Abstract:
The flexible connector for high density circuit applications comprises a multilayer flexible substrate upon which are formed a plurality of contact pads, in a density required by a particular application. This density may exceed two hundred contact pads per square inch. Contact pads of similar size and configuration are formed on the surface of another device, i.e., circuit board, and provision made to align the contact pads of the connector with those of the circuit board. Micro-pads are formed on the surface of the contact pads on the connector such, that when the connector is brought into contact with the circuit board, and sufficient pressure is applied, the micro-pads make actual electrical contact with the pads of the circuit board. Since the total surface area in contact, namely the sum of the surface areas of the micro-pads, is a small fraction of the total area of the connector, a large pressure is provided at the electrical contact interface even when low pressure is provided to the connector as a whole.

Description:
TECHNICAL FIELD 
     The present invention relates to the field of data transmission in high performance computer systems. 
     BACKGROUND OF THE INVENTION 
     In the field of computer processing, there are certain high performance computers that require multi-trace connector cables to carry signals and data from one location to another. Because of the circuit density of such machines, standard sockets and connectors are inadequate for the task, inasmuch as the number of individual electrical connections required on the circuit boards and modules would occupy an unacceptably large portion of the available space. 
     It is desirable that connectors used be capable of densities exceeding two or three hundred contacts per square inch of occupied space on the surface of a circuit board. 
     SUMMARY OF THE INVENTION 
     According to the principles of the present invention, micro bumps on the electrodes of an electrical connector are provided. The connector comprises a piece of flexible circuit material, of a size and shape required for the particular application. Disc-shaped electrodes are provided on the surface of the connector. Via the electrodes, the electrical connector carries the signals from a circuit board or other electronic device to another location, such as another circuit board. These electrodes are in electrical contact with electrical traces formed in various layers of the circuit material of the connector. 
     Each of the electrodes on the connector is provided with a plurality of tiny bumps, or micro-pads on its surface. Each electrode, having the plurality of micro-pads therein is a contact pad for carrying the electrical signal. 
     The circuit board or other electronic device is provided with a plurality of disc shaped contact pads, of a size and configuration that corresponds to that of the connector. The pads are formed concurrently with the formation of other features of the circuit board, by employing known manufacturing techniques. 
     When the connector is placed in correct alignment relative to the contact pads on the circuit board, and modest pressure is applied, solid electrical contact is achieved between each pad on the connector and each corresponding pad on the board via the micro bumps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of the flexible connector according to one embodiment of the invention. 
     FIG. 2 is an enlarged view of a section of the flexible connector, showing the configuration of the contact pads, together with electrical traces. 
     FIG. 3 is a plan view of a single contact pad on the flexible connector. 
     FIG. 4 is a cross section of a single contact pad, together with the underlying structure, along the line  4  of FIG.  3 . 
     FIG. 5 shows a contact pad on a circuit board in electrical connection with the contact pad of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an electrical connector  8 . The electrical connector  8  includes a flexible circuit material  10  having contact pads  12  thereon. In one embodiment of the invention, this flexible substrate, or “flex” is a composite of materials commercially available, i.e., Dupont Pyralux Series. Contact pads  12  are provided on the surface of the flexible substrate and configured such that they correspond to a configuration of contact pads on the surface of a circuit board or other device to which the connection is to be made. When these two surfaces are correctly aligned and brought into contact, each pad on the electrical connector  8  makes direct contact with a corresponding pad on the circuit board. 
     In one embodiment of the invention, this alignment is achieved through the use of alignment holes  34  in the connector. These holes  34  are precisely positioned in relation to the contact pads  12 , and correspond to alignment features on the circuit board. A variety of fasteners and methods of alignment between the electrical connection and circuit board may be designed for use with this invention, and variations in such fall within the scope of this invention. 
     The shape and size of the electrical connector  8  is determined by the specific application. The version shown in FIG. 1 represents one embodiment of this invention. Other applications will require other connector sizes and shapes, also within the scope of this invention. 
     In one embodiment of the invention, a plurality of flexible connectors are formed on a master sheet, then individually cut to the appropriate size and shape. In other embodiments of the invention the fasteners may be manufactured individually or concurrently with other devices. These are considered to be encompassed by the principles of the invention. 
     FIG. 2 shows an enlarged view of a portion of the surface of the connector  8 , on which the contact pads  12  are formed. According to one embodiment of the invention, the distance D between the pads is 0.05 inches, which achieves a density of 400 contacts per inch. Other configurations and densities are possible and may be preferable for specific applications. 
