FIELD

[0001] The present disclosure relates to peelable thin foils such as those used in the fabrication of printed circuit boards, and in particular to a double-sided peelable thin foil composite.

BACKGROUND

[0002] This section provides background information related to the present disclosure which is not necessarily prior art.

[0003] Peelable thin foils, comprising a thin conductive film on a carrier strip are well known, and widely used in the fabrication of high density printed circuit boards. The thin foil, typically of copper or a copper alloy is mounted on a carrier, typically a thicker foil, which may also be of copper or a copper alloy. The foil is applied to an insulating circuit board material (in solid or liquid form), and laminated thereto. The carrier strip can then be removed, leaving a copper foil layer clad to the circuit board. This copper layer can then be processed to remove unwanted copper after applying a temporary mask, followed by etching, leaving only the desired copper traces forming the circuit.

[0004] Sometimes the lamination occurs in bulk by forming a stack of alternating layers of foil and circuit board material, simultaneously bonding a layer of copper foil to the outer layers of circuit board in the stack. The carriers are stripped away leaving copper clad circuit board material ready for further processing into circuit boards. Release layers are sometimes provided to facilitate the separation of the foil from the carrier layer,

SUMMARY

[0005] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0006] Embodiments of the present invention provide a double-sided peelable thin foil composite. The composite comprises a carrier having first and second sides, and a peelable thin foil layer disposed on each side. A release layer may be disposed between the carrier and at least one of, and preferably each of the foil layers. This would allow the printed circuit processing to be conducted on both outer layers simultaneously, thus, doubling the productivity. When the printed circuit process is completed, the thin foils and the finished circuitries on both sides of the carrier strip can be separated from the carrier.

[0007] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

[0008] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. [0009] Fig. 1 is a transverse cross-sectional view of a first preferred embodiment of a double sided peelable thin foil composite in accordance with the principles of this invention;

[0010] Fig. 2 is a transverse cross-sectional view of a second preferred embodiment of a double sided peelable thin foil composite in accordance with the principles of this invention;

[0011] Fig. 3 is a transverse cross-sectional view of a third preferred embodiment of a double sided peelable thin foil composite in accordance with the principles of this invention;

[0012] Fig. 4 is a transverse cross-sectional view of a fourth preferred embodiment of a double sided peelable thin foil composite in accordance with the principles of this invention;

[0013] Fig. 5 is a transverse cross-sectional view of a fifth preferred embodiment of a double sided peelable thin foil composite in accordance with the principles of this invention;

[0014] Fig, 6 is a schematic diagram showing part of a stack of the double sided peelable thin foil composite in accordance with the principles of this invention used to simultaneously apply thin foil to opposite faces of circuit board material; and

[0015] Fig. 7 is a schematic diagram showing an assembly of two panels of circuit board material joined by a double sided peelable thin foil composite in accordance with the principles of this invention. [0016] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

[0017] Example embodiments will now be described more fully with reference to the accompanying drawings.

[0018] A first embodiment of a double sided peelable thin foil composite is indicated generally as 20a in Fig. 1. As shown in Fig. 1 the composite 20a comprises a carrier layer 22 having first and second sides, and first and second foil layers 24 and 26 disposed on the first and second sides of the carrier, respectively. The carrier layer 22 may be a metal foil layer, and is preferably between about 15 pm and about 400 μητι thick. Copper foil is a desirable carrier due to its high electrical conductivity since high plating currents are often applied to the manufacturing process. It is also beneficial to use copper foil that has a high strength or softening resistance to facilitate the handling and processing after heated lamination and cure processes. Certain elements, such as titanium, zirconium, silicon, magnesium, barium, niobium, and calcium in the copper alloy foil are also known to mitigate the blister formation between the thin foil and the carrier as disclosed in Brenneman et al, U.S. Patent 7,132,158. The carrier layer 22 preferably comprises a copper or copper alloy foil. The copper alloy foil preferably includes one or more of Si, Zr, Ti, Mg, Ba. Nb, Ca, Ag, Fe, P, Al, Ni, Sn, Zn, and Co, and may be C110, C102, C7025, C7026, C199, C151, C654, C638, C18080, C7035, C1094, C1093, C194, C195, C197, C706, C715, C728, C260, NK120 or other suitable alloy. The carrier layer 22 could also be made of aluminum, aluminum alloys, steel, and stainless steel, or other suitable material. The carrier layer may be an electrodeposited foiling, but it is preferably made of a rolled foil, which has smooth surfaces with a surface roughness of less than about 1.0 μΐτι Ra (arithmetic average roughness value), and preferably less than 0.4 μιη Ra.

[0019] The first and second foil layers 24 and 26 preferably comprise copper or copper alloy foil, such as CuNi, CuZn, CuAg, CuCo, and CuSn. The first and second foil layers 24 and 26 are preferably electrodeposited onto the carrier layer 22. The thickness of the foil layers 24 and 26 is preferably the same, but in some embodiments the thickness could be different. In the fabrication of some circuit boards, it may be desirable to apply a thicker foil layer on the one side of the circuit board material than on the other. The thickness of the foil layers 24 and 26 is preferably between about 0.2 pm and about 10 pm thick, and more preferably between about 1 pm and about 5 pm thick.

[0020] A second preferred embodiment of a composite according to the principles of this invention is indicated generally as 20b in Fig. 2. Composite 20b is similar to composite 20a, and corresponding parts are identified with corresponding reference numerals. As shown in Fig. 2, composite 20b comprises a release layer disposed between the carrier layer 22 and at least one first and second foil layers 24, 26. In this preferred embodiment, there is a release layer 28 between the carrier layer 22 and the first foil layer 24, and a release layer 30 between the carrier layer and the second foil layer 26. The release layer or layers may be selected from a group of elements, such as one or more of Cr, Mo, W, Mn, Ni, Co, Fe, as metal, metal oxides, or a mixture of metal/metal oxides, or could be a layer of organics. In some embodiments, the first foil layer 24 on the first side of the carrier layer 22 releases from the carrier at a lower force than the second foil layer 26 on the second side of the carrier. One way this can be accomplished is by differences in the release layers 28 and 30. This could be a difference in thickness of the layer, or identity difference in the release layer composition.

[0021] Various release layers are known and could be applied to the foils on the composite, including for example the release layers disclosed in Chen, et al., U.S. Patent Nos. 6,346,335, 6,569,543 and 6,689,268, Copper Foil Composite Including A Release Layer, the disclosures of which are incorporated herein by reference.

[0022] A third preferred embodiment of a composite according to the principles of this invention is indicated generally as 20c in Fig. 3. Composite 20c is similar to composite 20b, and corresponding parts are identified with corresponding reference numerals. As shown in Fig. 3, the surface of at least one of the first and second foil layers 24 and 26 opposite from the carrier 22 has an adhesion enhancement layer. As shown in Fig. 3. the surface of the first foil layer has an adhesion enhancement layer 32, and the surface of the second foil layer opposite from the carrier has an adhesion enhancement layer 34 The adhesion enhancement layers 32 and 34 may be made of Cu, CuNi, CuZn, CuCo, CuNiCo, CuNiZn, and/or CuCoZn. The adhesion enhancement layers 32 and 34 may be the same or they may be different.

[0023] Various peel strength or bond enhancement treatments are known and could be applied to the foils on the composite, including for example the treatments disclosed in Lin, U.S. Patent No. 5,071 ,520 on Method of Treating Metal Foil to Improve Peel Strength; and Chen et al.„ U.S. Patent No. 6,837,980 on Bond Enhancement Antitarnish Coatings; Chen et al., U.S. Patent Nos. 6, 569,543 and 6,893,742 on Copper Foil with Low Profile Bond Enhancement; Polan, U.S. Patent No. 4,468,293 and 4,515,671 on Electrochemical Treatment of copper for Improving its Bond Strength; and Chen et al., U.S. Patent No. 5,800,930 on Nodular Copper/Nickel Alloy Treatment for Copper Foil, the disclosures of which are incorporated herein by reference.

[0024] Of course the surface could also be physically altered to enhance its ability to bond with the circuit board material.

[0025] A fourth preferred embodiment of a composite according to the principles of this invention is indicated generally as 20d in Fig. 4. Composite 20d is similar to composite 20c, and corresponding parts are identified with corresponding reference numerals. As shown in Fig. 4, the surface of at least one of the first and second foil layers 24 and 26 opposite from the carrier 22 has an antitarnish layer. As shown in Fig. 4, the surface of the first foil layer has an antitarnish layer 36, and the surface of the second foil layer opposite from the carrier has an antitarnish layer 38. The antitarnish layers 36 and 38 may be made of selected from a group of elements, such as one or more of Zn, Cr, Ni, Si, P, W, and Mo, as metal, metal oxides, or a mixture of metal/metal oxides. The antitarnish layers 36 and 38 may be the same or they may be different. A double-sided peelable thin foil composite comprises a rolled foil carrier between about 15 pm and about 400 pm thick, having first and second sides, and first and second copper or copper alloy foil layers between about 0.2 pm and about 10 pm thick disposed on the first and second sides of the carrier, respectively.

