Electrical connection system

Abstract: A method for electrically connecting planar element substrates (12) to form an array (10) by forming conductive bridges (16) between metal pads (14) located on the surface of array elements (12). The conductive bridges (16) are formed to be nearly coplanar with the planar elements (12) and are made to connect the end faces (14b) of pads (14) which are fused to the planar substrate (12). Metal wire (16a), solder (16b-c), a conductive polymer (16d), or a suspension of conductive particles in paste (16e) are used to form the bridges. The bridges (16) have a low profile, occupy a very small area and reduce the need for highly accurate alignment of adjacent substrates (12) within the tiled array (10) before electrical connections are formed. These low profile bridges (16) are especially advantageous tn that they allow a protective plastics or similar cover sheet, or a liquid crystal laminate, to be surface mounted on the adjacent array (10) without causing detrimental surface blemishes or ridges. The small area of the bridges (16) greatly reduces the non-transmitting area of the visual display. The bridges (16), being substantially narrower than the pads (14) which they connect, are more likely to connect the intended pads (14) and less likely to "short" pads (14) which are not in accurate alignment.

Description: The present invention relates to a method and apparatus for providing electrical connections More particularly, this invention relates to the wireless connection between the butting side faces of adjacent planar tiles in a multi-tiled visual display system. Flat screen visual displays, because of their small volume and relative light weight, are of great interest in the computer and television industries Some uses for flat screen displays include portable computer displays, high definition television (HDTV) and conventional television screens and wall mounted visual displays Many large flat screen displays are assembled from a series of smaller displays which are linked together electrically and mechanically. There are several reasons why large displays are manufactured in this manner; many manufacturers do not have the capability to produce large flat displays as single units, and the yield of good displays is often low. Low yields mean that it is more economically attractive to piece together a large display from selected good smaller panels than to risk losing a large and costly panel during production. One conventional method of connecting visual display panels employs discrete wire bonding between adjacent panels. The present invention replaces these wire bonds with a wireless connection employing one of several conductive media. The quality of a large visual imaging system built of smaller displays depends, to some extent, on the percentage of usable display area. The area of each display devoted to an interconnection between itself and the adjoining display is unusable as a visual display. Conventional wire bonds occupy a relatively large area of the visual displays. An objective of wireless interconnections, along with electrical integrity, is to reduce the visual display area they occupy and increase the area available for visual output. Flat screen visual displays are often protected from mechanical damage by a transparent cover. This cover may integrate a liquid crystal or other display system. The cover ideally lies directly on the screen elements If the screen is a composite of smaller elements, however, and conventional wirebonding is used to connect abutting elements, the wires may create bumps or ridges under the cover. Thus, conventional wire bonds could prevent the cover from adhering smoothly to the panel faces and cause disturbing visual distortions. DE-A-36 35 558 discloses a device which is made from several adjacent substrates having electrical connections between two substrates in which a thin layer of conducting strips replace conventional wire connections. In one application, an electro-optical active substrate, which may be part of an array, is arranged between two dielectric substrates and coupled to the end faces of the conducting strips. The conducting strips are passed over an insulator, such as of polyimide or glass, which is placed under the conducting strips in the area where they pass from one substrate to the other over the abutting side faces of the substrates. The substrate faces to which the conducting strips and insulators are affixed lie generally in the same plane. Consequently, the insulators over which the conducting strips pass add thickness to the conducting strip layer, creating bumps or ridges under the flat screen cover. It is a further aim, therefore, of the present invention to provide an inter-panel connection which is nearly planar with the adjacent panels it connects. This allows a protective cover or display cover to lie flush and smooth over the display tiles which make up the array. The problem of providing small area, low volume interconnections has presented a major challenge to designers of flat display panels. The development of a reliable connection, occupying a small area between adjacent panels, represents a major technological advance in flat panel output performance The enhanced performance obtained by using such interconnections satisfies a long felt need in the field of flat display devices. The present invention is defined in independent claims 1 and 9. Electrical butting of tiled arrays by wireless bonds solves the problem of devoting a distractingly large area of tiled visual displays to interconnections. The present invention further solves the problem of interconnections between visual displays interfering with a display cover's or protective cover's ability to lie flush on the displays. When edges of flat display panels must be electrically connected, this system utilizes a smaller area of the visual panel to form the joint than other available