Surface-treated copper foil for high-frequency circuit, its production method and use thereof in PCB manufacturing

Abstract

The invention proposes a surface treated copper foil for a high-frequency circuit as well as a corresponding method of treating a copper foil. The copper foil comprises two opposite sides, wherein each one of a first side and a second side are coated with a respective treatment stack comprising, in this order: a first layer comprising oxides of Mo and of Zn deposited on the respective side of the copper foil, wherein said first layer is free of Ni; and a coupling agent layer. The first layer comprises the oxides of Mo and of Zn in a quantity of between 3.0 and 30 mg / m2 calculated as Mo and Zn; and each treatment stack has a roughness Rz JIS of 1.2 μm or less. A method of manufacturing a multi-layer printed circuit board is also disclosed, which uses an inner layer core that comprises a substrate layer clad on both sides with the present surface treated copper foil.

Description

[0001] P-CIRCUI-023 / WO 1

[0002] Surface-treated copper foil for high-frequency circuit, its production method and use thereof in PCB manufacturing

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a surface-treated copper foil for a high-frequency circuit and more particularly relates to a surface-treated copper foil, which is excellent in adhesiveness with an insulating substrate for a high-frequency circuit and also excellent in transmission characteristics in a high-frequency region.

[0005] BACKGROUND OF THE INVENTION

[0006] Data is growing at an exponential rate and is not going to slow down, due to the popularization of information terminals like smartphones and laptops as well as social networking services and video-sharing platforms. This leads to increasing demands for transmitting massive data, which requires ever increasing signal transmission speeds between components on circuit boards. To achieve these speeds, frequency ranges are necessarily increasing from the MHz range to, 1 GHz, 10 GHz or even higher. In these higher ranges, the electrical currents flow mostly near the surface of the conductors due to the well-known “skin effect”, which is the tendency of high frequency current density to be highest at the surface of a conductor and to decay exponentially towards the center.

[0007] The skin depth, where approximately 67% of the signal is carried, is inversely proportional to the square root of the frequency. Accordingly, at 1 MHz the skin depth is 65 pm, at 1 GHz it is 2.1 pm, while at 10 GHz the skin depth is only 0.65 pm. At the higher frequencies, the surface topography or roughness of the conductor becomes ever more important since a roughness in the order of, or greater than, the skin depth will impact the signal transmission through scattering. A surface roughness, Rz, on the roughened surface in the order of several pm is typical and will impact any transmission in the GHz range. The conventional design of copper foil for printed circuit boards (PCBs) is therefore constrained by the conflicting need for a roughness high enough to ensure a sufficient adhesion to a substrate, and low enough to minimize transmission loss. P-CIRCUI-023 / WO 2

[0008] In this connection, it may be noted that in conventional printed circuit boards (PCBs), the surface of the conductor tracks is intentionally roughened to enhance adhesion characteristics to the resin layer used in the laminated PCB structures. US 10,772,199 B discloses a copper foil with microscale nodular treatment to ensure high bondability with the substrate, and US 2021 / 321514 A1 suggests increasing the microroughness of the foils to ensure high bondability. Conventional roughening treatments comprise the deposition of nodules (nodular treatment) on the copper foil surface; or attacking the surface of the copper by means of an acidic solution, thus forming a so-called brown-oxide.

[0009] For multilayer PCB constructions, high bondability with the resin is required on both sides of copper foil. While the adhesion performances on the electrolyte side of a copper foil is conventionally ensured by deposition of a nodular treatment and the so-called passivation / coupling agent technology (such as disclosed e.g. in JP 2006 / 210689 A1 ), the adhesion on the untreated side is mainly obtained by a roughening treatment based on attacking the surface of the copper foil by means of an acidic solution (so called brown - black oxides treatment), applied by the PCB manufacturer.

[0010] For multilayers PCB constructions, the signal integrity performances depend on the copper profile displayed on both sides. The roughening treatment usually applied on the untreated side of the copper foil by a PCB manufacturer is therefore detrimental to get high signal integrity performances (and in particular low transmission loss) at high frequency.

[0011] Moreover, treated copper foils with enhanced thermal resistance or bondability of both their sides have been developed, such as disclosed e.g. in JP 2001 / 199006 A or CN 107645852 A. However, double-side treatment methods generally impact the roughness of the copper foil, either through a roughening treatment or through the deposition of a nodular treatment on both sides, which is detrimental to get high signal integrity performances (and in particular low transmission loss) at high frequency. Furthermore, treatments devolved to enhance thermal resistance of the copper foil may involve a metallic P-CIRCUI-023 / WO 3 passivation, such as disclosed e.g. in JP 2001 / 199006 A, which is detrimental for signal transmission at high frequency.

[0012] Despite various approaches proposed in the prior art, there still remains a need for copper foils with controlled properties for high-frequency circuits, in particular showing good adhesion, thermal resistance and chemical resistance, as well as the desired low transmission loss, and eliminating the need for PCB manufacturers to perform a brown - black oxide treatment.

[0013] OBJECT OF THE INVENTION

[0014] It is an object of the present invention to provide an improved surface treated copper foil to be used in a high-frequency circuit without the afore mentioned problems.

[0015] SUMMARY OF THE INVENTION

[0016] In order to achieve the above-mentioned object, the present invention provides a surface treated copper foil as claimed in claim 1 and a method of treating a copper foil as claimed in claim 19.

[0017] According to the present invention, a surface treated copper foil for a high- frequency circuit comprises two opposite sides, wherein each one of a first side and a second side are coated with a respective treatment stack comprising, in this order: a first layer comprising oxides of Mo and of Zn deposited on the respective side of the copper foil, wherein said first layer is free of Ni; and a coupling agent layer; wherein the first layer comprises the oxides of Mo and of Zn in a quantity of between 3 and 30 mg / m2calculated as Mo and Zn; and wherein said treatment stack has a roughness Rz JIS of 1 .2 pm or less.

[0018] The present invention proposes a surface treated copper foil that - according to first results - meets the requirements for application in high frequency circuits, P-CIRCUI-023 / WO 4 particularly in terms of adhesion, heat resistance, chemical resistance and low transmission loss.

[0019] Both sides of the inventive treated copper foil exhibit a surface roughness meeting the prescribed range of the roughness parameters Rz JIS. Ideally, the values of Rz are the same on both sides, or at least as similar as possible. In practice, however, due to manufacturing tolerances, there may be a difference in Rz values from one side to the other. Furthermore, in the case of electrolytic copper foils, the matte (electrolyte) side and the drum side may have a different initial roughness profile, which will influence the roughness of the treated sides.

[0020] In practice, similar treatments may be applied to both surfaces of the copper foil. However, the two sides of the treated copper foils may present different surface roughness due to variations of the initial surface roughness of the (untreated) copper foil, while still being within the prescribed ranges. It should be observed that this difference mainly reflects the initial variations irrespectively of the surface treatment performed to obtain the surface treated copper foil.

[0021] Surprisingly, the inventors found out that providing a treatment stack on each side (i.e. on both sides) of the copper foil eliminates the need to perform a brown / black oxide treatment on the conventionally untreated side of commercially available treated copper foils while providing the required bondability to a substrate and without impacting the signal transmission at high frequency. As there is no need to perform a brown / black oxide treatment, the PCB manufacturing process will be simplified and shortened, thereby increasing productivity.

[0022] The first treatment stack is coated (i.e. formed or deposited) onto the first side of the copper foil while the second treatment stack is coated (i.e. formed or deposited) onto the second side of the copper foil. The treatment stacks on the first side and the second side may be similar or different. Preferably, the treatment stack on the first side is different from the treatment stack on the second side, however both the first and the second treatment stacks satisfy the prescribed features of the invention, i.e. each one of the first and the second treatment stack comprise a first layer and a coupling agent layer as defined above. P-CIRCUI-023 / WO 5

[0023] The first layer (of each one of the first and second treatment stacks) comprises or consists of oxides of Mo and of Zn and provides a first passivation layer normally directly formed on one side of the copper foil. The first layer may be formed by an electrolytic co-deposition process. The first layer may include a variety of oxide forms (various oxidation states), namely oxides of Zn, oxides of Mo, or mixed oxide forms of Zn and Mo. In particular, the oxides may be formed which comprise at least one oxygen atom bound to Mo, resp. to Zn, at one or more oxidation states. Without willing to imply any limitation, first analyses have shown that the oxides contain, for zinc mainly Zn2+and for molybdenum Mo6+, Mo5+and / or Mo4+. Within the first layer, Mo allows improving heat resistance of the copper foil. Zn is used to permit the deposition of Mo, i.e. to operate codeposition of Mo and Zn in an electrolytic cell. In other words, the first layer is mainly a layer of a binary alloy of Zn oxides and Mo oxides, including mixed oxides, where Zn and Mo may be found in one or several oxidation states.

[0024] The herein prescribed amounts of Mo and Zn for the first layer (expressed with respect to the element itself, i.e. Mo resp. Zn - not the oxide forms) are selected to provide good thermal resistance as well as high chemical resistance.

[0025] A merit of the present invention is therefore to provide a double-side treated electrodeposited foil providing improved signal integrity at high frequency, by combining low roughness electrodeposited copper foil, nodular free treatment and metallic free passivation while ensuring high thermal and chemical resistance and high peel strength on PPE (polyphenylene ether) I PPO (polyphenylene oxide) and PTFE (polytetrafluoroethylene) material on both copper foils side without the application of brown - black oxides.

[0026] Another merit of the invention is that the production process of printed circuit boards comprising such a surface treated copper foil process is simplified by the deposition of (non-roughening) treatment (i.e. preferably roughening-free treatment) on the conventionally untreated side of the copper foil simultaneously to deposition of a surface treatment on the other, opposite side. The surface treatment deposited on both sides provide adhesion as well as thermal and chemical resistance performances, thereby removing the need to perform a P-CIRCUI-023 / WO 6 brown / black oxide treatment during PCB manufacturing on this conventionally untreated side and without degrading the surface of the copper foil (i.e. increasing rugosity) thereby without impacting the signal integrity performances.

[0027] Surprisingly, the described surface treatment provides for the required adhesion (as of peel strength) even with small contact surface areas. Also surprisingly, the adhesion property of the surface treated copper foil is maintained even after the etching of the copper foil during the PCB manufacturing process. The inventive double-side surface treated copper foil achieves adhesion performance comparable to traditional foils submitted to e.g. a brown oxide treatment or a black oxide treatment, without the need for those conventional treatments. As a result, it provides an efficient solution that preserves adherence and peel strength, offering significant advantages in applications where minimizing treatment steps and thus production time is desirable.

[0028] In other words, the inventive surface treated copper foils streamline the production process of e.g. a printed circuit board (PCB) as:

[0029] - there is no need to perform additional treatment of the copper foil, resulting in a gain in production time and no increase in insertion loss;

[0030] - both sides of the copper foil present similar treatments and properties, which ensures a more uniform lamination process, and remove the need for PCB manufacturer to keep traces of how which face was treated;

[0031] - for double-sided PCB, having both sides treated ensures that they both adhere equally well to the substrate, enhancing the mechanical integrity and performance of the board.

[0032] In embodiments, the weight ratio of Mo to Zn in the first layer of the first treatment stack is between 0.3 and 1 .5.

[0033] The first layer is free of Ni. Indeed, Ni is not desired in the first layer and there is no voluntary Ni addition in the bath forming said first layer. Impurities or traces may however exist, typically not more than 0.2 mg / m2

[0034] Similarly, the first layer is preferably free of Co. Impurities or traces may however exist, typically not more than 0.05 mg / m2. P-CIRCUI-023 / WO 7

[0035] Preferably, the first layer comprises more than 80 wt.% of oxides of Mo and of Zn, more preferably more than 85 wt.%, in particular more than 90 or 95 wt.%. Further to heat and chemical resistance, the use of oxide forms of Mo and of Zn, allows reducing insertion loss compared to the metallic forms (metallic passivation).

[0036] In some embodiments, the first layer may comprise a small amount of other metal(s), for example Cr, in particular in oxide forms.

[0037] The first layer may possibly comprise traces of species other than the desired Mo and Zn oxides that come from the electrolyte solution.

[0038] If the respective first layer on each side are too thin (i.e. does not comprise enough Mo and Zn), the desired thermal properties (i.e. thermal resistance) are not achieved. On the contrary, if the layers are too thick (i.e. comprise too much Mo and Zn), chemical resistance is reduced. In preferred embodiments, the first layer deposited on the first side of the copper foil comprises the oxides of Mo and of Zn in a quantity of between 5 and 30 mg / m2calculated as Mo and Zn, and the first layer deposited on the second side of the copper foil comprises the oxides of Mo and of Zn in a quantity of between 3 and 15 mg / m2calculated as Mo and Zn.

[0039] In embodiments, a second layer is deposited over the first layer on at least one side, preferably both sides, of the copper foil underneath the coupling agent layer, the second layer comprising Cr oxides. In other words, at least one treatment stack formed (i.e. deposited) on one side of the copper foil comprises a layer comprising chromium oxides arranged between the first layer and the coupling agent layer.

[0040] The second layer provides a second passivation, normally directly formed on the first layer. This second layer is provided to further improve the chemical stability of the first layer as well as prepare for the deposition of the coupling agent. The second layer may comprise 80 to 100 wt% of chrome oxide, in particular 95 to 100%. The second layer may include one or more oxides forms of Cr, in particular with Cr at oxidation state III and / or other oxidation state(s).

[0041] Preferably, the second layer of the first treatment stack (i.e. on the first side) comprises the Cr oxides in a quantity of between 2.0 and 10 mg / m2, preferably P-CIRCUI-023 / WO 8 between 3 and 7 mg / m2calculated as Cr. In embodiments, the second treatment stack also comprises a layer comprising (or even consisting) of Cr oxides, deposited over the first layer, i.e. between the first layer and the coupling agent layer. Advantageously, the amount of Cr oxides in the second layer of the second treatment stack may be similar to the quantity of Cr oxides in the first treatment stack, i.e. comprised between 2.0 and 10.0 mg / m2, preferably between 2.0 and 7.0 mg / m2calculated as Cr.

[0042] If the Cr amount is too small, the thermal properties are not achieved. If it is too important, the layer will be too thick and chemical resistance is reduced.

[0043] The coupling agent layer is formed on the first layer. In the present text, the term “formed on” means that the coupling agent layer is formed on a side of the first layer which is opposite the copper foil. The coupling layer may be formed directly or indirectly onto the first layer. In some embodiments, the coupling agent layer is formed directly on the respective first layer while in other embodiments, the coupling agent layer is formed indirectly on the respective first layer, i.e. at least one further layer (such as e.g. the second layer comprising oxides of Cr) may be formed between the first layer and the coupling agent layer. The coupling agent layer provides desired adhesion property to the resin / polymer substrate during lamination.

[0044] In that respect, it may be appreciated that the above-mentioned first and second layers, with their prescribed design, provide good adhesion to the third, coupling agent layer, which in turn provides adhesion to the resin / polymer substrate. This is of relevance since there is no roughening treatment to improve adhesion to the resin / polymer substrate.

[0045] In embodiments, the coupling agent layer comprises a functionalized silane coupling agent, which may comprise an aminosilane, an epoxy-silane, vinyl- si lane, methacrylate silane, or a mixture thereof. In some embodiments, each one of the coupling agent layer of the first and second treatment stack may comprise between 0.5 and 5 mg / m2of silane. If the functionalized silane coupling agent is less concentrated, the bonding performance are decreased and not enough to P-CIRCUI-023 / WO 9 satisfy the expected requirement. If the concentration of functionalized silane coupling agent is higher, there is no peel strength increase.

[0046] The coupling agent layer of the first treatment stack and the coupling agent layer of the second treatment stack may be similar or different. In some preferred embodiments, they are similar, preferably substantially identical.

[0047] The present surface treated copper foil has a very low surface roughness Rz JIS of 0.7 pm or less for the electrolyte side of the foil. For the drum side, the surface roughness Rz JIS is less than 1 .2 pm. The indicated roughness is that of the free surface of the treatment stack (i.e. the free surface of coupling agent layer, opposite the second layer).

[0048] It may be noted that the treatment stack does essentially not change the surface roughness of the copper foil side on which it is formed. In other words, the layers of the treatment stack tend to follow I reproduce the surface roughness of the underlying layer, and ultimately the one of the base copper foils. The functional layers of the treatment stack are rather thin layers that do not sensibly modify the surface roughness of the treated copper foil, which is mainly determined by the initial roughness of the base copper foil.

[0049] In preferred embodiments, the copper foil on which the treatment stacks are formed is an electrodeposited copper foil. The first treatment stack is generally applied on the electrolyte side while the second treatment stack is generally applied on the drum side.

[0050] Preferably, the side of the copper foil on which the first treatment stack is formed is a low roughness side, having preferably a roughness Rz JIS of 0.7 pm or less, e.g. 0.6, 0.5 or 0.4 pm. The SDR of the same (first) side is normally 0.3% or less, in particular 0.2 or 0.1 % or less. In preferred embodiments the second side of the copper foil, on which the second treatment stack is formed, has a surface roughness in the same range, e.g. a roughness Rz JIS of 1 .2 pm or less, such as 1.0 pm or less: however, the surface developed ratio (SDR) of the second side may be higher and up to 3.5% such as e.g. 3.4%, 3.2%, 3.0%. P-CIRCUI-023 / WO 10

[0051] It should be appreciated that such low SDR (surface developed ratio) expresses the fact that the treatment stack is a smooth stack of layers and that the copper foil does not include any roughening layer. Bondability I adhesion properties of one side of the inventive surface treated copper foil is thus ensured by its respective treatment stack, in particular the third (coupling agent) layer.

[0052] In this context, it is believed that 3D parameters, and more particularly the surface developpe ratio (SDR), is more suitable for accurately characterizing the surface roughness of treated copper foils, in particular surface treated copper foils which may be used for high-frequency application and present low insertion loss, than two-dimensional (or 2D) surface parameters.

[0053] The surface developed ratio (SDR), or developed interfacial area ratio, sometimes also referred to as the complexity of the surface, corresponds to the ratio between the area of the real developed surface and the area of the projected surface. The real surface is the interfacial area of the surface treated copper foil while the projected surface is the surface of a corresponding flat, completely smooth foil. The SDR can be calculated based on the following equation (or equivalent computation):

[0054] Real developped surface — Projected surface

[0055] SDR = xlOO

[0056] Projected surface

[0057] In other words, SDR is expressed as the percentage of additional surface area contributed by the texture (presence of peaks and valleys at the surface of the copper foil as well as copper nodules and / or filaments of dendritic copper) as compared to an ideal plane surface. As SDR is affected by both the texture (number and size of peaks and valleys, of nodules and / or of filaments of dendritic copper) and the spatial disposition thereof, this parameter advantageously further differentiates surfaces of similar roughness as expressed using 2D parameters, such as Rz. Typically, SDR will increase with the spatial intricacy of the texture, whether or not Rz changes.

[0058] In this regard, it should be noted that copper foils having been submitted to a roughening treatment, such as e.g. the electrodeposition of fine copper nodules or dendritic copper, might present a surface roughness as of Rz JIS similar to the P-CIRCUI-023 / WO 11 one of a (surface treated) copper foil without any roughening treatment, while the SDR value would be much higher for a roughened (surface treated) copper foil.

[0059] Preferred copper foils have a thickness in the range of 9 to 70 pm.

[0060] In summary, the present invention provides a surface treated copper foil containing a first layer of Zn and Mo oxides providing a first passivation with a non-metallic alloy, which has the advantage of improving the thermal resistance without impacting the signal integrity at high frequency.

[0061] Moreover, both sides of the (as produced) copper foil being coated with a treatment stack according to the invention (i.e. the copper foil being a double side treated copper foil), there is no need to perform a brown / black oxide treatment contrary to conventional one side treated copper foils. PCB manufacturing process will thus be simplified and shorten, thereby increasing productivity.

[0062] Compared to prior art foils, the surface treatment of the inventive surface treated copper foil does not include any nodules / nodular treatment, nor any other kind of roughening treatment. In other words, at least one and preferably both of the first side and the second side of the copper foil is free of roughening treatment, preferably free of nodular treatment. Indeed, in conventional treatment lines for copper foils, at least one side of the foil is usually roughened in the treater line, i.e. submitted to a treatment enhancing I increasing the rugosity of its surface. On the contrary, the surface treated copper foil according to the present invention is preferably not submitted to any roughening treatment, neither on its first side nor on its second side. According to preferred embodiments, the first layer of each one of the first and second treatment stacks is directly formed (i.e. coated or deposited) onto the respective side of the copper foil, i.e. without prior application of e.g. a nodular treatment.

[0063] That is to say, according to preferred embodiments, the first side of the copper foil is free of roughening treatment, preferably the first side is free of nodular treatment. Additionally, or alternatively, the second side of the copper foil is free of roughening treatment, preferably the second side is free of nodular treatment. P-CIRCUI-023 / WO 12

[0064] In the present text, the term ‘roughening treatment’ is to be understood as a treatment designed to increase the roughness of a copper foil and that is applied to a copper foil after it has been removed from the electroplating cell in which it has been formed. In particular, the roughening treatment may refer to a nodular treatment i.e. the electrodeposition of fine copper nodules (or sometimes dendritic copper) on the base copper foil, or a so-called brown oxide treatment or black oxide treatment. During a brown I black oxide treatment, the surface of the copper foil is micro-etched to a depth of about 1 -2 pm, in order to create microroughness on the copper surface and simultaneously convert superficial copper into a layer of an organo-metallic structure that will help for the adhesion. Such oxide treatment is typically also used in the PCB manufacturing process, being applied at the end of the copper patterning to improve adhesion of the copper tracks.

[0065] In other words, the prescribed surface treatment is directly applied onto the side of a copper foil, i.e. in the absence of any roughening treatment of the side of the copper foil, that is to say directly applied to a copper foil after it has been removed from the electroplating cell in which it has been formed without any intervening roughening treatment, which advantageously allows to minimize transmission losses at high frequency and to achieve the required low transmission loss for applications at frequencies of 1 GHz and above, such as for the Fifth-Generation of mobile communication (5G), without detrimental effect on the adhesion, heat resistance and chemical resistance of the surface treated copper foil.

[0066] This surprisingly goes against common practice in the field, wherein a roughening treatment is conventionally applied onto the copper foil to enhance its bondability and adhesion properties, as disclosed by JP S61 288095 A and EP 3 882 378 A1 . However, such roughening treatment impacts the profile roughness of the foil and negatively impact signal integrity at high frequency.

[0067] On the contrary, according to some embodiments, the inventive copper foil being free of roughening treatment, signal transmission losses at high frequency are minimized. P-CIRCUI-023 / WO 13

[0068] Once again, it shall be appreciated that the low surface roughness of the treatment layer, reflecting the low surface roughness of the underlying copper foil induces lower signal loss in high speed / high frequency applications. This is due to the fact that at high frequency the signal is propagated only at the surface of the conductor (skin effect). On a smooth conductor the propagation route of the signal is therefore shorter, inducing lower loss. This enable the fabrication of effective transmission lines for applications at frequencies of 1 GHz and above (5G, etc.).

