Abstract:
An electrical connector with electrically lossy materials bridging ground members. The lossy conductive members may be formed by filling a settable binder with conductive particles, allowing the partially conductive members to be formed through an insert molding process. Connectors assembled from wafers that contain signal conductors held within an insulative housing may incorporate lossy conductive members by having filled thermal plastic molded onto the insulative housing. The lossy conductive members may be used in conjunction with magnetically lossy materials. The lossy conductive members reduce ground system do resonance within the connector, thereby increasing the high frequency performance of the connector.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This patent application is a continuation of U.S. patent application Ser. No. 10/955,571, now U.S. Pat. No. 7,371,117, filed Sep. 30, 2004, the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND OF INVENTION 
   1. Field of Invention 
   This invention relates generally to an electrical interconnection systems and more specifically to improved signal integrity in interconnection systems. 
   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”) which are then connected to one another by electrical connectors. A traditional arrangement for connecting 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 the increased bandwidth. 
   As signal frequencies increase, there is a greater possibility of electrical noise being generated in the connector in forms such as reflections, cross-talk and electromagnetic radiation. Therefore, the electrical connectors are designed to control cross-talk between different signal paths and to control the characteristic impedance of each signal path. Shield members are often used for this purpose. Shields are placed adjacent the signal contact elements. 
   Cross-talk between distinct signal paths can be controlled by arranging the various signal paths so that they are spaced further from each other and nearer to a shield, which is generally a grounded plate. Thus, the different signal paths tend to electromagnetically couple more to the shield and less with each other. For a given level of cross-talk, the signal paths can be placed closer together when sufficient electromagnetic coupling to the ground conductors are maintained. 
   Shields are generally made from metal components. However, U.S. Pat. No. 6,709,294 (the “294 patent”), which is assigned to the same assignee as the present application, describes making shields in a connector from conductive plastic. The &#39;294 patent is hereby incorporated by reference in its entirety. 
   Electrical connectors can be designed for single-ended signals as well as for differential signals. A single-ended signal is carried on a single signal conducting path, with the voltage relative to a common reference conductor being the signal. 
   Differential signals are signals represented by a pair of conducting paths, called a “differential pair.” The voltage difference between the conductive paths represents the signal. In general, the two conducing paths of a differential pair are arranged to run near each other. No shielding is desired between the conducting paths of the pair but shielding may be used between differential pairs. 
   One example of a differential pair electrical connector is shown in U.S. Pat. No. 6,293,827 (“the &#39;827 patent”), which is assigned to the assignee of the present application. The &#39;827 patent is incorporated by reference herein. The &#39;827 patent discloses a differential signal electrical connector that provides shielding with separate shields corresponding to each pair of differential signals. U.S. Pat. No. 6,776,659 (the &#39;659 patent), which is assigned to the assignee of the present application, shows individual shields corresponding to individual signal conductors. Ideally, each signal path is shielded from all other signal paths in the connector. Both the &#39;827 patent and the &#39;659 patents are hereby incorporated by reference in their entireties. 
   While the electrical connectors disclosed in the &#39;827 patent and the &#39;659 patent and other presently available electrical connector designs provide generally satisfactory performance, the inventors of the present invention have noted that at high speeds (for example, signal frequencies of 1 GHz or greater, particularly above 3 GHz), electrical resonances in the shielding system can create cross talk and otherwise degrade performance of the connector. We have observed that such resonances are particularly pronounced in ground systems having a shield member per signal contact or per differential pair. 
   My prior patent, U.S. Pat. No. 6,786,771, now published as US 2004/0121652A1, which is hereby incorporated by reference in its entirety, describes the use of lossy material to reduce unwanted resonances and improve connector performance. It would be desirable to further improve connector performance. 
   SUMMARY OF INVENTION 
   In one aspect, the invention relates to a wafer for an electrical connector having a plurality of wafers. The wafer has a plurality of first type contact elements, positioned in a column; a plurality of discrete conductive elements each disposed adjacent at least one of the first type contact elements; insulative material securing at least the plurality of first type contact elements; and electrically lossy material bridging the discrete conductive elements. 
   In another aspect, the invention relates to an electrical connector that has a plurality of regions. Each region has insulative material; a plurality of signal conductors, each signal conductor having a contact tail and a contact portion and an intermediate portion there between, and at least a part of the intermediate portion of each of the signal conductors secured in the insulative material; a plurality of shield members, each shield member having an intermediate portion adjacent an intermediate portion of a signal conductor; and electrically lossy material positioned adjacent the intermediate portion of the each of the shield members. 