methods. Therefore, the usable area for visual display is at a maximum. Additionally, the wireless connection method maintains near coplanarity between the display panels and the electrical connections, allowing a protective or display cover sheet to lie smoothly on the panel faces The invention solves two problems inherent in wire bonding between flat panel displays: the problem of devoting too great an area to connections, and the problem of preventing a cover sheet from lying flat over the panels. The array of flat electronic display devices must be linked electrically to each other. Particularly, the semiconductors are part of a flat visual display screen. The apparatus of the present invention connects leads, or metal pads, which are located around the periphery of individual tiled elements of an array with leads in adjacent tiled elements in the composite array. The connections are made using small conductive bridges as opposed to discrete wires Each conductive bridge extends to touch bevel faces provided in the body of adjacent substrates on which electrical conductors are to be interconnected. The bevel faces provide an additional volume below the substrate face on which the conductors are affixed, which supports the conductive bridge and enhances the electrical and mechanical connection. Conductive bridges have several major advantages over discrete wires in this application; the bridges have a low profile and therefore do not cause bumps under a liquid crystal laminate or cover sheet which may be laid over the array of tiles; wireless connections occupy a smaller surface area of the overall tiled array than do discrete wire bonds and therefore leave a greater area free to transmit visual or other signals. These wireless electrical connections are a significant improvement over discrete wire bonding. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of two abutting substrates electrically connected together in accordance with the present invention; Figure 2 is a partial cross section, on a larger scale of the conductive bridge shown in Fig 1, and Figure 3 is a plan view of two substrates connected together in accordance with the present invention Figure 1 is a cross-sectional view of a tiled array 10 of visual displays. Each substrate 12 of each tile in the array is of dielectric material, such as glass or polyimide, and provides a support base for metal pads 14 having face 14a attached to the substrate face 12a. Both the faces 14a and 12a extend perpendicularly to the plane of the paper in Figure 1. The metal pads 14 may be aluminum, copper, gold, nickel, or any material which is capable of conducting electrical signals or power. The metal pads 14 may be affixed to the substrate base 12 by one of several methods, which include, but are not limited to: electroless plating, electro-plating, chemical vapor deposition or physical deposition The pad end face 14c terminates at or near a bevelled corner face 12c of the substrate body The bevel angle is preferably between 25 and 75 degrees to face 12a. The distance between pairs of pad faces 14c to be joined electrically is about 25 µm. A conductive bridge 16 carries electrical signals and power between the metal pads 14. Figure 2 is a cross-sectional view showing the conductive bridge 16. The upper faces 14b of the metal pads 14 lie in a common plane 20. The plane 20 is a reference plane for the preferred contact surface between the tiled array 10 and a protective plastics cover sheet or liquid crystal laminate which may be attached to the array 10. The plane 20 may pass through the conductive bridges 16 since the bridges may project above the plane 20. The conductive bridge projections may lie in a plane 22 spaced by up to about six µm of the ideal contact plane 20. This height limitation applies only to the portion of conductive bridge which is not disposed between the tiles of the adjacent array 10 The conductive bridge 16 is connected to the pad end faces 14c and may be connected to the upper pad faces 14b. The bridge 16 also may extend to touch the substrate bevel faces 12c and the substrate side faces 12b. Figure 3 is a plan view of two substrates 12, six metal pads 14, and three conductive bridges 16 which comprise a tiled array 10. The two substrates 12 are arranged side-by-side and are joined by the conductive bridges 16 The pitch of the metal pads is 2-40 pads per mm. Figure 3 is a schematic drawing which includes a generic representation of the various conductive bridges 16a-16e which may be employed. These bridges, 16a, b, c, d, and e, are all functionally interchangeable and are all equally satisfactory for the performance of this invention. The conductive bridges 16 in Figure 3 may be formed using a conductive ball bond 16a formed of gold or other conductive wire about 12 to 25 µm in diameter. More than one conductive ball bond 16a may be used to connect a pair of metal pads 14. The resulting conductive ball bonds 16d may be either melted or mechanically compressed as necessary to regulate the conductive bridge maximum height surface 22. The conductive balls may be bonded together ultrasonically The view shown is a broken-away section of a tiled array 10 which may extend in any direction parallel to the plane of the paper in Figure 3 Figure 3 also represents a functionally similar embodiment of the invention in which the conductive bridges 16b are formed from solder The solder may be any alloy with a low melting point and high conductivity, but the preferred embodiment uses one of the commercially available lead-tin alloys A flux, either integral to the solder form, or applied separately, is used to clean the surfaces which will be joined When fabricating a conductive bridge 16b, solder and a flux are applied in a pattern so that when the tiled array 10 is generally heated, the solder will melt to form conductive bridges 16b. The solder pattern can be achieved through a variety of techniques Some methods of applying solder in a pattern include: printing directly onto the metal pads 14, either using a transfer tape fabricated by screen printing or lithographic patterning, or using a lift-off technique to define solder areas, or, alternatively, by employing a spacer to contain the solder and by preventing it from bridging between unintended pairs of metal pads 14 By applying many thin solder stripes across the gaps between pairs of metal pads 14 in the tiled array 10, the risk of electrically connecting pairs of metal pads 14 other than those intended, and the need for highly accurate alignment of those pairs, can be significantly reduced. Figure 3 further represents another functionally similar embodiment of the invention, employing conductive bridges 16c. The solder which forms the conductive bridges 16c, however, is applied in a different manner from that in bridges 16b. Solder and a flux used to form bridges 16c are applied over and around the areas where conductive bridges 16c are desired. Then, the solder is melted only in those areas where conductive bridges 16c are needed. A laser or hot jet of air may be used to melt the solder where desired. Again, by melting only narrow stripes of solder at a time, the need for extremely accurate alignment between pairs of metal pads 14 may be reduced. Finally, any excess solder and flux are removed, and the conductive bridges 16c are all that remain. Figure 3 additionally represents the use of the following, functionally similar, yet physically different conductive bridges, here labeled 16d. The conductive bridges 16d are of a conductive organic paste The conductive paste may be a polymer. It also may be a thermally curable or other epoxy resin Dots of the conductive organic paste forming conductive bridges 16d may be about 100 µm in diameter, and positioned within about 12 µm of each other. Figure 3 also represents conductive bridges 16e, which are similar to those labeled 16d except that the paste which forms each conductive bridge 16e is not itself the conductive medium. The paste may be a polymer, epoxy resin or any material capable of forming a structural bridge Electricity, however, is transmitted through a suspension of conductive particles in the paste. The structural paste and conductive particles together form the conductive bridges 16e. These conductive particles may be a conductive polymer or epoxy material or conductive spheres, flakes or powder. The various conductive bridges 16a-16e represented in Figure 3, though sometimes different from each other in physical appearance, are all interchangeable without affecting the function of this invention. The schematic representation of the conductive bridges 16a-16e in Figure 3 demonstrates only their physical relationship to the other components of this invention, and does not limit the many variations that may be utilized to practice the present invention.

Claims: Electrical apparatus comprising: an array of substrates (12) of dielectric material arranged side-by-side, each substrate having a major face (12a), at least one side face (12b), and a bevelled corner face (12c) contiguous with the other faces, a plurality of electrical conductors (14) spaced apart from each other and fixed to the major face of each substrate, each conductor having a face (14c) at each end and being aligned with the end face of a conductor on the adjacent substrate, the conductors on each substrate having their surfaces remote from the respective major face lying in a common plane (20), and a bridge (16) of electroconductive material mechanically and electrically connecting the end faces (14c) ot an aligned pair of conductors on adjacent substrates, each bridge being in contact with the respective pair of bevelled corner faces (12c). Apparatus as claimed in claim 1, in which each conductor is a pad (14) of metal. Apparatus as claimed in claim 1 or 2, in which the conductive bridges (16) project above the common plane by a distance of up to six µm. Apparatus as claimed in any preceding claim, in which at least one bridge is formed by a conductive ball bond (16a). Apparatus as claimed in any preceding claim, in which at least one bridge is formed of a low melting-point alloy (16b) which has been melted and solidified to form the bridge. Apparatus as claimed in any preceding claim, in which at least one bridge comprises a generally-applied solder paste (16c) which has then been selectively melted from between the conductors to leave at least one bridge. Apparatus as claimed in any preceding claim, in which at least one bridge is formed of a curable electroconductive paste (16d) of organic material Apparatus as claimed in claim 7, in which the paste has within it a suspension of electroconductive particles. A method of fabricating a plurality of electrical connections between a plurality of electrical devices, including the steps of: affixing a plurality of electrical devices to an array of substrates (12) arranged side-by-side, with each substrate having a major face (12a), at least one side face (12b), and a bevelled corner face (12c) contiguous with the two faces; providing a plurality of spaced-apart electrical conductors (14) fixed to the major face, each conductor having a face (14b) at each end contiguous with the adjacent end of the respective corner face, and with the conductors on each substrate having their surfaces remote from the major face lying in a common plane, and interconnecting the aligned end faces of a pair of conductors on adjacent substrates with a bridge (16) of electroconductive material which is also in contact with the respective pair of bevelled corner faces, and with the contiguous side faces.

Inventors: BRUCE RICHARD, HO JACKSON, MEULI WILLIAM, MIKKELSEN JAMES C, BRUCE, RICHARD, HO, JACKSON, MEULI, WILLIAM, MIKKELSEN, JAMES C.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1991
Main classification: H05K3
IPC codes: H05K3, H01R4, H01R12, G09F9, H05K1