Patent Publication Number: US-6702587-B2

Title: Separable electrical connector using anisotropic conductive elastomer interconnect medium

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority of Provisional application Ser. No. 60/311,178, filed on Aug. 8, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to electrical interconnect devices. 
     BACKGROUND OF THE INVENTION 
     Anisotropic Conductive Elastomer (ACE) is a composite of conductive metal elements in an elastomeric matrix that is normally constructed such that it conducts along one axis only. In general, ACE is made to conduct through its thickness. One form of ACE material is made by mixing magnetic particles with a liquid resin, forming the mix into a continuous sheet, and curing the sheet in the presence of a magnetic field. This results in the particles forming a large number of closely spaced columns through the sheet thickness. The columns are electrically conductive. The resulting structure has the unique property of being both flexible and anisotropically conductive. These properties provide for an enabling interconnection medium which, when combined with other technologies, make it possible to realize new interconnect capabilities. 
     ACE materials require that they be compressed between top and bottom conductors to provide the interconnection. This is normally done by compressing the system using a backing plate and spring arrangement. One example of such is shown in FIG.  1 . This system can be used only with boards  12  which are designed to accommodate the backing plate  22 , and has holes for the connection hardware  28 ,  29 , to pass through, to connect spring plate  24  (with spring force adjustment screw  26 ) to pressure member  15 . Device  14  is electrically interconnected to board  12  using ACE  20 . Heat sink  30  is optional in the system. This can result in a conflict with components mounted under the device and on the back of the board. 
     ACE material can be married with flexible circuits to provide more dynamic range to an ACE interconnect. The flexible circuit consists of an electrically-insulating material such as polyimide, with opposing conductive pads on the surfaces. The pads are vertically interconnected by plated-through holes. Mounting such a flex circuit to ACE material can provide more vertical compliance to the ACE material. This allows the ACE material to be used in assemblies that are not planar, such as circuit boards with solder mask, in which the circuit board pads are lower than the top of the solder mask, thus creating a small well around each pad and into which the ACE material-based interconnect must protrude in order to make electrical contact with the pads. 
     BGA (ball grid array) devices are electronic components with solder balls placed in a grid and used for final installation (through a solder reflow process) on a printed circuit board. There are other types of electronic packages in which the invention is relevant, including Land Grid Arrays (LGA). Column Grid Arrays (CGA) have solder columns in place of the ball or pad. Factors such as cost, environment and population density determine which geometry is used. 
     SUMMARY OF THE INVENTION 
     The present invention combines ACE with an adapter board that eliminates the need for a specially modified main board in order to mechanically compress the ACE. 
     This invention features an electrical connector for separably, electrically interconnecting an electrical device to a main circuit board using anisotropic conductive elastomer (ACE) as part of the interconnect, the electrical connector comprising an adapter board coupled to the main board on one side, the other side of the adapter board defining connecting lands; a layer of ACE on the other side of the adapter board, the other side of the ACE in contact with the device; and a mechanical compression structure coupled to the adapter board, and that provides a compressive load on the device, the ACE and the adapter board, to accomplish a separable electrical connection between the device and the main board, through the ACE and the adapter board. 
     The connector may further comprise a heat sink coupled to the compression structure and in communication with the device, for removing heat from the device. The connector may further comprise a thermal conducting medium between the device and the heat sink. The ACE material may be formed by magnetically aligned particles that form columns extending between the top and bottom surface of the ACE. 
     The connector may further comprise a flex circuit between the device and the ACE. The flex circuit may comprise ball seats on one surface and lands on the other surface. The connector may further comprise an insulating adhesive backfill between the adapter board and the main board, to enhance the stiffness of the adaptor board. The backfill may be epoxy. The connector mechanical compression structure may comprise a plate. The mechanical compression structure may further comprise hinge members and a latch for moving and latching the plate. The mechanical compression structure may comprise a compressive spring element. The mechanical compression structure may further comprise a mechanical member for applying a variable compressive load. The connector may further comprise a spacer frame between at least a portion of the adapter board and the main board, to further thicken and stiffen the adapter. 
     Also featured is an electrical connector for separably, electrically interconnecting an electrical device to a main circuit board using anisotropic conductive elastomer (ACE) as part of the interconnect, the electrical connector comprising: an adapter board coupled to the main board on one side, the other side of the adapter board defining connecting lands; a layer of ACE on the other side of the adapter board, the other side of the ACE in contact with the device; and a mechanical compression structure coupled to the adapter board, and that provides a compressive load on the device, the ACE and the adapter board, to accomplish a separable electrical connection between the device and the main board, through the ACE and the adapter board; wherein the mechanical compression structure comprises a compressive spring element and a mechanical member for applying a variable compressive load. 
     The ACE material may comprise conductive particles embedded in an elastomer, the conductive particles defining the conductive columns. The flexible circuit element conductive pathways may comprise conductor-lined openings extending between the opposing faces of the flexible circuit element. The flexible circuit element may further define conductive pads on both faces of the flexible circuit element and in electrical contact with a conductive pathway, to provide electrical contact areas on the flexible circuit element, one contact area for interfacing with the ACE material, and the other contact area for interfacing with an electrical device. 
     The conductive pads on one face of the flexible circuit element may be annular, each surrounding a conductive pathway. The electrical device may comprise an electrical package with a series of electrical contact members protruding from a face thereof. The protruding electrical contact members may define an external peripheral shape, and the inside of the annular conductive pads may define the same shape, so that a protruding electrical contact member contacts a conductive pad about the entire periphery of the protruding electrical contact member. 
     The electrical package may comprise a ball grid array (BGA) with a series of external, partially-spherical contact members, and the inside of the annular conductive pads may define a circle having a diameter smaller than that of the spherical contact member, so that the contact member seats in the pad. The contact member may contact the pad such that the angle defined by coplanar radii from the contact member center to the contact member pad contact locations is approximately 90°. 
     The electrical package may comprise a land grid array (LGA) with a series of external rectangular contact members, and the conductive pads may be rectangular, to provide effective electrical contact therebetween. The electrical package may comprise a column array (CGA) with a series of external projecting column contact members. The conductive pads on the face of the flexible circuit element in contact with the ACE may be continuous, in order to maximize contact with the conductive columns in the ACE. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiments, and the accompanying drawings in which: 
     FIG. 1 is a schematic diagram of a prior art ACE compression system; 
     FIG. 2 is a schematic diagram of a preferred embodiment of the separable electrical connector of this invention; 
     FIG. 3 is a greatly enlarged partial view of a BGA package coupled to ACE material using a flexible circuit according to one aspect of this invention; 
     FIG. 4 is a schematic expanded view of a single solder ball interconnected to a board using the flexible circuit interconnect shown in FIG. 3; and 
     FIG. 5 is a side view of another embodiment of the invention with an easily separable connector using a pivot and latch mechanism. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     FIG. 2 provides a view of one preferred embodiment of the present invention. In this invention, adapter board  54  is attached to the main board  52  using conventional technology such as BGA (Ball Grid Array)  76 . For descriptive purposes, FIG. 2 indicates both pin/through-hole mounting  74  and BGA surface mounting  76 . In practice, one or the other is most likely to be used. Further rigidity of the attachment of adapter board  54  can be provided by a spacer frame  70  rigidly affixed to adapter board  54  and placed between adapter board  54  and printed circuit board (PCB)  52 . Frame  70  which when present is attached to adapter  54 , will thicken adapter  54  and provide stiffness. This may only be required for large area devices. Adapter board  54  can have BGA on one side and flat lands on the other side, to provide a mechanical translation from the BGA solder format to the flat land format optimally used for the ACE material  56 . Adapter  54  can also be used to rearrange the electrical contacts on board  52  to better match those on the device being connected  58 . This allows the use of what is referred to as a mother board to daughter board interconnection. Adapter board  54  can be made of an appropriately rigid insulator such as epoxy or ceramic. It can contain metal members to provide additional stiffness. 
     Adapter board  54  serves as the platform for the compressive interconnection between ACE material  56  and device  58 . In the preferred embodiment shown in FIG. 2, mechanical connectors such as retaining pins  66  are provided at appropriate positions along the edge and/or surface of adapter board  54  to affix an upper plate  62  which serves as the spring member which compresses device  58  through ACE  56  to adapter board  54 . In one preferred embodiment upper plate  62  can incorporate heat sink  60  and a heat spreader (a thermal conducting medium)  82 . 
     The structure described above and shown in FIG. 2 provides a mechanical couple between upper plate  62  and adapter board  54  which compresses ACE  56  between device  58  and adapter board  54 , resulting in a quality, separable interconnect. By using adapter board  54 , which is typically stiffer than main board  52 , less stress is placed on the solder joint interconnecting adapter board  54  to PCB  52 . Additional stiffness can be added by injecting an insulating (non conducting) adhesive substance to backfill the spaces between solder joints, such as epoxy underfill  72 , between the bottom of adapter  54  and PCB  52 . This encapsulates the solder joints, helps to bond boards  52  and  54 , and relieves additional stress from the reflowed solder joints. 
     When a BGA type device  58   a , FIG. 3, is being attached to the PCB using the adapter, a flex circuit  82  can be added between the balls  80  of BGA  58   a  and ACE  56 . This can be designed with large radius plated holes  84  on the upper surface of flex circuit  82 , to protect the bottom of solder ball  80  as indicated in FIG.  3 . The bottom side of the flex preferably has continuous pads (flat lands) at the plated through hole, providing an optimum interconnection surface for ACE  56 . 
     In another preferred embodiment, FIG. 5, a quick attachment mechanism using a latch and a pivot can be employed to facilitate the quick insertion and removal of devices from the adapter board. Main PCB  102  has an adapter board, ACE and the electrical device on top of it, within housing member  104 , that itself is attached to board  102  by bolts  103  and  105 . Upper housing member  106  is pivotably, removably coupled to member  104  at pivot bar  108 . Latching member  110  engages with end  112  of member  104  to hold member  106  on member  104  in the down (engaged) position (not shown in the drawing). Pivoting arm  114  with cam  116  is enabled to compress and release a spring (not shown) in housing  106 , which is held against the top of heat sink  118 . When the spring is released, it pushes down on heat sink  118 . Since heat sink  118  lies against the device when the assembly is closed, this force pushes down on the device and the ACE, to create the good electrical connection between the device and board  102 . With this system, the device can be engaged with board  102  by placing the device and ACE in member  104 , and then bringing member  106  down, and releasing the spring that pushes heat sink  118  against the top of the device, to thereby accomplish the electrical connection from the device to board  102  through the ACE. 
     Although specific features of the invention are shown in some drawings and not others, this is for convenience only as some feature may be combined with any or all of the other features in accordance with the invention. 
     Other embodiments will occur to those skilled in the art and are within the following claims: