Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2001-11-26
2004-02-17
Aftergut, Jeff H. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S275700, C257S778000, C257S783000
Reexamination Certificate
active
06692611
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for laminating layers or articles, the method comprising curing a photopolymerizable composition through a colored, opaque, or reflective substrate. The article can be, for example, an electronic component, a printed circuit board, or the layers can be two opposing faces of a compact disc.
BACKGROUND OF THE INVENTION
Photopolymerization of monomers using UV light to prepare adhesive compositions is an established part of polymer chemistry. Numerous photopolymerizable compositions, for example, those comprising ethylenically-unsaturated monomers and at least one photoinitiator, have been photopolymerized using UV irradiation. A necessary condition for these photopolymerizations is that the photopolymerizable compositions must be directly exposed to the UV irradiation in order for the photoinitiating component to generate the free radicals required to initiate the photopolymerization process. Numerous industrial processes rely on selective UV photopolymerization, wherein a mask is used to block UV irradiation to specified areas of a surface or substrate so that photopolymerization takes place only in the exposed areas.
In the electronics industry, numerous methods have been used to bond electronic components together for purposes of forming multilayer components or simply adhering a component to a substrate. Methods involving photopolymerization have generally been limited to situations where at least one of the substrate or the component is essentially transparent to UV irradiation. However, in the case of bonding components to a printed circuit board (PCB), this seldom is possible, since PCBs are often made of opaque and colored materials and they usually are at least partially covered with metallic circuit traces. In addition, the electronic component itself, e.g., a chip, typically is completely non-transmissive to electromagnetic radiation. In general, the electronics industry has turned to thermally polymerizable adhesives when reinforcement of solder bonds is desired. This is not entirely satisfactory because of the lengthy thermal curing cycles required. In addition, some electronics components may be heat sensitive and free-radical thermal polymerizations in general do not lend themselves to patterned or selective activation.
Photopolymerization has been used in the bonding of electronic components. For example, surfaces to be joined have been coated with a photopolymerizable adhesive, followed by irradiating the adhesive, then placing the two parts together and allowing the irradiated adhesive to completely cure. Alternatively, irradiation at the peripheral edges of parts to be joined can result in a bond of sufficient strength to temporarily hold a component in place. Often, heating the joined components is necessary for complete curing.
Curing of photopolymerizable adhesives by UV irradiation through a substrate is known. For example, DE 3939628 discloses bonding of electronic components to aluminum oxide or aluminum nitride ceramic substrates that are up to 1500 &mgr;m thick by UV irradiation of at least 50 mW/cm output density. Transmission of UV light through an aluminum oxide ceramic substrate of 1016 &mgr;m thickness is reported to be about 0.6%.
U.S. Pat. No. 4,656,314 discloses curing of a conductive metal-coated UV-curable ink on a translucent PCB by UV irradiation from both the top and bottom of the PCB, wherein at least some of the UV light passes through the substrate from the underside to assist in the complete curing of the ink. The substrate is characterized as a sheet of polyester or polycarbonate that must be at least partially translucent, preferably more than 50% translucent to UV light. Conventional print treated MYLAR (Dupont) is described as an effective commercially available substrate.
U.S. Pat. No. 5,065,505 discloses a method of connecting circuit boards wherein a photocurable adhesive is coated onto a light-transmissive circuit board on which electrodes have been formed. Light is irradiated through the circuit board from the side opposite the coated side, curing the adhesive in areas not shaded by the electrodes. Exemplified photoinitiators have absorption peak wavelengths ranging from 240 nm to 365 nm. Photoinitiators useful in the visible light range are not described. Suitable circuit board substrates include polyimide resin, polyester resin, and the like.
Japanese Kokai Application JP 7-106723 discloses curing of adhesives through a flexible circuit board that is transmissive to 5% or more UV light having a wavelength between 350 and 400 nm. Base films, through which UV-curing takes place, can include poly(etherimide), poly(ethersulfone), polyethylene naphthalate, polyether ether ketone, polycarbonate, and polyethylene terephthalate.
Japanese Examined Application JP 7-81114 discloses curing a photohardenable adhesive in the presence of a diketone photoinitiator and a dialkylamino benzophenone photosensitizer by irradiation through a semitransparent substrate using irradiation wavelengths up to 436 nm.
U.S. Pat. No. 5,607,985 discloses a photopolymerization initiator for visible-light polymerizing adhesives comprising a photopolymerization initiator, an aliphatic tertiary amine and a radical polymerizing monomer. Adhesion of a sandwich construction comprising two opaque glass pieces, each having 10% light transmissivity at 510 nm and 0% light transmissivity between 490 and 200 nm, on exposure for two minutes to a metal halide lamp, is described.
U.S. Pat. No. 5,798,015 discloses generating reactive species (adhesives) by providing a wavelength-specific sensitizer in association with a reactive species-generating photoinitiator and irradiating the wavelength-specific sensitizer. The method is used to laminate at least two layers together by coating the adhesive between the layers and irradiating to effect polymerization thereof, providing that at least one of the layers is a cellulosic or polyolefin nonwoven web or film and the sensitizer is one of a set of specified arylketoalkene moieties.
Optical recording discs, such as compact discs and CD-ROMs, often comprise two or more layers of a polymeric base substrate, each of which comprises a recording layer, bonded together by an adhesive with both recording layers facing each other. Typically, the recording layers comprise an opaque metal foil. Uniform curing of the adhesive between the foils is difficult. U.S. Pat. No. 5,360,652 discloses such an optical recording disc, wherein the adhesive is a photocurable adhesive. In order to adhere the two discs together, the recording medium is configured not to extend to the periphery of the discs so that UV irradiation rapidly cures the adhesive around the edges of the disc, allowing the masked adhesive under the recording medium to cure only by contact with initiators in the irradiated region.
U.S. Pat. No. 5,785,793 discloses one- or two-sided irradiation of an optical recording disc having one or two back-to-back storage medium layers. Curable adhesive is used on the side opposite from the recording medium, in either case, so that UV irradiation must pass through at least the recording medium in order to cure the adhesive. Heat management is an issue for optical recording disc manufacture, since the discs are easily warped and the recording medium is typically a low-melting metal, such as aluminum. Xenon flash lamps are preferred for irradiating the curable adhesive and bonding the discs together.
Bonding of DVD (Digital Versatile Disk) substrates is described by D. Skinner, “UV Curing Through Semi-transparent Materials—The Challenge of the DVD Bonding Process,”
RadCure Letter
, April, 1998, p. 53-56, wherein UV light of 320-390 nm wavelengths is shown to penetrate a 40 nm thick coating of aluminum on a polycarbonate substrate with 91% transmissivity. Curing of an adhesive under these conditions is not disclosed.
SUMMARY OF THE INVENTION
Briefly, the present invention provides a method of laminating a structure comprising the steps of:
a) providing a structure comprising at least two la
Hogerton Peter B.
Kropp Michael A.
Oxman Joel D.
3M Innovative Properties Company
Aftergut Jeff H.
Gover Melanie G.
Kilkenny Todd J.
Lown Jean A.
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