Abstract:
A connector, for example an LGA connector to be used in coupling a circuit board to a chip or a multi-chip module, includes an insulating carrier with a through-holes corresponding to the connections to be made above and below the carrier, and contacts in the through-holes. The contacts include wadded-wire wads that protrude above and below the carrier to make contact with the chip or circuit board, but each also includes a solid conductive member that extends over a major portion of the length of the through-hole and is in electro-mechanical connection with the wadded-wire wad or wads in the hole at numerous points along the length of the through-hole. The solid conductive member includes barbs or projections that penetrate the wadded wire. The solid conductive members thus provide a low-resistance parallel path and can also hold the wadded-wire wad or wads in place. A method of making and system are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to wadded-wire contacts, especially such contacts for use in an LGA (Land Grid Array) connector used to couple a circuit board to an electronic chip or multi-chip module. 
   2. Description of the Related Technology 
   An LGA connector is used for making contact between a system (circuit) board, having an array of contacts, and a substrate, having a corresponding array of LGA-pad contacts, where each of the LGA contacts is aligned with a respective one of the system board contacts. Various types of connectors are known and used for LGA connectors, including wadded-wire contacts. These wire buttons or wads are placed into through-holes in an insulating carrier (plate or sheet) to form the LGA connector. The wads protrude from each end of the hole in the insulating carrier plate to touch and electrically connect with the contacts above and below the insulating carrier, or with mating electrical circuits. 
   Recently, in order to increase the number of contacts on the chip or multi-chip module (MCM) with a given contact spacing, the area of the side of the MCM facing the LGA connector insulating carrier has been increased by use of the so-called “shallow grind.” The shallow grind removes less material around the edge of the MCM and therefore increases the thickness of the ground edge which is clamped in a C-ring to mount the MCM. The shallow grind is essential if the known and reliable hermetic sealing system, described below, is still to be used. 
     FIG. 1  shows a shallow-ground edge. ( FIG. 1  is not a “prior art” figure because it depicts the invention, but it also depicts an exemplary environment of the invention, including the shallow-ground edge.)  FIG. 1  shows an MCM substrate  10  that is held, by clamping its upper and lower ground edges (described below) between a base ring  22  and an upper plate  24  (the clamp is not illustrated in  FIG. 1 ). The uppermost surface  11  of the substrate  10  makes contact with solder balls  31  on the bottom of a chip  30 , which forms a thermal interface with the upper plate  24 ; meanwhile, on the lower surface of the substrate  10  are a plurality of LGA pads (electrical contact areas)  17 , which make contact with a system board  50  underneath through an LGA connector  100  and its contacts  120 . As is further described below, the LGA connector  100  comprises a sheet or plate of insulating material  110  (a carrier), usually of a plastic material, with individual metallic contacts  120  making electrical contact from one side of the connector  100  to the other side, at points corresponding to the locations of the pads  17 . In this way the pads  17  on the bottom of the substrate  10  are electrically coupled to the system board  50 . 
   To the left of the solder balls  31  in  FIG. 1  is a decline  12  which leads to a lower, ground surface  13 . Directly below the decline  12  in  FIG. 1 , on the underside of the substrate  10 , is an incline  15  which connects the lower surface  16  of the substrate  10  to a ground surface  14 . The small height difference between the lower surface  16  and the ground surface  14  is what defines the “shallow grind.” 
     FIG. 1  shows that the lower surface of the base ring  22  overlaps the upper surface of the system board  50 , so that the height difference of the surfaces  14  and  16  must approximate the thickness of the inwardly-protruding portion of the base ring  22 . In the earlier deep grind version (not shown), the incline  15  was longer and farther from the lateral edge of the substrate  10 , making for a greater height difference between the surfaces  14  and  16  so that the connector  100  could be relatively thin, but also making for a smaller area of the lower surface  16  and hence a smaller number of LGA pads  17 . The use of the illustrated “shallow” grind, as opposed to the “deep” grind, requires that the thickness of the LGA connector  100  be increased from about 0.8 mm to about 2.5 mm, which of course increases the thickness of the carrier  110  and the length of the contacts  120  that pass through the carrier  110 . 
   Between the lower ground surface  13  and the underside of the upper plate  24  is a C-ring  40 , which acts as an hermetic seal. Also shown in  FIG. 1  are a cushion  41 , which acts to distribute the C-ring compression force, and an alignment pin  43  that passes through the carrier plate  110  of the connector  100  into the base ring  22 . These represent the conventional known and reliable hermetic sealing system mentioned above. 
