Background of the Invention The present invention relates to a method for protecting the surface of a metal foil or other substrate sheet and to a laminate sheet product. More particularly, the present invention especially relates to a method for protecting an etchable conductive metal foil using an electron beam cured coating to protect the foil from damage during handling and subsequent processing into a printed circuit board. Printed circuit boards having a high density of electrical interconnections and circuitry have been developed from multilayer printed circuit board laminates, such as epoxy resin prepregs, to meet the needs for miniaturized electronic components. The multilayer printed circuit board laminates are made from suitable reinforced resin materials and metal foils, particularly copper foils, which are processed together at high temperature and pressure. The laminates are then used in the manufacture of printed circuit boards.

A printed circuit board is made from a laminate by etching away portions of the conductive foil from the laminate surface to leave distinct patterns of conductive lines and formed elements on the surface of the etched laminate. Additional laminates and/or laminate materials are then packaged together with the etched product to form a multilayer circuit board package. Additional processing, such as hole drilling and component attaching completes the printed circuit board package. While printed circuit boards are known, printed circuit board technology continues to advance to provide higher density boards with smaller printed circuit lines. Miniaturization has advanced to the point that surface contamination of the raw materials has now become a significant problem, and generally cannot be tolerated in commercial application. A clean environment is one way to avoid, or at least minimize, contamination problems. Another way to avoid or minimize problems is to use protective films as is, for example, disclosed in U.S. Patent 5,120,590

which issued June 9, 1992 to Savage et al for "Protective Conductive Foil and Procedure for Protecting an Electrodeposited Metallic Foil During Further Processing".

The present invention provides a further alternative way to protect surfaces of sheets and to avoid, or at least minimize, contamination problems in the manufacture of circuit boards and the like, in accordance with the present invention it has been found that electron beam cured adhesive coatings are especially advantageous in bonding a protective film to a foil laminate to be made into a printed circuit board. The electron beam cured adhesives have high temperature stability and are characterized by low tack and high cohesive strength. Thus, the present invention provides a foil laminate having a foil protecting film adhered to the foil by an electron beam cured adhesive. The film can be readily removed from the foil surface after it has served its protective purpose for subsequent processing of the foil. No visible residue is left on the foil. Furthermore, the adhesive used in this invention can be applied across the entire surface of the protective film without concern of film placement over the foil surface, nor of the effects of elevated temperature and pressure on the chemical structure of said adhesive.

SUMMARY OF THE INVENTION

The present invention relates to a method for protecting a substrate sheet such as a conductive foil suitable for use in the manufacture of printed circuit boards, comprising the steps of:

(A) applying an electron beam curable adhesive coating to one surface of a protective film, said protective film having a release coating on the other surface thereof;

(B) curing said adhesive coating; and

(C) laminating said protective film substrate to a substrate sheet.

The present invention also relates to a protected sheet laminate comprising:

(A) a substrate sheet;

(B) a protective film laminated to said substrate sheet, said 5 film having a release coating on one surface and an electron beam cured adhesive coating on the other surface, said other surface being adhesively secured to said substrate sheet by means of said electron beam cured adhesive coating.

Preferably, the substrate sheet is a foil laminate suitable for use in the

10 manufacture of a printed circuit board the electron beam cured adhesive is a continuous coating on the foil, and the release coating is either an electron beam cured release coating or a vacuum deposited aluminum release coating.

Brief Description of the Drawing 15 Figure 1 is a somewhat schematic view illustrating a preferred embodiment of the process of -the present invention.

Description of the Preferred Embodiments

Now referring to the Figure, Figure 1 illustrates a preferred embodiment of the process of the present invention and shows a cross-section of a

20 preferred product of the present invention, indicated generally by the numeral

10. While the preferred embodiment of the present invention is described in terms of foil laminates used in making printed circuit boards, it will be appreciated by those skilled in the art that, broadly speaking, this invention is applicable to substrate sheets other than foil laminates and, hence, to other

25 products. Thus, the protected substrate sheet can be glass, polycarbonate,

) acrylic, or any other desired material within the broad scope of this invention.

