JP2023134352A - Copper foil, laminate, and flexible printed wiring board - Google Patents

Abstract

To provide a copper foil having excellent electrical conductivity as a high frequency circuit copper foil.SOLUTION: A copper foil has a developed interfacial area ratio (sdr) of 0.0030 or less in at least one of the surfaces.SELECTED DRAWING: Figure 1

Description

The present invention relates to a copper foil, a laminate, and a flexible printed wiring board. In particular, the present invention relates to a copper foil having excellent conductivity as a copper foil for high-frequency circuits, and a laminate and a flexible printed wiring board including such a copper foil.

Printed wiring boards have made great progress over the past half century and are now used in almost all electronic devices. In recent years, as electronic devices have become smaller and the need for higher performance has increased, components have become more densely packaged and signals have higher frequencies, and printed wiring boards are required to have excellent high frequency support.

High-frequency circuit boards are required to reduce transmission loss in order to ensure the quality of output signals. Transmission loss mainly consists of dielectric loss caused by the resin (board side) and conductor loss caused by the conductor (copper foil side). Dielectric loss decreases as the dielectric constant and dielectric loss tangent of the resin decrease. In high-frequency signals, the main cause of conductor loss is that as the frequency increases, the cross-sectional area through which current flows decreases due to the skin effect, in which current flows only on the surface of the conductor, and the resistance increases.

As a technique aimed at reducing the transmission loss of copper foil for high-frequency circuits, for example, Patent Document 1 (Japanese Patent No. 4161304) discloses that silver or silver alloy is coated on one or both sides of the surface of the metal foil. However, a metal foil for high frequency circuits is disclosed, in which a coating layer other than silver or silver alloy is formed on the silver or silver alloy coating layer to be thinner than the silver or silver alloy coating layer. According to this document, it is stated that it is possible to provide a metal foil with reduced loss due to the skin effect even in an ultra-high frequency region such as that used in satellite communications.

Furthermore, in Patent Document 2 (Patent No. 4704025), the integrated intensity (I ₍₂₀₀₎ ) of the (200) plane determined by X-ray diffraction on the rolled surface after recrystallization annealing of rolled copper foil is Compared to the integrated intensity (I ₀₍₂₀₀₎ ) of the (200) plane determined by X-ray diffraction of powdered copper, I ₍₂₀₀₎ /I ₀₍₂₀₀₎ > 40, and the rolled surface was roughened by electrolytic plating. The arithmetic mean roughness (hereinafter referred to as Ra) of the roughened surface after the treatment is 0.02 μm to 0.2 μm, and the ten point average roughness (hereinafter referred to as Rz) is 0.1 μm to 1. A roughened rolled copper foil for high frequency circuits, which has a thickness of 5 μm and is a material for printed circuit boards, is disclosed. According to this document, it is stated that a printed circuit board that can be used at high frequencies exceeding 1 GHz can be provided.

Furthermore, Patent Document 3 (Japanese Unexamined Patent Publication No. 2004-244656) describes an electrolytic copper foil characterized in that a part of the surface of the copper foil has an uneven surface consisting of knob-like protrusions with a surface roughness of 2 μm to 4 μm. is disclosed. And, according to this, it is described that an electrolytic copper foil having excellent high frequency transmission characteristics can be provided.

Further, Patent Document 4 (Japanese Patent Application Laid-open No. 2017-193778) describes a copper foil having a roughening treatment layer, in which the roughening treatment layer has a primary particle layer, and the surface roughness of the surface on the primary particle layer side is A copper foil is disclosed in which Ra is 0.12 μm or less and the average particle size of primary particles in the primary particle layer is 0.10 to 0.25 μm. By forming such a predetermined roughened particle layer and controlling the surface roughness Ra of the surface on the side of the roughened particle layer and the average particle size of the roughened particles, transmission when used in a high frequency circuit board can be improved. It is described that it is effective in suppressing loss.

