Abstract:
A high speed connector assembly includes a first surface-mount connector (SMC) and a second SMC. The first SMC includes a first flexible printed circuit (FPC) that has conductors that extend from a first FPC edge to a second FPC edge. The first edge includes surface-mount contact structures for surface mounting to a first printed circuit board. The second SMC includes a second FPC that has conductors that extend from a first FPC edge to a second FPC edge. The first edge includes surface-mount contact structures for surface mounting to a second printed circuit board. A set of contact beams is disposed along the second FPC edge. The first and second SMCs are mateable such that the contact beams make electrical contact between conductors in the first FPC and conductors in the second FPC. The FPC of the second SMC flexes to adjust for misalignments between the first and second SMCs.

Description:
TECHNICAL FIELD 
   The present invention relates generally to high speed connectors. 
   BACKGROUND INFORMATION 
   Electrical connectors are used in electronic equipment and devices to communicate electrical signals from one printed circuit board to another. As operating speeds of the electronics of such electronic equipment and devices have increased, the communication of the electrical signals in a noise-free fashion has become more important and more difficult to achieve. If, for example, an electrical signal is transmitted down a conductor and if there are discontinuities in the characteristic impedance of the conductor, or if the conductor is not properly terminated, then electrical reflections may be generated. These reflections are undesirable and may obscure the desired signal that was to be conducted down the conductor. If, for example, two conductors extend parallel and close to one another for a long distance, a signal propagating down one of the conductors may induce a signal into the other conductor. Again, the induced signal is undesirable and may obscure a desired signal that was to be conducted down the other conductor. If, for example, an adequately long segment of a conductor is left unshielded and if a high frequency signal is present on the segment, then the segment may act as an antenna and radiate electromagnetic radiation or receive electromagnetic radiation. This is undesirable as well. As the operating speeds of the electronics within the electronic equipment and devices have increased over time, the need to minimize reflections, cross-talk and the radiation of electromagnetic energy in the conductors within electrical connectors has become more important. 
     FIG. 1  (Prior Art) is a simplified perspective view of a piece of electronic equipment  1  such as a router or computer. Equipment  1  includes a first printed circuit board  2  extending in a first plane and a second printed circuit board  3  extending in a second plane perpendicular to the first printed circuit board. The first printed circuit board is often referred to as a motherboard or a backplane. The second printed circuit board is often referred to as a daughterboard or line card or expansion board. Although not illustrated in  FIG. 1 , there are typically many daughterboards within the piece of electronic equipment. 
   Electrical signals are communicated between first printed circuit board  2  and second printed circuit board  3  across a right angle connector assembly. The connector assembly includes a first connector  4  disposed on the motherboard and a second connector  5  disposed on the daughterboard. The first connector  4  is often referred to as the motherboard connector and the second connector  5  is often referred to as the daughterboard connector. The assembly is called a right angle connector because the two printed circuit boards are disposed at right angles with respect to one another. 
     FIG. 2  (Prior Art) is an expanded perspective view of motherboard  2 , motherboard connector  4 , daughterboard  3 , and daughterboard connector  5 . To couple the daughterboard to the motherboard, the daughterboard is moved with respect to the motherboard in the direction of arrow  6  such that female daughterboard connector  5  mates with male motherboard connector  4 . Individual signal conductors within daughterboard connector  5  are thereby coupled to corresponding individual signal conductors within motherboard connector  4 . 
     FIG. 3  (Prior Art) is a cross-sectional diagram showing how motherboard connector  4  is mechanically and electrically coupled to motherboard printed circuit board  2 . Daughterboard connector  5  is coupled to daughterboard  3  in similar fashion. Motherboard connector  4  is a male connector that includes an insulative housing  7  and a plurality of metal pins  8  and  9 . Each pin has a first end for mating with female daughterboard connector  5  and a second press-file contact tail end. Each press-fit contact tail extends into a corresponding through hole in the printed circuit board. There are two press-fit contact tails  10  and  11  illustrated in  FIG. 3 . Each contact tail has a hollow eye which allows the contact tail to be compressed by the sidewalls of the through hole as the contact tail is forced into the through hole when connector  4  is fixed to motherboard  2 . The contact tail presses back out against the sidewalls of the through hole and thereby holds the contact tail and pin in place. All the contact tails of the all the pins in turn hold the connector  4  in place on the printed circuit board. 
     FIG. 4  (Prior Art) is an end view of male motherboard connector  4 . Insulative housing  7  includes a first sidewall portion  12  and a second sidewall portion  13 . The ends of pairs of numerous signal pins are seen extending upward toward the viewer from the plane of the page. Pins  8  and  9  are one such pair. The signal pins are disposed in pairs because differential electrical signals are conducted over the signal conductors. The electric signal being communicated is a differential signal between a signal on the first signal pin of the pair and the second signal pin of the pair. 
