Abstract:
The application discloses an improved interposer assembly with a molded plastic plate and stamp-formed metal contacts inserted in through passages in the plate. The contacts have redundant separate metal circuit paths extending between opposed contact points to reduce inductance and contact resistance.

Description:
BACKGROUND OF THE DISCLOSURE 
       [0001]    Interposer assemblies with molded plastic plates and inserted metal contacts in the plates are used for forming electrical connections between contact pads on opposed substrates. The contacts are spaced very close together in land grid array rows and columns to establish a large number of differential pair signal and ground connections extending through the plate. 
         [0002]    Increased circuit speed requires transmission of differential signals through interposer plates at signal frequencies of 10 or more gigahertz. Transmission of high-frequency signals through conventional interposer plates with short, closely spaced single circuit path contacts and very fast rise times for the signals increases signal impedance and degrades signal strength. High-frequency signaling can cause cross-talk between adjacent pairs of signal circuit path contacts.  FIGS. 10 and 11  illustrate a conventional interposer assembly with contacts forming single circuit paths between pads on opposed substrates. 
         [0003]    The prior art interposer assembly shown in  FIGS. 10 and 11  uses contacts  200  with single circuit paths  202  extending between contact points  204 . The portions or stubs  206  of the contacts  200  extending from points  204  to free ends  208  do not carry current. These stubs extend outside of the electrical current path or loop for the contact and act as antennas. The antennas radiate energy and increase signal loss, particularly at high signal frequencies. 
       SUMMARY OF THE DISCLOSURE 
       [0004]    An interposer assembly is disclosed with improved contacts for forming redundant electrical connections between contact pads on opposed substrates. Each contact in the assembly has two separate circuit paths between contact points which engage opposed substrate pads. The two circuit paths in each contact reduce inductance and contact resistance. The redundant circuit path eliminates the electrical contact stub that significantly contributes to signal degradation at high speeds. 
         [0005]    The improved contacts with redundant circuit paths are inserted into a conventional dielectric plate without the need to tool and manufacture a specialized plate. Manufacturing cost of the improved assembly is reduced by use of the conventional plate. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a top view of interposer assembly; 
           [0007]      FIG. 2  is a sectional view illustrating a contact in position to be inserted in a through passage in an interposer plate; 
           [0008]      FIG. 3  is a sectional view taken along line  3 - 3  of  FIG. 1  showing the contact in the plate in a gravity down position; 
           [0009]      FIG. 4  is a sectional view similar to  FIG. 3  showing the assembly positioned on a lower substrate with the contact raised up in a through passage; 
           [0010]      FIG. 5  is a view similar to  FIG. 4  showing an upper substrate on the top of the plate supported by contacts with the contact partially compressed; 
           [0011]      FIG. 6  is a sectional view similar to  FIG. 5  showing the upper and lower substrates sandwiched onto the plate and the contact fully compressed to form two circuit paths; 
           [0012]      FIGS. 7 ,  8  and  9  are top and opposed side views of the contact; and 
           [0013]      FIGS. 10 and 11  are sectional views of a prior art interposer assembly. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Interposer assembly  10  has a flat dielectric plate  12  with parallel top and bottom surfaces  14  and  16 , a uniform thickness  18  and closely spaced contact passages  20  arranged in intersecting land grid array rows and columns. The plate may have a thickness of 1.22 mm. Formed metal contacts  22  are positioned in contact passages  20 . 
         [0015]    In plate  12 , each contact passage  20  has a wide end wall  24  and an opposed narrow end wall  26 . End wall  24  is flat and extends perpendicularly between the parallel top and bottom surfaces  14  and  16 . Narrow end wall  26  includes a contact retention protrusion  28  which extends into passage  20 . The protrusion  28  has two flat and inwardly angled cam surfaces  30  and  31  and a tip  32  at the intersection of surfaces  30  and  31  located equidistant between top and bottom surfaces  14  and  16 . 
         [0016]    The flat cam surfaces  30  and  31  on wall  26  extend from tip toward the top and bottom surfaces  14  and  16  at a shallow outward angle. As shown in  FIG. 2 , wall  24  is perpendicular to surfaces  14  and  16 , and the width of passage  20  increases to either side of tip  32 . 