   In yet another aspect, the invention relates to an electronic system with a plurality of printed circuit boards, each printed circuit board having a plurality of ground structures and a plurality of signal traces. Electrical connectors are mounted to the plurality of printed circuit boards. Each connector has a first plurality of conducting members, each connected to a ground structure in at least one of the plurality of printed circuit boards; a second plurality of conducting members, each connected to at least one of the plurality of signal traces in at least one of the plurality of printed circuit boards, the second plurality of conducting members being positioned in groups with at least two conducting members of the first plurality of conducting members positioned adjacent conducting members of the second plurality of conducting members in each group; and a plurality of partially conductive members, each connecting the at least two conducting members of the first plurality of conducting members positioned adjacent conducting members of the second plurality of conducting members in a group. 

   
     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 connector assembly showing a first electrical connector about to mate with a second electrical connector; 
       FIG. 2  is an exploded view of the first electrical connector of  FIG. 1 , showing a plurality of wafers; 
       FIG. 3  is a perspective view of signal conductors of one of the wafers of the first electrical connector of  FIG. 2 ; 
       FIG. 4  is a side view of the signal conductors of  FIG. 3  with an insulative housing formed around the signal conductors; 
       FIG. 5   a  is a side view of shield strips of one of the wafers of the first electrical connector of  FIG. 2 ; 
       FIG. 5   b  is a perspective view of the shield strips of  FIG. 5   a;    
       FIG. 6  is a side view of the shield strips of  FIG. 5   a  formed on two lead frames, with each lead frame holding half of the shield strips; 
       FIG. 7  is a side view of the shield strips of  FIG. 5   a  with an insulative housing formed around the shield strips; 
       FIG. 8   a  is a perspective view of an assembled one of the wafers of the first electrical connector of  FIG. 2 ; 
       FIG. 8   b  is a front view of a portion of the assembled wafer of  FIG. 8   a , showing first contact ends of the signal conductors and the shield strips configured for connection to a printed circuit board; 
       FIG. 9   a  is a cross section to the wafer illustrated in  FIG. 8   a  taken along the line  9   a - 9   a;    
       FIG. 9   b  is a cross section of an alternative embodiment of the wafer shown in  FIG. 9   a;    
       FIG. 9   c  is a cross section of an alternative embodiment of the wafer shown in  FIG. 9   a.    
       FIG. 10   a  is a plan view of a wafer formed according to an alternative construction method; 
       FIG. 10   b  is a cross sectional view of a portion of the wafer of  FIG. 10   a  taken along the line  b - b;    
       FIG. 11  is a cross sectional view of a wafer according to an alternative embodiment; 
       FIG. 12  is a cross section of a wafer formed according to a further alternative embodiment; and 
       FIG. 13  is a cross section of a wafer formed according to a further alternative embodiment. 
   

   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,” or “having,” “containing,” “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 , there is shown an electrical connector assembly  10 . The electrical connector assembly  10  includes a first electrical connector  100  mateable to a second electrical connector  200 . Electrical connector  100  may be used as a daughter card connector and electrical connector  200  may be used as a backplane connector. However the invention may be broadly applied in many types of connectors. 
   The second electrical connector  200  may be as described in the above referenced U.S. Pat. No. 6,776,659. 
   The first electrical connector  100 , which is shown in greater detail in  FIGS. 2-13 , includes a plurality of wafers  120 , with each of the plurality of wafers  120  having a housing  122 , a plurality of signal conductors  124  (see  FIG. 3 ) and a plurality of shield strips  126  (see  FIGS. 5   a  and  5   b ). For exemplary purposes only, the first electrical connector  100  is illustrated with ten wafers  120 , with each wafer  120  having fourteen single-ended signal conductors  124  and corresponding fourteen shield strips  126 . However, as it will become apparent later, the number of wafers and the number of signal conductors and shield strips in each wafer may be varied as desired. 
   The first electrical connector  100  is also shown having alignment modules  102  on either end, with each alignment module  102  having an opening  104  ( FIG. 2 ) for receiving a guide pin (which may also be referred to as a corresponding rod)  204  from member  202  of the second electrical connector  200 . Each alignment module  102  further includes features  105  ( FIG. 2 ),  106  to engage slots in stiffeners  110 ,  111 , respectively. Likewise, the insulative housing  122  of each wafer  120  provides features  113 ,  114  to engage the slots in stiffeners  110  ( FIG. 2 ),  111 , respectively. 
   Each signal conductor  124  has contact end  130  connectable to a printed circuit board, a contact end  132  connectable to the second electrical connector  200 , and an intermediate portion  131  there between. Each shield strip  126  ( FIG. 5   a ) has a first contact end  140  connectable to the printed circuit board, a second contact end  142  connectable to the second electrical connector  200 , and an intermediate portion  141  there between. 
   In the embodiment of the invention illustrated in  FIGS. 1-8   b , the first contact end  130  of the signal conductors  124  includes a contact tail  133  having a contact pad  133   a  that is adapted for soldering to the printed circuit board. The second contact end  132  of the signal conductors  124  includes a dual beam structure  134  configured to mate to a corresponding mating structure of the second electrical connector  200 . The first contact end  140  of the shield strips  126  includes at least two contact tails  143 ,  144  having contact pads  143   a ,  144   a , respectively, that are adapted for soldering to the printed circuit board. The second contact end  142  of the shield strips  126  includes opposing contacting members  145 ,  146  that are configured to provide a predetermined amount of flexibility when mating to a corresponding structure of the second electrical connector  200 . While the drawings show contact tails adapted for soldering, it should be apparent to one of ordinary skill in the art that the first contact end  130  of the signal conductors  124  and the first contact end  140  of the shield strips  126  may take any known form (e.g., press-fit contacts, pressure-mount contacts, paste-in-hole solder attachment) for connecting to a printed circuit board. 
   Still referring to  FIGS. 5   a  and  5   b , the intermediate portion  141  of each shield strip  126  has a surface  141   s  with a first edge  147   a  and a second edge  147   b , at least one of the first edge  147   a  or the second edge  147   b  being bent out of the plane of surface  141   s . In the illustrated embodiment, the first edge  147   a  is bent substantially perpendicular to the surface  141   s  of the shield strip  126  and extends through to the end of the second contact end  142  (but not through to the end of the first contact end  140 ). 
     FIG. 4  is a side view of the signal conductors  124  of  FIG. 3 , with the signal conductors  124  disposed in a first insulative housing portion  160 . Preferably, the first insulative housing portion  160  is formed around the signal conductors  124  by injection molding plastic. To facilitate this process, the signal conductors  124  are preferably held together on a lead frame (not shown) as known in the art. Although not required, the first insulative housing portion  160  may be provided with windows  161  adjacent the signal conductors  124 . These windows  161  are intended to generally serve multiple purposes, including to: (i) ensure during an injection molding process that the signal conductors  124  are properly positioned, (ii) provide impedance control to achieve desired impedance characteristics, and (iii) facilitate insertion of materials which have electrical properties different than housing  160 . 
     FIG. 7  is a side view of the shield strips  126  of  FIGS. 5   a  and  5   b , with the shield strips  126  disposed in a second housing portion  170 . As will be described in greater detail below, housing portion  170  may be formed from one or more materials that provides insulation, conductivity, lossy conductivity or magnetic lossiness. 
   Housing portion  170  may be formed in whole or in part by injection molding of material around shield strips  126 . To facilitate the injection molding process, the shield strips  126  are preferably held together on two lead frames  172 ,  174 , as shown in  FIG. 6 . Each lead frame  172 ,  174  holds every other of the plurality of the shield strips  126 , so when the lead frames  172 ,  174  are placed together, the shield strips  126  will be aligned as shown in  FIGS. 5   a  and  5   b . In the embodiment shown, each lead frame  172 ,  174  holds a total of seven shield strips  126 . 
   The lead frame  172  includes tie bars  175  that connect to the second contact ends  142  of its respective shield strips  126  and tie bars  176  that connect to the first contact ends  140  of the shield strips  126 . The lead frame  174  includes tie bars  177  that connect to the second contact ends  142  of its respective shield strips  126  and tie bars  178  that connect to the first contact ends  140  of the shield strips  126 . These tie bars  175 - 178  are cut during subsequent manufacturing processes. 
   The first insulative housing portion  160  may include attachment features (not shown) and the second housing portion  170  may include attachment features (not shown) that correspond to the attachment features of the first insulative housing portion  160  for attachment thereto. Such attachment features may include protrusions and corresponding receiving openings. Other suitable attachment features may also be utilized. 
