Compositions – Electrically conductive or emissive compositions – Metal compound containing
Reexamination Certificate
1998-07-20
2001-03-06
DelCotto, Gregory R. (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Metal compound containing
C252S512000, C252S514000, C252S519320, C252S519330, C106S001130, C106S001150, C106S001210
Reexamination Certificate
active
06197222
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The present invention is directed to an electrically conductive paste useful in forming electroconductive connections between electroconductive members. The invention is further directed to a method of forming electroconductive connections between electroconductive members utilizing the aforementioned electrically conductive paste.
2. Background of the Invention
Electrically conductive compositions for forming electroconductive connections between conductive members, i.e., termination pads on a chip and pads associated with circuits printed on a circuit board, are well known in the art. Indeed, conventional solder compositions which comprise eutectic alloys, such as lead-tin alloys, have long been known in the art. Such materials, however, require the inclusion of a flux composition to remove oxides resident on the surface of the solder metals. These flux materials which are acidic in nature, are corrosive to the highly valuable electrical components that are so electrically connected, and are usually removed by the use of solvents. However, traditional flux removal solvents, such as CFCs, are environmentally objectionable and their use complicates the manufacturing procedures wherein flux materials are utilized. Thus, flux materials find less and less application in electrical assemblies requiring the connection of expensive electrical components used in modern electrical and electronic devices such as computers and the like.
New electrically conductive pastes have been developed in view of this known disadvantage of eutectic alloys. Typically, these pastes comprise electrically conductive metal powders and a thermosetting resin material. Such materials are applied to the electrical components then heated to form an electrical connection. During that heating the thermosetting resin cures. Although it is known to utilize a non-oxidizing metal as the electrically conductive powder in these new pastes, thus avoiding the necessity of utilizing a flux, such materials are characterized by the disadvantageous property of not being remeltable. Those skilled in the art are aware that upon curing of a thermosetting resin, a crosslinked structure is formed. Polymers having a crosslinked structure cannot be remelted or dissolved. Thus, such electrically connected components, which can be quite expensive, cannot be easily disconnected and reconnected, when necessary, by merely heating or redissolving the conductive paste to disconnect the components and thereafter reconnecting them using the previously used or fresh identical paste.
A recent development, which provides a significant advance in the art, is described in U.S. Pat. No. 5,062,896 which is incorporated herein by reference. The '896 patent is directed to a composite polymer-solder paste which included a eutectic metal alloy powder filler, said alloy having a melting point below 200° C., present in a concentration of about 85% to 93% by weight, based on the total weight of the paste.
The paste composition of the '896 patent included, as a second component, a thermoplastic polymer. It is the presence of that thermoplastic polymer which represents the advance in the art provided by this patent, The thermoplastic polymer of the paste of the '896 patent permits remelting of the paste. As those skilled in the art are aware, a thermoplastic polymer does not cure into a three-dimensional network. Rather, the thermoplastic polymer, which in the '896 patent is preferably a poly(imide siloxane), because it essentially does not crosslink, can be remelted or redissolved.
A third component included in the paste of the '896 patent is a volatile organic solvent having a boiling point above the melting point of the alloy powder but below the maximum reflow temperature of the composite paste composition.
Finally, a fourth and last essential component of the '896 composite paste is a transient flux material which is an aliphatic monocarboxylic acid having a boiling point between about 140° C. and 200° C. It is noted, however, that an aromatic monocarboxylic acid, such as 2-methoxybenzoic acid, may also be utilized. The flux material is generally present in an amount of between about 0.5% and 1.5% by weight.
Although the paste composition of the '896 patent, which may include optional components such as surfactants and the like, represents a significant advance in the art, solving as it does the problem associated with remelting of metal powder-polymer composites, does not overcome the other major problem associated with the materials of the prior art. That is, the problem associated with acidic fluids, which evolve from the fourth essential transient flux material and have a detrimental corrosive effect upon the expensive electronic components which are electrically connected by the paste, is unaddressed.
A flux component must be included in the paste of the '896 patent because of the inclusion therein of oxidizable metals, as made clear by the use therein of metals having a melting point of less than 200° C. As such, the advance in the art provided by the paste of the '896 patent does not address all the problems associated with the electrically conductive interconnecting materials of the prior art.
SUMMARY OF THE INVENTION
A new electrically conductive paste has now been developed which overcomes the problems associated with electroconductive compositions of the prior art used to electrically connect electroconductive members. Thus, the electroconductive paste of the present application not only permits remelting of the paste but, in addition, solves the problems associated with acidic fluxes by eliminating a flux material therein.
In accordance with the present invention, an electrically conductive paste is provided. The paste includes a thermoplastic polymer, a conducting metal powder, and an organic solvent system.
In further accordance with the present invention, an electrically conductive composite is described. The composite comprises at least 30% by volume, based on the total volume of the composite, of a conducting metal powder.
In still further accordance with the present invention, a method of circuitizing a flexible substrate to create a flexible circuit is provided. In this method, electronic components, such as integrated chips and surface mount devices, are bonded to a flexible substrate by means of the conductive paste of the present invention. The paste, under the influence of heat, is converted into the conductive composite of the instant invention.
In yet still further accordance with the present invention, a method of interconnecting multilayered structures, alternately described as multichip module laminates, is taught. In this method, multilayered structures, which may be thermoplastic, e.g., a polyamide, are electrically interconnected by means of vias, i.e., through holes or blind holes, from pairs of multilayered structures which are fused together under appropriate, heat and pressure utilizing the conductive paste of the present application.
In even further accordance with the present invention, a method of surface mounting an integrated circuit (IC) chip in a lead-frame to a circuit board is disclosed. In this method, the electrically conductive leads of the lead-frame or a surface mount device, such as a capacitor or a resistor, are electrically connected to pads of a circuit board by means of the paste composition of the present invention which is thereupon converted to the conductive composite under the application of heat.
In final accordance with the present invention, a method of directly bonding a bare integrated circuit chip to a circuit board is set forth. In this method a bare integrated chip is disposed, in upside down configuration, such that its terminal I/O pads contact conductive pads on the surface of a circuit board. An effective amount of the paste of the present invention is placed between the I/O pads of the chip and the conductive pads of the circuit board. The paste is heated to form an electrically conductive compo
Booth Richard Benton
Cooper Emanuel I.
Gaynes Michael Anthony
Ostrander Steven Paul
Roldan Judith Marie
Delcotto Gregory R.
International Business Machines - Corporation
Scully Scott Murphy & Presser
Trepp, Esq. Robert M.
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