BUTTON BOARD CHIP INTERCONNECTION SYSTEM

Abstract: The external contact pads of an integrated circuit chip are connected to an overlying printed circuit board by means of wrapped wire connector conductive buttons held in place in a button board in registration with individual ones of the chip contact pads and contact areas on the overlying printed circuit board. The printed circuit board is pressed in place against the integrated circuit chip so as to elastically compress the con­ nector buttons between the corresponding individual contact areas on the circuit board and contact pads on the clip and thereby maintain a high quality electrical interconnection therebetween.

Description: BUTTON BOARD CHIP INTERCONNECTION SYSTEM Cross-Reference to Co-Pendinq Application This application is related to a co-pending application by the same inventor, entitled ZMulti-Element Circuit Construction" filed with this application and given serial number (unknown). Technical Field This invention relates to techniques for connecting an integrated circuit chip onto a printed circuit board. Background of the Invention Integrated circuit chips are typically mounted on a ceramic chip package which in turn is connected to a multi-layer printed circuit board. The multi-layer printed circuit board is typically plugged in at one edge to a larger printed circuit board, commonly called a "smother board". Each integrated circuit chip (hereinafter "chip") may be comprised of thousands of microscopic transistors which are ultimately connected to a plurality of conductive pads located along the periphery of the chip and providing electrical contact to the outside world. This contact is made by soldering extremely fine wires to the chip pads, the wires in turn being connected individually to pins located on the periphery of the ceramic chip package. The pins on the chip package in turn plug into matching receptacles located on the multi-layer printed circuit board. One problem with the foregoing conventional method of connecting a chip to a large scale integrated system is that the bonding of the fine wire between the chip pads and the chip package pins requires expensive tooling and special labor which is typically done only in the factory. The wires themselves increase the interconnect inductance while the presence of the ceramic chip package increases the overall capacitance of the system, which inhibits operation at high speeds. Also, while a heats ink may be attached to the ceramic chip package, the chip package inhibits heat transfer between the chip and the heatsink, thus raising the operating temperature of the chip. Furthermore, the interconnection between each pad and the multi-layer printed circuit board requires four interconnections, namely the two ends of each wire soldered to the pad and the two ends of each pin on the chip package, which limits the quality of the overall connection. Finally, the required space in-between the chip package pins requires a chip package having an area much greater than the chip itself, on the order of three or four times. Therefore, the number of chips on a given multi-layer printed circuit board is necessarily limited. Summary of the Invention In the present invention, reliable mounting and interconnection between a chip and a multi-layer printed circuit board is realized without a ceramic package, soldered wires, special tooling or special labor. In the present invention, the chip is mounted on a housing, which may be metal and may serve as both a heats ink and a hermetic seal. The housing has a recess in which the chip is snugly held. The top face of the chip, whereon is formed the peripheral contact pads and the thin film integrated circuits, faces away from the housing. A button board, comprising insulating material, overlies the top face of the chip and has a plurality of cylindrical openings therethrough which are in registration with individual ones of the contact pads along the chip periphery. Each cylindrical button board opening holds a fuzz button or wrapped wire connector element comprising a cylinder of finely compressed wrapped wire. The connector elements or "fuzz buttons are preferably each formed as a single strand of wadded wire that makes electrical contact at multiple points when compressed against a contact area. Connectors of this type have significant advantages over soldered or wire-wrapped connections, and provide connections of high integrity and reliability. In contrast to a soldered connection, this mechanical connector element has very few associated variables that can affect the quality of the connection. In fact the only significant variables are the size of the element, which can be fairly accurately controlled, and the compressive force used to make the connection, which is also easily controllable. The cylindrical fuzz button is greater in length than the thickness of the button board so that it protrudes beyond both the top and bottom faces of the button board and makes contact with the underlying chip pad. Overlying the button board and its plurality of fuzz buttons is the multi-layer printed circuit board which has specially patterned conductive elements on its bottom surface facing the button board, individual ones of these contact areas being in registration with individual ones of the fuzz buttons held in the button board openings. Accordingly, the multi-layer printed circuit board is electrically interconnected with each of the chip pads through he fuzz buttons. Excellent ohmic contact is assured by means of a clamp overlying the multi-layer printed circuit board. The clamp is fastened to the underlying housing so that each of the fuzz buttons is compressed between the underlying pad and the overlying printed circuit board contact area. The compression causes the fuzz buttons to deform so as to acconmodate the compression and form a very high quality electrical connection. Preferably, the deformation of the buttons is elastic so that, upon release, the buttons return to their original shape. The advantages of the invention include the elimination of the ceramic package and its attendant soldered wires. As a result, no special tooling or labor is required to connect the chip to the printed circuit board, and in fact the chip can be replaced outside of the factory by a user with very simple tools and no special labor. Furthermore, the interconnect inductance of the soldered wires and the dielectric capacitance of the ceramic chip package are eliminated. There is only one interconnection between each chip pad and the multi-layer printed circuit board, namely the corresponding fuzz button which results in a much higher quality electrical connection. Elimination of the ceramic chip package eliminates the main source of resistance to heat transfer between the chip package and the heats ink comprising the housing. Finally, there is no interconnection fanout between the chip pads and the ultimate connection to the multi-layer printed circuit board, so that the density of chips mounted on a multi-layer printed circuit board in this invention is three or more times greater than that of the prior art. Probably the most significant advantage of the invention is the savings in cost due to the elimination of special tooling and special labor connected with soldering the individual wires between the chip pads and the ceramic package pins of the prior art. It is expected that the invention will find use for connecting integrated circuits in computers, toys, military electronics and consumer electronics. Description of the Drawings The invention is best understood by reference to the accompanying drawing, Figure 1, which is an exploded perspective view illustrating the button board chip interconnection system of this invention. Detailed Description of the Invention Referring to Figure 1, a housing (2) may serve as both a hermetic seal and a heatsink. The housing (2) may comprise, for example, a planar aluminum substrate of approximately .055 inches in thickness and includes a recess (4) formed in the top surface thereof of about .035 inch in depth in the example of Figure 1. An integrated circuit chip (6) fits snugly in the recess (4), the chip being on the order of .025 inches in thickness in this example. The top surface of the chip (6), which includes the thin film integrated circuits and a plurality of peripheral contact pads (8), faces away from the housing (21. A button board (10) also fits snugly in the recess (4) over the top surface of the chip (6) and has a thickness on the order of .010 inches in this example. Preferably, the recess (4), the chip (6) and the button board (10) are virtually congruent to assure a tight fit. The bottom board (10) includes a plurality of openings (12), each of which can hold a single fuzz button (14) or wrapped wire connector element protruding from both ends of each opening (12). The fuzz button (14) comprises very fine wire which is wrapped into a solid form. Preferably each of the openings (12) comprises a circular aperture of about .004 inch in diameter extending through the thickness of the button board (10) in registration with individual ones of the peripheral contact pads (8), and each fuzz button (14) comprises beryllium-copper alloy wire of about .0003 to .0005 inch in diameter wrapped in a cylindrical shape of about .004 inch in diameter (so that it fits snugly in an opening (12)) and approximately .016 inches high. The fuzz button (14) may be formed of alternative materials such as a topper-8ilver alloy comprising 98.5% copper and 1.5% silver or may comprise a phosphor-bronze alloy wire. The wire is pressed into a solid shape (such as the preferred cylinder) to form a fuzz button of about 30% volume density. A circuit board to6). preferably comprising a multi-layer printed circuit board, overlies the housing (2), the chip (6) and the button board (10), and is pressed toward the housing (2) by means of a clamp (20) and a bolt (22). The printed circuit board (16) includes a plurality of individual conductive contact areas (18) on the bottom surface thereof, which are in registration with individual ones of the fuzz buttons. The fuzz buttons (14) are compressed by the clamp (20) from their original .016 inch length to about the 010 inch thickness of the button board (10) between the chip pads (8) and the individual conductive contact areas (18). In this manner, a high quality electrical connection is formed by each fuzz button (14) between the corresponding chip pad (8), in contact with one end of the fuzz button, and the corresponding circuit board contact area (18), in contact with the other end of the fuzz button (14). Preferably, the compression of the fuzz buttons (14) is completely elastic so that, when the compressive force of the clamp (20) is removed, the fuzz buttons (14) return to their original shape. The chip (6) may be removed and replaced while the fuzz buttons (14) and the button board (10) may be reinstalled and used again. Typically, the chip contact pads (8) comprise square areas of thin film metallization on the chip periphery, each pad being .004 inch on a side located at .005 inch centers along the chip periphery. The foregoing detailed description is exemplary in nature and describes what is the preferred mode of the invention. However, other variations are possible. For example, the fuzz buttons may be square in cross-section rather than circular so that the openings (12) would also be square. Furthermore, the dimensions given are exemplary and other dimensions, such as the thickness of the housing (2), the depth of the recess (4), the thickness of the chip (6) and the thickness of the button board (10) may differ from the foregoing example without departing from the scope of the invention. Finally, it is not necessary that the housing (2) be of aluminum metal or that it be a heatsink. For example, only a portion of the housing (2) which is in contact with the chip may be a heatsink element, the rest of the housing (2) being thermally passive. Furthermore, arrangements other than the recess (4) and the clamp (20) may be provided for registering the fuzz buttons (14) with the contact pads (8). The button board (10) may be formed of a rigid plastic material such as epoxy. The only requirement of the multi-layer printed circuit board is that it have the circuit board contact areas (18) on its bottom surface in registration with the fuzz buttons (14), the button board openings (12) and the chip contact pads (8). Furthermore, the board (16) does not necessarily have to be a multi-layer printed circuit board. In case of chip malfunction, it is contemplated that the system comprising the printed circuit board (16) may be repaired by simply replacing individual chips (6) that are connected to it. The operation is simple and merely requires releasing the clamp bolt (22), removing the clamp (20), the printed circuit board (16), the button board (10) (leaving the plurality of fuzz buttons in their respective button board openings (12)) and removing and replacing the chip (6) and reversing the foregoing procedure.

