Abstract:
A novel Land Grid Array (LGA) interposer with adhesive-retained contacts and method of manufacture provide improved reliability in LGA mounting applications. A flexible adhesive is used to secure LGA interposer contacts to the walls of voids through an interposer frame. The contacts may be spring contacts or “fuzz button” type contacts. The use of a flexible adhesive provides for floating movement of the contacts within the voids so that thermal expansion stresses do not cause unbalanced compression of the contacts that could otherwise occur with a fixed attachment of the contacts to the frame. The resulting interposer can provide reliable electrical connection from LGA lands on an integrated circuit package to lands on an electronic assembly that is highly tolerant of thermal expansion differences, while eliminating migration of the contacts out of the voids that could otherwise cause shorting or disconnection.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to integrated circuit mounting systems, and more particularly, to an interposer for mounting Land Grid Array (LGA) integrated circuits to an electronic assembly and a method of manufacturing the interposer. 
   2. Background Information 
   Land Grid Array (LGA) integrated circuits are in widespread use in electronic systems. The LGA interconnect scheme provides a high interconnect-density electronic interface that also provides a highly-reliable socket interconnect structure that permits mounting of semiconductor packages having a large thermal expansion with respect to an electronic assembly mounting area such as a Printed Circuit Board (PCB). For example, ceramic semiconductor packages generally have a much lower coefficient of thermal expansion than PCBs and so typical solder-ball mounting such as with Ball Grid Array (BGA) leads to fracture of the solder ball connections when temperatures vary widely, causing failure of the electronic system. 
   The LGA interconnect also permits interchangeable integrated circuit mounting without using pin-based interconnects. For example, in present-day computing systems, processing components are often mounted using a socket configuration rather than mounted via a solder interconnect so that the processing components may be selected and installed after an electronic assembly such as a “motherboard” has been fabricated. 
   In order to provide a reliable and low resistance electrical connection between the LGA integrated circuit and an electronic assembly such as a printed wiring board (PWB), LGA interposers have been developed that typically use a loaded spring contact or a “fuzz button” contact at each contact position that extend from the lands on the integrated circuit to a land on the electronic assembly. Both types of contacts in present use are typically gold-plated to provide a low-resistance connection that is resistant to environmental degradation such as oxidation. 
   The fuzz buttons or spring contacts are inserted into a mounting frame that is generally a rectangular plastic interposer frame having circular holes at each contact position. After the contacts are inserted, the interposer assembly is complete. In use, the interposer is placed and aligned between the LGA integrated circuit and the electronic assembly and the LGA integrated circuit is secured with a mounting mechanism that applies pressure between the LGA integrated circuit and the electronic assembly. The pressure compresses the fuzz buttons or spring contacts, providing a reliable electrical interconnect between the LGA lands on the integrated circuit and the lands on the electronic assembly. 
   However, the retention of either fuzz button contacts or spring contacts in the interposer frame has presented a manufacturing and post-manufacturing handling and reliability problem, as the contacts may fall out of or move from their proper positions in the interposer frame during handling of the interposer or assembly of the LGA to the electronic assembly. Even after assembly, vibration or other stresses may cause movement of the contacts causing a failed interconnect or a short due to lateral contact of the LGA interposer contacts above or below the surfaces of the interposer frame. 
   Also, rigid securing of LGA interposer contacts is not desirable, as ideally, the contacts should be permitted to “float” so that thermal expansion differences between the LGA integrated circuit package and the electronic assembly do not cause an interconnect failure due to compression unloading of one end of a contact or possible overloading and deformation of a contact that will subsequently disconnect when the temperature changes. One securing scheme that has been implemented for retaining LGA contacts uses hourglass-profiled voids in the interposer frame into which the contacts are inserted. The “neck” points of the voids slightly compress the contacts laterally, so that the contacts are more securely retained in the interposer frame. However, the contacts may still move or fall out of the interposer if sufficient force is applied. 
   Therefore, it would be desirable to provide a mechanism and method of manufacture for securely retaining both fuzz button and spring contacts in a LGA interposer frame that still allows the contacts to float. 
   SUMMARY OF THE INVENTION 
   The objective of providing a secure mechanism for retaining LGA interposer contacts in an interposer frame is accomplished in a new LGA interposer and method of manufacture. 
   The interposer includes an interposer frame having a grid of holes that may be cylindrical or may have other profiles. A contact, which may be a spring contact or a fuzz button type contact is inserted in each hole and is secured with an elastomeric adhesive. The elastomeric adhesive provides sufficient flexibility so that the contacts may still float in response to an applied force, providing a secure electrical connection, but prevents permanent migration of the contacts from the desired position. In the case of spring contacts, a self-healing adhesive may be deposited within the holes and the spring contacts inserted after curing of the adhesive. 
   The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein like reference numerals indicate like components, and: 
       FIG. 1A  is a pictorial diagram depicting a cross-section of an LGA interposer assembly in accordance with an embodiment of the present invention. 
       FIG. 1B  is a pictorial diagram depicting a cross-section of an LGA interposer assembly in accordance with another embodiment of the present invention. 
       FIG. 2A  is a pictorial diagram depicting a perspective view and detail of LGA interposer  5  of  FIG. 1A . 
       FIG. 2B  is a pictorial diagram depicting a perspective view and detail of LGA interposer  5  of  FIG. 1B . 
       FIG. 3  is an illustration depicting a manufacturing process for making an LGA interposer in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention will now be described in more detail by way of example with reference to the embodiments shown in the accompanying figures. It should be kept in mind that the following described embodiments are only presented by way of example and should not be construed as limiting the inventive concept to any particular physical configuration. 
   Further, if used and unless otherwise stated, the terms “upper,” “lower,” “front,” “back,” “over,” “under,” and similar such terms are not to be construed as limiting the invention to a particular orientation. Instead, these terms are used only on a relative basis. 
   The present invention is directed toward an interposer for land grid array (LGA) semiconductor package mounting that provides improvement in retention of contacts within the interposer frame. 
   With reference now to the figures, and in particular with reference to  FIG. 1A , there is depicted an LGA interposer  5  in accordance with an embodiment of the present invention. Interposer  5  is used to interconnect a semiconductor package  16  having a plurality of LGA terminals  17  to lands  15  on a system substrate  14 , such as a printed circuit board. Interposer  5  provides electrical connection between each of LGA terminals  17  and a corresponding land  15  via a plurality of electrically-conductive contacts  11 . In the depicted embodiment, contacts  11  are c-shaped spring contacts formed from stamped, bent and plated metal. Contacts  11  are held in position along the plane of the grid array by an interposer frame  10  that has a through-hole (void)  12  for each contact  11  and into which each contact  11  is inserted. Pressure is provided between semiconductor package  16  and system substrate  14  by a clamping arrangement such as spring clip  18 . The pressure on contacts  11  causes compression of contacts  11 , providing a secure electrical connection. 
   However, as described above with respect to the prior art, contacts  11  may not be under uniform pressure, are subject to vibration and may move prior to securing interposer  5  between semiconductor package  16  and system substrate  14 , causing contacts  11  to fall out completely or extend so far as to bend and cause intermittent connections, missing connections or shorts to adjacent contacts. If contacts  11  were rigidly secured to interposer frame  10 , differential thermal expansion or differences in mechanical pressure across interposer  5  could lead to missed wiping of contacts  11  on lands  17  or  15 . Therefore, an elastic adhesive  13  is introduced to retain contacts  11  within holes  12  so that contacts  11  may move in a direction perpendicular to the primary plane of interposer frame  10  while keeping contacts  11  from falling out or moving far enough to cause shorting to adjacent contacts or lands. 
   Elastic adhesive  13  is generally a silicone or urethane adhesive that is either a two-part adhesive mixed at the time of application, or may be a UV or temperature curable adhesive. 
   While the above-described use of LGA interposer  5  is in a connection between an LGA semiconductor package and a printed circuit board land pattern, it should be understood that other LGA or grid array applications are possible such as board-to-board interconnects. Further, while LGA is a particular technology subject to certain standards in the art, it should be understood that the present invention applies to interposers and sockets having floating contacts that are not typically mechanically secured to the frame in order to provide a wide range of thermal and pressure variation between two subsystems that are connected. The present invention provides a means and method by which such interposer and socket systems may be improved in reliability and ease of handling/installation. 
   Referring now to  FIG. 1B , there is depicted a Land Grid Array (LGA) interposer  5 A in accordance with another embodiment of the present invention. As described above with respect to  FIG. 1A , Interposer  5 A is used to interconnect a semiconductor package  16  having a plurality of LGA terminals  17  to lands  15  on a system substrate  14 , such as a printed circuit board. Interposer  5 A provides electrical connection between each of LGA terminals  17  and a corresponding land  15  via a plurality of electrically-conductive contacts  11 A. In the depicted embodiment, contacts  11 A are “fuzz buttons” formed from a single strand of molybdenum wire spun into a compressible cylindrically-shaped electrically conductive contact. Contacts  11 A are held in position along the plane of the grid array by an interposer frame  10 A that has a through-hole (void)  12 A for each contact  11 A and into which each contact  11 A is inserted. Holes  12 A are depicted as having an hourglass-shaped cross-section that is typically used to improve retention of contacts  11 A. However, with the advantages of the present invention, it is not necessary to provide such an hourglass profile in order to retain contacts  11 A, although assembly may be simplified. As in the embodiment depicted in  FIG. 1A , pressure is provided between semiconductor package  16  and system substrate  14  by a clamping arrangement such as spring clip  18 . The pressure on contacts  11 A causes compression of contacts  11 A, providing a secure electrical connection. 
   However, as described above with respect to  FIG. 1A , contacts  11 A may not be under uniform pressure, are subject to vibration and may move prior to securing interposer  5 A between semiconductor package  16  and system substrate  14 , causing contacts  11 A to fall out completely or extend so far as to bend and cause intermittent connections, missing connections or shorts to adjacent contacts. Further, fuzz button contacts such as contacts  11 A can be deformed or may actually unwind, increasing the likelihood of shorting if an end of the wire forming a fuzz button leaves the area of void  12 A. Elastic adhesive  13 A is introduced to retain contacts  11 A within holes  12 A so that contacts  11 A retain their shape and do not unwind. Also, elastic adhesive  13 A provides movement in a direction perpendicular to the primary plane of interposer frame  10 A while keeping contacts  11 A from falling out or moving far enough to cause shorting to adjacent contacts or lands. The combination of preventing unwinding and restricting movement of fuzz button contacts  11 A, substantially eliminates the possibility of shorting due to migration or unwinding of contacts  11 A. 
   Referring now to  FIG. 2A , interposer  5  is shown in further detail. In the depiction, the location of holes  12  through interposer frame  10  forming a grid array can be seen. Callout  30  shows a magnified detail of a contact  11  secured by adhesive  13  within hole  12 . While holes  12  are of a dumbbell shape for accepting c-shaped spring contacts  11 , it should be understood that the shape of holes  12  is incidental to the purpose of the present invention, although in the depicted embodiment, the dumbbell-shaped holes may provide better retention of contacts  11  by adhesive  13  when adhesive is disposed within the body of contacts  11  by shortening the distance between the body of contacts  11  and the walls of holes  12 . 