   Although not shown in detail in  FIG. 1 , the contacts  120  of the connector  100  include wadded-wire portions. Wadded-wire contacts are intrinsically reliable because of the numerous points at which they touch their intended contact surfaces (thought to number three to seven coupling points for each wad), and also themselves (at various points along the length of the wire), which provides multiple current paths, redundancy, and reliability. Statistically, they out-perform other types as to failure rate and signal integrity. However, wadded-wire contacts do not operate as well when they are made long, in part because the axial spring constant drops as the length increases. (This happens by an elementary property of springs; for a spring of constant cross-section, the longer it is the lower the spring constant of the whole spring.) Also, the resistance increases with length, and the resistivity of spring metal is typically significantly higher than the resistance of pure copper. 
   When wadded-wire contacts need to be long, they are conventionally combined with solid-plunger contacts that take up some of the length so that the wadded-wire wads can stay short. Then, electrical contact is made through the wadded-wire wad or wads and the solid plunger or plungers, which are deployed in series within the through-holes. There are various combinations of usually alternating wads and plungers. An example of this prior-art approach is shown in prior-art  FIG. 5 , where a solid plunger  270  and a wadded-wire wad  250  are both inserted into a through-hole  212  in a plastic carrier plate  210  of a connector  200 . The solid plunger  270  provides length and the wad  250  provides resiliency. 
   This combined plunger-wad contact has the following drawbacks: 
   First, there is a decrease in reliability (signal integrity or SI), at least in part because a solid plunger introduces another pair of separable interfaces, causing it to be less reliable, and a change in electrical impedance. When they are arranged in series mechanically and electrically, the reliability of the combination cannot be any higher than that of a single contact. The overall contact failure rate can be approximated as the individual contact failure rate multiplied by the number of interfaces. 
   Second, the DC resistance increases and causes a voltage drop through the MCM-card assembly. Although the solid plunger  270  has low internal resistance, this resistance is in series with that of the wadded wire and the two resistances are additive. 
   In addition, the center-to-center spacing may be somewhat higher in the case of plunger-wad combinations like that illustrated in  FIG. 5 . A wad alone typically has a diameter of 0.020 inches and center spacing of 0.040 inches, while a single-wad, single-plunger combination might have the same diameter but a center spacing of 0.050 inches. The same might be true of a plunger-wad-plunger arrangement or a wad-plunger-wad arrangement. 
   Thus, there has been a need for a longer contact that retains the advantages of the shorter wadded-wire contact. 
   SUMMARY OF THE INVENTION 
   It is, therefore a principal object of this invention to provide an enhanced wadded-wire contact having a greater length but having the characteristics of a shorter wadded-wire contact, and a contact assembly, method, and system incorporating this contact. 
   It is another object of the invention to provide an enhanced contact assembly that solves the above-mentioned problems, and a contact assembly, method, and system thereof. 
   These and other objects of the present invention are accomplished by the contact, contact assembly, method, and system disclosed herein. 
   In order to meet the object set out above, and other objects apparent from the discussion below, an aspect of the invention is a parallel-path solid conductor, with which a wadded wire is in contact at places along the mutual length of the solid conductor and the wadded wire. The solid conductor can take various forms, including a conductive tube in which a wadded wire wad or wads are held, and/or a central spear. 
   This combination provides very high reliability, due to the numerous current paths through the wadded wire and the solid conductor, and also a very low resistance even when the current passes through a great length of wadded wire. The reason for the low resistance is that the solid conductor has a resistance several orders of magnitude lower than an equivalent length of wadded wire. A one-millimeter length of wadded wire has a resistance around 40 milli-ohms, while the resistance of the solid conductor is approximately 40 micro-ohms. According to computer simulations, the resistance of the combination is about that of a short wadded-wire wad, and the increased resistance of a long wadded-wire wad is avoided. 
   Preferably, the parallel solid conductor includes some means for retaining the wadded wire in place, and/or for increasing the resistance of some central portion of the wadded wire to axial motion, whereby the spring constant at the protruding wad ends may be increased. For example, the spear may include barbs (projections) that penetrate into the wadded wire bundle and hold it axially. In the tubular embodiment, the interior or the tube may include internally-protruding members or a central cross member. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       FIG. 1  is an elevational view of the invention in its environment. 
       FIG. 2  is a schematic elevational, partially cross-sectional view of the invention. 