         [0017]    Passages  20  have opposed sidewalls  34  extending between end walls  24  and  26 . Each sidewall  34  includes a flat portion  36  extending perpendicularly between the top and bottom surfaces  14  and  16  and perpendicularly from one edge of wide end wall  24 . Flat, inwardly tapered sidewall portions  38  extend from the edge of narrow end wall  26  away from end wall  26  to portion  36 . Portions  36  and  38  are perpendicular to top and bottom surfaces  14  and  16 . As illustrated in  FIG. 1 , each passage  20  has a maximum width between wall portions  36 . The width of the passage is reduced along portions  38  to a minimum width at narrow end wall  26 . 
         [0018]    Plate  12  is identical to the plate used in the prior art interposer assembly shown in  FIGS. 10 and 11 . 
         [0019]    Metal contacts  22  are stamp formed from uniform thickness gold coated beryllium copper strip stock. The stock may have a thickness of 0.043 mm. Each contact has a rigid vertical spine  40  with rounded upper contact support  42  and rounded lower contact support  44  at the ends of the spine. 
         [0020]    A first flat spring arm  46  angles upwardly and inwardly from support  42  to contact point  48  at the top of contact  22 . A second flat spring arm  50  angles downwardly and outwardly from point  48  to support bend  52  which faces away from spine  40 . End strip  54  extends downwardly and inwardly from bend  52  to rounded end  56 . The contact is vertically symmetrical to either side of the center of spine  40  so that the lower half of the contact has flat spring arm  58 , lower contact point  60 , flat spring arm  62 , support bend  64 , end strip  66  and rounded end  68 . A pair of contact tips  70  are provided on the opposed edges of contact points  48  and  60 . Tips  70  are disclosed in U.S. Pat. No. 6,905,343. 
         [0021]    Flat spring arm  58  extends downwardly and inwardly from lower contact support  44  to lower contact point  60 , opposite from point  48 . Flat spring arm  62  extends upwardly and outwardly from contact point  60  to rounded support bend  64 . Contact end strip  66  extends up and in from bend  64  to rounded end  68 . The widths of arms  46  and  58  decrease away from spine  40  to distribute stresses when the arms are stressed. 
         [0022]    Contact  22  is formed from a long and relatively narrow length of uniform thickness metal strip stock bent in the form of an elongate circumferential band  72  with a gap  74  in the band  72  between contact rounded ends  56  and  68 . The band  72  has a continuous circumferential inner surface  76  and a continuous circumferential outer surface  78 . Surfaces  76  and  78  are perpendicular to a transverse plane  80  extending through the center of the band, illustrated in  FIG. 4 . As shown in  FIG. 2 , the minimum width  82  of passage  20  at tip  32  is less than the maximum width  84  of contact  22 . 
         [0023]    Contacts  22  are inserted into passages  20  by positioning the contacts above the passages, as shown in  FIG. 2 , and then lowering the contacts down into the passages to the positions shown in  FIG. 3 . The contacts are positioned with spines  40  above wide end walls  24  and arms  50 ,  54 ,  62  and  66  above narrow end walls  26 . During insertion of the contacts, the spine supports  42  and  44  move down along walls  24 , and arms  62  are moved down to engage the upper cam side  30  of narrow end wall  26 . The arms  58  and  62  are flexed by cam side  30  and are moved over tip  32  and return to the uncompressed position with the contact inserted in the passage, as shown in  FIG. 3 . The contact has a loose fit in the passage  20  with limited free vertical movement in the passage. Supports  42  and  44  rest on the walls  24 . As shown in  FIG. 3 , the contact  22  is in a gravity down position with support bend  52  on upper cam side  30  and support bend  64  spaced inwardly from lower cam side  31 . Upper contact  48  is adjacent plate top surface  14 . Lower contact  60  is spaced a greater distance below lower plate surface  16 . 
         [0024]    Assembly  10  with contacts inserted and in the gravity down position as shown in  FIG. 3  is placed on lower substrate  86  as illustrated in  FIG. 4 . The substrate has a contact pad (not illustrated) located below each contact for engagement with lower contacts  60 , as illustrated. When the assembly is placed on substrate  86 , the plate  12  is lowered until the contact points  60  engage the pads on the substrate and cam surfaces  31  engage support bends  64 , as shown in  FIG. 4 . The contacts  22  are raised up in passages  20  to an elevated position. Support bend  52  is above cam side  30 . The plate  12  is held in position of  FIG. 4  by gravity and is supported by engagement between contact support bends  64  and lower cam surfaces  31 . Plate  12  is a distance  88  above the substrate  86  as illustrated in  FIG. 4 . The contact  22  is raised up in passage  20  with contact point  48  a distance  90  above the top of the plate which is greater than the distance  88  contact  60  is below the bottom of the plate. 
         [0025]    With the contacts in the position of  FIG. 4 , an upper substrate  92  is placed on the top of plate  12 . The upper substrate has contact pads (not illustrated) which engage the upper contacts  48 . The upper substrate  92  is lowered toward the lower substrate  86  to compress the raised contacts  22  into passages  20 . The contacts are in the position shown in  FIG. 4  when first engaged by upper substrate  92 . Supports  42  and  44  slide down along walls  24 . Arms  46  are bent down into passages  20  to move bends  52  into engagement with upper cam sides  30  and, with further lowering of substrate  92 , down along cam sides  30 . At this time, arms  58  are bent upwardly, and bends  64  are correspondingly moved up along lower cam sides  31 . The bending of arms  46  and  58  moves contact points  48  and  60  along the contact pads on the upper and lower substrates  92  and  86  to form wiped electrical connections between the pairs of contact points  48  and  60  on contact  22  and the pads. 
         [0026]    As contacts  22  are compressed, the support bends  52  and  64  are moved inward along cam sides  30  and  31  to bend the upper and lower portions of contact  52  toward wide end wall  24  until rounded contact ends  56  and  68  engage each other and, with further lowering of the upper substrate  90  towards the lower substrate  86  and collapse of the contact, the contact is bent toward wall  24  and out of engagement with wall  26 . See  FIG. 5 . 
         [0027]    Further downward movement of substrate  90  collapses the substrates against the top and bottom surfaces  14  and  16  of plate  12  as shown in  FIG. 6 . During this movement, the contact  22  is further compressed and the point of physical engagement between ends  56  and  68  is moved inwardly along the adjacent surfaces of the ends. Compare  FIGS. 5 and 6 . The electrical connection between the redundant contact ends  56  and  68  establishes a second or redundant electrical circuit path between contact points  48  and  60 . 
         [0028]    A first continuous metal circuit path between the contact points extends from upper contact point  48  through arm  46 , spine  40  and arm  58  to lower contact point  60 . A second or redundant continuous metal circuit path extends from contact point  48  through arm  50  and strip  54 , ends  56  and  68  and arms  66  and  62  to contact point  60 . 
         [0029]    During compression of the contacts in passages  20 , the bends  52  and  64  rotate out of engagement with cam sides  30  and  31 . The contacts are free to move vertically in the passage with the spine supports  42  and  44  sliding along wall  24 . This freedom of vertical movement desirably equalizes the contact pressures at the top and bottom of the contact to ensure that a reliable electrical connection is formed between each contact point  48 ,  60  and the pad on the adjacent substrate. 
         [0030]    The redundant circuit paths between the two contact points  48  and  60  reduce electrical resistance between the contacts and reduce high speed inductance between contacts in plate  12 . 
         [0031]    During collapse of contacts  22  and formation of the second, redundant electrical connections between points  48  and  60 , variables inherent in the manufacture of interposer assemblies, their components and the location and movements of the components affect the connections between the contact ends  56  and  68 . Mating with contact pads on the opposed substrates is never perfectly symmetrical. Parts have dimensional tolerances which affect mating. The result of these variables is that during collapse of the contacts and formation of the connections between ends  56  and  68 , the support bends  52  and  64  engage the cam sides  30  and  31  at different times so that the rounded ends  56  and  68  move along each other to form wiped pressure connections between the ends. The wiped connections at ends  56  and  68  reduce resistance in the second or redundant contact path formed between points  48  and  60  shown in  FIG. 6 . Depending upon the order in which the bends  52  and  64  engage sides  30  and  31 , the wiped contact path may be formed by rounded end  56  sliding inwardly along end  68  or end  68  sliding inwardly along rounded end  56 . In the disclosed assembly  10 , the wipe distance between ends  56  and  68  may be 0.15 mm. 
         [0032]    As illustrated in  FIG. 6 , compressed contact  22  includes a first circuit path  94  extending across the thickness of plate  12  from contact point  48  to contact point  60 . The compressed contact also includes a second circuit path  96  extending across the height of plate  12  between points  48  and  60 . The two paths  94  and  96  substantially extend through the full length of the metal contact  22  to eliminate any substantial antenna or stub portion of the contact outside of the electrical loop for the contact. Elimination of the stubs prevents radiation of current passing through the contact and, as a result reduces signal loss at the contact.