Patent No. 4161304 Patent No. 4704025 Japanese Patent Application Publication No. 2004-244656 Japanese Patent Application Publication No. 2017-193778

In recent years, with further advances in signal frequencies, there has been an increasing need to manufacture printed wiring boards with better transmission characteristics in high frequency ranges such as 60 GHz and above, and further improvements to the copper foil that is the raw material for these boards are required. . In particular, as the signal frequency increases, the conductivity of copper foil decreases due to the skin effect, which becomes a factor in worsening transmission loss. It may be difficult to respond to In particular, the invention according to Patent Document 4 focuses on the fact that the roughness of the copper foil surface is the main cause of conductor loss, and that the smaller the roughness, the lower the transmission loss. This is an excellent invention in that it can both suppress transmission loss and achieve adhesion to the resin by controlling the Ra and the average particle size of the roughening particles, but the conditions of the roughening treatment must be strictly controlled. There is also the problem of not having to do so. Furthermore, due to the presence of roughening particles, an increase in resistance due to the skin effect as described above is unavoidable.

The present invention was completed in view of the above problems, and in one embodiment, an object of the present invention is to provide a copper foil having excellent conductivity as a copper foil for high frequency circuits. Moreover, an object of the present invention, in another embodiment, is to provide a laminate including such a copper foil.

As a result of extensive studies, the present inventor has found that controlling the developed interface area ratio (sdr) is an effective characteristic of copper foil for suppressing the above-mentioned skin effect and suppressing the decrease in conductivity during high frequency transmission. I discovered that. The present invention was completed based on the above findings, and is exemplified below.
[1]
A copper foil having a developed interface area ratio (sdr) of at least one surface of 0.0030 or less.
[2]
The copper foil according to [1], wherein the developed interface area ratio (sdr) on the at least one surface is 0.0025 or less.
[3]
The copper foil according to [1] or [2], further comprising a surface treatment layer on 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].

According to one embodiment of the present invention, it is possible to provide a copper foil having excellent electrical conductivity as a copper foil for high frequency circuits. Moreover, according to another embodiment of the present invention, a laminate including such a copper foil can be provided.

This is a graph plotting sdr on the horizontal axis and conductivity on the vertical axis based on Examples and Comparative Examples.

Next, embodiments of the present invention will be described. It is understood that the present invention is not limited to the following embodiments, and that design changes, improvements, etc. may be made as appropriate based on the common knowledge of those skilled in the art without departing from the spirit of the present invention. Should.

(Composition of copper foil)
There are no particular limitations on the form of copper foil that can be used in this embodiment. Further, typically, the copper foil used in the present invention may be either an electrolytic copper foil or a rolled copper foil. Generally, electrolytic copper foil is manufactured by electrolytically depositing copper onto a titanium or stainless steel drum from a copper sulfate plating bath, and rolled copper foil is manufactured by repeatedly undergoing plastic working with rolling rolls and heat treatment. Rolled copper foil is often used in applications that require flexibility.

Copper foil materials include high-purity copper such as tough pitch copper and oxygen-free copper that are commonly used as conductor patterns for printed wiring boards, as well as copper containing Sn, copper containing Ag, P, Cr, Zr, or Mg. Copper alloys such as Corson-based copper alloys containing copper alloys such as Ni and Si can also be used. Note that in this specification, when the term "copper foil" is used alone, it also includes copper alloy foil.

Further, the thickness of the copper foil does not need to be particularly limited, but for example, 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. It is.

The copper foil of this embodiment can be suitably used for high frequency circuit applications. Here, the high frequency circuit is defined as a circuit whose signal frequency transmitted through the circuit is 60 GHz or higher.

The copper foil of this embodiment has a developed interface area ratio (sdr) of at least one surface of 0.0030 or less. The developed interface area ratio is an index showing how much the developed area of the defined region increases with respect to the area of the defined region, and the sdr of a completely flat surface with no unevenness is 0. Since the surface is uneven and the surface area (developed area) is larger than that of a flat surface, the sdr is larger than 0. When sdr becomes large, the conductivity at high frequencies decreases due to the skin effect described above, so it is preferably small. From this viewpoint, the developed interface area ratio (sdr) is preferably 0.0028 or less, more preferably 0.0025 or less, and even more preferably 0.0020 or less. Although the lower limit of sdr is not limited from the viewpoint of conductivity, it is assumed that scratches on the surface of the copper foil become more noticeable, so it is desirable that it is 0.0001 or more, more preferably 0.0002 or more.

