Abstract:
Multiple small conductive and flexible hollow rings, each of which is made from a pliable material, provide a flexible connection medium for use between a substrate and a microelectronic device package. Each ring is soldered to both the substrate and the device. A portion of the sidewall of each ring is not soldered thus insuring that at least part of the ring stays flexible. The rings accommodate elevation differences on a substrate and electronic device package. They also provide a vibration resistant and flexible joint.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. provisional patent application Nos. 60/575,347 and 60/575,348, both of which were filed May 28, 2004.  
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to high-density electrical interconnections structures and more particularly to such an interconnection structure that may be used in interposer applications and to connect electrical devices to circuit boards.  
         [0003]     Microelectronic devices such as state-of-the-art microprocessors require large numbers of reliable connections in increasingly-small areas. As the number of connections between an electronic device and a substrate to which the device is to be mounted increases, the likelihood that just a single connections will not be made or will fail increases.  
         [0004]     In “wave soldering,” an electronic components is soldered to a substrate by flowing molten solder over a substrate in which electronic components are mounted. A substrate, to which electronic components are to be soldered, is passed over the flowing, molten solder such that exposed metal and fluxed surfaces on the lower surface of the substrate surface wick the molten solder upward from the solder bath. As the substrate with the wicked, molten solder moves away from the molten solder bath, the solder cools and solidifies, establishing an electrical connection between electronic devices and soldered surfaces of the substrate.  
         [0005]     As connection density increases in the electronic arts and lead lengths from electronic devices decreases, the increasing number of connections that must be made make it statistically more likely that even a single connection will not be made or will fail. Even minor irregularities in a substrate&#39;s planarity can cause connection problems.  
         [0006]     One problem with prior art soldering techniques arises when the contact surfaces of a substrate and an electronic device are separated from each other by different distances. For example, if one or two contact leads or one or two contact surfaces of a microprocessor are more widely separated from a planar substrate than the other contact leads or contact surfaces, the molten solder might not wick between the substrate and the more-distant contact surfaces of the electronic device. Prior art soldering techniques suffer from an inability to make a connection when the spacing or distance between contact surfaces of two devices or surfaces to be joined, varies by more than a small amount.  
         [0007]     When even a single connection between an electronic device and its supporting substrate is either not made at the time of manufacture, or fails while in use, the cost to identify a failed electrical connection and to repair it can often exceed the cost to manufacture the product in which the electronic device and supporting substrate operates. Improving the manufacturability of electrical connections and improving the reliability of electrical connections after manufacture would be an improvement over the prior art.  
         [0008]     The present invention is directed to a connector structure that is suitable for use in high-density applications, is easy to manufacture and which provides a reliable contact force while avoiding the aforementioned shortcomings.  
       SUMMARY OF THE INVENTION  
       [0009]     It is a general object of the present invention to provide a connector device that has a plurality of flexible, conductive rings arranged in an array so as to contact conductive pads on a circuit board and contacts or contact pads of an opposing electronic device.  
         [0010]     Microelectronic devices are electrically connected and mounted to a circuit board or other planar surface using small conductive hollow rings between electrical contacts of an electronic device and a circuit board or substrate. Each ring is a band of pliant conductive material that extends around a center point. An axis of rotation extends through each ring. Each ring&#39;s axis of rotation is substantially parallel to the other axes or rotation and to the plane of the substrate and the plane of the electronic device.  
         [0011]     Each ring acts as a small, round spring-type of contact which will deform when a force is directed toward the interior of the ring from any direction. When the force is removed, the ring will return to its original shape. The resilient behavior of the rings provide a small, flexible interconnection which can accommodate variations in the planarity of opposing surfaces. Each ring&#39;s flexibility also accommodates circuit board or substrate flexing as well as impacts and vibration.  
         [0012]     These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     In the course of this detailed description, the reference will be frequently made to the attached drawings in which:  
         [0014]      FIG. 1  is a perspective view of a discrete conductive hollow ring constructed in accordance with the principles of the present invention, and which is suitable for connecting an electronic device to a circuit board or other substrate;  
         [0015]      FIG. 1A  shows the deformation of the discrete conductive hollow ring of  FIG. 1  in response to an externally-applied force;  
         [0016]      FIG. 2  is a side elevation of a microelectronic device and a plurality of conductive hollow mounting rings mounted to a substrate;  
         [0017]      FIG. 3  is a side elevation of a conductive mounting ring filled with a resilient, non-conductive material and the ring being soldered to a substrate;  
         [0018]      FIG. 4  is a side elevation of a substrate and a plurality of conductive hollow mounting rings mounted to an electronic device; and,  
         [0019]      FIG. 5  shows a conductive ring and the space between an electronic device and a substrate filled with non-conductive resilient material.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]      FIG. 1  is a perspective view of a discrete conductive hollow ring  10  which is constructed in accordance with the principles of the present invention and which may be used for the mounting of an electronic device to a circuit board or other substrate. In the embodiment shown, the conductive hollow ring  10  has a diameter D substantially the same as the length L of the ring  10 , but other configurations may be used .  
         [0021]     The ring is preferably made up of a band of pliant conductive material, such as a copper or gold alloy or a spring steel coated or plated with a good conductor such as copper or gold. Alternate embodiments can include resilient plastics that are either plated or otherwise conductively coated. Regardless of its material, as is true of all rings, the material from which it is made is centered about a point in space  12  through which extends an axis of rotation  14  for the ring  10 . A force, F, exerted on the ring  10  from the exterior, and directed radially inward of the ring, will cause the ring  10  to deflect as shown in  FIG. 1A . As is well-known, as the force F increases past the material&#39;s elastic limit, the ring will collapse but as long as the applied force F remains below the elastic limit of the ring material, the ring  10  will act as a spring, and return to its original shape when the applied force is removed. The spring-like action of the ring  10 , when used as in array of rings will provide a connection that can accommodate planarity differences between a substrate  8  and an electronic device  6 . It can also provide a connection that can be flexed and which will be more tolerant of impact and vibration. The improved physical robustness is provided by the flexible material from which the ring  10  is made, a portion of which between the substrate  8  and device  4  is not soldered. The rings may be easily by electro forming or electro-discharge machining to maintain the tolerances down to critical sizes and diameters, such as 500 micrometers and the like.  
         [0022]     The ring  10  illustrated is provided with two strips, or bands, of nickel-plating  20 ,  22  that run along the side of the ring  10  from one open end to the other. The nickel plating bands  20  and  22  act as and are referred to herein as solder barriers  20  and  22 . As shown, they are substantially opposite to each other on the exterior surface of the ring  10 . They prevent solder from wicking all the way up and around the circumference of the ring, thereby insuring that at least part of flexible ring side wall will not be soldered to the substrate  8  or an opposing surface, but rather will still remain pliant.  
         [0023]     As shown in  FIG. 3 , when the ring  10  is attached to a substrate  8 , molten solder will only wick upwardly until it reaches the solder barriers  20  and  22 . Solder that wicks upward along the exterior of the ring  10  will form fillets  24  between the ring&#39;s  10  lower curvature ( FIG. 10 ) and the top of the substrate  8  as part of the normal soldering process. The solder barriers  20  and  22  insure that flexible material from which the ring  10  is made will not be completely coated with solder during a soldering process, insuring that the ring  10  will retain flexibility.  
         [0024]     In a preferred embodiment as shown in  FIG. 1 , the ring  10  side wall cross-section is substantially planar or rectangular. In an alternate embodiment, the ring side wall cross-section can be circular, oval or other shape although non-rectangular side wall shapes might tend to be more rigid. Inasmuch as a circle and an oval are both special case ellipses, the more general side wall shape is referred to herein as elliptical.  
         [0025]      FIG. 2  is a side elevation of a microelectronic device  4  positioned just above a plurality of conductive hollow mounting rings  10 , the assembly of which comprise a connector  2  for mounting the electronic device  4  to a circuit board or other substantially planar substrate  8 . Each of the rings  10  in  FIG. 2  is substantially the same as the ring  10  shown in  FIG. 1  albeit in  FIG. 2 , the solder barriers  20  and  22  are not visible.  
         [0026]     The mounting rings  10  in  FIG. 2  are aligned to that each of their axes  14  are parallel to each other and extending into the plane of the figure. In an alternate embodiment, the rings  10  can have their axes co-linear.  