   A first insulative housing portion  160  and the second housing portion  170  may be attached to form a wafer  120 . As shown in  FIGS. 8   a  and  8   b , each signal conductor  124  is positioned along the surface  141   s  adjacent a corresponding shield strip  126 . The bent edge  147   a  of the surface  141   s  is directed toward the corresponding signal conductor  124 . The bent edge  147   a , in combination with surface  147   s , creates shielding on two sides of the adjacent signal conductor  124 . 
   The first electrical connector  100  may also be configured to carry differential pairs of signals. In this configuration, the signal conductors may be organized in pairs. The surface  141   s  of each shield strip is preferably wider than the width of a pair to provide sufficient shielding to the pair. 
     FIG. 9   a  shows a wafer  120  in cross section taken along the line  9   a - 9   a  in  FIG. 8   a . Intermediate portions  131  of signal conductors  124  are embedded within an insulative housing  160 . A portion of shield strips  126  are held within housing portion  170 . The shield strips  126  are held with first edge portions  147   a  projecting between adjacent intermediate portions  131 . The surface  141   s  of each shield strip is held within housing portion  170 . Housing portion  170  may be molded around shield strips  126  and first insulative housing  160  may be molded around signal conductors  124  prior to assembly of wafer  120 . 
   In the illustrated embodiment, housing portion  170  is made of two types of materials. Housing portion  170  is shown to contain a layer  910  and a layer  912 . Both layers  910  and  912  may be made of a thermoplastic or other suitable binder material such that they may be molded around shield strips  126  to form the housing  170 . Either or both of layers  910  and  912  may contain particles to provide layers  910  and  912  with desirable electromagnetic properties. 
   In the example of  FIG. 9   a , the thermoplastic material serving as the binder for layer  910  is filled with conducting particles. The fillers make layer  910  “electrically lossy.” 
   Materials that conduct, but with some loss, over the frequency range of interest are referred to herein generally as “electrically 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. 
   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 0.01 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. Examples of materials that may be used are those that have an electric loss tangent between approximately 0.04 and 0.2 over a frequency range of interest. 
   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 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 a filler that contains conductive particles to a binder. Examples of conductive particles that may be used as a filler to form an electrically lossy materials 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. 
   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 material are 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. As used herein, the term “binder” encompasses a material that encapsulates the filler or is impregnated with 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. 
   In one contemplated embodiment, layer  910  has a thickness between 1 and 40 mils (about 0.025 mm to 1 mm). The bulk resistivity of layer  910  depends on its thickness as well as its surface resistivity. The bulk resistivity is suitable to allow the layer to provide some conduction, but with some loss. Bulk resistivity of an electrically lossy structure used herein may be between about 0.01 Ω-cm and 1 Ω-cm. In some embodiments, the bulk resistivity is between about 0.05 Ω-cm and 0.5 Ω-cm. In some embodiments, the bulk resistivity is between about 0.1 Ω-cm and 0.2 Ω-cm. 
   Layer  912  provides a magnetically lossy layer. Layer  912  may, like layer  910 , be formed of a binder or matrix material with fillers. In the pictured embodiment, layer  912  is made by molding a filled binder material. The binder for layer  912  may be the same as the binder used for layer  910  or any other suitable binder. Layer  912  is filled with particles that provide that layer with magnetically lossy characteristics. The magnetically lossy particles may be in any convenient form, such as flakes or fibers. Ferrites are common magnetically lossy materials. Materials such as magnesium ferrite, nickel ferrite, lithium ferrite, yttrium garnet or aluminum garnet may be used. 
   The “magnetic loss tangent” is the ratio of the imaginary part to the real part of the complex magnetic permeability of the material. Materials with higher loss tangents may also be used. Ferrites will generally have a loss tangent above 0.1 at the frequency range of interest. Presently preferred ferrite materials have a loss tangent between approximately 0.1 and 1.0 over the frequency range of 1 Ghz to 3 GHz and more preferably a magnetic loss tangent above 0.5. 
   It is possible that a material may simultaneously be a lossy dielectric or a lossy conductor and a magnetically lossy material. Such materials can be formed, for example, by using magnetically lossy fillers that are partially conductive or by using a combination of magnetically lossy and electrically lossy fillers. 
   Layer  912  plays the role of absorptive material as described in my prior U.S. Pat. No. 6,786,771, which is incorporated herein by reference. Layer  912  reduces resonance between shields in adjacent wafers  120 . 
   Layer  910  provides “bridging” between the individual shield strips  126  within the wafer  120 . The bridging provides an electrically lossy path between conducting members over the frequency range of interest. The bridging may be provided by a physical connection to the conducting members that are bridged. In addition, over the frequency range of interest, signals may couple between structures capacitively or otherwise without direct physical contact between the structures. Accordingly, “bridging” may not require direct physical contact between structures. 
   With bridging in place, each of the shield strips  126  is less likely to resonate independently from the others. Preferably, layer  910  is sufficiently conductive that the individual shield strips do not resonate independently but sufficiently lossy that the shield strips and the bridging do not form a combined structure that, in combination with similar structures in another wafer, support resonant modes between adjacent wafers. 
     FIG. 9   b  shows an alternative embodiment of the wafer  120 . In wafer  120 ′, intermediate portions  131  of signal conductors  124  and shield strips  126  are held within an insulative housing  160 ′. Insulative housing  160 ′ may be formed in any convenient manner. It may be formed in a single molding step or in multiple molding steps. Layer  914  is formed on top of insulative housing  160 ′. Layer  914  is an electrically lossy layer similar to layer  910 . 
   In contrast to layer  910 , surfaces  141   s  of shield strips  126  are not embedded in layer  914 . In the embodiment shown, surfaces  141   s  are not in direct contact with layer  914 . The surfaces  141   s  are separated from layer  914  by a small portion of insulative housing  160 ′. Each of the surfaces  141   s  is capacitively coupled to layer  914 . In this way, layer  914  provides a partially conductive path at the frequencies of interest bridging the individual shield strips  126  in wafer  120 ′. Similar to the configuration in  FIG. 9   a , partially conductive layer  914  reduces resonances between the shield strips  126  within wafer  120 ′. 
   Wafer  120 ′ may optionally be formed with a magnetically lossy material, such as a layer  912  shown in  FIG. 9   a.    
     FIG. 9   c  shows a further embodiment. Wafer  120 ″ includes an insulative housing  160  as shown in  FIG. 9   a . Surfaces  141   s  of the shield strips  126  are held within a partially conductive layer  916 . Layer  916  may be a partially conductive layer formed in the same fashion as layer  910 , thereby bridging the shield strips  126 . Regions  918  within layer  916  are formed from magnetically lossy material. Regions  918  may be formed of the same material as is used to form layer  912 . Regions  918  may be formed in a separate step or may be formed by adding magnetically lossy particles during the formation of layer  916 . 
     FIGS. 9   a  and  9   c  show the use of electrically lossy and magnetically lossy materials in combination. In the described embodiments, both the magnetically lossy and electrically lossy materials are formed by the addition of particles to a binder. It is not necessary that the particles be added to binders forming distinct structures. For example, magnetically lossy and conductive particles may be intermixed in a single layer, such as layer  914 , shown in  FIG. 9   b.    
   It is also not necessary that bridging between shield strips in a wafer be formed from particles encapsulated in the binder.  FIG. 10   a  shows an alternative construction of a wafer  120 ′″. Wafer  120 ′″ has inserts  950   a  and  950   b  inserted in openings in a surface of wafer  120 ′″. Preferably, the openings are sufficiently deep that they expose surfaces  141   s  of the shield strips within the wafer. 
     FIG. 10   b  shows a cross section of a portion of wafer  120 ′″ taken along the line  b - b  in  FIG. 10   a . In  FIG. 10   b , insert  950   a  is seen in cross-section. Insert  950   a  may, for example, be a lossy conductive carbon filled adhesive preform such as those sold by Techfilm of Billerica, Mass., U.S.A. This preform includes an epoxy binder  952  filled with carbon flakes. The binder surrounds carbon fiber  956 , which acts as a reinforcement for the preform. When inserted in a wafer  120 ′″, preform  950   a  adheres to shield strips  126 . In this embodiment, preform  950   a  adheres through the adhesive in the preform, which is cured in a heat treating process. Preform  950   a  thereby provides electrically lossy bridging between the shield strips. Various forms of reinforcing fiber, in woven or non-woven form, may be used. Non-woven carbon fiber is one suitable material. 
   In alternative embodiments, the preforms could be made to include both conductive and magnetically lossy filler. The conductive and magnetically lossy filler may be intermixed in a continuous binder structure or may be deposited in layers. 