       FIG. 3  is an elevational, partially cross-sectional view of the invention according to  FIG. 2  in a first variation on the shape of the solid conductor. 
       FIG. 4  is an elevational, partially cross-sectional view of the invention according to  FIG. 2  in a second variation on the shape of the solid conductor. 
       FIG. 5  is an elevational, partially cross-sectional view of the prior art. 
       FIG. 6  is a plan view according to  FIG. 3 . 
       FIG. 7  is a perspective view. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As discussed above in the Background section,  FIG. 1  shows the invention in an exemplary environment of use as an LGA connector  100 . The contact portion  120  of the connector  100  is shown in more detail in  FIG. 2 , which corresponds to a partial view according to  FIG. 1 . 
   In  FIG. 2 , the main body of the connector  100  is a carrier plate or sheet  110  of insulating material, which is shown in a cross section taken perpendicular to the surfaces of the carrier  110  and also parallel to the length of a through-hole  112 , which contains the illustrated contact  120 . The carrier  110  plate is preferably plastic, for example G10, RYTON, ULTEM, or NORYL. The through-holes might, for example, have a length of about two and one-half millimeters. Preferably, they extend in a direction generally perpendicular to a surface of the insulating carrier. Typically, they might be deployed in a rectangular array pattern, but can be deployed in any arrangement. 
   The illustrated contact  120 , which may be identical to all of the other contacts in the carrier  110 , has two main elements: one or more wadded-wire bundles (wads)  150 ; and a solid conductive member  130  which extends in general along the length of the through-hole  112  and is electrically and mechanically connected to the wadded-wire wad  130  by numerous contact points. 
   Preferably, the solid conductive member extends over a major portion of the length of the respective through-hole, as is illustrated in  FIG. 2 . The solid conductive member  130  is preferably longitudinally fixed within the through-hole  112 , so that it cannot move longitudinally (parallel to the length of the through-hole  112 ). Preferably in some cases, it may also be fixed so that it cannot rotate within the through-hole  112 . The illustrated member  130  preferably may be in contact with the side of the through-hole  112 , held against the hole interior by friction (e.g., press fit), glue, welding, fasteners (including fasteners integral with the member  130 , such as the barbs described below), the wadded wire, or any other means (no particular means is illustrated in  FIG. 2 ). Alternatively, the member  130  may also be out of contact with the sides through-hole  112 , so that it is located with respect to the through-hole  112  by other contact with the carrier  110 , or, by its contact with the wadded wire  150  (which can be fastened to the through-hole  112 ). 
   The solid conductive member  130  preferably includes one or more “barbs” (projections, fasteners, or the like, which need not be sharp or pointed)  135  that may penetrate into the mass of wadded wire of the wad  150  for improved mechanical and/or electrical contact between these parts. The barbs  135  may be integral with the body or with body portions of the solid conductive member  130 , but may also be separable or permanently attached. If the solid conductive member  130  is formed of sheet metal, the barbs  135  may be formed by bending a tab portion of the solid conductive member  130  over at an angle from the rest of the sheet of metal. The barbs  130  may be pointed, rounded, or of any shape or configuration. 
   Preferably, the wadded-wire wad  150  protrudes from the through-hole above the surface of the insulating carrier  110 , on each side, by a distance sufficient to make electrical contact with parts such as the LGA pads  17  of  FIG. 1 , which typically are larger than the wads they contact. (Depending on the environment of the invention, protruding wads might not be necessary.) 
     FIG. 3  shows the same embodiment as  FIG. 2  but the solid conductive member  130  is in contact with the sides of the through-hole  112  and is preferably held there by friction, although adhesive and/or any other locating means can be employed. The solid conductive member  130  of  FIG. 3  may be embodied as a hollow cylinder, tube, or sleeve of conductive material, optionally with a compressive resilience feature such as a longitudinal gap or fold. A smooth outer cylindrical surface on a conductive member minimizes coupling and electrical noise, and may be preferable. 
   The solid conductive member  130  can be fixed into the carrier  110  by being molded in place, in which case the outside of the solid conductive member  130  can be formed to provide a firm mechanical connection between them. The solid conductive member  130  can also be fixed into the carrier  110  by including a flange or other protrusion that rests on at least one of the upper and lower surface of the insulating carrier  110  (not shown). 
   The barbs  135  may include a central spacer  133  with distinct wads on either side and/or a retention rim  137  that helps to keep the main body of the wadded-wire wad  150  in place, as well as simple barbs  135  protruding from points along the inside of the sleeve. 