     Electrical traces  36  are positioned on the electrical connector  8  to connect the pads  20  to other terminals and locations on the connector  8 . FIG. 3 shows a plan view of a single contact pad  12 . In one embodiment of the invention, the contact pads are round and have a diameter of 0.031 inches. Tiny bumps, also called micro-pads,  20  are provided on the surface of the contact pad  12 , and comprise a part thereof. In one embodiment of the invention there are four such micro-pads, 0.004 inches in diameter, configured in a circle whose center is common with the center of the pad, and whose diameter A is 0.015 inches. In other embodiments of the invention the size or shape of the contact pads may vary, and the size shape or configuration of the micro-pads may vary. Such variations fall within the scope of this invention. 
     FIG. 4 shows a cross section view, at line  4  of FIG. 3, of a single contact pad  12 , together with the flexible circuit material  10  upon which it is formed. 
     In one embodiment of the invention, a commercially available flexible circuit material is used. A first non-conductive substrate layer  28  is processed, using known methods, to form highly conductive pads  14  on its first surface. For example, non-conductive substrates can be purchased having highly conductive layers affixed to both sides thereof. These substrates can be purchased in various thicknesses, such as 0.001 inches, 0.002 inches, or any acceptable thickness. According to one technique, a commercial substrate is obtained having a conductive layer on both sides thereof. The conductive layer is then patterned and etched on each side, so as to form electrical traces at the desired locations. For example, one side thereof may become the ground plane and the other side thereof may be etched to form pad  14 , and electrical traces connected thereto. The pads  14  may be copper and may have a thickness of 700 μinches. A second nonconductive substrate layer  32  is processed, using known methods, to form electrical traces  36  on its second surface. The traces may be copper and may have a thickness of 700 μinches. Using a non-conductive bonding layer  30 , the first substrate layer  28  and the second substrate layer  32  are bonded together forming a composite flexible substrate, comprising the flexible circuit material  10 . On those places where a grounding plane is desired, an electrically conducted ground plane may be present between first substrate layer  28  and second substrate layer  32 . Connection to such a ground plane may be made by the same techniques by which connection is made to the electrical trace  36  as described later herein. Their positioning is such that the appropriate electrical traces  36  are directly opposite the corresponding pads  14 . In various embodiments of the invention it is appropriate to form additional electrical traces on other surfaces or layers of the flexible circuit material. This may include the formation of electrical traces concurrently, and on the same surface as the conductive pads. In other embodiments of the invention additional substrate layers may be used to accommodate the trace pattern and density desired. The thickness of the individual substrate layers may be 0.001 inches, 0.002 inches, or any thickness or combination of thicknesses deemed appropriate. 
     According to one embodiment of the invention, holes  22  are drilled through the entire assembly, centered in each pad  14 . The holes may have a diameter of 0.0065 inches. Using known processes, a highly conductive layer  24  is plated on the pads  14 , the insides of the holes  22 , and that part of the traces  36  that lies directly opposite the pads, providing an electrically conductive path from the pad  14  to the trace  36 . This conductive layer may be copper or other conductor. Using known processes, a non-conductive cover layer  26  is applied to both surfaces of the substrate  10 . The thickness of this layer is sufficient to provide insulation and protection from abrasion during normal handling and operation. This covers the traces  36  and the pads  14  as desired. According to one embodiment, this layer may have a thickness of 0.002 inches. An opening is provided in the layer  26  over the pad  14 , centered over the hole  22 . The opening may be produced by any appropriate techniques such as masking during formation, etching, milling, photo processing steps or the like. 
     The cover layer  26  forms a mask through which a conductive layer  16  is plated, primarily onto the surface of the preceding conductive plating  24 . According to one embodiment of the invention, this layer is copper or copper alloy, and is coplanar with the surface of the cover layer  26 . 
     Using known plating techniques, micro-pads  20  are formed on the conductive layer  16 . The thickness of the micro-pads may be 0.001 inches. According to one embodiment of the invention, a finish layer  18  is formed on the surface of the plate layer  16  and the micro-pads  20 . The finish layer  18  comprises three successive layers: 150 μinches of nickel, 15 μinches of palladium, and 3-7 μinches of gold in one embodiment. For example, one acceptable technique for forming micro-pads  20  is to provide a mask over each contact pad  12  and then, using appropriate electroplating techniques, plate the micro-pads  20  onto the conductive layer by electrical connection via the respective trace connected to the individual contact pads. An alternative technique is to form a layer thereover and then, using appropriate mask and etching techniques, remove the layer so as to provide the micro-bumps in the final shape as shown. 
