Copper foil, laminates, and flexible printed circuit boards

Abstract

To provide a copper foil that features reduced conduction loss as a copper foil for high frequency circuits.SOLUTION: The present invention provides a copper foil, where at least one surface of the copper foil has a root mean square gradient (Sdq) of 0.120 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to copper foil, laminates, and flexible printed circuit boards. In particular, it relates to copper foil having excellent transmission characteristics as copper foil for high-frequency circuits, and to laminates and flexible printed circuit boards equipped with such copper foil. [Background technology]

[0002] Printed circuit boards (PCBs) have made significant progress over the past half-century and are now used in almost all electronic devices. In recent years, the increasing need for miniaturization and higher performance in electronic devices has led to higher density mounting of components and higher frequency signals, requiring PCBs to have superior high-frequency capabilities.

[0003] High-frequency circuit boards require reduced transmission loss to ensure the quality of the output signal. Transmission loss mainly consists of dielectric loss due to the resin (substrate side) and conductor loss due to the conductor (copper foil side). Dielectric loss decreases as the dielectric constant and dielectric loss tangent of the resin decrease. In high-frequency signals, conductor loss is mainly caused by the skin effect, where as the frequency increases, the cross-sectional area through which the current flows decreases, resulting in increased resistance.

[0004] As a technology aimed at reducing the transmission loss of copper foil for high-frequency circuits, for example, Patent Document 1 (Japanese Patent No. 4161304) discloses a metal foil for high-frequency circuits in which one or both sides of the surface of the metal foil are coated with silver or a silver alloy, and a coating layer other than silver or a silver alloy is applied on top of the silver or silver alloy coating layer, with the coating layer being thinner than the thickness of the silver or silver alloy coating layer. It is stated that this makes it possible to provide a metal foil with reduced loss due to the skin effect even in the ultra-high frequency range used in satellite communications.

[0005] Furthermore, Patent Document 2 (Japanese Patent No. 4704025) describes the integral intensity of the (200) plane (I) obtained by X-ray diffraction on the rolled surface of rolled copper foil after recrystallization annealing. (200)) is the integral intensity of the (200) plane (I) determined by X-ray diffraction of fine copper powder. 0(200) ) vs. I (200) / I 0(200) A roughened rolled copper foil for high-frequency circuits is disclosed, characterized in that the rolling surface has a roughening treatment of >40, the arithmetic mean roughness (hereinafter referred to as Ra) of the roughened surface after roughening treatment by electroplating is 0.02 μm to 0.2 μm, and the ten-point mean roughness (hereinafter referred to as Rz) is 0.1 μm to 1.5 μm, and it is a material for printed circuit boards. It is stated that this makes it possible to provide a printed circuit board that can be used at high frequencies exceeding 1 GHz.

[0006] Furthermore, Patent Document 3 (Japanese Patent Application Publication No. 2004-244656) discloses an electrolytic copper foil characterized by having an uneven surface with a surface roughness of 2 μm to 4 μm, where a part of the surface of the copper foil consists of knob-like protrusions. It is stated that this can provide an electrolytic copper foil with excellent high-frequency transmission characteristics.

[0007] Furthermore, Patent Document 4 (Japanese Patent Publication No. 2017-193778) discloses a copper foil having a roughened layer, wherein the roughened layer has a primary particle layer, the surface roughness Ra of the surface on the primary particle layer side is 0.12 μm or less, and the average particle size of the primary particles in the primary particle layer is 0.10 to 0.25 μm. It is described that forming such a predetermined roughened particle layer and controlling the surface roughness Ra of the surface on the roughened particle layer side and the average particle size of the roughened particles is effective in suppressing transmission loss when used in a high-frequency circuit board. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 4161304 [Patent Document 2] Patent No. 4704025 [Patent Document 3] Japanese Patent Publication No. 2004-244656 [Patent Document 4] Japanese Patent Publication No. 2017-193778 [Overview of the project] [Problems that the invention aims to solve]

[0009] In recent years, with the further advancement of signal frequency, there has been a growing need to manufacture printed circuit boards with superior transmission characteristics in high-frequency ranges such as 10 GHz and above, and further improvements to the copper foil used as the raw material are required. In particular, as the signal frequency increases, the conductivity of copper foil decreases due to the skin effect, which worsens transmission loss. However, the technologies described in Patent Documents 1 to 4 may have difficulty in handling high-frequency circuit boards such as 10 GHz and above. In particular, the invention described in Patent Document 4 is an excellent invention in that it focuses on the fact that surface roughness of copper foil is the main cause of conductor loss, and that transmission loss decreases as the roughness decreases. By controlling the surface roughness Ra of the roughened particle layer side surface and the average particle size of the roughened particles, it is possible to achieve both suppression of transmission loss and adhesion with the resin. However, there is a problem in that the conditions for the roughening treatment must be strictly controlled. Furthermore, as mentioned above, the presence of roughened particles makes it unavoidable that resistance will increase due to the skin effect.

[0010] This invention was completed in view of the above-mentioned problems, and in one embodiment, aims to provide a copper foil with low transmission loss for use in high-frequency circuits. In another embodiment, this invention aims to provide a laminate equipped with such a copper foil. [Means for solving the problem]

[0011] As a result of intensive studies, the present inventors have found that, as a characteristic of a copper foil for suppressing the above skin effect and suppressing transmission loss in high-frequency transmission, it is effective to control the root mean square slope (Sdq) of the surface of the copper foil before surface treatment. In particular, when a surface treatment that is expected to increase conductor loss is applied to the surface of the copper foil, by controlling the Sdq of the copper foil before surface treatment within an appropriate range, even when a surface treatment containing a magnetic metal is applied, it is possible to enjoy the merits of the surface treatment while suppressing an increase in transmission loss in high-frequency transmission. The present invention has been completed based on the above findings and is illustrated below. [1] A copper foil having a root mean square slope (Sdq) of 0.120 or less on at least one surface. [2] The copper foil according to [1], wherein the root mean square slope (Sdq) on the at least one surface is 0.100 or less. [3] The copper foil according to [1] or [2], further including a surface treatment layer on the at least one surface. [4] The copper foil according to any one of [1] to [3], which is a rolled copper foil. [5] A laminate formed by laminating the copper foil according to any one of [1] to [4] and a resin substrate. [6] A flexible printed wiring board using the laminate according to [5]. [Effect of the Invention]

[0012] According to one embodiment of the present invention, it is possible to provide a copper foil with low transmission loss as a copper foil for high-frequency circuits. Further, according to another embodiment of the present invention, it is possible to provide a laminate provided with such a copper foil. [Brief Description of the Drawings]

[0013] [Figure 1] FIG. 1 is a graph plotting the root mean square slope (Sdq) on the horizontal axis and the transmission loss on the vertical axis based on Examples and Comparative Examples.

Best Mode for Carrying Out the Invention

[0014] Next, embodiments of the present invention will be described. It should be understood that the present invention is not limited to the following embodiments, and design changes, improvements, etc. can be appropriately added based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention.

[0015] (Composition of Copper Foil) There is no particular limitation on the form of the copper foil that can be used in this embodiment. Typically, the copper foil used in the present invention can be either an electrolytic copper foil or a rolled copper foil. Generally, electrolytic copper foil is produced by electrolytically depositing copper on a drum of titanium or stainless steel from a copper sulfate plating bath, and rolled copper foil is produced by repeatedly performing plastic processing and heat treatment using rolling rolls. Rolled copper foil is often applied to applications where flexibility is required.

[0016] As the copper foil material, in addition to high-purity copper such as tough pitch copper and oxygen-free copper that are usually used as conductor patterns of printed wiring boards, copper alloys such as copper containing Sn, copper containing Ag, copper alloys added with P, Cr, Zr or Mg, and Colson-based copper alloys added with Ni and Si can also be used. In this specification, when the term "copper foil" is used alone, it shall include copper alloy foil.

[0017] Also, the thickness of the copper foil does not need to be particularly limited, but for example, it is 1 to 1000 μm, or 1 to 500 μm, or 1 to 300 μm, or 3 to 100 μm, or 5 to 70 μm, or 6 to 35 μm, or 9 to 18 μm.

[0018] The copper foil of this embodiment can be suitably used for high-frequency circuit applications. Here, a high-frequency circuit is defined as a circuit in which the signal frequency transmitted through the circuit is 10 GHz or higher.