[0069] Surprisingly, the mechanical properties, and in particular the adhesion, of the surface treated copper foil were not negatively impacted by the absence of roughening treatment, due to the specific combination of the layers with prescribed composition forming the treatment layer.

[0070] In the present text, the term Toughening treatment’ is to be understood as a treatment designed to increase the roughness of a copper foil and that is applied to a copper foil after it has been removed from the electroplating cell in which it has been formed. In particular, the roughening treatment may refer to a nodular treatment i.e. the electrodeposition of fine copper nodules (also sometimes referred to as dendritic copper) on the base copper foil, or a so-called brown oxide treatment or black oxide treatment. During a brown I black oxide treatment, the surface of the copper foil is micro-etched to a depth of about 1-2 pm, in order to create microroughness on the copper surface and simultaneously convert superficial copper into a layer of an organo-metallic structure that will help for the adhesion.

[0071] Furthermore, the treatment layer has a very smooth roughness profile; the treatment stack does essentially not change the surface roughness of the copper foil side on which it is formed. It may be noted in that respect that the low surface roughness is reflected by the Rz and SDR values.

[0072] The present invention proposes a copper foil that is smooth to ensure low insertion loss at high frequencies, despite any roughening treatment, and which exhibits good performance having regard to criteria such as peel strength, chemical attack and blistering. P-CIRCUI-023 / WO 14

[0073] As a further benefit, the passivation on a smooth copper foil typically requires less material than a copper foil with nodules.

[0074] The present invention thus solves the problem of high transmission loss at high frequency by a surface treatment with the herein prescribed combination of layers, provided on a low roughness copper foil, with a non-metallic passivation and without nodular / roughening treatment.

[0075] According to another aspect, the present invention relates to a method of treating a copper foil, the method comprising providing a copper foil having two opposite sides and simultaneously coating a first side of the copper foil with a first treatment stack and a second side of the copper foil with a second treatment stack, said coating comprising the steps of: a) in a first bath, electrodepositing a first layer of oxides of Zn and of Mo on each side of the copper foil, said first bath comprising between 1 and 9 g / L of Mo and between 1 and 8 g / L of Zn; b) in a second bath, forming a coupling agent layer over the first layer.

[0076] The present method is adapted to provide a treatment layer on a copper foil as disclosed above. Technical features, explanations and advantages disclosed in relation to the herein disclosed surface treated copper foil apply mutatis mutandis to the present method.

[0077] It should be appreciated that both sides of the copper foil are preferably treated simultaneously, so that the time required for producing the double-side treated copper foil of the present invention is advantageously not greater than for producing a foil treated on only one side.

[0078] Accordingly, a conventional treater (or treatment line) configured to treat (i.e. apply passivation and silane layer) on only one side need to be adapted to apply e.g. the third layer of silane coupling agent simultaneously on both sides.

[0079] The first bath may be an aqueous acidic solution comprising Mo and Zn in the prescribed amounts, or containing only Mo and Zn in addition to the acidic species (typically sulfuric acid or equivalent). P-CIRCUI-023 / WO 15

[0080] Preferably, the first bath is free of Ni, as Ni is not desired in the first layer to be formed by the present method, and there is no voluntary addition of nickel in the bath. Impurities or traces may however exist, typically not more than 0.5 mg / m2

[0081] In embodiments, the first bath may comprise between 1.5 and 7 g / L of Mo and between 1 .0 and 5 g / L of Zn. The first bath may have a pH between 3.0 and 4.5, preferably between 3.5 and 4.

[0082] The electrodeposition process in the first bath may be carried out using two distinct sets of anodes applying different current densities. In some embodiments, a first set of anodes may apply a current density in the range of 0.1 to 2.2 A / dm2, preferably 0.2 to 1 .4 A / dm2, to deposit the first layer on the first side of the copper foil and a second set of anodes may apply a current density in the range of 0.1 to 2.0 A / dm2to deposit the first layer on the second side of the copper foil.

[0083] Using different sets of anodes to deposit the first layer on each side, i.e. using a first set of anodes to deposit the first layer on the first side and a second set of anodes to deposit the first layer on the second side, allows a more flexible control of the simultaneous deposition of layers having different compositions.

[0084] Moreover, using a set of anodes to deposit a respective first layer on a respective side allows a more flexible control of the co-deposition process. It may be noted that whereas Mo alone is difficult to deposit in aqueous solution, the present approach relying on co-deposition does provides a working solution to form a layer of oxides of Mo and of Zn. Hence the present invention contrasts with the state of the art where Mo has been co-deposited with ferromagnetic elements such as Ni or Co, which have a negative effect at high frequencies.

[0085] The electrodeposition in the first bath is advantageously realized to form a first layer on the first side comprising the oxides of Mo and of Zn in a quantity of between 5 and 30 mg / m2calculated as Mo and Zn elements, preferably between 15 and 25 mg / m2, while preferably simultaneously forming a first layer on the second side comprising the oxides of Mo and of Zn in a quantity of between 3 and 30 mg / m2calculated as Mo and Zn elements, preferably between 5 and 15 mg / m2. P-CIRCUI-023 / WO 16

[0086] In embodiments, the method may also comprise a step c) of electrodepositing, in a third bath, a layer of Cr oxide over the first layer and underneath the coupling agent layer on the first side of the copper foil, wherein step c) is carried out between step a) and step b). In other words, a layer comprising chromium oxides is preferably formed in the first treatment stack between the first layer and the coupling agent layer.

[0087] The third bath preferably comprising between 0.5 and 4 g / L of Cr, more preferably between 1 and 2 g / L. The third bath is typically an acidic solution (sulfuric acid) in which chromium oxide (e.g. CrO3) is added to meet the prescribed concentration. The third bath may have a pH between 1 and 4.

[0088] In embodiments, the electrodeposition process in the third bath is carried out such that the second layer comprises the oxides of Cr in a quantity of between 3.0 and 10 mg / m2calculated as Cr (not the oxide form). The third bath may have a pH between 1 and 4.

[0089] In embodiments, the second bath comprises a functionalized silane coupling agent at a concentration between 0.5 and 5 wt.%, wherein said functionalized silane coupling agent preferably comprises an aminosilane, an epoxy-silane, vinyl-silane, methacrylate silane, or a mixture thereof.

[0090] Preferably, the second bath, in case of an aminosilane coupling agent, has a pH of 9 to 12, in particular about 10.5.

[0091] Prior to dipping in the first bath, the copper foil advantageously undergoes a cleaning step to remove any oxides, grease, etc. The cleaning step may e.g. involve dipping the copper foil in an acidic bath.

[0092] It may be noted that the process does not involve any nodular treatment, nor any other kind of roughening treatment of the surface of the copper foil onto which the treatment layer is formed. The process is conducted to have a smooth surface treated side. Preferably, the surface treated copper foil, after electrodepositing in the second bath, has a roughness Rz JIS of 0.7 pm or less on its first side and a roughness Rz JIS of less than 1.2 (preferably less than 1.0 pm) on its second P-CIRCUI-023 / WO 17 side. The surface roughness on each side is measured from the exposed side of the treatment stack (i.e. the side not in contact with the copper foil).

[0093] According to another aspect, the invention relates to a copper clad laminate comprising a surface treated copper foil as disclosed herein laminated onto a substrate at 200°C for 2h. The substrate may generally be a polymer, in particular a resin or a prepreg. Such copper clad laminate exhibits following properties: the copper foil has a peel strength superior or equal to 0.40 N / mm, preferably at least 0.45 or 0.50 N / mm; a peel strength drop of 10 % or less after a HCI test (chemical resistance); and is able to resist a blistering test (thermal resistance) at a temperature of 270°C, or 275°C or more.

[0094] According to another aspect, the invention also concerns the use of the inventive surface treated copper foil in the manufacturing of multi-layer PCBs, in particular as copper layers of an inner core layer.

[0095] A further benefit of the inventive surface treated copper foil is its good adherence properties in the context of multi-layer PCB manufacturing. Indeed, it has been surprisingly found that the inventive surface treated copper foil, when used in an inner layer core, has sufficient adhesion in the multi-layer build up. Accordingly, the inner layer core using the inventive surface treated copper foil can be processed in further lamination steps directly after patterning, without the need for black / brown oxide treatment of the copper tracks.

[0096] Accordingly, the invention also relates to a method of manufacturing a multi-layer printed circuit board as claimed in claim 16. Preferred embodiments are recited in claims 17 and 18.

[0097] Such method typically employs at least one so-called inner layer core, consisting of a substrate layer clad on both sides with a copper layer. The inner layer core can be laminated with additional inner layer core(s) via intermediate substrate layers. Generally, the top and bottom copper foils are laminated, via intermediate substrate layers, on the inner layer core(s) to form the outer layers.

[0098] At least one of the inner layer core includes the surface treated copper foil according to the present disclosure. Preferably, at least one inner layer core has its substrate layer clad on both sides with a surface treated copper foil according P-CIRCUI-023 / WO 18 to the present disclosure. More preferably, all inner layer cores used in the process are made from the present surface treated copper foils.

[0099] As explained, the use of the present surface treated copper foils in a multilayer PCB manufacturing process eliminates the need for performing any surface adhesion enhancing treatment (such as e.g. brown oxide treatment or black oxide treatment) after formation of the copper tracks, which simplifies and speeds up the manufacturing process.

[0100] BRIEF DESCRIPTION OF THE DRAWINGS

[0101] Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings, in which:

[0102] Figure 1 : is a principle diagram of an embodiment of the present surface treated copper foil;

[0103] Figure 2: is a principle diagram of a surface treatment line for implementing the present process;

[0104] Figure 3: is a graph showing the insertion loss as a function of the frequency for an example and two comparative examples;

[0105] Figure 4: is a principle diagram of a copper clad laminate comprising the surface treated copper foil of Fig.1 ; and

[0106] Figure 5 is a principle diagram of the manufacturing process of a multi-layer printed circuit board.

[0107] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0108] The present invention addresses issues specific to copper foils for high-frequency circuits. Specifically, the invention provides in the following embodiments electrodeposited copper foils providing improved signal integrity at high frequency, by combining low roughness electrodeposited copper foil, nodular free P-CIRCUI-023 / WO 19 treatment and metallic free passivation while ensuring high thermal and chemical resistance and high peel strength on PPE I PPO and PTFE material.

[0109] In conventional processes, low profile foils copper foils are treated on at least one side with either microscale nodular treatments or roughening treatments to ensure high bondability to the substrate, impacting the signal transmission at high frequency.

[0110] Indeed, skin effect is experienced by resistors at high frequency. At low frequency, the distribution of the current is uniform throughout the resistor. However, as the frequency increases, the current distribution becomes non- uniform and is concentrated on the surface of the resistor. The current is confined only to the surface at RF frequency. At high frequency, alternative current has higher current density on the edges of the conductor and the current flows within the “skin depth”. Therefore, at these frequency ranges, signal integrity is mainly affected by the profile of the foils.

[0111] Losses of signal integrity at high frequency are therefore related to the high profile of the foil. Reduction of the profile of the foil will improve the signal integrity at high frequency, but can impact bondability. While some prior art processes involve the deposition of microscale nodular treatments or roughening treatments which impact the profile of the foil, the present process does not change or affect the profile of (basis) low roughness copper foil, while keeping bondability.

[0112] Improvements of the heat resistance is usually achieved by deposition of some metallic elements, such as Ni or Co, deposited in metallic state, which have a negative impact on signal integrity at high frequencies. While some patents are teaching the use of these elements to improve heat resistance, the present invention is only using an alloy in its non-metallic form which does not negatively influence signal transmission at high frequency.

[0113] Figure 1 schematically illustrates a surface treated copper foil 10 according to an embodiment of the present invention. It includes a (untreated) copper foil 12, in particular an electrodeposited copper foil, having two opposite sides, namely a drum side 12.1 (also referred to as shiny side) and an electrolyte side 12.2 (also referred to as matte side). P-CIRCUI-023 / WO 20

[0114] The electrolyte side 12.2 is coated with a treatment stack, generally indicated 14.2, that includes three layers:

[0115] - a first layer 16.2 comprising oxides of Mo and of Zn;

[0116] - a second layer 18.2 of chromate oxides; and

[0117] - a third layer 20.2, referred to as coupling agent layer.

[0118] The drum side 12.1 is coated with a treatment stack - or treatment arrangement, generally indicated 14.1 , that includes two layers:

[0119] - a first layer 16.1 comprising oxides of Mo and of Zn; and

[0120] - a second layer 18.1 of chromate oxides; and

[0121] - a third layer 20.1 , referred to as coupling agent layer.

[0122] The respective first and second layers 16.1 , 16.2, 18.1 , 18.2 are passivation layers, whereas the respective third layer 20.1 , 20.2 is provided for improving adhesion to polymer / resin.

[0123] It may be noted that the three layers are, in practice, formed one after another on a side of the copper foil (so to speak one on top of another). Accordingly, they are herein described and represented as three separate layers. However, due to the small deposited amounts of material in each layer there may be somewhat intermingled.

[0124] The manufacture of the copper foil is not the purpose of the present invention. Any appropriate copper foil may be used. The copper foil is preferably an electrodeposited copper foil. Preferred characteristics of the copper foil are:

[0125] - thickness in the range of 9 to 70 pm

[0126] - roughness Rz JIS: drum side: 0.6 - 1 .2 pm - electrolyte side: 0.4 - 0.7 pm

[0127] - SDR: electrolyte side: < 0.3% ; drum side < 3.5%

[0128] Preferably, the foil has a copper purity of at least 99.8%. The tensile strength may typically be in the range of 31 to 38 kgf / mm2

[0129] The inventive surface treated copper foil 10 results from a specific combination of layers having prescribed compositions. It has good results in terms of heat resistance, peel strength, chemical resistance and exhibits low transmission loss. P-CIRCUI-023 / WO 21

[0130] < Surface treatment process >

[0131] The present copper foil is obtained by submitting the foil to a treatment process comprising three baths 22, 24, 26 contained in separate recipients 28i - referred to as treaters, one for forming each of said layers 16, 18 and 20. The process is typically continuous, i.e. the copper foil is dipped in a continuous manner through the series of treaters 28i. This is illustrated in Fig. 2. The untreated (as produced) copper foil 12 is unrolled from a support drum 30 and guided, by means of guide rolls 32, through the various treaters 28i. The obtained surface treated copper foil 10 is finally rolled on a receiving drum 34.

[0132] During storage of the untreated copper foil 12, copper oxides may form locally. Accordingly, before forming the treatment stack, the copper foil 12 is preferably cleaned. This optional cleaning step may be carried out by dipping in an acidic bath 36 in first treater 28i. The acidic bath 36 may comprise sulfuric acid at a concentration between 60 and 100 g / L.

[0133] The cleaned copper foil 12 then enters the second treater 282 containing the first, passivating bath 22. First bath 22 is an acidic solution comprising 1.0 to 9.0 g / L of Mo and 1.0 to 8.0 g / L of Zn. It may be prepared from I^MoO^FW and ZnSO4-7H2O. Concentrations given herein for the various baths relate to the metal ions in the solution.