   For spring contact interposers in accordance with the present invention such as interposer  5 , interposer frame  10  may be prepared with elastic adhesive  13  prior to insertion of contacts  11 . By using a self-healing adhesive compound and curing the compound prior to insertion of contacts  11 , contacts  11  may be retained by insertion displacement of adhesive  13  without adhesive  13  adhering (bonding) to contacts  11 . Adhesive  13  may be applied via a paste and wipe operation, but may be alternatively applied only within interposer frame  10  by injection in a central portion of holes  12  so that adhesive  13  does not reach the top or bottom surface of interposer frame  10 . Alternatively, contacts  11  may be inserted through uncured adhesive  13  that has been injected into holes  12  and adhesive then cured, so that adhesive  13  adheres to contacts  11 . 
   As another alternative, interposer  5  may be assembled by loading contacts  11  in holes  12  and then injecting adhesive  13  in holes  12  in order to bond adhesive  13  to both the inner walls of holes  12  and to contacts  11 . Generally, for a c-shaped contact  11  such as those depicted in  FIG. 1A , adhesive  13  is injected so that the central void in the body of contact  11  is filled with adhesive  13  that is also bonded to the walls of hole  12 . However, adhesive  13  may be applied only to the periphery of hole  12  or a portion thereof that is adjacent a surface of contact  11 , depending on the amount of resistance to displacement of contact  11  that is needed. 
   Referring now to  FIG. 2B , interposer  5 A is shown in further detail. In the depiction, the location of holes  12 A through interposer frame  10 A forming a grid array can be seen. Callout  32  shows a magnified detail of a contact  11 A secured by adhesive  13 A within hole  12 A. 
   For fuzz button contact interposers in accordance with the present invention such as interposer  5 A, interposer  5 A may be assembled by loading contacts  11 A in holes  12 A and then injecting adhesive  13 A in holes  12 A in order to bond adhesive  13 A to both the inner walls of holes  12 A and to contacts  11 A. Alternatively, adhesive may be injected into holes  12 A and contacts  11 A inserted before curing adhesive  13 A. Generally, for a fuzz button contact  11 A, adhesive  13 A is injected around the periphery of the walls of hole  12  at least near the surfaces of the interposer frame  10 A, which helps maintain the shape of contacts  11 A as described above. The above-described process can be performed on prefabricated fuzz button interposers. However, adhesive  13 A may be applied only to the interior of hole  12 A, depending on the amount of resistance to displacement of contact  11 A that is needed and the amount of control of the shape of contact  11 A near the outside surfaces of interposer frame  10 A. 
   Referring now to  FIG. 3 , the methods of manufacture of the various interposers described above are illustrated. An adhesive dispensing system is used to introduce elastic adhesive into interposer frame  10  holes  12 . Interposer frame  10  is generally held in an X-Y positioning table with a fixed dispensing needle location or may be held in a fixed position and the dispensing needle is moved as will be illustrated. The adhesive dispensing system comprises a positioner  40  for moving a dispenser  44  that terminates in a needle  42  for injecting adhesive into holes  12 . By controlling the position of needle  42  and amount of adhesive dispensed at each hole  12  the proper amount of resistance to displacement (and deformation in the case of fuzz button contacts) can be maintained. When adhesive is injected prior to insertion of contacts, a needle of appropriate size for quickly filling each hole  12  to between 10% and 100% of the hole volume at each step position (hole location) of positioner  40  is used. An adhesive of fairly high viscosity is used to retain the adhesive in holes  12  prior to curing. But, when injecting adhesive after insertion of contacts, a small needle (approximately 30 ga) is used so that adhesive can be injected around or into the contacts. A low viscosity adhesive is also necessary when contacts are pre-inserted so that the adhesive can wick into the open volumes within holes  12 . 
   It should be understood, however, that the invention is not necessarily limited to the specific process, arrangement, materials and components shown and described above, but may be susceptible to numerous variations within the scope of the invention. 
   It will be apparent to one skilled in the art that the manner of making and using the claimed invention has been adequately disclosed in the above-written description of the preferred embodiments taken together with the drawings. 
   It will be understood that the above description of the preferred embodiments of the present invention are susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.