[0019] The copper foil of this embodiment has a root mean square slope (Sdq) of 0.120 or less on at least one of its surfaces. The root mean square slope is a parameter calculated by taking the root mean square of the slope at all points in the defined domain, and the Sdq of a perfectly flat surface without irregularities is 0. On an uneven surface, the surface has a slope, so the Sdq will be greater than 0. Also, as the local slope angle increases, the Sdq tends to increase. Therefore, Sdq is an index that quantifies the roughness (steepness of the slope) of the surface irregularities. When the Sdq is large, the transmission loss at high frequencies increases due to the aforementioned skin effect, so it is preferable for the Sdq to be small. In particular, when surface treatment is applied to copper foil, the surface profile tends to increase significantly as the metal of the surface treatment layer adheres to the steep surface irregularities, and from this viewpoint, copper foil with gentle surface irregularities is desirable.

[0020] From the above perspective, it is preferable that the root mean square slope (Sdq) is 0.100 or less, and more preferably 0.060 or less. The lower limit of Sdq is not limited from the standpoint of transmission loss, but since scratches on the copper foil surface are expected to become more noticeable, it is desirable that it be 0.001 or more, more preferably 0.002 or more.

[0021] While arithmetic mean height (Sa) and maximum height (Sz) are more common indicators of surface roughness, they are not considered suitable for representing the degree of increase in transmission distance when a signal flows only near the copper foil surface due to the skin effect. This is because maximum height (Sz) is an indicator whose value fluctuates greatly due to singularities such as scratches scattered on the copper foil surface, and is therefore considered unsuitable as an indicator that represents the characteristics when a signal flows across the entire surface of the copper foil as a high-frequency circuit. Furthermore, arithmetic mean height (Sa) is considered unsuitable because it is a concern that fine undulations that contribute to the increase in transmission distance may be hidden by larger undulations. Therefore, the root mean square slope (Sdq), which directly represents the degree of increase in transmission distance when a signal flows only near the copper foil surface due to the skin effect, is considered an appropriate indicator that accurately reflects the essence of the problem.

[0022] The root mean square slope (Sdq) is calculated after measuring the surface roughness data of the copper foil using a Keyence VK-X1000 (controller unit) / 1050 (head unit) laser microscope or an equivalent device, in accordance with ISO 25178-2:2012, as described below. As mentioned above, the root mean square slope (Sdq) is an important parameter from the perspective of suppressing the increase in transmission loss due to surface treatment, so when surface treatment is applied to copper foil, the root mean square slope (Sdq) should be measured for the copper foil before surface treatment.

[0023] Furthermore, at least one surface of the copper foil may be provided with a surface treatment layer. The surface treatment layer may also be a roughening treatment layer. Roughening treatment usually refers to a process in which nodular electrodeposition is formed on the surface of the degreased copper foil, specifically on the surface that adheres to the resin substrate, i.e., the surface-treated side, with the aim of improving the peel strength of the laminated copper foil. Electrolytic copper foil has irregularities at the time of manufacture, but roughening treatment can increase the protrusions of the electrolytic copper foil, making the irregularities even larger. Roughening treatment can be performed, for example, by forming roughening particles with copper or a copper alloy. The roughening treatment may be very fine. The roughening treatment layer may be a layer made of any single element or an alloy containing one or more of the elements selected from the group consisting of copper, nickel, cobalt, phosphorus, tungsten, arsenic, molybdenum, chromium, and zinc. In addition, after forming roughening particles with copper or a copper alloy, a roughening treatment can be performed to further provide secondary or tertiary particles with nickel, cobalt, copper, zinc, or other elements or alloys.

[0024] Furthermore, the surface treatment layer may be one or more layers selected from the group consisting of a roughening treatment layer, a heat-resistant layer, a rust-preventive layer, a chromate treatment layer, and a silane coupling treatment layer. These layers can be provided using known methods. Also, as will be described later, since the copper foil of the present invention can control the aforementioned root mean square slope (Sdq) without surface treatment, in one embodiment the copper foil does not include a surface treatment layer in the sense that the manufacturing method is simple.

[0025] The surface treatment layer preferably contains magnetic metals such as nickel or cobalt from the viewpoint of corrosion resistance and heat resistance. The skin depth during high-frequency signal transmission is expressed by the following formula, and as the permeability (μ) increases, the skin depth decreases, and the skin effect due to the increase in surface profile becomes more pronounced, so the effects of the present invention also become more pronounced. Permeability is high for magnetic metals, so when a surface treatment with a magnetic metal as one component is applied, the difference in high-frequency transmission loss between the copper foil of the present invention and conventional copper foil becomes even more pronounced.

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[0026] The copper foil of the present invention can be bonded to a resin substrate to produce a laminate. The resin substrate is not particularly limited as long as it has properties applicable to printed circuit boards and the like, but for example, polyester films such as polyethylene terephthalate (PET), polyimide films, liquid crystal polymer (LCP) films, fluororesin films, etc. can be used.

[0027] The lamination method involves bonding a polyimide film or other substrate to a rolled copper foil using an adhesive, or laminating and bonding the polyimide precursor to the substrate under high temperature and pressure without using an adhesive, or by applying, drying, and curing the polyimide precursor to produce the laminate.

[0028] Furthermore, a laminate can be used to construct a printed circuit board, particularly a flexible printed circuit board. Therefore, in another aspect, the present invention discloses a method for manufacturing a printed circuit board, particularly a flexible printed circuit board, using the rolled copper foil and the laminate of the present invention.

[0029] (Manufacturing method) The copper foil of the present invention is not particularly limited in its manufacturing method as long as the aforementioned root mean square slope (Sdq) can be controlled within the scope of the present invention. However, as a method that can be easily manufactured without surface treatment, a technique can be adopted in which the equivalent amount of oil film thickness in the final rolling pass of the final cold rolling process is appropriately adjusted.

[0030] In this embodiment, the method for manufacturing rolled copper foil involves first melting the raw materials in a melting furnace to obtain molten metal of the desired composition. Then, this molten metal is cast into an ingot. Subsequently, hot rolling, cold rolling, and annealing are performed as appropriate to finish the foil to a predetermined thickness. After heat treatment, surface pickling or polishing may be performed to remove the surface oxide film formed during heat treatment. In the final cold rolling, the heat-treated material is repeatedly passed through a rolling mill to finish it to a predetermined thickness. In this embodiment, it is important to perform the final rolling pass of the final cold rolling process with an oil film thickness equivalent of 10,000 to 40,000.

[0031] By controlling the equivalent oil film thickness, the root mean square slope of the copper foil surface can be suppressed. If the equivalent oil film thickness exceeds 40,000, the effect of flattening the copper foil surface diminishes, and the reduction of the root mean square slope becomes insufficient. On the other hand, if the equivalent oil film thickness falls below 10,000, the oil film between the copper foil and the rolling roll becomes thin, making it easier for scratches and dirt on the rolling roll surface to transfer to the copper foil surface, which can cause the root mean square slope to increase.

[0032] Here, the equivalent oil film thickness is defined by the following formula. Oil film thickness equivalent = {(Rolling oil viscosity [cSt]) × (Entry plate speed [mm / s] + Roll peripheral speed [mm / s])} / {(Roll engagement angle [rad]) × (Rolling load per unit area [kg / mm])} 2 ])}

[0033] The rolling oil viscosity was measured in accordance with JIS K2283. The inlet sheet passing speed was calculated from the peripheral speed of the take-up reel on the outlet side, taking into account the rolling reduction of the rolling pass. The roll peripheral speed was calculated from the peripheral speed of the take-up reel on the outlet side, assuming no slip between the roll and the material. Also, the roll bite angle and the rolling load per unit area were calculated using the following formulas. The Young's modulus and Poisson's ratio of the roll were the values described in the roll manufacturer's catalog and the literature values of the roll material, respectively. The radius of the roll was measured by a roll diameter measuring machine capable of measuring in 0.0005 mm units. The rolling load was calculated from the pressure measured by the hydraulic sensor of the hydraulic cylinder of the rolling mill, using the diameter and number of cylinders. The reduction in thickness was calculated from the material sheet thickness before and after passing the sheet. The material sheet thickness can be measured, for example, in accordance with the mass thickness measurement method of JIS C6515. Roll bite angle = {(Reduction in thickness [mm]) / (Flat radius of roll [mm])} 0.5 Rolling load per unit area = (Rolling load [kg]) / {(Average sheet width [mm]) × (Contact arc length [mm])} Flat radius of roll = (Radius of roll [mm]) × [1 + 16 × {1 - (Poisson's ratio of roll) 2} / {π × (Young's modulus of roll [kg / mm 2 )} × (Rolling load [kg]) / {(Average sheet width [mm]) × (Reduction in thickness [mm])}] Contact arc length = {(Reduction in thickness [mm]) × (Flat radius of roll [mm])} 0.5 Note that the numerical values of the oil film thickness equivalent in Table 1 are rounded to the nearest 100.

[0034] To control the oil film thickness equivalent, known methods such as using a low-viscosity rolling oil or reducing the inlet sheet passing speed may be used.

Examples

[0035] Hereinafter, the present invention will be specifically described by way of examples, but the description here is for the purpose of mere illustration and is not intended to be limited thereto.

[0036] First, copper ingots with the copper content listed in Table 1 were manufactured and hot-rolled. After that, cold rolling and annealing in an annealing furnace set to a temperature of 300-800°C were repeated at least once, followed by cold rolling to obtain rolled plates with a thickness of 0.1-1.0 mm. These rolled plates were annealed in an annealing furnace set to a temperature of 300-800°C to recrystallize, and then subjected to final cold rolling. The copper foil thickness of Example 1 was 12 μm, and the copper foil thickness of Comparative Examples 1 and 2 was 18 μm. At this time, in the final cold-rolling process, the oil film thickness equivalent was changed in the final rolling pass as shown in Table 1 to manufacture the copper foils of the Example and Comparative Examples.

[0037] The samples from the examples and comparative examples prepared as described above were evaluated as follows. The results are shown in Table 1.

[0038] (Measurement of the root mean square slope (Sdq)) Shape measurements were performed using a Keyence VK-X1000 (controller unit) / 1050 (head unit) laser microscope. Subsequently, the root mean square slope (Sdq) was measured using the Keyence VK-X1000 (controller unit) / 1050 (head unit) analysis software. At this time, a 250 μm × 200 μm area (specifically, 50,000 μm) was measured using a 50x objective lens in the laser microscope. 2 Measurements of the root mean square slope (Sdq) were performed at 10 arbitrary locations, and the root mean square slope (Sdq) at each location was calculated. The arithmetic mean of the 8 Sdq values ​​obtained from the 10 locations, excluding the maximum and minimum values, was used as the Sdq value. In order to eliminate the influence of the tilt and waviness of the surface of the object being measured when it was placed during the shape measurement, the Sdq was calculated after performing surface shape correction (planar tilt correction and waviness removal (cutoff wavelength 0.08 mm)) on the measurement area using analysis software. The ambient temperature for measuring the Sdq using the laser microscope was 20-25°C. The main settings for the laser microscope and analysis software are as follows. <Measurement settings> Measurement Mode: Simple Measurement Scan Mode: Laser Confocal Measurement Size: Standard (1024×768) Measurement Quality: High Precision Measurement Pitch: 0.26μm RPD: ON Brightness 1: About 6500 - 7500 (varies with focus fine adjustment) Brightness 2: Auto (9500) Illumination Filter 1: About 10 - 30% (varies with focus fine adjustment) Illumination Filter 2: Auto (About 10 - 30% (varies with focus fine adjustment)) Double Scan: ON Average Number of Times: 1 time Do not acquire color image: OFF Fine Mode: ON Enable processing of noise area: OFF <Illumination> Coaxial Incidence: 100 Ring Proof: OFF <Z - axis> Z - axis Mode: Recommended Setting Measurement Upper Limit: About 9900 - 10100μm (varies with focus fine adjustment) Measurement Lower Limit: About 10000 - 10100μm (varies with focus fine adjustment) Z - measurement Distance Fixed: OFF Z - measurement Distance: About 13 - 14μm (varies with focus fine adjustment) Auto Upper and Lower Limits·Auto Brightness: ON <Head> Head: R Objective Lens Name: Plan(CF IC EPI Plan 50X) Objective Lens Magnification: 50X Lens NA: 0.800 WD: 0.54mm Image Plane Curvature Correction: ON Light Quantity Eccentricity Correction: ON Ring Illumination Correction: ON XY Calibration: 268.846nm / pixel Z calibration: 1.000 (0.100 nm / digit) <Camera Settings> Brightness mode: Auto Brightness (Auto): Approximately 120-140 (may vary depending on focus fine-tuning) Brightness (Manual): 2 Edge enhancement: 5 <Laser settings> γ coefficient (γ correction value): 0.45 γ offset: 0% Black and white inversion: OFF Edge enhancement type: None Edge enhancement direction: horizontal Edge emphasis strength: weak <Other conditions shown in the worksheet> Measurement mode: Surface shape Optical zoom magnification: 1.0x Filter: OFF Camera gain: 0dB Shutter speed: Auto White balance mode: Manual White Balance R:0 White balance B:0 Light reception intensity correction mode: γ correction Head type: VK-X105 Vibrancy: 5 Contrast: 5 Brightness: 0 AI noise removal: OFF Slope noise filter: OFF <Analysis conditions> Image processing: Surface shape correction performed (plane tilt correction, undulation removal (cutoff wavelength 0.08 mm)) Surface roughness setting (filter setting) Filter type: Gaussian S-filter (low-pass filter): None F-Operation (Shape Correction): None L-filter (high-pass filter): None End effect correction: ON