Claims: CLAIMS 1. A button board interconnect system for an integrated circuit chip having a plurality of contact pads on its top surface, comprising: a button board overlying the top surface of the chip and having a plurality of openings therethrough in registration with said plurality of contact pads on said chip; a plurality of wrapped wire connector buttons in said button board openings and extending near the top and bottom surfaces of said button board in registration with individual ones of said contact pads; a printed circuit board overlying said button board and having a plurality of circuit board contact areas in registration with and facing said plurality of wrapped wire connector buttons and being in contact therewith; ; means for compressing said chip toward said printed circuit board so as to compress said connector buttons between said circuit board contact areas and said contact pads and thereby deform said connector buttons. 2. A button board integrated circuit chip interconnection system, comprising: a housing having a recess in its top surface; an integrated circuit chip snugly resting in said housing recess and having a top surface whereon are formed a plurality of chip contact pads facing away from said housing; a button board overlying said chip and snugly resting in said housing recess, said button board having a plurality of openings extending therethrough which are in registration with individual ones of said chip contact pads; a plurality of wrapped wire conductive connector buttons in said plurality of button board openings, said buttons extending near the top and bottom surfaces of said button board; ; a printed circuit board overlying said button board and overlying said housing, said printed circuit board having circuit board contact areas on its bottom surface facing said button board and being in registration with corresponding ones of said buttons, button board openings and chip contact pads; a clamp overlying said printed circuit board, including means for pressing said clamp toward said housing so that said buttons are elastically deformed and compressed between said circuit board areas and said chip contact pads. 3. The device of claims (1) or (2) wherein said housing comprises a hermetic seal around said chip and a heats ink for said chip. 4. The device of claims (1) or (2) wherein said buttons comprise finely wrapped alloy wire on the order of .0003 to .0005 inch in diameter of one of the following alloys: beryllium copper, silver copper or phosphorous bronze, said buttons being about 30% dense. Se The device of Claims 1 or 2 wherein said buttons are of a cylindrical shape, and wherein said button board openings comprise corresponding circular apertures, the thickness of said button board being less than the length of said buttons along their cylindrical axis. 6. An integrated circuit chip interconnector system, comprising: a heat conductive housing comprising a conductive body having a top surface and a polygonal recess formed in said top surface; an integrated circuit chip having a top surface and a bottom surface and individual conductive contact pads located on its top surface, said top and bottom surfaces of said chip having a virtually congruent polygonal shape, wherein at least a portion of said polygonal recess is virtually congruent with said polygonal shape of said chip and said chip is snugly nested in said polygonal recess portion; ; a button board comprising an insulating gasket having a polygonal shape congruent with at least a portion of said polygonal recess and being nested therein, said button board including a plurality of openings extended therethrough which are in registration with individual ones of said chip contact pads; a plurality of compressible wrapped wire connector buttons having a cross section corresponding to that of said button board openings and nested therein, each of said buttons having an uncompressed axial length such that it axially protrudes beyond the button board opening at both of its ends; a printed circuit board with a bottom surface facing said button board and having a plurality of conductive contact areas on said button board bottom surface, said contact areas being in registration with individual ones of said connector buttons; and means for pressing said circuit board toward said housing so that said connector buttons are compressed between said chip contact pads and said circuit board contact areas. 7. A device for connecting the contact pads of an integrated circuit chip to a circuit board, comprising: a plurality of conductive contact areas on the surface of said circuit board facing said chip contact pads and being in registration therewith; a plurality of wrapped wire connector buttons; means for holding said buttons between said pads and said circuit board contact areas in registration therewith; and means for pressing said chip and said circuit board together so as to compress said connector buttons between said contact areas and said chip pads. 8. A device for connecting the contact pads of an integrated circuit to a plurality of contact areas on a substrate in registration with said pads, comprising: a plurality of compressible wrapped wire connector elements; and means for compressing each of said connector elements between a corresponding one of said pads and a corresponding one of said contact areas. 9. The device of Claims 6 or 7 wherein said connector buttons comprise a cylinder of wrapped wire elastically compressed along its longitudinal axis between said contact areas and said chip pads.

Inventors: SMOLLEY, ROBERT
Applicants: TRW INC.
Application year: 1984
Main classification: H05K7
IPC codes: H05K7, H05K1, H01L21