Resilient electrically conductive terminal assemblies
BACKGROUND OF THE INVENTION 
     1. Field Of the Invention 
     The subject invention relates to resilient electrically conductive terminal assemblies for use in high density circuit applications, such as connecting high density memory modules to a circuit board. 
     2. Description Of the Prior Art 
     Memory modules include a generally planar rectangular ceramic body with an integrated circuit chip centrally therein. Electrically conductive leads extended from the chip to the periphery of the ceramic body. Until recently, memory modules were substantially as shown in FIG. 1. More particularly, the prior art memory module 10 of FIG. 1 has electrically conductive pins 12 extending outwardly from the ceramic body. The pins 12 are generally L-shaped, and include a first leg projecting from a side edge of the ceramic body generally parallel to the plane of the circuit board 14, and a second leg projecting downwardly approximately orthogonally to the rectangular chip. The pins 12 project through apertures 16 in the circuit board and are soldered to electrically conductive paths printed or otherwise disposed on the circuit board 14. The soldered connections between the pins 16 and the electrically conductive paths on the circuit board 14 are visible and accessible. Thus, the prior art assembly shown in FIG. 1 enables the quality of the soldered connections to be optically assessed. 
     The prior art memory module typically is the most expensive element on the board. It is not uncommon for a prior art memory module to cost between $50.00 and $100.00. The entire board, prior to mounting the memory module thereto also might cost $50.00-$100.00. The completed board invariably is tested prior to final installation into a computer or other piece of electronic equipment. If possible, any observed defect would be corrected, rather than discarding the entire board. For example, if a memory module was found the be defective, the accessible soldered connections might be desoldered. The defective memory module would then be discarded and a new memory module would be soldered to the board. If the board was found to be defective, the memory module could be desoldered and used on another board. 
     Memory modules have steadily become more complex and sophisticated without corresponding increases in size. Initially, the greater complexity led to more leads extending from the side edges and a corresponding increase in the number of apertures in the circuit board. However, the increase in the number of apertures was found to cause local weaknesses in the circuit board. In response to these problems surface mount memory modules were developed for mounting directly to the surface of a circuit board without a dense array of through holes. With reference to FIG. 2, the prior art surface mount memory module 10a has either short leads 12a projecting from side edges or contact pads along side edges that are soldered to contact pads 16a on the surface of the circuit board 14a. The prior art surface mount memory module 10a enables somewhat greater circuit densities without weakening the board 14a. The prior art surface mount memory module 10a still enables optical inspection of soldered connections and permits desoldering when necessary. 
     Memory modules have continued to increase in complexity without corresponding increases in size. The greater circuit densities enabled by the more complicated memory modules could not readily be accommodated along the peripheral edges of the memory module. Furthermore, connections along peripheral edges of the memory module require a bigger circuit &#34;footprint&#34; which offsets the miniaturization being achieved within the memory module. As a result, memory modules were developed with conductive paths leading to the bottom surface for mating with a corresponding array of conductive paths on the circuit board. An example of such a prior art high density memory module is illustrated schematically in FIGS. 3 and 4. In particular, the memory module 10b includes a plurality of conductive dots 12b on the bottom face thereof. The circuit board 14b includes a corresponding array of conductive pads 16b. Current technology permits the dots 12b and pads 16b to be disposed at center-to-center spacings, as indicated by dimension &#34;a&#34; in FIG. 3 of approximately 0.050 inch, and further miniaturization is possible. The prior art memory module 10b is accurately positioned such that the conductive dots 12b contact the conductive pads 16b. The circuit board is then subjected to wave soldering, or other known soldering techniques, to permanently connect the memory module 10b to the circuit board 14b as shown in FIG. 4. However, in contrast to the prior art embodiments depicted in FIGS. 1 and 2, the soldered connections in FIG. 4 are not visible and cannot be optically checked. Furthermore, the soldered connections in FIG. 4 are not accessible and hence the memory module 10b cannot readily be removed if a defect is subsequently observed in either the memory module 10b or the circuit board 14b. The more complex and sophisticated memory modules shown in FIGS. 3 and 4 often are significantly more costly than the prior art memory modules depicted in FIGS. 1 and 2. It is not uncommon for a memory module to cost more than $100.00, and some cost as much as $500.00. Additionally, the circuit boards for these sophisticated memory modules also are more complex, and hence more costly than their simpler predecessors. The difficulties of desoldering the inexcessible connections shown in FIG. 4 may force a computer manufacturer to discard both a memory module and a circuit board. Often either the discarded memory module or the discarded board will be perfectly functional. In some instances both the discarded memory module and the discarded board will be functional, and the defect will merely exist in a soldered connection between the two. The component manufacture would prefer not to discard a perfectly good memory module costing several hundred dollars, nor a good circuit board costing in excess of $100.00. 
     In view of these problems, the prior art has developed a high density memory module socket assembly as shown schematically in FIGS. 5 and 6. The prior art memory module socket assembly uses the circuit board 14b and the memory module 10b described and illustrated above. However, the prior art socket assembly further includes a base 18 having an array of apertures 20 extending therethrough and registered with the contact pads 16b on the circuit board 14b. The apertures 20 are filled with a jumbled array of very thin conductive wire 22 resembling a small steel wool pad. A jumbled wire array 22 is urged into each aperture 20, and is dimensioned to extend beyond the opposed surfaces of the prior art base 18. Thus, the wire 22 in the aperture 20 will engage a conductive pad 16b on the circuit board 14b and will engage a corresponding conductive pad 12b on the memory module 10b to provide electrical connection therebetween. Solder is entirely avoided, and mechanical means are used to hold the memory module 10b and the prior art base 18 in proper registration on the circuit board 14b. The memory module 10b can be removed and replaced or repositioned for any reason, such as an observed defect in either the memory module 10b or the circuit board 14b. 
     The connector assembly shown in FIGS. 5 and 6 overcome several of the disadvantages described with respect to the soldered connection depicted in FIGS. 3 and 4. However, the prior art connector assembly shown in FIGS. 5 and 6 also has drawbacks. One such drawback is cost. Prior art connectors, as shown in FIGS. 5 and 6, often cost between nine cents and fifteen cents per connection. Thus, a memory module with 500 conductive pads would have a connector costing $45.00-$75.00. Second, it is difficult to ensure that the jumbled array of wire 22 will exert the specified pressures against both the walls of the aperture 20 through the base 18 and on the conductive pads 12b and 16b on the memory module and board respectively. The entire jumbled array of wire 22 will fall out of the aperture 20 if the engagement forces are too low. Similarly, poor electrical connection will be achieved if the contact forces between the jumbled array of wire and the memory module for the board are too low. Furthermore, the jumbled array of wire 22 is not well suited to making plural make and break connections. Thus, if a defect in the memory module is observed or if it is desired to merely change to a different memory module, the jumbled array of wire 22 may not resiliently return a sufficient amount to make a good second connection. 
     In view of the above, it is an object of the subject invention to provide a connector for a high density memory module. 
     It is another object of the subject invention to provide a memory module connector that enables repeated connection and disconnection of high density memory modules therefrom. 
     It is a further object of the subject invention to provide an electrically conducted terminal assembly for connecting the contact pad of a memory module to the contact pad of a circuit board. 
     SUMMARY OF THE INVENTION 
     The subject invention is directed to an electrical connector assembly and to resilient electrically conductive terminals for use therein. The electrical connector assembly of the subject invention includes a base having a plurality of apertures extending therethrough for registration with conductive pads on a circuit board and conductive pads on a memory module. The base may further include means for mounting the base to the circuit board and means for receiving a memory module thereon. Additionally, the base may include means for receiving a cover for holding the memory module in secure electrical contact with the conductive pads on the circuit board as explained herein. The mounting means for securing the base and/or the cover in fixed relationship to the circuit board may merely include bolts or screws passing through the base and/or the cover and connected to the circuit board. The bolts may be configured to achieve secure engagement or disengagement in response to a quarter turn. 
     The resilient terminal assemblies of the subject invention comprise a board contact, a module contact and an elongate flexible connector extending therebetween. The board contact and the module contact may have surfaces coated or otherwise treated to have miniature spike-like surface features and corresponding multiple contact points. For example, the board contact surface and the module contact surface may be provided with a dendritic contact interface similar to the dendritic interface available through IBM-Endicott. The connector may define a flexible braided wire electrically connected to the contacts by soldering, crimping or the like. However, in a preferred embodiment, the contacts and the connector are unitarily stamped and formed from a strip of conductive metal. The connector of the resilient terminal assembly is configured to be selectively contracted and/or expanded and to undergo plural cycles of resilient compression and expansion. For example, the connector of the resilient terminal assembly may be formed into a generally sinusoidal wave shape or a coiled configuration. 
     The resilient terminal assembly of the subject invention further comprises an elastomeric plug surrounding the connector and portions of the contacts. The plug and the terminal may be joined by insert molding, such that the plug defines a unitary matrix of elastomeric material surrounding the connector of the terminal assembly and portions of the contacts. The plug of the terminal assembly defines a cross-sectional configuration which enables the terminal assembly to be frictionally retained in an aperture of the base without gravitationally falling from the aperture. The plug also is cross-sectionally dimensioned to permit a controlled axial contraction of the plug in the aperture as opposed contacts are urged toward one another. Thus, for example, the plug may include an annular bead extending therearound and defining a diameter sufficient for frictional engagement of the plug in an aperture. However, portions of the plug on either side of the annular bead may define smaller diameters which permit transverse expansion of the plug as the opposed ends of the terminal assembly are contracted inwardly and toward one another. 
     The terminal assembly defines a length as measured between the oppositely facing contacts which is greater than the thickness of the base. Thus, the terminal assemblies can be frictionally mounted in apertures of the base with the oppositely facing contacts projecting beyond opposed faces of the base. With these relative dimensions, the terminal assembly can be compressed by both the conductive pads on the circuit board and the conductive pads on the memory module, and the terminal assembly will exert selected quantifiable contact forces against the circuit board and the memory module. The magnitude of the contact forces and the amount of deformation can be controlled precisely by carefully selecting the cross-sectional dimensions of the elastomeric plug and the aperture in the base. In this regard, the aperture through the base can be dimensioned to control the cross-sectional or transverse expansion of the plug that necessarily occurs as the plug is being axially compressed. The amount of axial contraction and hence the contact forces also can be controlled by standoffs molded into the base. The standoffs can positively control the amount of compression permitted in the terminal assembly. 
     The terminal assembly provides several distinct advantages over the prior art. First, the frictional forces between the plug of the terminal assembly and the walls of the aperture through the base can be easily controlled to prevent the terminal assemblies from falling out of the holes in the base. Similarly, the relative dimensions of the plug and the apertures through the base can be selected to achieve narrowly specified contact forces. Contact forces also can be controlled by the selection of the elastomer to be incorporated into the plug. Still further, the terminal assemblies are well suited to automated insertion into the apertures, and hence enable significant cost efficiencies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a first prior art memory module and circuit board assembly. 
     FIG. 2 is a perspective view of a second prior art memory module and circuit board assembly. 
     FIG. 3 is an exploded perspective view of a third prior art memory module and circuit board assembly. 
     FIG. 4 is a perspective view of the assembled prior art memory module and circuit board assembly. 
     FIG. 5 is an exploded perspective view of a prior art memory module, connector assembly and circuit board in accordance with the subject invention. 
     FIG. 6 is a cross-sectional view taken along line 6--6 in FIG. 5. 
     FIG. 7 is a perspective view of a connector assembly in accordance with the subject invention. 
     FIG. 8 is a side elevational view of a resilient electrically conductive terminal assembly used in the connector assembly of FIG. 7. 
     FIG. 9 is a top plan view of a stamped blank for the conductive portions of the terminal assembly. 
     FIG. 10 is a side elevational view of the blank formed for use in the connector assembly. 
     FIG. 11 is a cross-sectional view taken along line 11--11 in FIG. 8. 
     FIG. 12 is a cross-sectional view taken along line 12--12 in FIG. 7. 
     FIG. 13 is a cross-sectional view similar to FIG. 12 but showing the connector assembly in electrical contact with a circuit board and a memory module. 
     FIG. 14 is a cross-sectional view taken alongline 14--14 in FIG. 13. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The connector assembly in accordance with the subject invention is identified generally by the numeral 24 in FIGS. 7 and 12-14. The connector assembly 24 includes a base 26 which may be unitarily molded from a thermoplastic material. The base 26 is of substantially rectangular planar configuration with opposed top and bottom faces 28 and 30 respectively which define a thickness &#34;b&#34; of approximately 0.062 inch. The base 26 is provided with an array of apertures 32 drilled therethrough or molded therein to extend entirely from the top face 28 to the bottom face 30 thereof. The apertures 32 are at center-to-center spacings &#34;c&#34; corresponding to the spacing of conductive pads on a circuit board and on a memory module with which the connector 24 is employed. For example, the apertures 32 may be disposed at center-to-center spacings &#34;c&#34; approximately equal to 0.050 inch. The base 26 further includes mounting flanges 34 projecting therefrom and having apertures 36 for receiving bolts to enable secure mounting of the base 26 to a circuit board as explained further herein. 
     The connector assembly 24 further includes resilient electrically conductive terminal assemblies 38 mounted respectively in the apertures 32. Each terminal assembly 38 includes a terminal 40 insert molded into a generally cylindrical elastomeric plug 42. The terminal 40, as shown in FIGS. 8-10, is stamped from a unitary strip of beryllium copper alloy having a thickness of approximately 0.003 inch. The terminal 40 is initially stamped to define an elongate planar blank having a length &#34;d&#34; of approximately 0.275 inch, as illustrated in FIG. 9. The blank of the terminal 40 includes an elongate connecting portion 44 defining a width &#34;e&#34; of approximately 0.025 inch. First and second generally round contacts 46 and 48 are disposed at opposite ends of the connecting portion 44. The contacts 46 and 48 define diameters &#34;f&#34; of approximately 0.048 inch. Additionally, the contacts 46 and 48 may be gold plated on one side. The contacts 46 and 48 may further be provided with surface treatments to define miniature spike-like structures 70 with multiple contact points thereon. These features may be defined by a dendritic contact interface similar to that available through IBM-Endicott. 
     The blank of the terminal 40 is initially formed to include contact dimples 50 and 52 on the contacts 46 and 48 respectively. The elongate connecting portion 44 then is formed to define a plurality of resiliently deflectable generally sinusoidal bends 54, with the contacts 46 and 48 being substantially parallel. Sides of the contacts 46 and 48 opposite the dimples 50 and 52 define pressure bearing surfaces that will compress an adjacent elastomer, as explained herein. In this initially formed condition, the terminal 40 defines a height &#34;g&#34; of approximately 0.105 inch. 
     The terminal 40 and the plug 42 are insert molded such that the elastomer of the plug 42 defines a unitary matrix surrounding and engaging the elongate connecting portion 44 of the terminal 40. The insert molding is carried out such that the contact dimples 50 and 52 are exposed for making electrical contact with the circuit board and the memory module respectively. However, the opposed pressure bearing surfaces are embedded in the elastomer. The molding cavity in which the terminal assembly 38 is formed is dimensioned to slightly compress the formed terminal 40 to define an overall axial length &#34;h&#34;. As a result, the formed terminal 40 will be under a slight preload. The overall axial length &#34;h&#34; of the formed terminal will be a function of the thickness of the base 26 and the amount of resilient deformation desired for the particular circuit board and memory module. In the illustrated example, the base 26 defines a thickness of approximately 0.062 inch, and the height &#34;h&#34; of the terminal assembly 38 equals approximately 0.100 inch. 
     The diametric dimensions of the plug 42 are a function of the diameter of each aperture 32 in the base 26 and a function of the maximum amount of compression desired for the terminal assembly 38. In the illustrated embodiment, each aperture 32 in the base 26 defines a diameter &#34;i&#34; of approximately 0.060 inch. In this embodiment, the plug 42 defines a diameter &#34;j&#34; of approximately 0.050 inch along a major portion of its length. However, the plug 42 includes an annular rib 60 extending thereabout at a central position between the ends 56 and 58 of the plug 42. The rib 60 defines an outside diameter &#34;k&#34; which is approximately equal to 0.065 inch, or slightly greater than the diameter of the aperture 32. Thus, as shown in FIG. 12, the rib 60 will require deformation for insertion of the terminal assembly 38 into the aperture 32. As a result, the resiliently deformed annular rib 60 will exert pressure against portions of the base 26 defining the aperture 32 for preventing unintended separation or removal of the terminal assembly 38. Also with reference to FIG. 12, portions of the plug 42 on either side of the rib 60 will be disposed in spaced relationship to the walls of the aperture 32. The radial distance between the plug 42 and the walls of the aperture 32 are selected to control the amount of permissible compression of the terminal assembly 38. More particularly, as shown in FIGS. 13 and 14, sufficient compression of the terminal assembly 38 will cause the plug 42 to entirely fill the aperture 32. Upon such complete filling of the aperture 32, the elastomer of the plug 42 will have no room for deformation, and hence further deformation will be substantially prevented. In the embodiment depicted in FIG. 13, this maximum compression defines an overall axial length &#34;l&#34; of approximately 0.085 inch. Thus, the terminal assembly 38 will have undergone a maximum compression of 0.015 inch. In this compressed state, the entire terminal assembly, including the elastomer of the plug 42 and the resiliently deformed terminal 40 will exert forces in opposed axial directions for achieving a high quality contact with both the circuit board and the memory module. The maximum amount of compression can be varied, of course, by altering the relative diametrical dimensions of the aperture 32 and the plug 42. The amount of permissible compression also can be controlled by providing a standoff 62 on the base 26. The standoff 62 will positively control the relative positions of the memory module and circuit board relative to the base 26. For example, standoffs with a height of 0.0075 inch will ensure the compression of 0.015 inch desired for the illustrated embodiment. 
     The terminal assemblies 38 can be inserted easily into the apertures 32 of the base 26 by vacuum means. In particular, the terminal assemblies may be deposited on the base 26 in an apparatus for applying vibration to the entire base and for applying vacuum through the apertures 32. The vacuum will be of a sufficient strength to urge the respective terminal assemblies 38 into a corresponding aperture 32. The amount of insertion can be positively controlled by stop means in the vacuum apparatus to ensure that each terminal assembly 38 is centered relative to the oppositely disposed surfaces 28 and 30 of the base 26. 
     The connector 24, with the terminal assemblies 38 mounted in the base 26 is then positioned on the circuit board shown in FIG. 13, and the memory module is positioned on the connector 24. A cover can be threadedly engaged with the mounting tabs 34 to urge the memory module toward the circuit board. The amount of movement of the memory module toward the circuit board is positively controlled by the above described deformation of the plug 42 into the walls defining the respective apertures 32. The amount of movement can further be controlled by the particular connection means which may, for example, be limited to one quarter turn of a threaded screw. In this connected condition, as shown in FIG. 13, the compressed terminal assembly 38 will exert forces in opposed directions to ensure a high quality electrical contact with both the circuit board and the memory module. The entire circuit board then can be tested. If it is determined that either the memory module or the circuit board are defective, the memory module can easily be removed and either discarded or used elsewhere. Additionally, the connector 24 also can be reused with either a new memory module or a new circuit board. The elastomeric plug 42 is capable of more than the twenty cycles preferred by the industry. 
     While the invention has been described with respect to a preferred embodiment, it is apparent that various changes can be made without departing from the scope of the invention as defined by the appended claims. For example, the terminal may include a wire extending between contacts at the opposed ends of the elastomeric plug.