[0039] (Measurement of transmission loss) For each example and comparative example, a separate sample prepared in the same process was subjected to Ni plating under the following conditions. Specifically, a 90mm wide x 210mm long sample was degreased using an aqueous solution of GN87 (manufactured by JX Metal Trading Co., Ltd.), pickled with a 10% sulfuric acid aqueous solution, and then plated in a Watt bath (Ni 13g / L, pH 2.3, temperature 50℃) at a current density of 10A / dm². 2 The theoretical Ni deposition amount is 826 μg / dm 2 Ni plating was performed by adjusting the plating time to achieve the desired result. In addition, a copper-clad laminate was fabricated by hot pressing 50 μm of PI (product name: UPILEX, manufactured by Ube Industries) with copper foil facing the PI side on both sides. A microstrip line was obtained by circuit processing (etching) the copper foil on one side of the copper-clad laminate so that the width of the copper foil was approximately 110 μm. The transmission loss of the microstrip line at a circuit frequency of 10 GHz was measured using a network analyzer Keysight N5247A and a universal test fixture Anritsu Universal Test Fixture Model 3680V for converting between microstrip line and coaxial cable. The measurement results are shown in Table 1.

[0040] [Table 1]

[0041] (Consideration) In the example, the root mean square slope (Sdq) on the surface was 0.120 or less, thus suppressing transmission loss at high frequencies. On the other hand, in the comparative example, the oil film thickness equivalent of the final rolling pass in the final cold rolling was high, so the root mean square slope (Sdq) exceeded 0.120, and the transmission loss was greater than in the example. Furthermore, when the circuit frequency was changed to 20 GHz, 30 GHz, and 40 GHz and the transmission loss was measured for each frequency under the same conditions as described above, the example similarly showed better suppression of transmission loss than the comparative example.

[0042] The results of plotting the root mean square slope (Sdq) on the horizontal axis and the transmission loss on the vertical axis are shown in Figure 1. Furthermore, the result of calculating the approximate line using the linear approximation function of Microsoft® Excel is also shown in Figure 1. The approximation formula and the squared value of R are also displayed. The closer the squared value of R is to 1, the better the approximate line represents the trend of the experimental data.

[0043] Based on the approximation line in Figure 1, the transmission loss is estimated for cases where the root mean square slope (Sdq) is 0.100 and 0.120, as shown in Table 2. This shows that the transmission loss is suppressed when the root mean square slope (Sdq) is 0.100 and 0.120.

[0044] [Table 2]

Claims

1. A rolled copper foil having a root mean square slope (Sdq) of 0.120 or less on at least one surface, and a thickness of 5 to 300 μm.

2. The rolled copper foil according to claim 1, wherein the root mean square slope (Sdq) on at least one of the surfaces is 0.100 or less.

3. The rolled copper foil according to claim 1 or 2, further comprising a surface treatment layer on at least one of the aforementioned surfaces.

4. A laminate comprising rolled copper foil and a resin substrate as described in claim 1 or 2.

5. A flexible printed circuit board using the laminate described in claim 4.

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