material of relatively lower dielectric constant adjacent curved regions has the benefit of offsetting effects of different length conductors as those effects occur. Consequently, signal components associated with each signal conductor of the pair stay synchronized throughout the entire length of the differential pair. In such an embodiment, the differential pair may have an increased common mode noise immunity, which can reduce crosstalk. Of course, equalizing the total propagation delay through the signal conductors of a differential pair is desirable even if the signal components are not synchronized at all points along the differential pair. Accordingly, the material of relatively lower dielectric constant may be placed in any suitable location or locations. 
   In the embodiments described above, regions of relatively lower dielectric constant are formed by incorporating into the housing of wafer  720  regions of material that has a lower dielectric constant than other material used to form the housing. However, in some embodiments, a region of relatively lower dielectric constant may be formed by incorporating material of a higher dielectric constant outside of that region. 
   For example,  FIG. 9  shows a wafer  920  having a housing predominately formed of material  940 . Differential pairs  942   1  and  942   2  are incorporated within the housing of wafer  920 . In the example of  FIG. 9 , signal conductor  944   1 B is longer than signal conductor  944   1 A. Likewise, differential pair  942   2  has a signal conductor  944   2 B that is longer than signal conductor  944   2 A. To reduce the skew of the differential pairs  942   1  and  942   2 , regions  910   1  and  910   2  may be formed with a lower dielectric constant than material that surrounds the shorter signal conductors  944   1 A and  944   2 A. 
   However, in the embodiment illustrated, regions  910   1  and  910   2  are formed of the same material used to form the insulative portion of housing  940 . Nonetheless, regions  910   1  and  910   2  have a relatively lower dielectric constant than the material surrounding the shorter signal conductors because of the incorporation of regions  912   1  and  912   2 . In the embodiment illustrated, regions  912   1  and  912   2  have a higher dielectric constant than the material used to form the insulative portion  940 . 
   Regions  912   1  and  912   2  may be formed in any suitable way. For example, they may be formed by incorporating fillers or other material into plastic that is molded as a portion of the housing of wafer  920 . However, any suitable method may be used to form regions  912   1  and  912   2 . 
     FIG. 9  also illustrates some of the variations that are possible in constructing a connector according to embodiments of the invention. In the embodiment of  FIG. 9 , differential pair  942   2  is at the end of a column within wafer  920 . Signal conductor  944   2 B in the pictured embodiment may be too close to the edge of wafer  920  to allow incorporation of a material of lower dielectric constant adjacent signal conductor  944   2 B. Accordingly, altering the relative dielectric constants through the incorporation of regions  912   1  and  912   2  of higher dielectric constant may be desirable in an embodiment such as the embodiment of  FIG. 9 . 
   The embodiment of  FIG. 9  also illustrates that regions of relatively higher and relatively lower dielectric constant material may be formed even when differential pairs are not positioned between ground conductors. For example, differential pair  942   2  is adjacent ground conductor  930   2  but has no ground conductor on the opposite side of the pair. Thus, while it may be desirable in some embodiments to create regions of relatively higher or relatively lower dielectric constant between a differential pair and a ground conductor, the invention need not be limited in this respect. 
     FIG. 9  also demonstrates that embodiments may be constructed without incorporating lossy material. 
   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. 
   As one example, a connector designed to carry differential signals was used to illustrate selective placement of material to achieve a desired level of delay equalization. The same approach may be applied to alter the propagation delay in signal conductors that carry single-ended signals. 
   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. 
   Also, impedance compensation in regions of signal conductors adjacent regions of lower dielectric constant was described to be provided by altering the width of the signal conductors. Other impedance control techniques may be employed. For example, the signal to ground spacing could be altered adjacent regions of lower dielectric constant. Signal to ground spacing could be altered in an suitable way, including incorporating a bend or jag in either the signal or ground conductor or changing the width of the ground conductor. 
   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.