[0134] First bath 22 is an electroplating bath where Zn and Mo are co-deposited in oxide forms. Various oxide forms of Zn and Mo are deposited as well as possibly mixed oxides. Whereas deposition of Mo in aqueous solutions is difficult, the present approach based on co-deposition does allow forming a coherent layer of oxides of Zn and of Mo.

[0135] The pH may be adjusted by addition of sulfuric acid and / or sodium hydroxide to between 3.0 and 4.5. A pH greater than 4.5 tends to causes precipitation of Zn. At pH lower than 3, lower Zn amounts are deposited.

[0136] This electrodeposition step is conducted such that the first layer on the first side of the copper foil comprises the oxides of Mo and of Zn in a quantity of between 5 and 30 mg / m2and the first layer on the second side of the copper foil comprises the oxides of Mo and of Zn in a quantity of between 5 and 30 mg / m2, preferably P-CIRCUI-023 / WO 22 between 5.0 and 15.0 mg / m2This specific mass is calculated in respect of the Zn and Mo metals only.

[0137] Preferably, two separate sets of planar anodes are arranged in the bath, to which different current densities are applied. The current density at each set of anodes is adjusted depending on the desired amount of Zn and Mo to be deposited on each side of the copper foil, and within a set of anodes, the current density at each anode is adjusted depending on the desired respective amount of Zn and Mo to be deposited and the ratio Zn / Mo. The current density may typically vary between 0.1 and 2.2 A / dm2for the two sets of anodes depositing the respective first and the second layers on the two sides.

[0138] At the exit of treater 282 the copper foil is coated with the first layer 16 and enters the third bath 24 in treater 283. Third bath 24 is a chrome plating bath typically comprising a mixture of chromium trioxide (CrOs) and sulfuric acid. The concentration of Cr in the bath may be between 0.5 and 4 g / L. The pH of this passivation bath is preferably adjusted to about 2.0. Current density may be around 2 to 6 A / dm2.

[0139] Next the copper foil with the first layers 16.1 , 16.2 and the second layer 18.1 , 18.2 enters the last treater 284 containing the second bath 26. Bath 26 is an aqueous solution comprising a coupling agent, in particular a functionalized silane coupling agent, such as e.g. an aminosilane, an epoxy-silane, vinyl-silane, methacrylate silane, or a mixture thereof. The pH of the bath 26 is adapted depending on the type of coupling agent. For example, bath 26 is a basic solution when comprising aminosilane. Sprayers may be arranged above the treater 284 and configured to spray the second bath 26 over a side of the copper foil not immerged in the bath 26, typically over the second side 12.1.

[0140] The concentration of coupling agent in the second bath 26 may be between 0.5 and 5 wt.%. The pH of the aqueous solution may be adjusted by addition of sulfuric acid or sodium hydroxide.

[0141] The copper foil exiting the last treater 284 is thus coated on each side with three layers forming the present surface treated copper foil 10. P-CIRCUI-023 / WO 23

[0142] Before being rolled on the receiving drum 34, the surface treated copper foil 12 is dried in a drying tunnel 40, typically for about 15, 20 or 30 s, or greater.

[0143] < Printed circuit board manufacturing >

[0144] The inventive surface treated copper foil proves advantageous in the context of multi-layer PCB manufacturing. Multi-layer PCB manufacturing processes are well known in the art and only the first steps are shown in Fig. 5. They typically use one or more inner layer cores, to build a stack of copper layers separated by insulating substrate layers. The various components are assembled by lamination under heat and pressure.

[0145] Step A1

[0146] Step A1 corresponds to the manufacture of the inner layer core. Two surface treated copper foils 10 as disclosed herein are laminated on both sides of a substrate 101 of insulating material (e.g. prepreg material made from woven glass and epoxy). The lamination is carried out under heat and pressure, e.g. at 200 °C for 2h. A copper clad laminate 100 is obtained (also represented alone in Fig.4), which is used as inner layer core in a multi-layer PCB.

[0147] Step A2

[0148] In a second step A2, the inner layer core 100 is submitted to a patterning step to form copper tracks 105 (or traces) by:

[0149] - A2i) Applying a film of light sensitive resin 103 (photosensitive resist material) and hardening the resin where copper is to be left in place. This is typically done by selectively exposing the resin to light through a patterned mask;

[0150] - A2ii) Removing the non-hardened resin 103 so as to form an image of the desired copper pattern;

[0151] - A2iii) Etching the exposed copper surface to form copper tracks 105 (under the protective, hardened resin 103);

[0152] - A2iv) Removing the hardened resin 103.

[0153] Step A3 P-CIRCUI-023 / WO 24

[0154] Directly after formation of the copper tracks 105, laminating another inner layer core 100’ (or a copper foil) with an intermediate insulating substrate 109 on at least one of the patterned surfaces of the inner layer core 100.

[0155] The other inner layer core 100’ is obtained through similar steps A1 +A2.

[0156] On Fig.5, patterning step A2 and lamination step A3 are shown for the upper copper foil 10 of the inner layer core 100, but it is preferably done on both sides.

[0157] Two, three or more inner layer cores may be assembled in that manner, using intermediate substrate layers.

[0158] The multi-layer assembly is then further processed as known in the art to obtain a multi-layer printed circuit board.

[0159] A more detailed method may comprise the following steps S1 -S15.

[0160] Step S1

[0161] An inner layer core 100 is provided, see Fig.4. It is advantageously produced according to step A1 , by laminating the present double side surface treated copper foil 10 on both sides of a substrate layer (insulating material, e.g. a prepreg made from woven glass fiber and epoxy).

[0162] Step S2

[0163] A film of light-sensitive resist material (so-called resist) is applied on the exposed sides of the copper foils.

[0164] Step S3

[0165] On both sides, the resist is exposed to light through patterned masks, thereby creating a latent image of the desired pattern.

[0166] Step S4

[0167] The exposed or unexposed regions of the resist are then developed to reveal the underlying material. Typically, non-hardened film is washed out and the exposed copper is etched (chemically removed) thereby creating a pattern of copper tracks. Surface of the substrate is exposed where copper was etched away.

[0168] Step S5 P-CIRCUI-023 / WO 25

[0169] After etching, the hardened light-sensitive resist is chemically removed so as to expose the surface of the underlying copper tracks.

[0170] It is to be noted that in conventional PCB manufacturing process, the outer surface of the copper tracks is treated by a so-called brown oxide treatment or black oxide roughening treatment so as to improve adhesion of the copper surface to subsequent substrate layers (inside the PCB).

[0171] Such oxide treatment is unnecessary when using a surface treated copper foil as per the present invention, and the step of oxide treatment is thus not performed.

[0172] Step S6

[0173] The patterned inner layer core is then laminated (under heat and pressure) with one or more other patterned inner core layer(s) - obtained in the same manner - and using intermediate substrate layers. The outer layers are typically formed by simple copper layers (foils). This assembly I build up process is well known in the art and can be performed in the conventional manner, except for the brown / black oxide roughening steps that can be dispensed where the present copper foils are used.

[0174] From there, the PCB can be finalized according to the conventional process, including inter alia:

[0175] - hole drilling through the multi-layer assembly

[0176] - copper thickness reinforcement

[0177] - outer layers patterning through photolithography

[0178] - finalization of the PCB.

[0179] < Example and counter-examples >

[0180] A number of example and counter-examples (or comparative embodiments) will now be discussed hereinbelow. P-CIRCUI-023 / WO 26

[0181] In all of the example and counter-examples, the initial copper foil, to be surface treated, is an electrolytic copper foil produced to have a thickness of about 35 pm with the use of a titanium electrolytic drum, a cathode and an insoluble anode, and a cupric sulfate electrolyte. The surface roughness of the as produced electrolytic copper foil was lower than 1.2 pm Rz JIS on both sides (preferably lower than 0.7 pm on the first side).