   In addition to pairs of signal pins, a plurality of vertically oriented ground strips  15  is illustrated. Each ground strip includes a set of press-fit contact tails. The contact tails extend into through holes in the printed circuit board and make electrical contact with a ground plane in printed circuit board  2 . In the illustration of  FIG. 4 , the opposite strip bar side of each ground strip is seen extending upward toward the viewer from the plane of the page. The contact tails (not seen) of the ground strip extend into the plane of the page. Motherboard connector  4  is made by inserting the signal pins and ground strips into accommodating holes and slots in insulative housing  7 . See U.S. Pat. No. 6,872,085 for additional details. 
   To facilitate the design of transmission lines having constant characteristic impedances, signal conductors and dielectrics and ground planes are realized that have preset physical forms and orientations with respect to one another. One such set of forms and orientations is illustrated in cross-section in  FIG. 5  (Prior Art). The signal conductors  16  and  17  within the dielectric  18  of a printed circuit board are disposed between two ground planes  19  and  20 . In the diagram, two coupled stripline conductors  16  and  17  extend parallel to one another into the plane of the page. 
     FIG. 6  (Prior Art) illustrates another form and orientation called microstrip. In this form and orientation, there is one ground plane  20  disposed on one side of a pair of signal conductors  21  and  22 , and the signal conductors are embedded in dielectric material  23  of the printed circuit board. 
   The stripline and microstrip forms of signal conductors, dielectric and ground planes are employed in the design of male motherboard connector  4  of  FIG. 4 . Note the similarity in appearance between the ground strips and signal conductor pins of the connector of  FIG. 4  and the ground planes and signal conductors of the printed circuit boards of  FIGS. 5 and 6 . 
     FIG. 7  (Prior Art) is a simplified cross-sectional diagram that shows the female daughterboard connector  5  aligned with respect to the male motherboard connector  4 . Female daughterboard connector  5  includes an insulative housing  24  and a set of signal conductors. Signal conductor  25  is referred to as an example. Signal conductor  25  terminates at one end in a press-fit contact tail  26  that extends into an associated through hole in the printed circuit board of daughterboard  3 . Signal conductor  25  terminates at the other end in a pair of contact beams  27 . When the two connectors  4  and  5  of the assembly are mated, pin  8  of male connector  4  extends through a hole  29  in insulative housing  24  and slidingly engages contact beams  27  so as to make electrical contact with signal conductor  25 . Once mated, an electrical signal can pass from a conductor (not shown) within motherboard  2 , through the contact tail  10  of pin  8  of motherboard connector  4 , through pin  8  and to contact beams  27  of signal conductor  25 , through signal conductor  25  in daughterboard connector  5 , through the contact tail  26  and into a signal conductor (not shown) within daughterboard printed circuit board  3 . 
   Daughterboard connector  5 , in one embodiment, is made of multiple “wafers”. See U.S. Pat. No. 6,872,085 for further details. The signal conductors of one such wafer are illustrated in  FIG. 7 . 
     FIG. 8  (Prior Art) is an exploded view of one wafer. The wafer includes a shield plate of metal  31 , insulative housing  24  and signal conductors  33 . Signal conductor  25  is one of signal conductors  33 . The metal signal conductors can be made by stamping them out of a metal plate. The metal plate is typically a thick, approximately 0.2 millimeter thick, stiff sheet of copper or copper alloy. The stamped metal signal conductors  33  are pressed into accommodating slots in insulative housing  24 . Similarly, shield plate  31  can be stamped out of a sheet of metal and can be pressed into an accommodating recess in insulative housing  24 . Many such wafers are stacked together so that the holes (for example, hole  29 ) in the insulative housings of the wafers align to form a two-dimensional matrix of holes. The stack of wafers is held together in place by a conductive stiffener clip (not shown). See U.S. Pat. No. 6,872,085 for further details. 
   Although this type of connector assembly works well in many environments, there exist problems in certain applications due to mismatches between connectors when motherboard and daughterboard connectors are brought together when printed circuit boards of electronic equipment are to be connected to one another.  FIG. 9  (Prior Art) illustrates one such problem. Due to shortcomings in some printed circuit board fabrication techniques, a separation  28  between two daughterboard connectors  5  and  34  may vary in a range of plus or minus 0.1 millimeters. Similarly, a separation  30  between two motherboard connectors  4  and  35  may also vary in a range of plus or minus 0.1 millimeters. When daughterboard  3  and motherboard  2  are brought together, there can be a significant mismatch between connectors of each connector assembly. When the connectors are mated, the misalignment gives rise to mechanical stress between the connectors and the printed circuit boards to which they are attached. This mechanical stress must be absorbed satisfactorily without breaking the connectors or structures by which the connectors are attached to the printed circuit boards. 