     The diameter of the micro-pads is selected to provide good electrical contact under the appropriate pressure when the contact pad is acting as an electrode for connection to a printed circuit board. As a general rule, it will have a surface area considerably smaller than the surface area of the large contact pad. For example, the surface area of the micro-pads may be in the range of 1% to 5% of the surface area of the contact pad  12  thus providing correspondingly increasing pressure at their contact points than would be provided across the wide surface area of contact layer  12 . They may also be provided in other shapes, such as pointed at the tapered square, cone shaped, pyramid shape, or other acceptable shapes for providing solid electrical contact. 
     FIG. 5 shows the individual contact pad  12  of FIG. 4, together with a contact pad  38  formed on a circuit board  40 . According to the principles of this invention, contact pads are provided on the surface of the circuit board, in a size and configuration corresponding to those on the connector. These pads may be formed using the same processes as, and concurrently with, the formation of other features of the circuit board. 
     The contact pads  38  on the circuit board are produced according to known principles. In a common method of manufacture, a hole is drilled and plated to form the contact pad. A feature known in the industry as a knee  41  is formed on the edge of the hole during the plating process. This feature appears in FIG. 5 as a rise in the plate. The knee can interfere with a solid electrical connection by preventing the contact pads from making full contact. According to the principles of the invention, the use of micro-pads solves this problem by straddling the knee and allowing good contact between the respective contact pads. 
     The connector is aligned with the circuit board such that the contact pads  12  of the connector are in contact with the appropriate contact pads  38  of the circuit board  40 , and sufficient pressure applied to ensure a solid connection. The micro-pads  20  may, in some instances, bite slightly into the contact pad  38  to provide a high quality, low resistance electrical connection. The traces  36  of the connector form conductive paths to electrically connect the circuit board to other electronic devices. These may include other circuit boards, modules, printers, computers etc. The connection with these other devices may be made by any means appropriate for the application, including conventional connectors, or some embodiment of this invention. may be made by any means appropriate for the application, including conventional connectors, or some embodiment of this invention. 
     It should be noted that, according to the embodiment described, the surface  47  of the connector directly opposite the primary face of contact pad  12  forms a contact pad which, when the connector is in place on the circuit board  40 , is in electrical continuity with the contact pad  38 . Another connector, stacked on surface  47  would make contact, through the first connector, with the circuit board  40 . This feature allows the designer the liberty of using multiple connectors at the same site, permitting greater trace density or connection to multiple devices. 
     The current carrying requirement of the connectors is low as compared to that needed to power a computer. An important factor to consider is the security and dependability of the contacts. If the pressure between a flat contact pad of the connector and those of the circuit board is inadequate, the result may be either intermittent opening of the contact, which would interfere with the transmission of data, or increased resistance at the point of connection, raising the total impedance of the circuit to an unacceptable level. The design of fasteners and hardware used to exert force adequate to press the contacts of the connector onto those of the circuit board is an important consideration. The use of micro-pads  20  on the surface of the contacts on the connector can significantly reduce the pressure requirements on the board as a whole while providing higher pressure at the actual points of contact, thus making possible the use of lighter, smaller fasteners in high density applications. 
     For example, it may be desired in some designs that 50 pounds of pressure per square inch is required on each contact to assure a solid connection. Assuming a density of 400 contacts per square inch and a diameter of 0.03 inches per contact, then using smooth contacts the total surface area of the contacts would be 0.28 square inches per square inch of connector. This would require 14 pounds of pressure per square inch of connector. In contrast, by incorporating the micro-pads  20  on the surface of the connectors, the surface area is reduced significantly. One embodiment of the invention employs four micro-pads per contact, with a diameter of 0.003 inches each. In other applications, the micro-pads may be 0.001 to 0.01 inches in diameter. If one assumes the same density of contacts, the total surface area is 0.011 square inches per square inch of connector. To achieve the same security in the connection, the required pressure becomes 0.57 pounds of pressure per square inch. A fastener capable of exerting 14 pounds of pressure psi, evenly across hundreds of contacts is more complex and far bulkier than one requiring little more than half a pound. 
     The figures described in the above calculations represent a single possible embodiment of this invention. In other applications the current carrying requirements might be different, or the contact density could vary, but it is clear from the foregoing that the use of micro-pads provides a considerable advantage, and any such use is within the scope of this invention. 
     The use of the term circuit board in the foregoing description is for convenience only, and includes a broad range of electrical products. The connector and method described herein may be used to connect to any electronic device including circuit boards, modules, other connectors, peripheral devices etc. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.