Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2002-10-04
2004-06-01
Lorengo, J. A. (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S233000, C156S240000, C156S247000, C156S277000, C156S289000, C156S273300, C427S096400, C427S148000, C428S201000, C428S202000, C428S208000, C428S914000, C174S257000
Reexamination Certificate
active
06743319
ABSTRACT:
BACKGROUND OF THE INVENTION
In the microelectronics industry the basic method for forming circuit traces on a substrate involves a combination of photoresist and electroplating steps, which incorporate many hazardous and expensive compounds and solvents, and involves extensive processing of the circuit board/substrate. One method used to avoid the repeated processing of the substrate involves forming the circuit traces on a metallic board using the photoresist/electroplating processes or by die cutting the circuit patterns from a metal foil. An adhesive is then used to transfer the circuit to the substrate by transfer lamination. Another alternative is the “lift off” method. In this process an adhesive image of the circuit traces is formed on the substrate. A metal foil is then bonded to the adhesive image and the unwanted foil not bound to the adhesive image is then lifted off by an adhesive film. It would be desirable to eliminate the use of photoresist and electroplating steps and the need for an adhesive in a transfer lamination process.
There is substantial literature relating to transfer lamination of patterned circuits from one substrate to another. For example, Seeger, Jr. (U.S. Pat. No. 4,75,439) discloses “applying a slurry of a vaporizable solvent, metal particles and a small amount of binder in the shape of the circuit pattern desired to a removable layer, vaporizing the solvent, covering the powdered metal and binder with an adhesive to hold the powdered metal and carrier in place on the removable layer, laminating the hydrocarbon containing substrate with pressure and heat to cause compacting of said powder and bonding of said compacted powder to said substrate by adhesive layer(sic), said heat being insufficient to destroy said adhesive, substrate and removable layer, and separation of the removable layer.” Seeger, Jr., discloses that the adhesive is essential not only to bond the finished circuit to the final substrate but also to bond the metal particles together.
Seeger, Jr. discloses, “A metal slurry of metal particles, e.g. noble metals such as silver, palladium, gold and platinum, is preferably mixed with the combination of other metal particles such as nickel or tin. A vaporizable solvent is mixed therewith as well as a small amount of a curable plastic binder.” (Seeger, Jr. column 2, lines 15-20). A particular mixture is given as an example in Seeger, Jr. column 4, lines 8-18. These mixtures are very similar to mixtures known as “Ormet” and are described in Capote et al, U.S. Pat. Nos. 5,538,789 and 5,565,267, among others. The mixtures are described by Ormet Corp. (formerly Toronaga Technologies) as “Transient Liquid Phase” materials in that they function by heating the combination of high melting point and low melting point metal powders in a fluxing environment to the eutectic temperature at which the powders alloy and freeze again to form a continuous conductor. The mixture also includes an epoxy resin, which cures at the eutectic temperature and acts as a binder to fill the porosity between the metal particles and to adhere them to the substrate.
The mixtures of Seeger, Jr. also resemble conventional polymer thick film (PTF) mixtures of metal powders and epoxy or other polymer binders available from Acheson, DuPont and many other vendors, with the addition of the low melting tin powder. Polymer thick film materials are cured at approximately 150° C. for times ranging from a few minutes to an hour. The result is a conductor in which the epoxy binder is the continuous phase and electrical conductivity results from adventitious contact between metal particles. The conductivity typically is less than ten percent of the conductivity of bulk metal, usually much less, and is somewhat subject to changes with age and environmental stress. The traces are not solderable because of the epoxy. However, the cure conditions are compatible with most polymer substrates, and so there is no incentive to perform transfer lamination. Virtually all computer keyboards and other membrane touch switches are now typically made by printing PTF materials directly on polyester substrates.
Ormet type Transient Liquid Phase mixtures cure to metal traces in which the metal is the continuous phase and the epoxy binder fills the interstices. Electrical conductivity is substantially better than most PTF materials, but still only 10% of bulk metal because the material is an alloy as well as being porous. Lack of solderability is still a problem because of the epoxy binder, but the electrical performance is better than PTF, and there is an incentive to do transfer lamination because the Ormet cure temperature of 220° C. is well above the 125° C. to which polyester substrates typically are limited. Seeger, Jr's stated cure temperature of 177° C. (350° F., column 2, lines 33-43) is lower than quoted by Ormet, and will result in poorer metallurgical properties, but will still require transfer lamination.
A novel family of materials has been developed for printing on polymer substrates such as those used for printed wiring boards and flexible circuits. These compositions have been used to produce metal traces utilized in a transfer laminate process. They offer the advantage over polymer thick films, Ormet materials, and the like, by producing electrical conductors consisting of a pure, single-phase metal can be produced by a simple print-and-heat process instead of by the usual multi-step photolithographic etching process. This family of novel compounds is commercially available as Parmod® compositions from Parelec, LLC, Rocky Hill, N.J., USA, and are described in U.S. Pat. Nos. 5,882,722 and 6,036,889 (the total disclosure and contents of each patent being hereby incorporated by reference); as well as in co-pending PCT patent application WO98/37133 and in Applicants' co-pending U.S. application Ser. No. 09/367,783 filed Aug. 20, 1999 (the total disclosure in its entirety of each application being hereby incorporated by reference). These compositions can be formulated for use in a printing process, for example as inks, pastes, toners, etc. These formulations can be printed on a substrate and cured at a temperature below 450° C. to well-consolidated films or traces of pure metal in seconds. The fast curing capability of these Parmod® compositions, as well as their easy application, makes it possible to use them to create complex thin metal objects by very simple and low-cost processes. An example of such an object is a pattern of electrical conductors used as an antenna in a radio frequency identification tag. Another such application is as a TAB bonding decal for semiconductors. Electronic circuit patterns of many types can be produced by this process and bonded to various types of substrates and devices. The method can be used to produce strain gauges, thermocouples and other types of instrumentation. Many other such objects and applications will be evident to those skilled in the art.
These Parmod® compositions include printable inks and pastes, which comprise metal flakes and/or powders mixed with a Reactive Organic Medium (ROM). The compositions are printed on a substrate and heated. This decomposes the ROM, which chemically welds the particulate constituents together, and the residual organic material leaves as vapor. The result is a pure metallic deposit which can function as an electrical conductor with low resistivity and which is solderable. This capability is unique relative to all other options for printable electronics.
In contrast to the mixtures described in Seeger, Jr., the Parmod® compositions cure to a pure, single-phase metal trace with no organic content. This is demonstrated in FIG. 4 of U.S. Pat. No. 5,882,722 for example. The cure temperature of these mixtures is typically 200-300° C., which definitely requires transfer lamination to apply them to many polymer substrates, specifically polyester. The result, however, is a vastly superior product with electrical conductivity between 25 and 50% of that of bulk metal. Copper traces prepared from these mixtures
Lorengo J. A.
Nissim, Esq. Stuart H.
Paralec Inc.
Synnestvedt Lechner & Woodbridge LLP
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