   These simple barbs might be deployed around the inside of the sleeve along a helical path and may also be set to protrude from the inside generally at equal angles around the axis so that the barbs  135  resemble a spiral staircase. For example, there might be four barbs set at 90-degree intervals around the axis of the through-hole  112  and deployed at increasing depths. 
     FIG. 6  illustrates such an arrangement, showing two solid conductive members set into the surface of the insulating carrier  110 , each including four barbs  135 . 
   The contact  120  of  FIG. 3  can be fabricated with two wads, one above and one below the central spacer  133 , where conductive member  130  provides an electrical connection between wadded contacts and an electrical paths for the wads to contacts themselves. 
   The solid conductive member  130  can be fabricated from flat sheet metal, with the retention rim  137  and barbs  135  formed by stamping, for example, and then the flat piece rolled into a cylindrical shape. 
   The sleeve may have an inside diameter of 0.50 mm (20 mils) and an outside diameter of 0.56 mm (25 mils). The diameter of the through-holes in the invention is preferably 0.56 mm as compared to 0.50 mm in the prior art exemplified by  FIG. 5 . With the contact center-to-center spacing retained as in the prior art, this greater diameter increases the capacitance between neighboring contacts and is calculated to offset the greater inductance due to the increased length of the contacts. 
   The length of the solid conductive member  130  is about the same as the thickness of the insulating carrier  110 , which will typically be 2.5 mm with shallow grinding. Preferably, it is slightly less than the thickness of the carrier  110  in order to form a stop when the solid conductive member  130  is press-fitted or insert molded, as the plastic material of the insulating sheet  110  expands slightly over the end of the solid conductive member  130 . 
     FIG. 4  depicts a spear-like solid conductive member  130  that can be fabricated from stamped sheet metal. Unlike the solid conductive member  130  of  FIG. 3 , it is not rolled into a cylinder. Instead, it is left basically flat and is pressed or molded into a portion  113  of the insulating carrier  110  that protrudes into the through-hole  112 . The portion  113  is preferably located in the middle of the through-hole and bridges across; it may even close off the through-hole  112  so that it causes the through-hole to become two blind holes. 
   The spear-like solid conductive member  130  preferably has multiple barbs  135  to engage the wadded wire  150 . These barbs may, as illustrated, include pointed ends and have an inward slant. Other barb shapes, sizes, and arrangements to engage the wadded wire are also within the scope of the invention. Besides the generally flat stamped piece illustrated in  FIG. 4 , the invention includes such a piece with an axial twist, with sinuous or other types of bends, and other variations. 
     FIG. 7  shows a variation on the contact of  FIG. 4 . In  FIG. 7 , the spear-like conductive member  130  has a roll form surface, somewhat like continuously threaded rod on a miniature scale. The thread-like form constitutes a plurality of barbs  135  to engage the wadded wire  150 . Like the version shown in  FIG. 4 , the spear-like conductive member  130  of  FIG. 7  may be forced into a central portion  113  like that of  FIG. 4 , or it may be molded in place. Besides the thread-like barb configuration, other textured-surface configuration can be used in the invention: for example, flanges, points, a roughened surface, hooks, and so on. 
   As was mentioned above, the combination of the insulating carrier  110  and the solid conductive member  130  can be fabricated, for example, by molding the carrier around the conductive members or by inserting the conductive members  130  into holes pre-formed in the carrier  110 . Then the wad or wads of wadded wire  150  can be stitched onto or into the conductive members  130 . Other fabrication orders and methods can also be used. In the case of a flat sheet metal part that is then rolled into a cylinder or partial cylinder, the wadded wire can be inserted before rolling so that the barbs  135  penetrate into the wad as the sheet is rolled and it is held securely in the finished contact  120 . 
   The solid conductive member  130  is preferably high-conductivity copper or copper alloy, plated with gold at least on the inside surface. The wadded wire is preferably Mo, BeCu, or other conductive spring material. 
   In summary, the invention provides a long wadded-wire contact that keeps the advantages of shorter wadded-wire contacts, such as reliability, low electrical resistance, and relatively high spring constant. Because of this (as explained above), it permits an increase in the module LGA contact surface area and the number of contacts without degraded contact performance, and can actually lower contact bulk resistance through member  130 . These benefits are provided by the electrical connection between the wadded wire and the solid conductor, and also by their mechanical connection.