[0026] Various antitarnish treatments are known and could be applied to the foils on the composite, including for example the anti-tarnish treatment disclosed in Lin et al., U.S. Patent No. 5,057,193 on Anti-Tarnish Treatment Of Metal Foil, and Howell et al., U.S. Patent No. 6,852,427 on Chromium-Free Antitarnish Adhesion Promoting Treatment Composition, Chen, U.S. Patent No. 5,250,363 on Chromium-Zinc Anti-Tarnish Coating For Copper Foil Having A Dark Color, Chen et al., U.S. Patent Nos. 5,098,796 and 5,230,932 on Chromium-Zinc Anti-Tarnish Coating for Copper Foil, Lin et al., U.S. Patent No. 4,952,285 on Anti-tarnish Treatment of Metal Foil, the disclosures of which is incorporated herein by reference.

[0027] A fifth preferred embodiment of a composite according to the principles of this invention is indicated generally as 20e in Fig. 5. Composite 20e is similar to composites 20a-20d, and corresponding parts are identified with corresponding reference numerals. As shown in Fig. 5, the surface of at least one of the first and second foil layers 24 and 26 opposite from the carrier 22 has an adhesion layer and an antitarnish layer. As shown in Fig. 5, the surface of the first foil layer has an adhesion layer 32 and an antitarnish layer 36, and the surface of the second foil layer opposite from the carrier has an adhesion layer 34 and an antitarnish layer 38. The adhesion layers 32 and 34 and the antitarnish layers 36 and 38 may be as described above with respect to composites 20c and 20d, and the layers on the foils 24 and 26 may be the same or they may be different. Of course, rather than separate layers, a combination adhesion enhancement antitarnish layer could be provided on the foils layers 24 and 26 if desired.

[0028] One possible method of manufacturing the double-sided peelable thin foil is described below. A carrier strip, which could be made of copper, a copper alloy, aluminum, aluminum alloy, steel, or stainless steel, is first cleaned by using electroclean or immersion in a commercially available cleaning solution. Such solution typically contains sodium hydroxide, sodium carbonate, sodium metasilicate, and surfactants. The electroclean current density could vary from 10 to 300 ASF (amps per square foot) for 2 to 60 seconds. The solution temperature can vary from ambient temperature to about 160°F. After the strip is cleaned, it is rinsed in water, typically using spray nozzles to sweep off the chemicals on the strip surface.

[0029] A release layer may then be applied on one or both sides of the carrier layer. The composition of the release layers can be as described above. One preferred release layer comprises metal and/or metal oxide layers containing one or all of the elements such as Cr, Mo, or W. The release layers could be composed of more than one layers; for example, a layer of Ni, Co, Mn, or Fe can be deposited first onto the carrier strip, followed by the metal/metal oxide mixture of Cr, Mo, and/or W. The purpose of the first layer is to inhibit diffusion between the second layer and the carrier strip. A typical method of depositing the release layer is electroplating. An example of electroplating Cr containing release layer is to apply an anodic current density of between about 0.5 and about 3.5 ASF, followed by a cathodic current density of between about 0.5 and about 3.5 ASF for between about 5 and about 40 sec. The anodic current is not necessary to render a releasable thin foil, but helps to produce a consistent and uniform release. The cathodic release current contributes to the releasability of the thin foil. By adjusting the relative values of the anodic and cathodic currents, the desirable release force can be controlled. The release layer plating solution preferably contains about 10-50 g/l NaOH, 1-20 g/l sodium dichromate at a temperature of between about 90 and about140°F. After the release layer plating, the strip is rinsed in water to remove the chemicals.

[0030] A seed layer of copper can then be plated onto the release layer. In this step of process, a neutral or alkaline copper plating solution is generally preferred because it typically has a better covering power than the acid copper plating solution. The non-acidic solution is also less likely to attack the release layer, which in some cases could be made of metal oxides and can be attacked by acids. There are many commercially available alkaline copper plating solutions provided by companies such as Enthone, MacDermid, or

Electrochemical Products Inc. A typical alkaline Cu plating solution contains between about 50 and about 100 g/l copper pyrophosphate, between about 200 and about 350 g/l potassium pyrophosphate, about 3 and about 6 g/l potassium nitrate, between about 4 and about 11 ml/I concentrated ammonium hydroxide, a pH of between about 8.0 and about 8.7, a temperature of between about 110 and about 140°F, a current density between about 10 about 80 ASF (Metal Finishing Guidebook and Directory, Volume 106, Number 10A, page 192, 2008). The thickness of the Cu seed layer can vary from about 0.1 and about Ο.δμΐη, sufficient to entirely cover the release layer. The strip is rinsed afterwards.

