Abstract:
In accordance with the invention, an I-channel surface mount connector comprising a length of cylindrical rod having a generally I-shaped cross section is improved by providing an extended mounting flange. When a first circuit device is connected to a second circuit device with the extended flange extending outward of the first device, the flange can extend beyond the periphery of the first device. This extension has the advantage that the solder bond between the flange and the second device can be easily inspected from above using visual inspection equipment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/737,303 filed Dec. 15, 2000 by the present inventor and entitled “I-Channel Surface-Mount Connector”. The 09/737,303 application is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to devices for interconnecting circuit devices such as IC packages to circuit boards, circuit boards and modules to circuit boards or substrates, and substrates to substrates. Specifically, the invention relates to low impedance surface-mount connectors. In addition to having advantageous qualities of compactness, low interconnection resistance, low inductance, and mechanical compliance, the connectors permit easier inspection of the solder bonds between the circuit devices.  
         BACKGROUND OF THE INVENTION  
         [0003]    As electronic circuits become denser, faster and increasingly complex, devices for interconnecting them are subject to more demanding requirements. With the great increase in the density of active components, interconnection devices become large consumers of available volume. And increased density brings an increase in required currents and power dissipation, aggravating thermal mismatch between connected circuit devices. In addition, higher circuit device speeds place stricter constraints on tolerable interconnect inductance.  
           [0004]    U.S. patent application Ser. No. 09/737,303 describes a low impedance surface-mount connector comprising a length of cylindrical rod having an I-shaped cross section. The device permits interconnection by pick-and-place techniques. A first circuit device having one or more circuit components is interconnected with a second circuit device by surface mounting such connectors on the first circuit device, providing corresponding solder pads on the second circuit device, and mounting the connectors of the first circuit device onto the pads of the second.  
           [0005]    This interconnection has advantageous qualities of low resistance, low inductance, mechanical compliance and ease of manufacture. However, it does not permit vertical visual inspection of the solder bonds at the point of connection of the first circuit device to the second. One looking down from above cannot see the solder connection between the connector and the solder pad on the second device. Accordingly there is a need for interconnection devices that can be inspected more easily.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the invention, an I-channel surface mount connector comprising a length of cylindrical rod having a generally I-shaped cross section is improved by providing an extended mounting flange. When a first circuit device is connected to a second circuit device with the extended flange extending outward of the first device, the flange can extend beyond the periphery of the first device. This extension has the advantage that the solder bond between the flange and the second device can be easily inspected from above using visual inspection equipment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings:  
         [0008]    [0008]FIGS. 1A, 1B,  1 C and  1 D are perspective, end and side views of an I-channel surface mount connector with extended flanges in accordance with one embodiment of the invention;  
         [0009]    [0009]FIG. 2 illustrates an alternative embodiment employing a plurality of slots;  
         [0010]    [0010]FIG. 3 is a perspective view of an embodiment employing no slots;  
         [0011]    [0011]FIGS. 4A and 4B are perspective views showing surface mount connectors being mounted on a first circuit device.  
         [0012]    [0012]FIGS. 5A and 5B show the first circuit device being mounted on a second larger circuit device;  
         [0013]    [0013]FIG. 6 is a top view showing the surface mounted connectors after being mounted on. 
     
    
       [0014]    It is to be understood that these drawings are for illustrating the concepts of the invention and are not to scale.  
       DETAILED DESCRIPTION  
       [0015]    Referring to the drawings, FIGS. 1A, 1B and  1 C are views of a novel surface mount connector  10  comprising an elongated metallic body of an essentially “I-shaped” cross section, with extended flanges of unequal length. The connector comprises a longitudinally extending central beam section  11  having a first side  12 A and a second side  12 B, and laterally extending base sections  13 ,  14  at both edges, comprising flanges  13 A,  13 B,  14 A and  14 B extending from the first and second sides of the central beam section  11 . Flange  13 A extending from the first side  12 A of central beam section  11  is longer than flange  13 B extending from the second side  12 B of said central beam section. Flange  14 B extending from the second side  12 B of central beam section  11  is longer than flange  14 A extending from the first side  12 A of said central beam section. Flanges  13 A and  14 B can be of equal length. Flanges  13 B and  14 A can be of equal length.  
         [0016]    The longitudinal dimension of the connector can be divided into longitudinal sections  15 ,  16  by one or more slots  17 . The ends of the body can include recessed regions  18 , which can be formed as partial slots, as shown in FIG. 1D.  
         [0017]    With reference to the coordinate system shown in FIG. 1, the connector  10  extends longitudinally along the x-axis. The major surfaces of the beam section  11  are parallel to the x-z plane, and the major surfaces of the base sections  13 ,  14  are parallel to the x-y plane. In the embodiments shown herein, the connector mounting surfaces are the outer major surface  19 A of base  13  and the outer major surface  19 B of base  14 . The connector thus provides electrical and thermal connectivity in the z-direction.  
         [0018]    In general, the connector length A is determined by the level of acceptable impedance for the connector. The greater the length, the lower the inductance and resistance. The height B is chosen to be greater than the height of the tallest component on the interconnect side of the circuit devices to be interconnected so that contact between the two circuit devices is only through the connectors. The base width C is chosen to meet visual inspection requirements and tipping requirements for the connector. The tipping requirements prescribe the maximum angle that the base outer surface can make with a planar substrate without falling over. Preferably the width C is sufficient to provide a tipping angle of at least 30°. The cross sectional corners of the base sections are advantageously rounded, as by a 7 mil radius, in order to provide a good fillet when soldered and thus produce reliable solder joints.  
         [0019]    The presence and number of slots  17  is determined by the xy compliance requirements for the connector. A slot  17  will divide the beam section into two adjacent longitudinal sections  15 ,  16 . Slots  17  should be dimensioned and placed so that the longitudinal dimension of each section  15 ,  16  does not exceed its height dimension. Thus if the length of a connector is less than its height, no slot is needed. If the length is greater than the height but not greater than twice the height, one slot is desirable. Recessed end regions  18  can reduce the effective length of the connector, reducing the need for slots to provide xy compliance. The optimal shape for a slot is geometrically similar to that of the central beam section  11 , but rotated by 90°. A slot  17  can be confined to the beam section  11  as shown in FIG. 1 or can cut through one of the base sections.  
         [0020]    These connectors can be easily fabricated by extruding a metal rod of an essentially I-shaped cross section with extended flanges, punching the desired slots and cutting to desired length. The connectors can achieve very low impedance (electrical and thermal) because the rods can be extruded of soft metals of high electrical and thermal conductivity such as copper or silver. Preferably the formed connector is plated with a solderable coating of Ni/Au or Ni/solder (e.g. tin-lead solder). The Ni advantageously has a thickness at least 50 microinch, the Au at least 3 microinch or the solder, at least 200 microinch.  
         [0021]    Alternatively, the connectors can be fabricated as hollow cylinders of bent sheet material. The sheet material (e.g. 7 mil. sheet material) is punched to shape, folded into an essentially I-shaped cross section with extended flanges and the desired slots are punched. In this instance copper-based alloys such as Be—Cu or phosphor bronze are favored over soft copper or silver to provide rigidity at the cost of increased impedance.  
         [0022]    Typical lengths A are in the range 0.030″-0.300″. Typical heights B are in the range 0.040″-0.120″, and typical base widths C are in the range 0.045″ to 0.180″. The central beam section  12  typically has a thickness in the range 0.010″-0.030″, and the base sections  13 ,  14  typically have a thickness in the range 0.010″-0.030″.  
         [0023]    [0023]FIG. 2 is a perspective view of an alternative embodiment of a connector  20  similar to FIG. 1 except that the connector employs a plurality of slots  17 A,  17 B and  17 C. The advantage of plural slots is that the connector, while still preserving xy-compliance, can have a greater length, providing reduced resistive, inductive, and thermal impedance. Advantageously the number of slots is sufficient to maintain the longitudinal extent of adjacent areas less than their height.  
         [0024]    [0024]FIG. 3 is a perspective view of another embodiment of a connector  30  similar to FIG. 1 except that it is free of slots. This connector advantageously has a longitudinal dimension less than its height.  
         [0025]    [0025]FIGS. 4A and 4B are perspective views showing surface mount connectors  10  being mounted on a first circuit device  51 . The circuit device  51  comprises a substrate or circuit board  52 , one or more circuit components  53  and one or more mounting pads  54  for receiving connectors  10 . Advantageously pads  54  are pre-coated with solder. Preferably the height of connectors  10  is greater than the height of any circuit component  53 . The connectors can then be placed on the pads by standard pick-and-place techniques, such that the longer flanges of the base sections distal to the substrate  52  extend outward away from the edge of such substrate so as to overhang the substrate. The connectors can be soldered to the pads in a conventional solder reflow step.  
         [0026]    The next step shown in FIGS. 5A and 5B is to mount the first circuit device  51  onto a second circuit device  60 . Device  60  can also comprise a package or substrate or circuit board  61  and is preferably the larger area circuit device of the two. As a preliminary step, circuit device  60  is provided with solder pads  62  appropriate in size and distribution for receiving the connectors mounted on circuit device  51 . The pads  62  are preferably pre-coated with solder, and circuit device  51  can be applied on device  60  using pick-and-place techniques with connectors  10  in registration with pads  62 . The two circuit devices can then be interconnected by solder reflow. The result is an interconnected composite device having advantageous qualities of, compactness, low interconnection resistance, low inductance, mechanical compliance, and visual inspectibility of the solder joints between the connectors and the second circuit device.  
         [0027]    [0027]FIG. 6 shows surface mounted connectors  10  after being mounted on first and second circuit devices ( 51 , 60 ). As can be seen, the longer flanges of the base sections distal to the substrate  52  can be seen to extend beyond the edge of the substrate, facilitating visual inspection of the edges of the solder joints between the base sections proximate to substrate  61  that are created when the first circuit device is attached to a second circuit device by solder reflow.  
         [0028]    The nature and advantages of the invention will become clearer by consideration of the following specific examples.  
       EXAMPLES  
       [0029]    The principles of the invention were used to design devices of the type shown in FIG. 1 to meet the needs of a specific application. The application required a total of 8 surface-mount interconnects between a “circuit device” comprising a circuit board and a “motherboard” (a circuit board of larger area). The height of the tallest component on the interconnected side of the circuit device was H=0.070″. The impedance requirements, dictated by device performance needs, were that each interconnect have a maximum inductance of 0.30 nH, and a maximum resistance of 50 micro-ohm. The mechanical requirement was compliance in both X and Y directions, in the face of thermal stress caused by large differentials between the operating temperature of the circuit device and the motherboard. The visual inspection requirement was that the portion of the interconnect directly in contact with the motherboard extend at least 0.020″ beyond the edge of the circuit device. In addition, the area occupied by each interconnect needed to be very small, while still maintaining a minimum angle of tip of at least 30° for stability prior to reflow.  
         [0030]    The device height B was chosen to be B=0.082″, to meet the B&gt;H requirement. Flange  13 A was designed to be 0.044″ longer than flange  13 B, and flange  14 B was designed to be 0.044″ longer than flange  14 A. This ensured on proper placement, that a minimum of 0.021″ of a connector flange would extend horizontally beyond the edge of the circuit device so that it can be inspected from above. The base width C was selected as C=0.112″, with a 0.007″ radius at the cross sectional corners to facilitate adequate solder fillets. This combination of B, C and the radius yielded a minimum angle of tip of 38°. To provide necessary compliance in the Y direction and mechanical stability, the thickness of the central beam section was selected to be 0.015″, and that of the basesection was selected to be 0.014″.  
         [0031]    Once the above dimensions were fixed, the connector length A and width of the slot were determined on the basis of meeting the requirements for impedance and X-directional compliance. A length of A=0.100″ and a slot width of 0.020″ were found adequate, in conjunction with the selection of copper, USN nomenclature C11000, as the device material. The inductance was calculated to be 0.27 nH, and the resistance was 35 micro-ohm. The device footprint was 0.100″×0.112″, with a pad size of 0.116″×0.128″. Thus, all requirements of the application were satisfied.  
         [0032]    It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention.