Compositions: coating or plastic – Coating or plastic compositions – Coating repellent
Reexamination Certificate
2000-06-06
2002-08-13
Lovering, Richard D. (Department: 1712)
Compositions: coating or plastic
Coating or plastic compositions
Coating repellent
C106S033000, C106S243000, C252S364000
Reexamination Certificate
active
06432182
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a method of controlling the spread of an adhesive on a circuitized organic substrate. More particularly, this invention relates to a method of forming a protective coating on the surfaces of a circuitized organic substrate. Such protective coating reduces the spread of adhesives that contact or are disposed on the surfaces of the substrate.
BACKGROUND OF THE INVENTION
Semiconductor devices typically comprise an integrated circuit die and a plastic chip carrier that is comprised of an organic substrate, such as polyimide, having metallic circuitry and wire bond pads disposed on a surface thereof Frequently, one of the initial stages in assembly of the semiconductor device involves attachment of the chip carrier to a lead frame by means of a thixotropic organic adhesive such as, for example, an epoxy-based adhesive, an acrylic-based adhesive, or a silicone. During this process, which is referred to as the board-attach process, the adhesive is applied to the lead frame. Then the chip carrier is placed onto the adhesive. The assembly may then be heated to assist in cure of the adhesive, thereby strengthening the attachment between the organic substrate and the lead frame.
Adhesives are also used to attach the integrated circuit die to the circuitized surface of the chip carrier. Such adhesives, generally, contain an organic binder and an electrically or thermally conductive filler. The adhesive, which initially is in the form of a viscous liquid or paste, is distributed onto preselected regions on the circuitized surface. The preselected regions include chip pads which are formed by selectively plating gold on a nickel plated copper surface. Thereafter, the perimeter of integrated circuit die is aligned with and placed on the adhesive. The entire structure, including substrate, adhesive and die, is then baked to cure the adhesive, thereby strengthening the attachment between the integrated circuit die and the underlying substrate. In some cases, the integrated circuit die is then electrically connected to the electric circuitry of the carrier by wire bonding terminals on the chip to wire bond pads on the surface of the substrate.
Although the adhesives used during the various stages of assembling the resulting module are fairly viscous, the organic liquids in the adhesives have a propensity to bleed and spread out away from the point of attachment. For example, during the board attach process, these organic liquids often bleed out from the periphery of the chip carrier attachment site and spread up the edges of the chip carrier onto the circuitized upper surface, where the organic liquids can contaminate the wire bond pads and render them non-bondable. This condition, which is hereinafter referred to as “adhesive resin bleed”, can cause significant problems during later assembly steps when the bond sites are needed to complete necessary electrical connections. The spreading organic liquid can also contaminate any portions of a soldermask which may lie in the near vicinity of the wire bond pads.
Adhesive resin bleed is also encountered during other stages of assembling the resulting module. For example, the resin in the electrically conductive or non-electrically conductive adhesive that is used to attach the integrated circuit die to the chip carrier can also bleed out during cure. The heat and pressure encountered during cure causes the resin to bleed out from the periphery of the die attachment area and spread out over adjacent areas where electrical connections ultimately need to be made.
The problems associated with adhesive resin bleed are even more pronounced when the chip carrier is treated with a plasma containing oxygen and/or argon, prior to the application of the adhesive. Such plasma treatment is frequently employed prior to assembly of the semiconductor module to clean the wire bond pads and to roughen the surface of the substrate. Adhesive resin bleed becomes an even greater problem when the surface of the substrate is roughened by pumice treatment.
Various methods for reducing contamination of the electrical bonding sites by adhesive resin bleed have been developed. For example, the chip carrier surface may have a recess at the point of attachment of the chip, such that the back of the chip and the adhesive will be positioned below the adjoining circuitized surface of the chip carrier where electrical bonding sites are located. Unfortunately, not all integrated circuit assemblies provide the option of a recessed cavity in the carrier surface. Very large scale integrated assemblies require a large number of bonding sites, and these sites, typically, are on the same level of the carrier surface as the chip attachment site.
In some instances mechanical barriers have been used to control adhesive resin bleed onto the bonding sites. For example, in U.S. Pat. No. 5,409,863 issued on Apr. 25, 1995, a low profile barrier, such as a solder mask ring surrounds the chip attachment site. During cure, the ring prevents the spread of adhesive resin onto the adjacent bonding sites on the chip carrier. Unfortunately, the ring does not completely prevent adhesive resin bleed when the chip carrier is treated with an oxygen or argon containing plasma.
Accordingly, a chip carrier having a surface that is resistant to adhesive resin bleed is desirable. Furthermore, a method of imparting adhesive resin repellency to the surface of the chip carrier and to the metallic components on the surface is desirable. A method which does not reduce the bondability of the wire bonding sites is especially desirable.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method of treating the surface of an organic substrate, particularly a circuitized surface of a chip carrier, which method reduces the spread of adhesive resin that is subsequently deposited on the surface, is provided. This method comprises the steps of applying a treatment solution comprising a fatty acid compound, an alkalizing agent, and a solvent comprising water and from about 5% by volume to about 90% by volume of an organic solvent selected from the group consisting of alcohol, glycol ether, and combinations thereof to the surface; and then removing substantially all of the solvent from the solution to provide a thin film on the surface of said substrate. The film comprises the fatty acids that were present in the treatment solution. In a preferred embodiment the treatment solution further comprises a chelating agent.
The present invention also relates to a treatment solution for reducing adhesive resin bleed on the surface of a circuitized organic substrate. The present invention also relates to a circuitized organic substrate that is resistant to resin bleed. Such substrate has a film comprising a fatty acid compound disposed on the circuitized surface thereto. In accordance with the present invention, it has been determined that organic substrates having a film comprising a fatty acid compound on the surface thereof are less wettable with respect to the organic liquids that are normally present in the adhesives that are used to bond chip carriers to integrated circuit chips and to lead frames. It has also been determined that the presence of the film on the surface of the organic substrate reduces adhesive resin bleed. It has also been determined that the presence of the film on the metallic components of the circuitized surface of a chip carrier renders the metallic components less wettable to the organic liquids present in such adhesives. It has also been determined that the presence of the film does not adversely affect the subsequent bonding of wires to the metal components on the surface of a chip carrier.
DETAILED DESCRIPTION
In accordance with the present invention, a method for conditioning a surface of an organic substrate, particularly a circuitized surface of an organic substrate, to provide a surface that is resistant to adhesive resin bleed is provided. The method comprises the steps of applying a treatment solution to the surfaces of the
Konrad John Joseph
Papathomas Konstantinos I.
Welsh John A.
Driggs Lucas Brubaker & Hogg Co. LPA
Lovering Richard D.
Lucas James A.
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