[0031] The conductor layers can then be built up in thickness, to between about 0.2 and about 10μιτι. An acidic Cu plating solution is typically used since it provides a much faster plating rate by applying higher current densities. A typical acid Cu plating solution contains from about 45 to about 75 g/l Cu, from about 50 to about 200 g/l sulfuric acid at between about 100 and about 150°F. The current density and time can vary in the ranges of about 50 to about 200 ASF and about 10 to about 200 sec, depending on the desirable thickness of thin foil and the operating line speed. The strip is rinsed afterwards.

[0032] An optional bond enhancement treatment can be applied by plating a rough nodular deposit on the surface of the thin foils. This nodular deposit could be composed of Cu, CuNi, CuZn, CuCo, CuNiCo, CuNiZn, or CuCoZn. An example of a plating Cu nodule solution might contain from about 15 to about 25 g/l Cu as Cu sulfate, about 50 to about 120 g/l sulfuric acid, at between about 90 to about 110°F, using a current density of between about 150 and about 300ASF within about 4 and about 40 sec. The purpose is to apply a high current density in a solution containing a relatively low Cu concentration to promote the deposition of a rough Cu layer or nodules, which serve as the anchor points when laminated with a dielectric substance, such as FR4, BT resin, ABF, polyimide, etc. The strip is rinsed after the acid Cu plating.

[0033] An optional antitarnish coating can be applied to protect the surface of Cu so that the Cu foil does not tarnish during shipping, storage, and lamination process. The antitarnish coating could be composed of a combination of metal and/or metal oxides of elements selected from Zn, Cr, Ni, Si, P, W, and/or Mo. The treatment could be applied with either an immersion process or electrolytic process. An example of applying a Zn/Cr metal/oxide layer employs a solution of between about 0.2 and about 3.0 g/l Zn as Zn oxide, between about 0.2 and about 5.0 g/l Cr as sodium dichromate, between about 20 and about 45 g/l NaOH, at between about 100 and about 160°F, and a current density of from about 5 to about 150 ASF, for between about 2 and about 40 sec plating period. Optionally, a silane coating can be applied on top of the antitarnish layer by immersing the strip in a water solution containing between about 0.02 and about 1% of silane. Many silane solutions are available commercially to be used as a coupling agent between metal and polymers; for example, Dow Corning Z6020, Z6011 , and Z6137(amino-silanes) are known to promote bonding between metal and acrylic, Nylon, phenolics, PVC, melamines, urethanes, and nitrile rubber. The silane solution and either be rinsed off or left on the metal surface and dried. The entire process of making double-sided peelable foil could be either in the form of panels or in a reel-to-reel continuous strip processing, which generally results in a lower cost. In the latter, a coil of thin foil with the carrier strip can be sold and subsequently processed on a lamination line or the coil can be sheeted and sold as foil sheets.

[0034] In use a composite in accordance with the present invention, for example composite 20d is interleaved between sheets 40 of circuit board material as shown in Fig 6. (The layers are shown broken because the thickness would typically be much greater than the thickness of the composite). The stack is heated under pressure to laminate the foil layers 24 and 28 to the surfaces of the board material 40. When the lamination process is complete, the circuit board material 40 can be separated from the carrier strips 22 of the composite, leaving a foil layer 24 from one composite 20d and a foil layer 26 from another composite 20d on the opposite surfaces of the circuit board material 40. If the foil layers 24 and 26 are identical, then the circuit board material will have identical foil layers on opposite sides. If the foil layers 24 and 26 are different, then the circuit board material will have different foil layers on opposite sides. The release layers 28 and 30 can be identical, or they can be different so that one of the layers 24 and 26 separates more easily from the carrier strip, than the other, which can facilitate the separation of the circuit board material from the carrier strip.

[0035] Only two pieces of circuit board material are shown in Fig. 6, but additional layers of circuit board material and composites 20d can be used. As shown in Fig. 5, the layer 24 of the top-most composite 20d and the layer 26 of the bottom-most composite 20d do not laminate to a circuit board. However, these layers can be used in subsequent laminating processes, so that the foil layers do not go to waste. Alternatively, a single sided peelable thin foil composite can be used for the top and the bottom of the stack.

[0036] In another use a composite in accordance with the present invention, for example composite 20e is sandwiched between two panels of circuit board material 40. (The layers are shown broken because the thickness would typically be much greater than the thickness of the composite). Circuits can be formed on the opposite sides of the two panels 40, and when the processing is complete, the panels can be separate from the carrier strip 22 of the composite 20e, leaving a foil layer 24 embedded on the underside of one of the panels and the foil layer 26 embedded on the top side of the other of the panels.

[0037] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure