Electrolytic copper foil and method for manufacturing the same

Abstract

To provide a method of producing electrolytic copper foil, capable of reducing the transmission loss without relying on the surface roughness of the copper foil and the surface treatment of the copper foil, and electrolytic copper foil produced by the method.SOLUTION: The method of producing electrolytic foil comprises an electrolysis step of electrolyzing an electrolytic solution to produce electrolytic foil of a crystal diameter of 0.4 μm or less, and a heating step of heating the electrolytic foil to change the crystal diameter to 0.6 μm or larger. The electrolytic foil produced by the method has a crystal diameter of 0.6 μm or more and surface roughness Sa of 0.4 μm or below.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an electrolytic copper foil and a method for producing the same.

Background Art

[0002] Electrolytic copper foils are excellent in mass productivity compared to piezoelectric copper foils and have relatively low production costs, so they are used in various applications such as printed wiring boards. Conventionally, the Internet, which was mainly connected to IT-related devices such as personal computers and servers, is expanding to all kinds of things such as clothing (wearable devices), automobiles (smart cars), and houses (smart homes). Along with this, higher speed and larger capacity of communication are required.

[0003] To increase the speed or capacity of communication, the frequency of an electrical signal may be increased. However, as the frequency of the electrical signal increases, the loss of signal power (transmission loss) increases, making it difficult to read data. Transmission loss in an electronic circuit generally consists of two parts: loss due to copper foil (conductor loss) and loss due to resin substrate (dielectric loss). Conductor loss is due to the skin effect seen in an alternating current signal and is strongly affected by the roughness of the copper foil surface. This tendency becomes more prominent as the frequency of the alternating current signal increases. Therefore, in order to reduce conductor loss, it is desirable to reduce the surface roughness of the copper foil.

[0004] Dielectric loss is affected by an adhesive that adheres between the copper foil and the resin substrate. Therefore, it is desirable to adhere the copper foil and the resin substrate without using an adhesive. In order to adhere the copper foil and the resin substrate without using an adhesive, the adhesive surface of the copper foil may be roughened to enhance the adhesiveness between the copper foil and the resin substrate by the anchor effect. However, as described above, roughening the surface may increase conductor loss, especially in the high-frequency range. Thus, regarding the surface roughness of the copper foil, transmission loss and adhesion are in a trade-off relationship.

[0005] Patent Document 1 describes a surface-treated copper foil in which a roughening treatment layer, a rust-preventive treatment layer, and a silane coupling agent treatment layer are laminated in this order relative to the copper foil on at least one surface of the copper foil, wherein the value of the interface development area ratio Sdr, which is a composite parameter of the three-dimensional surface properties measured from the surface of the silane coupling agent treatment layer, is in the range of 8 to 140%, the value of the root mean square surface gradient Sdq is in the range of 25 to 70°, and the value of the surface property aspect ratio Str, which is a spatial parameter of the three-dimensional surface properties measured from the surface of the silane coupling agent treatment layer, is 0.25 to 0.79.

[0006] Patent Document 2 describes a material in which the carbon content is 5 ppm or less, the sulfur content is 3 ppm or less, the oxygen content is 5 ppm or less, the nitrogen content is 0.5 ppm or less, and the total content of carbon, sulfur, oxygen, nitrogen, and hydrogen is 15 ppm or less, and the number of crystal grains is 8.0 to 12.0 grains / μm 2 This is an electrolytic copper foil, which, when heated at 150°C for 1 hour, has a crystal grain count of 0.6 to 1.0 grains / μm 2 The electrolytic copper foil that changes to this is described. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 6462961 [Patent Document 2] International Publication No. 2020 / 121894 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Patent Document 1 describes that by surface treating electrolytic copper foil and sequentially laminating a roughening treatment layer, a rust prevention treatment layer, and a silane coupling agent treatment layer, and by keeping the interface development area ratio Sdr, the root mean square surface gradient Sdq, and the surface property aspect ratio Str, which are parameters representing the three-dimensional surface properties measured from the surface of the silane coupling agent treatment layer, within a predetermined range, excellent transmission loss at high frequencies can be achieved. However, there is a problem in that it is not easy to perform a surface treatment that satisfies all three parameters representing the three-dimensional properties of the copper foil surface.

[0009] Therefore, the present invention aims to provide an electrolytic copper foil and a method for manufacturing the same that can improve transmission loss without relying on the surface roughness of the copper foil or surface treatment of the copper foil. [Means for solving the problem]

[0010] To achieve the above objective, the present invention, in one aspect, provides a method for manufacturing electrolytic copper foil, comprising: an electrolytic step of obtaining electrolytic copper foil with a crystal grain size of 0.4 μm or less by electrolyzing an electrolyte; and a heat treatment step of changing the crystal grain size of the electrolytic copper foil to 0.6 μm or more by heating the electrolytic copper foil.

[0011] In the heat treatment step, it is preferable to heat the electrolytic copper foil to 100 to 500°C.

[0012] Preferably, the surface roughness Sa of the electrolytic copper foil before and after the heat treatment process is 0.4 μm or less.

[0013] Furthermore, in another embodiment of the present invention, an electrolytic copper foil is provided, wherein the crystal grain size is 0.6 μm or more and the surface roughness Sa is 0.4 μm or less. [Effects of the Invention]

[0014] Thus, according to the present invention, by heat-treating an electrolytic copper foil with a crystal grain size of 0.4 μm or less at a predetermined temperature, the crystal grain size of the electrolytic copper foil is coarsened to 0.6 μm or more. Surprisingly, this makes it possible to improve transmission loss without relying on the surface roughness of the copper foil or any surface treatment applied to the copper foil. [Brief explanation of the drawing]

[0015] [Figure 1] This is a scanning electron microscope (SEM) image showing a cross-section of the electrolytic copper foil before heat treatment in Example 1. [Figure 2] This is an SEM image showing a cross-section of the electrolytic copper foil after heat treatment in Example 1. [Modes for carrying out the invention]

[0016] An embodiment of the electrolytic copper foil and its manufacturing method according to the present invention is described below. However, the present invention is not limited to the embodiment described below.

[0017] The method for manufacturing electrolytic copper foil according to this embodiment mainly comprises an electrolytic step of obtaining electrolytic copper foil with a crystal grain size of 0.4 μm or less by electrolyzing an electrolyte solution, and a heat treatment step of changing the crystal grain size of the electrolytic copper foil to 0.6 μm or more by heating the electrolytic copper foil.

[0018] The electrolytic process can be carried out using a general electrolytic apparatus for forming electrolytic copper foil. For example, an apparatus can be used that comprises an electrodeposition drum (cathode) that rotates with approximately half of it housed in an electrolytic cell, and an insoluble electrode plate (anode) positioned in the electrolytic cell at a predetermined distance from the surface of the electrodeposition drum. The surface of the electrodeposition drum is formed from, for example, titanium, nickel, chromium, or stainless steel or its alloys. The insoluble electrode plate is formed from, for example, lead, platinum-coated titanium, iridium-coated titanium, etc.

[0019] As the electrolytic solution supplied to the electrolytic cell, for example, an aqueous solution in which a copper raw material is dissolved in sulfuric acid can be used. The sulfuric acid concentration is preferably 40 to 220 g / L, more preferably 40 to 100 g / L. The copper concentration in the electrolytic solution is preferably 60 to 400 g / L in terms of copper sulfate (CuSO4·5H2O), more preferably 200 to 400 g / L. In addition, various additives usually used in the electrolytic solution for forming electrolytic copper foil, for example, leveling agents, brightening agents, chloride ions, etc., as long as they are additives that do not form eutectic in the copper foil, can be used.

[0020] The current density in the electrolysis process is not particularly limited, but 0.5 to 100 A / dm 2 is preferable, and 30 to 50 A / dm 2 is more preferable. The liquid temperature of the electrolytic solution is not particularly limited, but 10 to 50 °C is preferable, and 15 to 30 °C is more preferable.

[0021] The thickness of the electrolytic copper foil obtained by the electrolysis process is preferably 2 to 50 μm, more preferably 6 to 35 μm. If the thickness is too thin compared to 6 μm, it may be difficult to handle the electrolytic copper foil. On the other hand, if the thickness is too thick compared to 35 μm, it may be disadvantageous for forming fine patterns when used for applications such as printed wiring boards. Since the thickness of the electrolytic copper foil is generally proportional to the electrolysis time, the electrolysis time can be adjusted according to the desired thickness of the electrolytic copper foil.

[0022] In this embodiment, the electrolytic copper foil obtained by the electrolytic process has a crystal grain size of 0.4 μm or less. The crystal grain size of the electrolytic copper foil is measured by cutting the electrolytic copper foil lengthwise and observing the cross-section. Observation can be performed using a scanning electron microscope (SEM) or the like. Electrolytic copper foil typically has a glossy surface ("S surface") that was in contact with the cathode and a copper-deposited surface ("M surface") on the opposite side. The crystal grain size of the electrolytic copper foil differs between the S surface and the M surface. For example, the crystal grain size on the S surface side (e.g., at a depth of 1 μm from the S surface) tends to be about 0.1 μm lower than the crystal grain size on the M surface side (e.g., at a depth of 1 μm from the M surface). In this embodiment, the electrolytic copper foil obtained by the electrolytic process preferably has a crystal grain size of 0.38 μm or less on the S surface side, and more preferably 0.28 μm or less. Furthermore, the grain size on the M-plane side is preferably 0.40 μm or less, and more preferably 0.38 μm or less. The lower limit of the grain size is not particularly limited, but for example, it is 0.1 μm or more.

[0023] Furthermore, the surface roughness Sa of the electrolytic copper foil usually differs slightly between the S side and the M side. For example, the surface roughness Sa of the S side tends to be about 0.1 μm lower than the surface roughness Sa of the M side. In the electrolytic copper foil obtained by the electrolytic process of this embodiment, the surface roughness Sa of the S side is preferably 0.01 to 0.4 μm, and more preferably 0.1 to 0.3 μm. The surface roughness Sa of the M side is preferably 0.01 to 0.4 μm, and more preferably 0.15 to 0.35 μm.

[0024] Surface roughness Sa, also known as arithmetic mean height, is measured according to ISO 25178 and represents the average of the absolute differences in height at each point relative to the average surface. A higher surface roughness Sa value indicates a rougher surface.

[0025] In the heat treatment process, the crystal grain size of the electrolytic copper foil obtained in the electrolytic process is coarsened to 0.6 μm or larger by heating it. The heating temperature of the electrolytic copper foil in the heat treatment process is preferably, for example, 100 to 500°C. More preferably, it is 200 to 500°C, and even more preferably, 300 to 500°C. By heating the temperature to 100°C or higher, the crystal grain size of the electrolytic copper foil can be significantly coarsened as described above. On the other hand, if the heating temperature is too high, oxidation will progress, and there is a problem that heat treatment under an inert gas atmosphere will be required, so it is preferable to keep it below 500°C. The heating time is not particularly limited as long as it is a sufficient time for the crystal grains of the copper foil to recrystallize, but for example, 30 minutes or more is preferred.

[0026] In electrolytic copper foil whose grain size has been coarsened by such a heat treatment process, the grain size on the S-side is preferably 0.60 μm or larger, more preferably 0.63 μm or larger, and even more preferably 0.66 μm or larger. Furthermore, the grain size on the M-side is preferably 0.65 μm or larger, more preferably 0.70 μm or larger, and even more preferably 0.75 μm or larger. There is no particular upper limit to the grain size, but for example, it is 2.0 μm or smaller.

[0027] Furthermore, by coarsening the crystal grain size of the electrolytic copper foil to 0.6 μm or more through this heat treatment process, it is surprisingly possible to significantly reduce transmission loss compared to before the heat treatment process, regardless of the surface roughness or surface treatment of the electrolytic copper foil.

[0028] Although the surface roughness Sa of the electrolytic copper foil tends to become slightly rougher after heat treatment, it does not undergo significant changes like those seen in crystal grain size. For example, the surface roughness Sa of the S surface of the electrolytic copper foil after the heat treatment process is preferably 0.01 to 0.4 μm, and more preferably 0.1 to 0.3 μm. Similarly, the surface roughness Sa of the M surface is preferably 0.01 to 0.4 μm, and more preferably 0.15 to 0.35 μm. Having the surface roughness Sa of the electrolytic copper foil after the heat treatment process within this range allows for adhesion between the foil and the resin substrate due to the anchoring effect, while also suppressing transmission loss due to the skin effect.

[0029] The surface of the electrolytic copper foil obtained in this way may be provided with a roughening treatment layer, a rust prevention layer, a chromate treatment layer, a silane coupling treatment layer, etc., as needed. The surface of the electrolytic copper foil on which these layers are provided may be the S surface or the M surface. [Examples]

[0030] The present invention will be described in more detail below with reference to examples and comparative examples. However, the present invention is not limited to the following examples.

[0031] [Example 1] Electrolytic copper foil was manufactured using an electrolytic apparatus with an insoluble electrode as the anode and a titanium electrode as the cathode. The electrolyte was an aqueous solution containing 100 g / L sulfuric acid, 100 g / L copper sulfate, and a small amount of leveling agent. The current was then measured at room temperature at 45 A / dm³. 2 Copper was electrodeposited onto the surface of the titanium electrode by electrolytic treatment at the specified current density for 75 seconds. The generated copper was peeled off to obtain a copper foil with a thickness of 12 μm.

[0032] The surface roughness Sa of the obtained copper foil was measured in accordance with ISO 25178 using a surface roughness measuring instrument (Tokyo Seimitsu Co., Ltd., model number: SURFCOM 1400G). As a result, the surface roughness Sa of the side in contact with the titanium electrode (S side) was 0.16 μm, and the surface roughness Sa of the opposite side (M side) was 0.28 μm.

[0033] Furthermore, in order to measure the grain size of this electrolytic copper foil, the copper foil was cut lengthwise, and the cross-section was observed using a scanning electron microscope (SEM) (Hitachi High-Tech, "SU1510"). The SEM image is shown in Figure 1. In the cross-sectional SEM image shown in Figure 1, a line was drawn parallel to the surface at a depth of 1 μm from the surface of the copper foil, and the number of grain boundaries crossing a 10 μm length portion of this parallel line was measured. The length of the parallel line for each grain boundary was calculated and defined as the grain size. As a result, the grain size was 0.27 μm on the S-plane and 0.38 μm on the M-plane.

[0034] Next, the electrolytic copper foil was heated at 400°C for 1 hour in an air atmosphere and then air-cooled to room temperature using a blower. The sample size of the electrolytic copper foil was 210 mm × 210 mm, and the equipment used for heating was a Denken Heidental tabletop muffle furnace (model: KDF-S100). The surface roughness Sa and grain size of the electrolytic copper foil after cooling to room temperature were measured in the same manner as above. As a result, the surface roughness Sa of the S side of the heat-treated electrolytic copper foil was 0.15 μm, and the surface roughness Sa of the M side was 0.35 μm. Figure 2 shows an SEM image of the cross-section of the heat-treated electrolytic copper foil. Based on this cross-sectional SEM image, the grain size was measured to be 0.66 μm on the S side and 0.77 μm on the M side, confirming the coarsening of the grain size due to heat treatment.

[0035] Next, the transmission loss of the electrolytic copper foil after this heat treatment was evaluated. A copper-clad laminate was obtained by laminating a 50 μm thick LCP film (Chiyoda Integre, Pericure LCP) to the S side of the electrolytic copper foil. Then, a microstrip line with a line width of 100 μm and a length of 100 mm was formed, and the transmission loss S21 at frequencies of 10 to 40 GHz was measured using a network analyzer (E8363B, KEYSIGHT). The results are shown in Table 1.

[0036] [Table 1]

[0037] [Example 2] A copper-clad laminate was fabricated using the same procedure as in Example 1, except that the copper foil surface bonded to the LCP film was the M-side. The transmission loss S21 of the microstrip line was then measured. The results are shown in Table 1.

[0038] [Comparative Example 1] A copper-clad laminate was fabricated using the same procedure as in Example 1, except that heat treatment was not performed on the electrolytic copper foil, and the transmission loss S21 of the microstrip line was measured. The results are shown in Table 1.

[0039] [Comparative Example 2] A copper-clad laminate was fabricated using the same procedure as in Example 2, except that heat treatment was not performed on the electrolytic copper foil, and the transmission loss S21 of the microstrip line was measured. The results are shown in Table 1.

[0040] As shown in Table 1, it was confirmed that the transmission loss S21 was significantly reduced at all frequencies from 10 to 40 GHz by performing heat treatment at 400°C for 1 hour to coarseen the crystal grain size of the electrolytic copper foil. In particular, at 28 GHz, which is expected to be used for high-speed transmission, the loss was reduced by approximately 0.2 to 0.3 dB regardless of whether the surface of the bonded electrolytic copper foil was the S-side or M-side, indicating that the transmission loss in high-frequency electronic circuits can be significantly improved.

Claims

1. An electrolytic process to obtain electrolytic copper foil with a crystal grain size of 0.4 μm or less by electrolyzing an electrolyte, A heat treatment step in which the electrolytic copper foil is heated at 300 to 500°C to change the crystal grain size of the electrolytic copper foil to a range of 0.6 μm to 2.0 μm. A method for producing electrolytic copper foil, including, A method for producing electrolytic copper foil in which the surface roughness Sa of the electrolytic copper foil before and after the heat treatment step is 0.4 μm or less.

2. Electrolytic copper foil having a crystal grain size in the range of 0.6 μm to 2.0 μm and a surface roughness Sa of 0.4 μm or less.

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