   Electrically lossy materials may also be used in connectors that do not have ground strips.  FIG. 11  shows in cross-section an example of a wafer  1120  that includes signal conductors with intermediate portions  131  embedded in the insulative housing  1160 . Wafer  1120  is designed for applications in which alternating signal conductors are connected to ground forming what it is sometimes referred to as a “checkerboard pattern.” For example, signal conductor  1126  is intended to be connected to ground. In wafer  1120 , a partially conductive layer  1170  is used to provide bridging between signal conductors  1126  that are grounded. Layer  1170  may be formed generally in the same fashion as layers  910  or  914 . 
     FIG. 12  shows a wafer  1220  designed for carrying differential signals. Wafer  1212  includes an insulative housing  1260 . Signal conductors such as  1231   a  and  1231   b  are arranged in pairs within insulative housing  1260 . Shield members  1226  separate the pairs. Shield strips  1226  are embedded in a housing  1270 . In wafer  1220 , housing  1270  includes a partially conductive layer  1210  and a magnetically lossy layer  1212 . Layers  1012  and  1210  may be formed generally as layers  910  and  912  described above in connection with  FIG. 9   a.    
     FIG. 13  shows a further embodiment of a wafer  1320  that may be used to form an electrical connector as pictured in  FIG. 1 . Wafer  1320  may be similar to wafer  1120 . It contains a plurality of conductors  131  held in an insulative housing  1360 . However, none of the signal conductors  131  in wafer  1320  is specifically designed to be connected to ground. 
   Layer  1370  is an electrically lossy material. It bridges all of the signal conductors  131 . Where the benefit of reducing resonances between the signal conductors acting as grounds outweighs any loss of signal integrity caused by attenuation of the signals carried on conductors, layer  1370  provides a net positive impact on the signal integrity of a connector formed with wafers  1370 . 
   In embodiments such as those shown in  FIGS. 9   b  and  13  in which the bridging material is not in direct contact with structures serving as ground contacts, there may be no direct electrical connection between the electrically lossy material and ground. Such a connection is not required, though may be included in some applications. 
   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, it is described that bridging may be provided by capacitively coupling an electrically lossy member to two structures. Because no direct conducting path need be provided, it is possible that the electrically lossy material may be discontinuous, with electrically insulating material between segments of electrically lossy material. 
   Alternatively, electrically lossy bridging may be formed by creating signal paths that include conductive and lossy materials. For example,  FIG. 11  shows a lossy layer  1170  that has vertical portions  1150  adjacent conductors  1126  and a horizontal portion  1152  joining the vertical portions. Portions  1150  and  1152  in combination create an electrically lossy path between contacts  1126 . On or the other of these portions may be formed from a conductive material, such as metal. For example, portions  1150  may be electrically lossy material molded into housing  1160  and portion  1152  may be implemented as a metal plate. Though portion  1152  would be conductive, the signal path between adjacent contacts  1126  would be electrically lossy. 
   Further, example embodiments show each of the signal conductors and ground conductors molded in an insulative housing, such as plastic. However, air is often a suitable dielectric and may be preferable to plastic in some applications. In some embodiments, the conductors within the wafer will be held in an insulative plastic housing over a relatively small portion of their length and surrounded by air, or other dielectric material, over the remainder of their length. 
   As another example, electrically lossy structures and magnetically lossy structures were described as being formed by embedding particles in a settable binder. Where molding is used, preferably features are provided in each region formed by a separate molding step to interlock the regions. 
   Partially conductive structures may be formed in any convenient manner. For example, adhesive substances which are inherently partially conductive may be applied to shield strips through windows in an insulative housing. As another alternative, conducting filaments such as carbon fibers may be overlaid on shield members before they are molded into a housing or they may be attached to the shield members with adhesive after the shield members are in place. 
   Further, lossy conductive material is shown in planar layers. Such a structure is not required. For example, partially conductive regions may be positioned only between shield strips or only between selective shield strips such as those found to be most susceptible to resonances. 
   Also, it was described that wafers  120  are formed by attaching a subassembly containing signal contacts to a subassembly containing shield members. It is not necessary that the sub-assemblies be secured to each other. However, where desired, the sub-assemblies may be secured with various features including snap fit features or features that engage through function. 
   Further, electrically and magnetically lossy materials are shown only in connection with a daughter card connector. However, benefits of using such materials is not limited to use in daughter card connectors. Such materials may be used in backplane connectors or in other types of connectors, such as cable connectors, stacking connectors, mezzanine connectors. The concepts may also be applied in connectors other than board to board connectors. Similar concepts may be applied in chip sockets in other types of connectors. 
   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.