         [0027]     Inasmuch as the axes  14  extend into the plane of  FIG. 2 , the axes  14  of the rings  10  also tend to extend parallel to the plane of the substrate  8  which also extends into the plane of  FIG. 2 , as well as the plane of the underside  6  of the device  4 . The side walls of each ring therefore “face” the substrate  8  and the underside  6  of the device  4 . The planes in which the ring  10  open ends lie are substantially orthogonal to the substrate surface  8  and the underside  6  of the electronic device  4 .  
         [0028]     The several discrete conductive hollow rings  10  each provide a redundant signal path along its body between conductive traces on the surface  8  of the substrate and connection points or nodes on the under side  6  of the electronic device  4 . Signals can traverse both sides of the ring to get from circuits on the device  4  to circuits on the substrate  8  below. This dual signal path also assist in reducing the inductance of the system in which such contacts are used. As shown in  FIG. 2 , the several conductive rings  10  are initially attached to the substrate  8  and provide a connector for the device  4 .  
         [0029]      FIG. 3  shows an alternate embodiment of a conductive ring  10  wherein the interior  18  of the ring  10  is filled with a resilient, non-conductive material  18 , such as silicone. The aforementioned solder fillets  24  mechanically and electrically attached the ring  10  to the substrate  8 . Filling the interior  18  space with a resilient material increases the strength of the ring  10  but also prevents solder from flowing into the interior space  18  by either wicking or capillary action.  
         [0030]      FIG. 4  shows a connector  2  for mounting an electronic device. In  FIG. 4 , the connector  2  is formed using the aforementioned discrete conductive rings  10 , but the connector  2  in  FIG. 4  includes a non-conductive under fill material  26  which holds the conductive rings  10  in place with respect to each other. The under fill material  26  can be a non-conductive silicone layer, the thickness of which is less than the outside diameter of the conductive rings  10 . When the electronic device  4  is urged downward, each of the rings will deform slightly. Because they are pliable, with each of them tending to oppose a downward compressive force, each conductive ring  10  will tend to make physical contact with the surface of the substrate  8  below it as well as the surface  6  of the electronic device  4  above it. Each ring will therefore provide a better electrical and physical contact than is otherwise possible with a straight pin used in the prior art.  
         [0031]      FIG. 5  shows a non-conductive, resilient under fill material  26  disposed between the device  4  and a substrate. It also shows the hollow conductive ring  10  filled with the under fill material, adding stiffness to the ring  10 .  
         [0032]     The connector  2  shown in  FIG. 4  can be initially attached to the substrate  8  or to the electronic device  4 . It can be wave soldered to either the substrate  8 , the device  4  or both of them simultaneously.  
         [0033]     As shown in  FIG. 2  and  FIG. 3 , each of the hollow contact rings  10  of the connector  2  shown in  FIG. 4  has solder barriers (not shown in  FIG. 4 ) which prevent molten solder from wicking all the way around the ring  10  thereby defeating the flexibility provided by the thin metal from which the rings are made.  
         [0034]     The hollow, conductive rings are preferably made from electronically conductive metals that will also accept a solder barrier. Copper, silver and gold are excellent conductors and can be alloyed with other metals that can provide good resilience; they can also be locally plated with solder-barrier metals such as nickel. The rings  10  can also be formed from metal-plated plastics.  
         [0035]     Those of skill in the art will appreciate that since each of the rings  10  can be slightly compressed from its original shape that the rings can overcome slight variations in the planarity of the substrate  8  and/or the electronic device  4 . By providing a solder barrier that prevents solder from wicking all the way around a ring, each ring&#39;s flexible side walls acts as a small round spring and will deform when a force is directed toward the interior of the ring. When the force is removed, the ring will return to its original shape. The resilient behavior of the rings provide a small, flexible interconnection which can accommodate variations in the planarity of opposing surfaces. Each ring&#39;s flexibility also accommodates circuit board or substrate flexing as well as impacts and vibration. The resulting connection between the substrate  8  and an electronic device  4  is more tolerant of substrate and/or device flexing. The connection is also less susceptible to shock or vibration-induced failure.  
         [0036]     While the preferred embodiment of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.