     FIG. 10  (Prior Art) is a cross-sectional diagram illustrating such stress. The pin that extends downward and terminates in contact tail  36  is strong and absorbs stress due to connector  37  being pushed in the direction of arrow  38  with respect to printed circuit board  39  being pushed in direction of arrow  40 . As signal frequencies increase, however, the length of such a contact tail and the associated plated through hole and the irregular shape and discontinuous electrical characteristics of the contact tail and plated through hole cause electrical reflections, cross-talk and/or electromagnetic radiation. Although strong and reliable, the structure of  FIG. 10  is undesirable due to its electrical characteristics. 
     FIG. 11  (Prior Art) is a simplified cross-sectional diagram of an alternative structure wherein connector  37  is surface mounted to printed circuit board  39 . A solder ball or surface mount connector pin  41  on connector  37  is soldered to a solder pad  42  on printed circuit board  39  by a solder joint  43 . This structure does not have the irregularly shaped contact tail of  FIG. 10 , but the structure does have a somewhat long and conductive plated through hole  44 . Plated through hole  44  may act as an antenna is an undesirable way. To help avoid this problem, a backdrilling step may be employed to remove much of the plated through hole  44 . The dashed line  45  in  FIG. 12  (Prior Art) illustrates the hole that results after back drilling. 
     FIG. 13  (Prior Art) illustrates another structure wherein expensive back drilling step is not needed. In the structure of  FIG. 13 , stacked blind vias or conductive plugs  46 ,  47  and  48  are built into printed circuit board  39  to connect surface mounted connector  37  to electrical conductor  49  within printed circuit board  39 . Although it may be desired to be able to have the improved electrical properties of the surface mount structures of  FIGS. 11–13  in a connector assembly design, stress due to the misalignment of connectors may cause solder joints between connector  37  and printed circuit board  39  to fail. The stress may lift the solder pad  42  off printed circuit board  39 . It is therefore difficult or impossible to employ the surface mount techniques in high speed connector assemblies involving many signal pairs where there may be multiple connectors on each printed circuit board. An improvement upon the connector assembly structure of U.S. Pat. No. 6,872,085 is desired. 
   SUMMARY 
   A high speed connector assembly includes a first surface-mount connector and a second surface-mount connector. The first connector may, for example, be a male motherboard connector. The first connector includes a first printed circuit (PC) portion that has a plurality of signal conductors. Each signal conductor extends from a location proximate to a first PC edge to a location proximate to a second PC edge. The first edge includes surface-mount contact structures for making connection with a printed circuit board. 
   The second surface-mount connector may, for example, be a female daughterboard connector. The second surface-mount connector includes a second PC portion. The second PC portion has a plurality of signal conductors. Each signal conductor extends from a location proximate to the first PC edge of the second PC to a second PC edge of the second PC portion. The first edge includes surface-mount contact structures for making connection with a second printed circuit board. A set of contact beams is disposed along the second PC edge such that there is a single contact beam coupled to the second edge end of each signal conductor in the second PC portion. 
   The first and second surface-mount connectors are mateable such that when the second edge of the PC portion of the first connector is pushed-into the second connector, the contact beams on the second edge of the second connector make electrical contact between signal conductors of the PC portion in the first surface-mount connector and corresponding signal conductors of the PC portion in the second surface-mount connector. 
   In some embodiments, the PC portion of the second surface mount connector is a flexible printed circuit (FPC) portion. The FPC portion is more flexible than a typical printed circuit board of similar dimensions and has a tensile modulus of five GPa or less. The FPC portion can flex to adjust for misalignments between the first and second connectors. 
   The second connector in one embodiment includes a head portion and a body portion, wherein the FPC portion extends from the body portion to the head portion. The FPC portion flexes so that the head portion is laterally displaceable with respect to the body portion. 
   By allowing the head portion of the second connector to be laterally displaceable with respect to the body portion of the second connector, the connector assembly can prevent stress from being transferred to the surface-mount connections between the first connector and the first printed circuit board and between the second connector and the second printed circuit board. By preventing or reducing this stress, damage to the surface mount connector-to-printed circuit board connections is reduced or avoided. Relatively fragile solder surface mount techniques and structures can therefore be employed to couple the connectors to their respective printed circuit boards without unacceptable high failure rates of the surface mount joints. 
   The contact beam and conductor structure of the mating PC portions in the connector assembly is fashioned to shield signal conductors and signal contact beams with ground conductors. By having a PC portion signal conduction path in one connector and a PC portion signal conduction path in the second connector, the same PC materials and conductor dimensions and ground planes are provided in both connectors. Changes in the characteristic impedance of the signal path as the signal path extends from one connector to the other connector is reduced, thereby reducing unwanted reflections. By using surface-mount structures (for example, solder balls or metal surface mount contacts) to surface-mount the first edges of the PC portions to their respective printed circuit boards, unwanted extending plated through holes need not be used in the printed circuit board. The extending conductors of contact tails of press-fit pins are also avoided. The associated cross-talk and electromagnetic radiation and reception due to extending plated through holes and contact tails are therefore eliminated due to the use of surface-mount connections to the printed circuit boards. 
   Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
       FIG. 1  (Prior Art) is a perspective view of a piece of electronic equipment within which a connector assembly is disposed. 
       FIG. 2  (Prior Art) is a perspective view showing the connectors on the motherboard and daughterboard in the piece of electronic equipment of  FIG. 1 . 
       FIG. 3  (Prior Art) is a cross-sectional diagram showing how the motherboard connector of  FIGS. 1 and 2  is attached to the motherboard. 
       FIG. 4  (Prior Art) is an end view of the motherboard connector of  FIGS. 1–3 . 
       FIG. 5  (Prior Art) is a diagram of a coupled stripline transmission line structure. 
       FIG. 6  (Prior Art) is a diagram of a microstrip transmission line structure. 
       FIG. 7  (Prior Art) is a cross-sectional side view of the motherboard connector and the daughterboard connector of the connector assembly of  FIGS. 1–4 . 
       FIG. 8  (Prior Art) is an expanded exploded perspective view of a wafer of the daughterboard connector of  FIG. 7 . 
       FIG. 9  (Prior Art) is a simplified side view that illustrates stress imposed on the connectors of the connector assembly due to misalignment of the connectors. 
       FIG. 10  (Prior Art) is a cross-sectional side view showing a pin and its press-fit contact tail extending into a through hole in a printed circuit board. 
       FIG. 11  (Prior Art) is a cross-sectional side view showing a surface mount solder attachment by which a connector can be connected to a printed circuit board. 
       FIG. 12  (Prior Art) is a cross-sectional side view of the surface mount attachment of  FIG. 11 , but where an extending portion of the plated through hole has been removed in a back drilling step. 
       FIG. 13  (Prior Art) is a cross-sectional side view of a stacked blind via structure within the printed circuit board that facilitates surface mount connection of the connector to the printed circuit board without a radiating extra portion of plated through hole. 
       FIG. 14  is a perspective view of a connector assembly in accordance with one novel aspect. 
       FIG. 15  is a perspective view of the daughterboard connector of the assembly of  FIG. 14 . 
       FIG. 16  is an exploded view of the daughterboard connector of  FIG. 15  showing its constituent parts. 
       FIG. 17  is a perspective view of the inside of third housing portion  109  of  FIG. 16 . 
       FIG. 18  is a cross-sectional view of the daughterboard connector of  FIG. 15  taken along sectional line A—A. 
       FIG. 19  is an expanded view of a portion of  FIG. 18 . 
       FIG. 20  is a perspective view of a flexible printed circuit board (FPC) portion of the daughterboard connector of  FIG. 16 . 
       FIG. 21  is a perspective view of the bottom surface mount surface of the daughterboard connector of  FIG. 15 . 
       FIG. 22  is a perspective view looking into the motherboard connector of  FIG. 14 .  FIG. 22  also includes an expanded view of the FPC portions within the motherboard connector. 
       FIG. 23  is a perspective view of the bottom surface mount portion of the motherboard connector of  FIG. 14 .  FIG. 23  also includes an expanded view of the solder balls on the bottom surface the connector. 
       FIG. 24  is an exploded view of the motherboard connector of  FIG. 14 . 
       FIG. 25  is an expanded perspective view of a portion of the FPC portions of  FIG. 24 . 
       FIG. 26  is a perspective view of the connector assembly of  FIG. 14  when the daughterboard connector is mated to the motherboard connector. 
       FIG. 27  is a cross-sectional side view taken along sectional line D—D of  FIG. 26 .  FIG. 27  includes an expanded view of the contact beams on the FPC portions of the daughterboard, where the contact beams make electrical contact with the FPC portions of the motherboard connector. 
       FIG. 28  is a side view showing two connector assemblies in accordance with a novel aspect, where the connector assemblies flex and distend to absorb a misalignment between the connectors connected to the daughterboard and the connectors connected to the motherboard. 
       FIG. 29  is an end view of the structure of  FIG. 28 . 
       FIG. 30  is a cross-sectional view of the daughterboard connector  102  when its constituent FPC portions are bent in the flexing region of the daughterboard connector when head portion  106  is pushed in the direction of arrow  151  with respect to body portion  107 . 
       FIG. 31  is a cross-sectional view of the daughterboard connector  102  when its constituent FPC portions are bent in the flexing region of the daughterboard connector when head portion  106  is pushed in the direction of arrow  153  with respect to body portion  107 . 
       FIG. 32  is a cross-sectional end view of an FPC portion within the connector assembly. 
       FIG. 33  is a cross-sectional side view that illustrates how a contact beam contacts a ground conductor within an FPC portion of the motherboard connector such that a ground conductor within an FPC portion of the daughterboard connector is connected to a ground conductor within an FPC portion of the motherboard connector. 