In accordance with the process of Figure 1 , protective film 12, for example, a polyester film, is unwound from roll 14 and is passed through offset gravure coating station 16. Any suitable protective film 12 can be used

in the present invention and suitable films 12 are well known in the art. At offset gravure coating station 16 an electron beam curable release coating material 18 is taken up by roller 20, transferred to roller 22, and applied to one side of film 12 as a continuous coating 24, thereon. Suitable electron beam curable release coatings are well known in the art. Such coatings enjoy substantially consistent release characteristics during subsequent processing steps. Film 12 with coating 24 is then passed through an electron beam curing apparatus 26 where an electron beam is applied to coating 24 which is thereby cured to form cured release coating 28. Suitable electron beam curing apparatus is commercially available and the amount of energy to be applied to effect curing is well within the skill of an artisan. Film 12 with now cured release coating 28 thereon is wound onto roll 30. Alternatively, a release coating is provided on a film 12 by vacuum metallizing a layer or coating of aluminum thereon. Roll 30 is then turned over as indicated at arrow 32 and film 12 with cured coating 28 thereon is unwound from roll 30 and an electron beam curable adhesive coating material 34 is applied by a "knife over roll" applicator 36 to the previously uncoated side of film 12. Suitable electron beam curable adhesive coating polymers are known in the art and include saturated copolyesters with terminal acrylic double bonds. Film 12 with adhesive coating 38 is then passed through electron beam curing apparatus 40 and an electron beam is applied to cure adhesive material 30 on film 12 to form cured coating 42. The electron beam cured adhesive is characterized by high temperature stability, low tack, and high cohesive strength. Film 12 is rewound onto roll 44 and the adhesive side thereof is applied to a copper foil sheet 46 between rolls 48 and 50. The now protected foil sheet 52 is cut at station 54 to provide foil sheet laminates 56 which are heated and pressed in press 58 with a dielectric epoxy prepreg sheet 60 to bond the sheets together to form laminate 10. Laminate 10 is allowed to cool, transported, then further processed by suitable conventional processes such as drilling at

64 and cutting at 66 to provide a circuit board laminate 68. Before etching of the foil film 12 is removed as shown at 70.

Further understanding of the present invention will be had from the following specific examples.

EXAMPLE 1

A polyester film (92ga Melinex 314, ICI Resins) was installed in a coating machine equipped with an electron processor, manufactured by Energy Sciences, Inc. An electron beam curable release coating, such as that disclosed in U.S. Patent 4,753,847, was applied by an offset gravure application station to one side of the polyester film at a coating weight of approximately 3 lbs. per 3000 sq. ft. and cured by electron beam at a dose a 3 megarads. The film was turned over, and Dynacoll A 2583 from Huls America, Inc., heated to suitable application viscosity (about 100°C), was applied by a "knife over roll" applicator to the other side of the polyester film, and cured at a dose of 3 megarads at 165KUs. The adhesive side of the resulting roll of film was easily releasable from the opposite side release layer and was applied to a copper foil sheet laminate, and pressed at 350°F at 500 psi for 90 minutes. Following the press cycle, the sample was allowed to cool, and the protective film was easily removed from the copper sheet. There was no visible evidence of surface contamination on the copper foil.

EXAMPLE 2 A roll of 48ga polyester film chemically treated on one side (ICI type 813) was converted in a commercial vacuum metallizer for the deposition of aluminum from induction heated curable onto the untreated side of the film to an aluminum thickness of about 3 ohms per square. The metallized polyester was then converted in accordance with Example 1 with Dynacoll A 2583 adhesive from a "knife-over-roll" device to the opposite side, i.e. the treated side, of the polyester film, and cured by electron beam radiation at a dose of 4 megarads.

The cured adhesive easily released from the metallized surface which acts as a release layer.