[0182] Example 1

[0183] Example 1 relates to a (double side) surface treated copper foil according to the present invention. The first bath 22 comprised 3.8 g / L of Mo and 2.0 g / L of Zn. The deposition was carried with a current density of 1 .0 A.drrr2at the first anode and 1.0 A.drrr2at the second anode, to achieve a specific Mo+Zn mass of 20 mg / m2on the electrolyte side and 20 mg / m2on the second side (drum side). The speed of the copper foil through the bath was between 10 and 20 m / min.

[0184] The third bath 24 contained 2.0 g / L of Cr. Deposition was carried out with one electrode at a current density of 2.0 A.dm-2.

[0185] The second bath 26 contained 0.5 wt.% of aminosilane as coupling agent.

[0186] Comparative example 1

[0187] The copper foil was treated according to a standard brown oxide treatment comprising a step of 2 pm etching, highly impacting the surface roughness.

[0188] Comparative example 2

[0189] The copper foil was treated according to an advanced brown oxide treatment comprising a step of 0.8 pm etching, slightly impacting the surface roughness.

[0190] < Test procedures >

[0191] To characterize the surface treated copper foils obtained according to the invention and according to the comparative embodiments, several tests were performed. These tests are generally known in the art and are only briefly presented below.

[0192] Peel test to determine peel strength P-CIRCUI-023 / WO 27

[0193] The copper foil is laminated on a resin substrate. The peel strength is measured at 90°. The test was carried out according to IPC-TM-650 Method 2.4.8.5.

[0194] A standardized specimen comprising copper foil laminated onto a resin substrate is prepared. The peel strength (PS) test is conducted using tensile testing machine. The copper foil is peeled away from the resin substrate at a controlled 90° angle and a consistent pulling speed of about 50 mm per minute. The force required to peel the copper foil from the resin is continuously measured and recorded, providing the peel strength expressed in units of force per unit width (N / mm).

[0195] Roughness measurements

[0196] The roughness Rz JIS is measured by means of a perthometer in accordance with IPC-TM-650 Method 2.2.17.

[0197] The roughness of the surface treated copper foil is measured, for the treatment stack, from the exposed surface of the treatment stack. In the presently disclosed example, this correspond to the free side of the coupling agent layer 20.1 , 20.2 respectively opposite the underlying the second passivation layer 18.1 , 18.2.

[0198] SDR - surface developed interfacial ratio

[0199] SDR, or developed interfacial area ratio, expresses the percentage of the definition area's additional surface area contributed by the texture as compared to the planar definition area. The SDR of a completely flat surface is 0. Where a surface has any peak or slope, its SDR value becomes larger. SDR parameter is measured by contactless measurement.

[0200] SDR was measured with 3D laser scanning microscope, namely model VK-X- 3100 by Keyence.

[0201] SDR parameter is measured using non-contact three-dimensional white light interferometry.

[0202] The principle is to divide a light beam in two paths, directing one to a reference mirror and the other one to the sample surface. This measurement beams travel different distances depending on the surface profile. The two waveforms are then recombined and create specific interference patterns depending on their phase P-CIRCUI-023 / WO 28 difference. Those patterns are analysed to calculate the height of the sample at each point (pixel) scanned. Roughness parameters are then calculated from this 3D profile.

[0203] The SDR is calculated by comparing the actual measured surface area (three- dimensional) to the projected surface area (two-dimensional). It is expressed by the following formula, wherein A is the projected surface area:

[0204] Here the surface roughness SDR refers to (i.e. is measured according to) ISO 25178-2:2021 and is typically measured on a 290 x 220 pm sample surface.

[0205] Thermal resistance (blistering test)

[0206] Thermal resistance is measured via the so-called blistering test.

[0207] For this test, a double-sided copper clad laminate is prepared. Then the laminate is placed in a dry oven at a known temperature for 1 hour. The laminate is then visually examined using binocular magnifier. The test is passed when no blisters is visible which indicates that no delamination occurred.

[0208] The result of the blistering test indicates the highest temperature at which no blister nor delamination is observed on the copper-clad laminates after 1 hour.

[0209] Chemical resistance (peel strength drop after HCI)

[0210] Chemical resistance is evaluated via the drop of Peel Strength measured after HCI test. For this test, a copper foil is laminated on a resin substrate. Copper tracks of 1 .5 mm width are then formed. To do that, several strips of 1 ,5mm wide tape are placed and the exposed copper is etched using an acidic copper chloride solution sprayer. After etching, the tape is removed. The tracks are divided in two groups:

[0211] - group 1 : The peel strength of the track is measured using IPC-TM-650 Method 2.4.8.5 standard test. P-CIRCUI-023 / WO 29

[0212] - group 2: The tracks are immersed in a 12% HCl solution during 30 minutes. After that, the peel strength is measured using IPC-TM-650 Method 2.4.8.5 standard test.

[0213] The loss of adherence (peel strength drop after HCl test) is then calculated using 100

[0214] Insertion Loss measurements

[0215] Insertion Loss measurements conducted from 10 MHz to 67 GHz made on PNA E8361 C on microstrip PCB design using following characteristics: Microstrip design on Astra77 material (Dk = 3.0); Copper thickness: 1.8 MIL - 18pm; Track width: 0.47pm; dielectric thickness: 8 MIL; Impedance = 50 Q; No soldermask; No plating finishing; Track length: 25 cm; Connectors ELF-67-002.

[0216] Determining the quantities of Mo and / or Zn

[0217] Mo and Zn concentrations may be measured using Agilent 5110 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). To perform this analysis, the copper foil is partially dissolved in nitric acid to extract Mo and Zn into solution.

[0218] The solution is then analysed by ICP-OES, wherein a plasma source excites the atoms of Mo and Zn, resulting in element-specific emission spectra. The intensity of these emission lines directly correlates with the concentration of each element. Quantification is achieved through calibration against certified reference standards, yielding accurate concentrations of Mo and Zn.

[0219] Certified reference standards are provided by a supplier following ISO 17025 standard and ISO Guide 34 standards for preparing the reference standards. Characterization of the copper foil (i.e determining quantities of Mo and / or Zn) is generally performed according to ASTM E1479 standard. P-CIRCUI-023 / WO 30

[0220] Determining the quantity of coupling agent calculated as Si

[0221] The surface weight of the coupling agent calculated as Si is measured using Glow Discharge Optical Emission Spectroscopy (GD-OES). The equipment used is a Horiba GD Profiler 2. This analytical technique involves sputtering the sample surface within a controlled glow discharge plasma environment to progressively remove material layers.

[0222] During sputtering, excited silicon atoms emit characteristic optical emission lines. The intensity of these emission lines is directly proportional to the silicon concentration in the analyzed layers. The elemental concentration is quantitatively determined by calibration against certified reference materials, providing precise measurement of Si concentration in the coupling agent coating.

[0223] The quantity of coupling agent calculated as Si is generally determined according to ISO 14707:2021 standard. <Results>

[0224] Table 1 summarizes surface treatment of the copper foils according to the invention and comparative embodiments. Foils of example and counter-examples were submitted to series of test, the results of which are summarized in Table 2.

[0225] [Table 1 ] P-CIRCUI-023 / WO 31

[0226] P-CIRCUI-023 / WO 32

[0227] [Table 2]

[0228] Surface treated copper foil according to the present invention, i.e. example 1 , is treated with a non-metallic first layer of Zn and Mo oxides. It presents a good peel strength on both sides (<0.8 N / mm) and low HCL loss (less than 10).

[0229] The inventive foils of example 1 provide high chemical resistance, corresponding to a PS (peel strength) drop of less than 10% after HCI test, as observed on foils with brown oxide treatment (etching) on the drum side, and thermal resistance similar to what is achieved for conventional foils with brown oxide treatment on the drum side (comparative examples 1 and 2, Table 2).

[0230] Moreover, the inventive foil of example 1 provides similar adhesion on PPE / PPO as measured for foils with brown oxide treatment (comparative examples - table 2).

[0231] However, the inventive foil of example 1 allows achieving a better signal integrity at high frequencies compared to foils with brown oxide treatment (comparative examples 1 and 2 - see Fig.3 and Table 2 showing lower insertion loss for the drum side of the example according to the present invention than for comparative examples).

Claims

P-CIRCUI-023 / WO 33Claims1. A surface treated copper foil for a high-frequency circuit, the copper foil comprising two opposite sides, wherein each one of a first side and a second side are coated with a respective treatment stack comprising, in this order: a first layer comprising oxides of Mo and of Zn deposited on the respective side of the copper foil, wherein said first layer is free of Ni; and a coupling agent layer; wherein the first layer comprises the oxides of Mo and of Zn in a quantity of between 3.0 and 30 mg / m2calculated as Mo and Zn; and wherein each treatment stack has a roughness Rz JIS of 1 .2 pm or less.

2. The surface treated copper foil according to claim 1 , wherein the first side is free of roughening treatment, preferably the first side is free of nodular treatment.

3. The surface treated copper foil according to claim 1 or 2, wherein the second side is free of roughening treatment, preferably the second side is free of nodular treatment.

4. The surface treated copper foil according to any one of the preceding claims, wherein the weight ratio of Mo to Zn in the first layer of the first treatment stack is between 0.3 and 1 .5.

5. The surface treated copper foil according to any one of the preceding claims, wherein the first layer of the first treatment stack comprises more than 80 wt.% of oxides of Mo and of Zn, in particular more than 85 or 90 wt.%6. The surface treated copper foil according to any one of the preceding claims, wherein the first layer deposited on the first side of the copper foil comprises the oxides of Mo and of Zn in a quantity of between 5 and 30 mg / m2calculated as Mo and Zn, andP-CIRCUI-023 / WO 34 wherein the first layer deposited on the second side of the copper foil comprises the oxides of Mo and of Zn in a quantity of between 3.0 and 15 mg / m2calculated as Mo and Zn.

7. The surface treated copper foil according to any one of the preceding claims, wherein a second layer is deposited over the first layer on at least the first side of the copper foil, preferably on both sides, underneath the coupling agent layer, the second layer comprising Cr oxides.

8. . The surface treated copper foil according to the previous claim, wherein the second layer comprises the Cr oxides in a quantity of between 2.0 and 10.0 mg / m2calculated as Cr, preferably between 2.0 and 7.0 mg / m2.

9. The surface treated copper foil according to any one of the preceding claims, wherein the coupling agent layer comprises a functionalized silane coupling agent.

10. The surface treated copper foil according to the previous claim, wherein the coupling agent layer comprises between 0.5 and 5 mg / m2of coupling agent calculated as Si.11 . The surface treated copper foil according to any one of the preceding claims, wherein said copper foil has a thickness in the range of 9 to 70 pm.

12. The surface treated copper foil according to any one of the preceding claims, wherein the treatment stack on the first side has a roughness Rz JIS of 0.7, 0.6, 0.5 or 0.4 pm.

13. The surface treated copper foil according to any one of the preceding claims, wherein the treatment stack on at least one side of the copper foil has a SDR of 0.3% or less, in particular not more than 0.2 or 0.1 %.

14. The surface treated copper foil according to any one of the preceding claims, wherein said copper foil is an electrodeposited copper foil.

15. The surface treated copper foil according to the previous claim, wherein said first side is an electrolyte side of said copper foil and said second side is a drum side of said copper foil.P-CIRCUI-023 / WO 3516. A method of manufacturing a multi-layer printed circuit board comprising the steps of: a. providing an inner layer core comprising a substrate layer clad on both sides with a copper foil, wherein at least one of said copper foils, preferably both, is a surface treated copper foil as claimed in any one of claims 1 to 15; b. patterning the surface treated copper foils of the inner layer core to form a plurality of copper tracks; c. laminating the patterned inner layer core with further substrate layers as well as copper foil(s) or further inner layer core(s).

17. The method according to claim 16, wherein patterning includes applying photosensitive resist material on the copper foils, selectively exposing the resist material to light through at least one patterned mask, followed by image developing, selective copper etching and stripping of resist material; and characterized in that step c) is performed directly after the stripping of resist material of step b).

18. The method according to claim 16 or 17, wherein no roughening treatment step is performed during steps b) and c) or between them.

19. A method of treating a copper foil comprising: providing a copper foil having two opposite sides; simultaneously coating a first side of the copper foil with a first treatment stack and a second side of the copper foil with a second treatment stack, said coating comprising the steps of: a) in a first bath, electrodepositing a first layer of oxides of Zn and of Mo on each side of the copper foil, said first bath comprising between 1 .0 and 9.0 g / L of Mo and between 1 .0 and 8.0 g / L of Zn; b) in a second bath, forming a coupling agent layer over the first layer.

20. The method according to claim 19, wherein the first bath comprises between 1 .5 and 7.0 g / L of Mo and between 1 .0 and 5.0 g / L of Zn.P-CIRCUI-023 / WO 3621. The method according to claim 19 or 20, wherein the first bath has a pH between 3.0 and 4.5, preferably between 3.5 and 4.

22. The method according to any one of claims 19 to 21 , wherein said electrodepositing in said first bath is carried out using two distinct sets of anodes applying different current densities, and wherein a first set of anodes applies a current density in the range of 0.1 to 2.2 A / dm2to deposit the first layer on the first side of the copper foil and a second set of anodes applies a current density in the range of 0.1 to 2.0 A / dm2to deposit the first layer on the second side of the copper foil.

23. The method according to any one of claim 19 to 22, wherein said electrodepositing in said first bath is carried out such that the first layer deposited on the first side of the copper foil comprises the oxides of Mo and of Zn in a quantity of between 5 and 30 mg / m2calculated as Mo and Zn and the first layer deposited on the second side of the copper foil comprises the oxides of Mo and of Zn in a quantity of between 3 and 15 mg / m2calculated as Mo and Zn.

24. The method according to any one of claim 19 to 23, further comprising the step c) in a third bath, electrodepositing a layer of Cr oxide over the first layer and underneath the coupling agent layer on at least one side of the copper foil; wherein step c) is carried out between step a) and step b) and wherein the third bath comprises between 0.5 and 4 g / L of Cr.

25. The method according to the previous claim, wherein the third bath has a pH between 1 and 4.

26. The method according to claim 24 or 25, wherein said electrodepositing in said third bath is carried out such that said second layer comprises the oxides of Cr in a quantity of between 3.0 and 10 mg / m2calculated as Cr.

27. The method according to any one of claim 19 to 26, wherein the second bath comprises a functionalized silane coupling agent at a concentration betweenP-CIRCUI-023 / WO 370.5 and 5 wt.%, wherein said functionalized silane coupling agent preferably comprises an aminosilane, an epoxy-silane, vinyl-silane, methacrylate silane, or a mixture thereof.

28. The method according to any one of claim 19 to 27, comprising a cleaning step before said first bath, in particular by dipping said copper foil in an acidic bath.

29. The method according to any one of claim 19 to 28, wherein said surface treated copper foil, after the second bath, has a roughness Rz JIS of 0.7 pm or less on its first side and a roughness Rz JIS of 1 .2 pm or less on its second side.

30. A copper clad laminate comprising a surface treated copper foil according to any one of claims 1 to 15 laminated onto a substrate at 200°C for 2h, wherein the copper foil has a peel strength superior or equal to 0.40 N / mm; a peel strength drop of 10 % or less after a HCI test and is able to resist a blistering test at a temperature of 270°C or more.

31. An inner layer core comprising a substrate layer clad on both sides with a surface treated copper foil according to any of claims 1 to 15.

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