       FIG. 34  is a cross-sectional side view that illustrates how a contact beam contacts a signal conductor within an FPC portion of the motherboard connector such that a signal conductor within an FPC portion of the daughterboard connector is connected to a signal conductor within an FPC portion of the motherboard connector. 
       FIG. 35  is a diagram of a right angle version of the novel connector assembly. 
       FIG. 36  is a diagram of a stacked version of the novel connector assembly. 
       FIG. 37  is a diagram of a side-by-side version of the novel connector assembly. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 14  is a perspective view of a right angle connector assembly  100  in accordance with one novel embodiment. Connector assembly  100  includes a first connector  101  and a second connector  102 . First connector  101  may, for example, be attached to a motherboard printed circuit board whereas second connector  102  may be attached to a daughterboard printed circuit board. First connector  101  is therefore hereinafter referred to as a motherboard connector and second connector  102  is hereinafter referred to as a daughterboard connector. To couple the two connectors  101  and  102  together, the second connector  102  may be moved in the direction of arrow  103  with respect to connector  101 . 
     FIG. 15  is a perspective view of daughterboard connector  102 . Ribs  104  of connector  102  slidingly engage corresponding guide groves  105  in connector  101  when the two connectors  101  and  102  engage one another. 
     FIG. 16  is an exploded perspective view of connector  102 . Connector  102  includes a first insulative head housing portion  106 , second insulative body housing portion  107 , a plurality of flexible printed circuit board portions (FPC portions)  108 , and a third insulative cap housing portion  109 . In one example, the insulated housing portions are made of Liquid Crystal Polymer (LCP) material that has a stable dielectric constant of approximately 3.5 to 4.0 and exhibits small mold shrinkage characteristics. 
   Each FPC portion includes a plurality of thin signal conductors disposed on a flexible insulative substrate. FPC portion  115  is the foremost FPC portion seen in  FIG. 16 . A main material of which printed circuit boards are customarily made is FR4 laminate. “FR” means flame retardant, and “4” indicates a woven glass reinforced epoxy resin. The FR4 material is made from glass fabric impregnated with epoxy resin and copper foil. The copper foil is usually formed by electrodeposition. This FR4 material is relatively stiff and has a tensile modulus of approximately eight to nine gigapascals (8.0–9.0 GPa). (The higher the tensile modulus value, the stiffer the material.) 
   Unlike an ordinary printed circuit board made of FR4, each FPC portion of daughterboard connector  102  is more flexible than an ordinary printed circuit board. Each FPC portion may, for example, have a tensile modulus of less than five GPa. In one embodiment the FPC portions have a tensile modulus in the range of from approximately 2.5 to 3.5 GPa. The FPC portions are flexible printed circuits where the conductors of the FPC portion are carried on a dielectric substrate layer. The dielectric substrate layer may, for example, be a polyimide layer (KAPTON®), a polyester layer (MYLAR®), or a TEFLON® layer. Each conductor of the FPC portion may, for example, be a 0.018 millimeter thick layer of copper or copper alloy. 
   A first end of each signal conductor terminates in solder ball pad. In the illustration of  FIG. 16 , the solder ball pads of FPC portion  115  are disposed along a first horizontal bottom edge  111  of FPC portion  115 . A second end of each signal conductor terminates in a contact beam. In the illustration of  FIG. 16 , the contact beams of FPC portion  115  are disposed along a second vertical side edge  110  of FPC portion  115 . When assembled, second edge  110  and its contact beams extend into slit-shaped, vertically oriented slot openings  112  in the face of first head housing portion  106 . First edge  111  and its solder ball pads extend downward into slit-shaped, horizontally oriented slot openings  113  in the bottom of second housing portion  107 . The FPCs and the first, second and third housing portions are formed such that the housing portions hold the FPCs in place and such that the third housing portion  109  snap fits onto the second body housing portion  107 . 
     FIG. 17  is a perspective view of third housing portion  109 . A comb of fingers  154  is seen extending downward from the inside ceiling of third housing portion  109 . A corresponding comb of fingers  155  is seen extending upward from the inside floor of second housing portion  107 . Each finger extending downward from the ceiling of third housing portion  109  makes contact with a corresponding finger extending upward from the floor of the second housing portion  107  so that the two fingers form an insulative rib that separates adjacent ones of the FPC portions  108 . There are grooves  156  in the ceiling surface and back inside surface of the third housing portion  109 . These grooves  156  together with fingers  154  hold the FPC portions  108  aligned in parallel with respect to one another. Similarly, there are grooves  157  in the inside back surface of second housing portion  107 . These grooves  157  together with fingers  155  and openings  113  hold the FPC portions  108  aligned in parallel with respect to one another. 
   When the first head housing portion  106 , second body housing portion  107 , third cap housing portion  109 , and FPC portions  108  are assembled together to form daughterboard connector  102 , extensions  158  on first head housing portion  106  slidably engage guide rails  159  on the inside of third cap housing portion  109 . There are similar extensions  160  that engage guide rails (not shown) on the inside of second insulative body housing portion  107 . The extensions and guide rails allow first head housing portion  106  to slide back and forth laterally in the direction of arrow  161 . The head portion  106  is therefore said to be laterally displaceable. 
     FIG. 18  is a cross-sectional perspective view taken along sectional line A—A in  FIG. 15 . The perspective view shows the FPC portions disposed in parallel with one another. 
     FIG. 19  is an expanded view of the portion within box  114  in  FIG. 18 . Exemplary FPC portion  115 , is shown with its vertical second edge  110  inserted into the slit-shaped opening within first housing portion  106 . A contact beam  116  is soldered to a signal conductor of FPC portion  115 . Contact beam  116  can flex in the direction of arrow  117  if another FPC were forced in the direction of arrow  118  and into connector  102 . 
     FIG. 20  is a larger perspective view of FPC portion  115 . Solder ball pads are disposed along horizontal first edge  111 . A solder ball pad is a site on a signal conductor of FPC  115  to which a solder ball can be attached. Contact beams (such as contact beam  116 ) are disposed along vertical second edge  110 . Tab  119  fits into a receiving slit in third housing portion  109 . 
     FIG. 21  is an enlarged exploded perspective view of connector  102 . There is a plurality of receiving slits in the face of first housing portion  106 . The receiving slits are oriented parallel to one another. 
   Box  120  is an expanded view of the detail of the portion of the face of connector  102  within box  121 . The contact beams of each FPC portion are seen on end disposed in a column along the edge of a receiving slit  122 . 
   Box  123  is an expanded view of the detail of the portion of the bottom of connector  102  within box  124 . The view of box  123  is a cross-sectional view taken along line B—B. A row of solder balls  125  is seen attached to solder ball pads along the bottom first edge of each FPC portion. The solder balls extend downward past the bottom surface of insulative housing portion  107 . 
   Connector  102  is manufactured by pushing the first edges of the FPC portions through slits or openings  113  in the bottom of housing portion  107  such that the solder ball pads on the first edges of the FPC portions are exposed in openings when housing portion  107  is viewed from below. Solder paste is applied to the pads. A ball of solder is then placed in each opening. The entire structure is then heated so that the solder balls are soldered to the solder pads while the FPC portions are disposed in their corresponding slits in housing portion  107 . Housing portion  106  is placed over the second edges of the FPC portions such that the extensions on housing portion  106  fit into the guide rails on housing portion  107 . Housing portion  109  is then slid down over the upward extending FPC portions so that the downward extending fingers on the inside of housing portion  159  slide down between adjacent FPC portions. The upward facing extensions  158  on housing portion  106  fit into a guide rail on the inside ceiling of housing portion  109 . A retaining latch on housing portion  109  clips down and over an edge on housing  107 , thereby fixing housing portion  109  in place to housing portion  107 . Housing portion  106  is prevented from falling off due to the extensions on housing portion  106  being retained by the guide rails of housing portions  107  and  109 . 
     FIG. 22  is a top-down perspective view of the inside of motherboard connector  101 . Multiple flexible printed circuit board (FPC) portions  125  are disposed parallel to one anther. Each FPC portion  125  is held in place by receiving grooves in the inside sidewall of insulative housing portion  126 . Box  127  is an expanded view of the portion of motherboard connector  101  within box  128 . Each FPC portion  125  of motherboard connector  101  includes ground conductors and signal conductors disposed on a flexible insulative substrate. Ground conductor  129  is one such ground conductor. Although each of conductors  132 ,  133  and  129  extends upward to locations proximate to second edge  130 , ground conductor  129  extends upward toward second edge  130  farther than do signal conductors  132  and  133 . 
     FIG. 23  is a perspective view of the bottom surface  134  (the surface that lies adjacent to the motherboard printed circuit board) of motherboard connector  101 . Box  135  is an expanded view of the portion of motherboard connector  101  within box  136 . Box  135  illustrates a cross-section of motherboard connector  101  taken along line C—C of  FIG. 23 . A row of solder balls  137  is seen attached to solder ball pads along the bottom first edge of each FPC portion. The solder balls extend downward past the bottom surface  134  of insulative housing portion  126 . 
   Connector  101  is manufactured by pushing the first edges of the FPC portions through slits  138  in the bottom of housing  126  such that the solder ball pads on the first edges of the FPC portions are exposed in openings when housing  126  is viewed from below. Solder paste is applied to the pads. A ball of solder is then placed in each opening. The entire structure is then heated so that the solder balls are soldered to the solder pads while the FPC portions are disposed in their corresponding slits in housing  126 . 
     FIG. 24  is an exploded perspective view of motherboard connector  101 . The upper second edges of the FPC portions extend upward through corresponding slits  138  in the bottom of insulative housing portion  126 . In this example, the FPC portions are made of the same FPC material as are the FPC portions of connector  102 . The dielectric thicknesses and dimensions and spacings of the conductors within the FPC portions in connector  101  are identical to the dielectric thicknesses and dimensions and spacings of the conductors with the FPC portions in connector  102  so that the characteristic impedance through the FPC portions of connector  101  will be the same as the characteristic impedance through the FPC portions of connector  102 . The characteristic impedance of each signal path through connector assembly  100  from the surface mount attachment solder balls on connector  102  to the surface mount attachment solder balls on connector  101  varies by less than plus or minus ten percent. 
     FIG. 25  is an expanded view of the portion of motherboard connector  101  within box  139  of  FIG. 24 . The upper second edges of the FPC portions are seen. There are multiple sets of conductors on each FPC portion. Each set includes one ground conductor and two signal conductors. A ground plane that is coupled to the ground conductor is disposed in the FPC portion in a plane behind the signal conductors. 
     FIG. 26  is a perspective view showing daughterboard connector  102  coupled to motherboard connector  101 . 
     FIG. 27  is a cross-sectional view taken along line D—D in  FIG. 26 . The portion within box  140  is shown expanded in box  141 . For each FPC portion in daughterboard connector  102  there is an associated FPC portion in motherboard connector  101 . FPC portion  142  is one such daughterboard connector FPC portion and FPC  143  is one such motherboard connector FPC portion. To connect the two connectors  101  and  102  together, the upward facing second edge of FPC portion  143  is forced into receiving slit  144  in the face of daughterboard connector  102 . This is usually accomplished by pushing second connector  102  into first connector  102 . Contact beam  145  in second connector  102  flexes as second edge of FPC portion  143  moves into the receiving slit  144  and past the contact beam. Contact beam  145  pushes back against FPC portion  143  so as to provide electrical contact between a conductor in FPC portion  143  and a conductor within FPC portion  142 . 
     FIG. 28  is a view that illustrates a daughterboard printed circuit board  146  upon which two daughterboard connectors  102  and  147  are attached. The daughterboard connectors  102  and  147  are surface mounted by soldering the solder balls of the daughterboard connectors to corresponding solder pads (now shown) on the printed circuit board  146 . 
   A motherboard printed circuit board  148  is also illustrated. Motherboard  148  has two motherboard connectors  101  and  149  surface mounted to it. Motherboard connectors  101  and  149  are likewise surface mounted by soldering the solder balls of the motherboard connectors  101  and  149  to corresponding solder pads (not shown) on printed circuit board  148 . The surface mount attachment structure of any one of  FIGS. 11–13  can be employed. Due to misalignments (for example, due to imperfections in the printed circuit board manufacturing process) between dimension A between connectors  102  and  147  and dimension B between connectors  101  and  149 , there may be a stress imposed on the connectors when the printed circuit boards  146  and  148  are brought together (the direction of arrow  150 ) when corresponding daughterboard and motherboard connectors are fit together. 
     FIG. 29  is an end view of the structure of  FIG. 28 . In accordance with a novel aspect, FPC portions  108  flex within the daughterboard connector  102  of the connector assembly. 
     FIG. 30  is a sectional view of daughterboard connector  102  wherein housing portion  106  is deflected a distance to the left in the direction of arrow  151  with respect to housing portion  107 . The FPC portions of daughterboard connector  102  are flexed in flexing region  152  of the connector. Adjacent FPC portions are separated from one another at the flexing boundary plane  162  of flexing region  152  by fingers  155 . 
     FIG. 31  is a sectional view of daughterboard connector  102  wherein housing portion  106  is deflected a distance to the right in the direction of arrow  153  with respect to housing portion  107 . The FPC portions of daughterboard connector  102  are flexed in flexing region  152  of the connector. Due to the ability of the connector assembly to flex and accommodate lateral displacement of the daughterboard connector with respect to the motherboard connector, mechanical stress on the surface mount attachment of the connectors to the printed circuit boards is reduced. Due to this reduced stress, surface mount attachment techniques having desirable electrical properties can be employed while at the same time providing adequate reliability of the connector the printed circuit board joints. 
     FIG. 32  is a cross-sectional end view of an FPC portion  200  in either the motherboard connector or the daughterboard connector. A ground plane  201  is coupled by conductive vias, plugs or through holes  202  and  203  to the surface of FPC portion  200  upon which a pair of differential signal conductors  204  and  205  is disposed. Material  206  is flexible insulative polyimide material or another flexible insulative material used to make flexible printed circuit boards. The signal conductors  204  and  205  are, in the cross-section illustrated, covered by a solder mask layer. Contact beams (not shown) for ground potential contact the ground pad portions atop or near vias  202  and  203  in situations where the FPC portion is part of a motherboard connector. Contact beams (not shown) for ground potential are fixed to the contact pads atop or near vias  202  and  203  in situations where the FPC portion is part of a daughterboard connector. Note that the ground plane and conductive vias surround the signal conductors on three sides in the view of  FIG. 32 . 
     FIG. 33  is a cross-sectional diagram showing a contact beam  300  that couples ground potential from a ground plane conductor  301  in FPC portion  302  of the motherboard connector  101  to a ground plane conductor  303  in FPC portion  304  of the daughterboard connector  102 . A plurality of conductive plated through holes  309 – 310  are provided to connect the ground plane conductor  303  to a strip of metal on the opposite side of FPC portion  304 . Contact beam  300  is connected to this strip of metal. More than one 0.2 millimeter diameter plated through hole is provided to reduce ground current bottlenecks in the ground current path between ground plane conductor  303  and contact beam  300 . Similarly, two 0.2 millimeter diameter plated through holes  311  and  312  are provided to reduce ground current bottlenecks in the ground current path between ground plane conductor  301  and contact beam  300 . 
     FIG. 34  is a cross-sectional diagram showing a contact beam  305  that couples a signal from a signal conductor  306  in FPC portion  302  of the motherboard connector  101  to a signal conductor  307  in FPC portion  304  of the daughterboard connector  102 . Note that the via and conductor structure of  FIG. 33  extends a grounded conductor to the rightmost end of FPC portion  302  in  FIG. 33  and also extends a grounded conductor to the leftmost end of FPC portion  304  in  FIG. 33 . The grounded conductor structure in this area helps shield the area of contact beam  305  of  FIG. 34 . The grounded conductor structure shown in cross-section in  FIG. 33  exists on either side (exists once in a plane behind the plane shown in the illustration of  FIG. 34 , and exists again in a plane in front of the plane shown in the illustration of  FIG. 33 ) of the signal conductor structure of  FIG. 34 . The free end of contact beam  305  extends in a direction away from the second edge of FPC  304 . Signal conductor  306  in FPC  302  only extends 1.0 millimeters beyond the contact point  308  where contact beam  305  makes contact with signal conductor  306 . Contact beam extends to a location proximate to the second edge of FPC  302 . The distance (2.0 millimeters) between the end of signal conductor  306  and the second edge should be less than the contact beam length (3.0 millimeters). 
     FIGS. 35–37  illustrate other forms of the connector assembly  100 . The connector assembly  100  is shown in  FIG. 35  in a right angle configuration connecting motherboard  148  to daughterboard  146 . The connector assembly  100  is shown in a parallel (sometimes called stacking) configuration in  FIG. 36 . In  FIG. 36 , the connector assembly connects two printed circuit boards  146  and  148  together so that the two printed circuit boards are oriented parallel to one another.  FIG. 37  illustrates connector assembly  100  in a horizontal (sometimes called side-by-side) configuration connecting motherboard  148  to daughterboard  146  such that the two printed circuit boards are disposed side by side. 
   Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Rather than attaching an FPC portion to a printed circuit board using solder balls, metal surface mount contacts can be attached to the FPC portions. To attach a connector using metal surface mount contacts to a printed circuit board, solder paste is applied to solder pads on the printed circuit board and the connector is placed on the printed circuit board such that the metal surface mount contact is in the solder paste. The connector and printed circuit board is then heated so that the solder paste melts and solders the metal surface mount contact of the connector to the solder pad of the printed circuit board. The tensile modulus of the FPC portions of the motherboard connector may be significantly greater (for example, eight GPa or more) than the tensile modulus of the FPC portions of the daughterboard connector (for example, 5.0 GPa or less). 
   In some embodiments, printed circuit boards are used in place of the FPC portions of the motherboard connector illustrated in  FIG. 24 . Where flexibility is not required in the connector assembly, printed circuit boards can be used in place of the FPC portions in both the motherboard and daughterboard connectors. Rather than using a flexible printed circuit in the connector with the laterally displaceable head portion, conductors that are stamped out of a sheet of metal can be used. These conductors can be supported by the insulative housing material of one of the connectors in places and not in other places so that they can flex within the connector, thereby preventing the buildup of stress between misaligned connectors of the assembly. Alternatively, the stamped conductors can be attached to or laminated to an insulative substrate layer. The resulting multi-layer structure is then used in place of the FPC portions in the embodiments described above. Rather than using a conductive contact beam to make electric contact between a signal conductor on one FPC portion and a signal conductor of another FPC portion, an insulative spring member can push on the backside of one FPC portion such that a conductor on the other side is forced against a conductor of another FPC portion. Conductors on the printed circuits of the motherboard and daughterboard connectors can be used to communicate single-ended signals, differential signals, and/or a combination of the two. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.