Arithmetic mean height (Sa) and maximum height (Sz) are more common indicators of surface roughness, but they also measure the degree of increase in transmission distance when a signal flows only near the copper foil surface due to the skin effect. It is considered unsuitable for representation. This is because the maximum height (Sz) is an index whose value fluctuates greatly due to singular points such as scratches scattered on the surface of the copper foil, and it is an index that represents the characteristics when a signal flows across the entire surface of the copper foil in a high-frequency circuit. I think it is inappropriate as such. Furthermore, the arithmetic mean height (Sa) is considered inappropriate because there is a concern that fine undulations that contribute to an increase in transmission distance will be hidden by larger undulations. Therefore, we believe that the developed interface area ratio (sdr), which directly represents the degree of increase in transmission distance when a signal flows only near the surface of the copper foil due to the skin effect, is a suitable index that shows the essence.

As described below, the developed interface area ratio (sdr) is calculated using a Keyence laser microscope VK-X1000 (controller part)/1050 (head part) or an equivalent device in accordance with ISO25178-2:2012. Calculate after measuring the roughness data of the foil surface.

Moreover, at least one surface of the copper foil can be provided with a surface treatment layer. The surface treatment layer may be a roughening treatment layer. Roughening treatment is usually performed to improve the peel strength of copper foil after lamination on the surface of the copper foil that will be bonded to the resin substrate, that is, on the surface treatment layer side. A process that forms lump-like electrodepositions on the surface. Although the electrolytic copper foil has irregularities at the time of manufacture, the roughening treatment can strengthen the convex portions of the electrolytic copper foil and make the irregularities even larger. The roughening treatment can be performed, for example, by forming roughening particles from copper or a copper alloy. The roughening treatment may be fine. The roughening treatment layer is a layer made of any single substance selected from the group consisting of copper, nickel, cobalt, phosphorus, tungsten, arsenic, molybdenum, chromium, and zinc, or an alloy containing one or more of them. Good too. Further, after forming roughening particles using copper or a copper alloy, a roughening treatment can also be performed to provide secondary particles or tertiary particles using nickel, cobalt, copper, zinc alone or an alloy, or the like. Furthermore, one or more layers selected from the group consisting of a heat-resistant layer, a rust-proofing layer, a chromate-treated layer, and a silane coupling-treated layer may be formed on the surface of the roughened layer.

In addition to the roughening layer, the surface treatment layer may be one or more layers selected from the group consisting of a heat resistant layer, a rust preventive layer, a chromate layer, and a silane coupling layer. These layers can be provided using known techniques. In addition, as will be described later, the copper foil of the present invention can control the developed interface area ratio (sdr) described above without surface treatment, so in the sense that the manufacturing method is simple, in one embodiment, Copper foil does not include a surface treatment layer.

A laminate can be manufactured by bonding the copper foil of the present invention to a resin substrate. The resin substrate is not particularly limited as long as it has characteristics applicable to printed wiring boards, etc., but examples include polyester films such as polyethylene terephthalate (PET), polyimide films, liquid crystal polymer (LCP) films, and fluorine resins. Film etc. can be used.

The bonding method is to use an adhesive on a base material such as a polyimide film, or to bond it to a rolled copper foil under high temperature and high pressure without using an adhesive, or to apply and dry a polyimide precursor. - A laminate can be manufactured by performing curing etc.

Moreover, a printed wiring board, particularly a flexible printed wiring board, can be constructed using the laminate. Therefore, in another aspect, the present invention discloses a method for manufacturing a printed wiring board, particularly a flexible printed wiring board, using the rolled copper foil of the present invention and the laminate of the present invention.

(Production method)
The method of manufacturing the copper foil of the present invention is not particularly limited as long as the above-mentioned developed interface area ratio (sdr) can be controlled within the range of the present invention. It is possible to adopt a method of appropriately adjusting the oil film thickness equivalent of the final rolling pass in the rolling process.

In the method for manufacturing rolled copper foil of this embodiment, raw materials are first melted in a melting furnace to obtain a molten metal having a desired composition. This molten metal is then cast into an ingot. Thereafter, hot rolling, cold rolling, and annealing are performed as appropriate to finish the foil into a foil having a predetermined thickness. After the heat treatment, the surface may be pickled, polished, etc. in order to remove the surface oxide film generated during the heat treatment. In the final cold rolling, the heat-treated material is repeatedly passed through a rolling mill to achieve a predetermined thickness. In the method for manufacturing rolled copper foil of this embodiment, it is important to perform the final rolling pass with an oil film thickness equivalent of 10,000 to 40,000 in the final rolling pass of the final cold rolling step.

By controlling the oil film thickness equivalent, the developed interface area ratio of the copper foil surface can be suppressed. When the oil film thickness equivalent exceeds 40,000, the effect of flattening the surface of the copper foil becomes weak, and the developed interface area ratio becomes insufficiently reduced. On the other hand, when the oil film thickness equivalent is less than 10,000, the oil film between the copper foil and the rolling roll becomes thinner, so scratches and dirt on the surface of the rolling roll are easily transferred to the copper foil surface, increasing the developed interface area ratio. Sometimes I end up doing it.

Here, the oil film thickness equivalent is defined by the following formula.
Oil film thickness equivalent = {(rolling oil viscosity [cSt]) x (inlet side passing speed [mm/s] + roll circumferential speed [mm/s])}/{(roll bite angle [rad]) x (Rolling load per unit area [kg/mm ² ])}

The rolling oil viscosity was measured in accordance with JIS K2283. The sheet passing speed on the inlet side was calculated from the circumferential speed of the take-up reel on the outlet side, taking into consideration the rolling degree of the rolling pass. The roll circumferential speed was calculated from the circumferential speed of the take-up reel on the exit side, assuming that no slip occurred between the roll and the material. Further, the bite angle of the rolls and the rolling load per unit area were calculated using the following formulas. For the Young's modulus and Poisson's ratio of the roll, the values listed in the roll manufacturer's catalog and the literature values of the roll material were used, respectively. The radius of the roll was determined by measuring the diameter with a roll diameter measuring machine capable of measuring in units of 0.0005 mm. The rolling load was calculated from the pressure measured by the oil pressure sensor of the oil pressure cylinder of the rolling mill using the diameter and number of cylinders. The reduction amount was calculated from the material thickness before and after threading. The material plate thickness can be measured, for example, in accordance with the mass thickness measurement method of JIS C6515.
Roll bite angle = {(reduction amount [mm])/(roll flat radius [mm])} ^0.5
Rolling load per unit area = (rolling load [kg]) / {(average plate 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 plate width [mm]) x (Reduction amount [mm])}]
Contact arc length = {(reduction amount [mm]) x (roll flat radius [mm])} ^0.5
Note that the oil film thickness equivalent values in Table 1 are rounded to the nearest 100.

In order to control the oil film thickness equivalent, known methods such as using low viscosity rolling oil or slowing down the plate passing speed on the entry side may be used.

Hereinafter, the present invention will be specifically explained with reference to Examples, but the explanation here is for the purpose of mere illustration and is not intended to be limited thereto.

First, a copper ingot having the copper proportion shown in Table 1 is produced, hot rolled, and then cold rolled and annealed in an annealing furnace set at a temperature of 300 to 800°C one or more times. Cold rolling was performed to obtain a rolled plate having a thickness of 0.1 to 1.0 mm. This rolled plate was annealed and recrystallized in an annealing furnace set at a temperature of 300 to 800°C, and finally cold rolled. 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 in the final rolling pass was changed as shown in Table 1, and copper foils of Examples and Comparative Examples were manufactured.

Various evaluations were performed as follows for each of the samples of Examples and Comparative Examples produced as described above. The results are shown in Table 1.

(Measurement of developed interface area ratio)
Shape measurement was performed using a Keyence laser microscope VK-X1000 (controller part)/1050 (head part). Thereafter, the developed interface area ratio sdr was measured using the analysis software of the Keyence laser microscope VK-X1000 (controller part)/1050 (head part). At this time, an area of 250 μm x 200 μm (specifically, 50,000 μm ² ) was measured at ten arbitrary locations using a 50x objective lens in a laser microscope, and the developed interface area ratio sdr at each location was calculated. The arithmetic mean value of the developed interface area ratio sdr obtained at 10 locations was taken as the value of the developed interface area ratio sdr. In order to eliminate the effects of slope and waviness on the surface of the workpiece when placing the workpiece in shape measurement, plane tilt correction and waviness removal (cutoff) are performed on the measurement area as surface shape correction during analysis using analysis software. The developed interface area ratio sdr was calculated after conducting the test (wavelength: 0.08 mm). Note that the ambient temperature for measuring the developed interface area ratio sdr using a laser microscope was 20 to 25°C. In addition, the main setting conditions for the laser microscope and analysis software are as follows.
<Shape measurement conditions>
Objective lens: CF IC EPI Plan 50X
Optical zoom magnification: 1x Average number of times: 1 time Z-axis mode: Recommended setting (R)
Shape measurement mode: Easy measurement (Scan mode: Laser confocal)
<Analysis conditions>
Image processing: Surface shape correction (plane tilt correction, waviness 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

(Measurement of high frequency conductivity)
The samples of each example and comparative example were etched into a disk shape with D=15.0 mm, and then annealed at 200° C. for 1 hr, simulating the temperature conditions during lamination of copper-clad laminates. In addition, in order to prevent the formation of an oxide film on the sample surface, annealing was performed in an Ar atmosphere. Thereafter, the conductivity in the high frequency range was determined using the balanced disc resonator method (BCDR method). More specifically, "Broadband Conductivity Measurement Technique at Millimeter-Wave Bands Using a Balanced-Type Circular Disk Resonator" (IEEE Transactions on Microwave Theory and Techniques, Volume: 69, Issue: 1, Jan 2021, pages: 861-873) The conductivity was determined by the method described in IV-C. For each sample, measurements were performed 10 times starting from attaching the sample to the measuring device. Table 1 shows the average value of the 10 measurements as the conductivity. The electrical conductivity in Table 1 indicates that the higher the value, the better the electrical conductivity. Further, the frequencies were measured at three points: 68 GHz, 81 GHz, and 107 GHz (all are rounded down to the nearest whole number).

(Consideration)
In the example, since the developed interface area ratio sdr on the surface was 0.0030 or less, a decrease in conductivity at high frequencies could be suppressed. In particular, the copper foils of the examples were able to suppress a decrease in conductivity even as the frequency increased, indicating that they have excellent conductivity as copper foils for high-frequency circuits. On the other hand, in the comparative example, since the oil film thickness equivalent of the final rolling pass in the final cold rolling was high, the developed interface area ratio exceeded 0.0030, and the electrical conductivity was lower than that of the example.

Note that the results plotted with sdr as the horizontal axis and conductivity as the vertical axis are shown in FIG. Further, the results of calculating an approximate straight line using the approximate straight line approximation function of Microsoft (registered trademark) Excel are displayed in FIG. The approximate equation of the straight line and the R-square value are also displayed. The closer the R-squared value is to 1, the more appropriately the approximate straight line represents the tendency of the experimental data. Moreover, each approximate straight line was as follows in the case of 68 GHz, 81 GHz, and 107 GHz.
For 68GHz (conductivity) = -14.2669 x (sdr) + 5.9763
For 81 GHz (conductivity) = -19.4499 x (sdr) + 5.8130
For 107 GHz (conductivity) = -13.6501 x (sdr) + 5.4665

Based on the approximate straight line in Figure 1, the electrical conductivity when sdr is 0.0030 and 0.0025 is estimated as shown in Table 2, and the electrical conductivity is excellent when sdr is 0.0030 and 0.0025. I can understand that.

Claims (6)

A copper foil having a developed interface area ratio (sdr) of at least one surface of 0.0030 or less. The copper foil according to claim 1, wherein the developed interface area ratio (sdr) on the at least one surface is 0.0025 or less. The copper foil according to claim 1 or 2, further comprising a surface treatment layer on the at least one surface. The copper foil according to claim 1 or 2, which is a rolled copper foil. A laminate comprising the copper foil according to claim 1 or 2 and a resin substrate. A flexible printed wiring board using the laminate according to claim 5.